Reactions of hydrocarbons table. Characteristic chemical properties of hydrocarbons. Reaction mechanisms. The structure and properties of hydrocarbons

Chemical properties of alkanes

Alkanes (paraffins) are non-cyclic hydrocarbons, in the molecules of which all carbon atoms are connected only by single bonds. In other words, there are no multiple, double or triple bonds in the molecules of alkanes. In fact, alkanes are hydrocarbons containing the maximum possible number of hydrogen atoms, and therefore they are called limiting (saturated).

Due to saturation, alkanes cannot enter into addition reactions.

Since carbon and hydrogen atoms have fairly close electronegativity, this leads to the fact that the CH bonds in their molecules are extremely low polarity. In this regard, for alkanes, reactions proceeding according to the mechanism of radical substitution, denoted by the symbol S R, are more characteristic.

1. Substitution reactions

In reactions of this type, carbon-hydrogen bonds are broken.

RH + XY → RX + HY

Halogenation

Alkanes react with halogens (chlorine and bromine) under the action of ultraviolet light or with strong heat. In this case, a mixture of halogen derivatives with different degrees of substitution of hydrogen atoms is formed - mono-, di-tri-, etc. halogen-substituted alkanes.

On the example of methane, it looks like this:

By changing the halogen/methane ratio in the reaction mixture, it is possible to ensure that any particular methane halogen derivative predominates in the composition of the products.

reaction mechanism

Let us analyze the mechanism of the free radical substitution reaction using the example of the interaction of methane and chlorine. It consists of three stages:

1. initiation (or chain initiation) - the process of formation of free radicals under the action of energy from the outside - irradiation with UV light or heating. At this stage, the chlorine molecule undergoes a homolytic cleavage of the Cl-Cl bond with the formation of free radicals:

Free radicals, as can be seen from the figure above, are called atoms or groups of atoms with one or more unpaired electrons (Cl, H, CH 3 , CH 2, etc.);

2. Chain development

This stage consists in the interaction of active free radicals with inactive molecules. In this case, new radicals are formed. In particular, when chlorine radicals act on alkane molecules, an alkyl radical and hydrogen chloride are formed. In turn, the alkyl radical, colliding with chlorine molecules, forms a chlorine derivative and a new chlorine radical:

3) Break (death) of the chain:

Occurs as a result of the recombination of two radicals with each other into inactive molecules:

2. Oxidation reactions

Under normal conditions, alkanes are inert with respect to such strong oxidizing agents as concentrated sulfuric and nitric acids, permanganate and potassium dichromate (KMnO 4, K 2 Cr 2 O 7).

Combustion in oxygen

A) complete combustion with an excess of oxygen. Leads to the formation of carbon dioxide and water:

CH 4 + 2O 2 \u003d CO 2 + 2H 2 O

B) incomplete combustion with a lack of oxygen:

2CH 4 + 3O 2 \u003d 2CO + 4H 2 O

CH 4 + O 2 \u003d C + 2H 2 O

Catalytic oxidation with oxygen

As a result of heating alkanes with oxygen (~200 o C) in the presence of catalysts, a wide variety of organic products can be obtained from them: aldehydes, ketones, alcohols, carboxylic acids.

For example, methane, depending on the nature of the catalyst, can be oxidized to methyl alcohol, formaldehyde, or formic acid:

3. Thermal transformations of alkanes

Cracking

Cracking (from the English to crack - to tear) is a chemical process occurring at high temperature, as a result of which the carbon skeleton of alkane molecules breaks with the formation of alkene and alkane molecules with lower molecular weights compared to the original alkanes. For example:

CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3 → CH 3 -CH 2 -CH 2 -CH 3 + CH 3 -CH \u003d CH 2

Cracking can be thermal or catalytic. For the implementation of catalytic cracking, due to the use of catalysts, significantly lower temperatures are used compared to thermal cracking.

Dehydrogenation

The elimination of hydrogen occurs as a result of breaking the C-H bonds; carried out in the presence of catalysts at elevated temperatures. Dehydrogenation of methane produces acetylene:

2CH 4 → C 2 H 2 + 3H 2

Heating methane to 1200 ° C leads to its decomposition into simple substances:

CH 4 → C + 2H 2

Dehydrogenation of other alkanes gives alkenes:

C 2 H 6 → C 2 H 4 + H 2

When dehydrogenating n-butane, butene-1 and butene-2 ​​are formed (the latter in the form cis- and trance-isomers):

Dehydrocyclization

Isomerization

Chemical properties of cycloalkanes

The chemical properties of cycloalkanes with more than four carbon atoms in the cycles are generally almost identical to those of alkanes. For cyclopropane and cyclobutane, oddly enough, addition reactions are characteristic. This is due to the high tension within the cycle, which leads to the fact that these cycles tend to break. So cyclopropane and cyclobutane easily add bromine, hydrogen or hydrogen chloride:

Chemical properties of alkenes

1. Addition reactions

Since the double bond in alkene molecules consists of one strong sigma bond and one weak pi bond, they are quite active compounds that easily enter into addition reactions. Alkenes often enter into such reactions even under mild conditions - in the cold, in aqueous solutions and organic solvents.

Hydrogenation of alkenes

Alkenes are able to add hydrogen in the presence of catalysts (platinum, palladium, nickel):

CH 3 -CH \u003d CH 2 + H 2 → CH 3 -CH 2 -CH 3

Hydrogenation of alkenes proceeds easily even at normal pressure and slight heating. An interesting fact is that the same catalysts can be used for the dehydrogenation of alkanes to alkenes, only the dehydrogenation process proceeds at a higher temperature and lower pressure.

Halogenation

Alkenes easily enter into an addition reaction with bromine both in aqueous solution and in organic solvents. As a result of the interaction, initially yellow solutions of bromine lose their color, i.e. discolor.

CH 2 \u003d CH 2 + Br 2 → CH 2 Br-CH 2 Br

Hydrohalogenation

It is easy to see that the addition of a hydrogen halide to an unsymmetrical alkene molecule should theoretically lead to a mixture of two isomers. For example, when hydrogen bromide is added to propene, the following products should be obtained:

Nevertheless, in the absence of specific conditions (for example, the presence of peroxides in the reaction mixture), the addition of a hydrogen halide molecule will occur strictly selectively in accordance with the Markovnikov rule:

The addition of a hydrogen halide to an alkene occurs in such a way that hydrogen is attached to a carbon atom with a larger number of hydrogen atoms (more hydrogenated), and a halogen is attached to a carbon atom with a smaller number of hydrogen atoms (less hydrogenated).

Hydration

This reaction leads to the formation of alcohols, and also proceeds in accordance with the Markovnikov rule:

As you might guess, due to the fact that the addition of water to the alkene molecule occurs according to the Markovnikov rule, the formation of primary alcohol is possible only in the case of ethylene hydration:

CH 2 \u003d CH 2 + H 2 O → CH 3 -CH 2 -OH

It is by this reaction that the main amount of ethyl alcohol is carried out in the large-capacity industry.

Polymerization

A specific case of the addition reaction is the polymerization reaction, which, unlike halogenation, hydrohalogenation and hydration, proceeds through a free radical mechanism:

Oxidation reactions

Like all other hydrocarbons, alkenes burn easily in oxygen to form carbon dioxide and water. The equation for the combustion of alkenes in excess oxygen has the form:

C n H 2n + (3/2)nO 2 → nCO 2 + nH 2 O

Unlike alkanes, alkenes are easily oxidized. Under the action of an aqueous solution of KMnO 4 on alkenes, discoloration, which is a qualitative reaction to double and triple CC bonds in molecules of organic substances.

Oxidation of alkenes with potassium permanganate in a neutral or slightly alkaline solution leads to the formation of diols (dihydric alcohols):

C 2 H 4 + 2KMnO 4 + 2H 2 O → CH 2 OH–CH 2 OH + 2MnO 2 + 2KOH (cooling)

In an acidic environment, a complete cleavage of the double bond occurs with the transformation of the carbon atoms that formed the double bond into carboxyl groups:

5CH 3 CH=CHCH 2 CH 3 + 8KMnO 4 + 12H 2 SO 4 → 5CH 3 COOH + 5C 2 H 5 COOH + 8MnSO 4 + 4K 2 SO 4 + 17H 2 O (heating)

If the double C=C bond is at the end of the alkene molecule, then carbon dioxide is formed as the oxidation product of the extreme carbon atom at the double bond. This is due to the fact that the intermediate oxidation product, formic acid, is easily oxidized by itself in an excess of an oxidizing agent:

5CH 3 CH=CH 2 + 10KMnO 4 + 15H 2 SO 4 → 5CH 3 COOH + 5CO 2 + 10MnSO 4 + 5K 2 SO 4 + 20H 2 O (heating)

In the oxidation of alkenes, in which the C atom at the double bond contains two hydrocarbon substituents, a ketone is formed. For example, the oxidation of 2-methylbutene-2 ​​produces acetone and acetic acid.

The oxidation of alkenes, which breaks the carbon skeleton at the double bond, is used to establish their structure.

Chemical properties of alkadienes

Addition reactions

For example, the addition of halogens:

Bromine water becomes colorless.

Under normal conditions, the addition of halogen atoms occurs at the ends of the butadiene-1,3 molecule, while π bonds are broken, bromine atoms are attached to the extreme carbon atoms, and free valences form a new π bond. Thus, as if there is a "movement" of the double bond. With an excess of bromine, one more bromine molecule can be added at the site of the formed double bond.

polymerization reactions

Chemical properties of alkynes

Alkynes are unsaturated (unsaturated) hydrocarbons and therefore are capable of entering into addition reactions. Among the addition reactions for alkynes, electrophilic addition is the most common.

Halogenation

Since the triple bond of alkyne molecules consists of one stronger sigma bond and two weaker pi bonds, they are able to attach either one or two halogen molecules. The addition of two halogen molecules by one alkyne molecule proceeds by the electrophilic mechanism sequentially in two stages:

Hydrohalogenation

The addition of hydrogen halide molecules also proceeds by the electrophilic mechanism and in two stages. In both stages, the addition proceeds in accordance with the Markovnikov rule:

Hydration

The addition of water to alkynes occurs in the presence of ruthium salts in an acidic medium and is called the Kucherov reaction.

As a result of the hydration of the addition of water to acetylene, acetaldehyde (acetic aldehyde) is formed:

For acetylene homologues, the addition of water leads to the formation of ketones:

Alkyne hydrogenation

Alkynes react with hydrogen in two steps. Metals such as platinum, palladium, nickel are used as catalysts:

Alkyne trimerization

When acetylene is passed over activated carbon at high temperature, a mixture of various products is formed from it, the main of which is benzene, a product of acetylene trimerization:

Dimerization of alkynes

Acetylene also enters into a dimerization reaction. The process proceeds in the presence of copper salts as catalysts:

Alkyne oxidation

Alkynes burn in oxygen:

C n H 2n-2 + (3n-1) / 2 O 2 → nCO 2 + (n-1) H 2 O

The interaction of alkynes with bases

Alkynes with a triple C≡C at the end of the molecule, unlike other alkynes, are able to enter into reactions in which the hydrogen atom in the triple bond is replaced by a metal. For example, acetylene reacts with sodium amide in liquid ammonia:

HC≡CH + 2NaNH 2 → NaC≡CNa + 2NH 3,

and also with an ammonia solution of silver oxide, forming insoluble salt-like substances called acetylenides:

Thanks to this reaction, it is possible to recognize alkynes with a terminal triple bond, as well as to isolate such an alkyne from a mixture with other alkynes.

It should be noted that all silver and copper acetylenides are explosive substances.

Acetylides are able to react with halogen derivatives, which is used in the synthesis of more complex organic compounds with a triple bond:

CH 3 -C≡CH + NaNH 2 → CH 3 -C≡CNa + NH 3

CH 3 -C≡CNa + CH 3 Br → CH 3 -C≡C-CH 3 + NaBr

Chemical properties of aromatic hydrocarbons

The aromatic nature of the bond affects the chemical properties of benzenes and other aromatic hydrocarbons.

A single 6pi electron system is much more stable than conventional pi bonds. Therefore, for aromatic hydrocarbons, substitution reactions are more characteristic than addition reactions. Arenes enter into substitution reactions by an electrophilic mechanism.

Substitution reactions

Halogenation

Nitration

The nitration reaction proceeds best under the action of not pure nitric acid, but its mixture with concentrated sulfuric acid, the so-called nitrating mixture:

Alkylation

The reaction in which one of the hydrogen atoms at the aromatic nucleus is replaced by a hydrocarbon radical:

Alkenes can also be used instead of halogenated alkanes. Aluminum halides, ferric iron halides or inorganic acids can be used as catalysts.<

Addition reactions

hydrogenation

Accession of chlorine

It proceeds by a radical mechanism under intense irradiation with ultraviolet light:

Similarly, the reaction can proceed only with chlorine.

Oxidation reactions

Combustion

2C 6 H 6 + 15O 2 \u003d 12CO 2 + 6H 2 O + Q

incomplete oxidation

The benzene ring is resistant to oxidizing agents such as KMnO 4 and K 2 Cr 2 O 7 . The reaction does not go.

Division of substituents in the benzene ring into two types:

Consider the chemical properties of benzene homologues using toluene as an example.

Chemical properties of toluene

Halogenation

The toluene molecule can be considered as consisting of fragments of benzene and methane molecules. Therefore, it is logical to assume that the chemical properties of toluene should to some extent combine the chemical properties of these two substances taken separately. In particular, this is precisely what is observed during its halogenation. We already know that benzene enters into a substitution reaction with chlorine by an electrophilic mechanism, and catalysts (aluminum or ferric halides) must be used to carry out this reaction. At the same time, methane is also capable of reacting with chlorine, but by a free radical mechanism, which requires irradiation of the initial reaction mixture with UV light. Toluene, depending on the conditions under which it undergoes chlorination, is able to give either the products of substitution of hydrogen atoms in the benzene ring - for this you need to use the same conditions as in the chlorination of benzene, or the products of substitution of hydrogen atoms in the methyl radical, if on it, how to act on methane with chlorine when irradiated with ultraviolet radiation:

As you can see, the chlorination of toluene in the presence of aluminum chloride led to two different products - ortho- and para-chlorotoluene. This is due to the fact that the methyl radical is a substituent of the first kind.

If the chlorination of toluene in the presence of AlCl 3 is carried out in excess of chlorine, the formation of trichlorine-substituted toluene is possible:

Similarly, when toluene is chlorinated in the light at a higher chlorine / toluene ratio, dichloromethylbenzene or trichloromethylbenzene can be obtained:

Nitration

The substitution of hydrogen atoms for nitrogroup, during the nitration of toluene with a mixture of concentrated nitric and sulfuric acids, leads to substitution products in the aromatic nucleus, and not in the methyl radical:

Alkylation

As already mentioned, the methyl radical is an orientant of the first kind, therefore, its Friedel-Crafts alkylation leads to substitution products in the ortho and para positions:

Addition reactions

Toluene can be hydrogenated to methylcyclohexane using metal catalysts (Pt, Pd, Ni):

C 6 H 5 CH 3 + 9O 2 → 7CO 2 + 4H 2 O

incomplete oxidation

Under the action of such an oxidizing agent as an aqueous solution of potassium permanganate, the side chain undergoes oxidation. The aromatic nucleus cannot be oxidized under such conditions. In this case, depending on the pH of the solution, either a carboxylic acid or its salt will be formed.

Limit hydrocarbons have in their molecules only low-polarity and weakly polarizing bonds, which are highly durable, therefore, under normal conditions, they are substances that are slightly chemically active with respect to polar reagents: they do not interact with concentrated acids, wholes, alkali metals, oxidizing agents. This was the reason for their name - paraffins. Parumaffinus is Latin for unrelated. Their chemical transformations proceed mainly at elevated temperatures and under the action of UV irradiation.

There are three main types of reactions of saturated hydrocarbons: substitution, oxidation and elimination. These reactions can proceed either by breaking the C-C bond (energy 83.6 kcal) or by breaking the C-H bond (energy 98.8 kcal/mol). Reactions often go with a break in the C-H bond, tk. it is more accessible to the action of the reagent, although the C-C bond requires less energy for cleavage. As a result of such reactions, very active species are intermediately formed - aliphatic hydrocarbon radicals.

Preparation and properties of aliphatic radicals

1. The formation of free radicals during the homolytic cleavage of C-C or C-H bonds occurs at a temperature of 300-700 ° C or under the action of free radical reagents.

2. The lifetime of free radicals (resistance) increases from primary to secondary and tertiary radicals:

b) Interaction with unsaturated compounds: addition occurs with the formation of a new radical as well:

CH3. + CH 2 \u003d CH 2 CH 3 -CH 2 -CH 2.

c) -decay - radicals with a long carbon chain decompose with a break in the C-C bond in the -position to carbon with an unpaired electron.

CH 3 - CH 2: CH 2 - CH 2 . CH 3 -CH 2 . + CH 2 \u003d CH 2

d) Disproportionation - redistribution of hydrogen associated with -decay along the C-H bond:

e) Recombination - the combination of free radicals with each other

CH 3 . + CH 3 . CH 3 -CH 3

Knowing the features of the behavior of free radicals, it is easier to understand the basic laws of specific reactions of saturated hydrocarbons.

I type. substitution reaction

1. Halogenation reactions. The most energetic reagent is fluorine. Direct fluorination results in an explosion. The reactions chlorination. They can proceed under the action of chlorine molecules in the light already at room temperature. The reaction proceeds according to the free-radical chain mechanism and includes the following main stages:

a) the first slow stage - chain initiation:

Cl:ClCl. +Cl.

R: H + . Cl HCl + R.

b) chain development - the formation of reaction products with the simultaneous formation of free radicals that continue the chain process:

R. + Cl: Cl RCl + Cl.

R:H+Cl. HCl+R.

c) open circuit:

Since CI. the reagent is active, it can attack the molecule of the already obtained chlorine derivative, as a result, a mixture of mono- and polyhalogenated compounds is formed. For example:

CH 4 + Cl 2 HCl + CH 3 Cl CH 2 Cl 2 CHCl 3 CCl 4

methyl chloride –HCl -HCl -HCl

methylene chloride chloroform four-

carbon chloride

Bromination reaction proceeds much more difficult, because bromine is less active than chlorine and reacts mainly with the formation of more stable tertiary or secondary radicals. In this case, the second bromine atom usually enters a position adjacent to the first, mainly at the secondary carbon.

iodination reactions practically do not leak, because HI reduces the resulting alkyl iodides.

2. Nitration- substitution of the H atom by the NO 2 group under the action of nitric acid. It goes under the action of dilute nitric acid (12%) at a high temperature of 150 ° C under pressure (Konovalov's reaction). Paraffins of isostructure react more easily, tk. substitution occurs more easily at the tertiary carbon atom:

The mechanism of the nitration reaction is associated with the intermediate formation of free radicals. The initiation is facilitated by a partially occurring oxidation process:

RH + HONO 2 ROH + HONO

nitrous acid

HONO + HONO 2 HOH + 2 . NO 2

CH 3 -C-CH 3 +. NO 2 CH 3 -C-CH 3 + HNO 2

CH 3 -C-CH 3 +. NO 2 CH 3 -C-CH 3

those. the radical reaction of nitration of hydrocarbons does not have a chain character.

II type. Oxidation reactions

Under normal conditions, paraffins are not oxidized either by oxygen or by strong oxidizing agents (KMnO 4 , HNO 3 , K 2 Cr 2 O 7 , etc.).

When an open flame is introduced into a mixture of hydrocarbon with air, complete oxidation (combustion) of the hydrocarbon to CO 2 and H 2 O occurs. Heating saturated hydrocarbons in a mixture with air or oxygen in the presence of catalysts for the oxidation of MnO 2 and others to a temperature of 300 ° C leads to their oxidation with the formation of peroxide compounds. The reaction proceeds by a chain free radical mechanism.

And: R: H R . +H. circuit initiation

R:R. + O: :O: R-O-O .

R-O-O. + R: H R-O-O-H + R .

alkane hydroperoxide

O:R-O-O. +R. R-O-O-R open circuit

alkane peroxide

The tertiary units are most easily oxidized, the secondary ones are more difficult, and the primary ones are even more difficult. The resulting hydroperoxides decompose.

Primary hydroperoxides when decomposed, they form aldehydes or a primary alcohol, for example:

CH 3 -C-C-O: O-H CH 3 -C-O. + . OH CH 3 -C \u003d O + H 2 O

ethane hydroperoxide acetaldehyde

CH 3 -CH 3

side

CH 3 -CH 2 OH + CH 3 -CH 2.

Secondary hydroperoxides form ketones or secondary alcohols upon decomposition, for example:

CH 3 -C-O:OH CH 3 -C-O. + . OH H 2 O + CH 3 -C \u003d O

CH 3 CH 3 CH 3

propane hydroperoxide

CH 3 -CH 2 -CH 3

side

CH 3 -CH-OH + CH 3 -. CH-CH 3

isopropyl alcohol

Tertiary hydroperoxides form ketones, as well as primary and tertiary alcohols, for example:

CH 3 CH 3 CH 3

CH 3 -C-CH 3 CH 3 -C: CH 3 +. OH CH 3 OH + CH 3 -C \u003d O

isobutane hydroperoxide

CH 3 -CH-CH 3

side

Isobutane

CH 3 -C-CH 3 + CH 3 -C-CH 3

tert-butyl alcohol

Any hydroperoxide can also decompose with the release of atomic oxygen: CH 3 -CH 2 -O-O-H CH 3 CH 2 -OH + [O],

which goes to further oxidation:

CH 3 -C + [O] CH 3 -C-OH

Therefore, in addition to alcohols, aldehydes and ketones, carboxylic acids are formed.

By choosing the reaction conditions, it is possible to obtain one of any product. For example: 2 CH 4 + O 2 2 CH 3 OH.

DIENE HYDROCARBONS (ALKADIENES)

Diene hydrocarbons or alkadienes are unsaturated hydrocarbons containing two double carbon-carbon bonds. The general formula of alkadienes is C n H 2 n -2.
Depending on the mutual arrangement of double bonds, dienes are divided into three types:

1) hydrocarbons with cumulated double bonds, i.e. adjacent to one carbon atom. For example, propadiene or allene CH 2 =C=CH 2 ;

2) hydrocarbons with isolated double bonds, i.e. separated by two or more simple bonds. For example, pentadiene -1.4 CH 2 \u003d CH–CH 2 -CH \u003d CH 2;

3) hydrocarbons with conjugated double bonds, i.e. separated by a single link. For example, butadiene -1,3 or divinyl CH 2 \u003d CH–CH \u003d CH 2, 2-methylbutadiene -1,3 or isoprene

2) dehydrogenation and dehydration of ethyl alcohol by passing alcohol vapor over heated catalysts (method of academician S.V. Lebedev)

2CH 3 CH 2 OH - - ~ 450 ° С; ZnO, Al2O3 ® CH 2 \u003d CH - CH \u003d CH 2 + 2H 2 O + H 2

Physical properties

Chemical properties

The carbon atoms in the 1,3-butadiene molecule are in the sp 2 hybrid state, which means that these atoms are located in the same plane and each of them has one p-orbital occupied by one electron and located perpendicular to the said plane.

p-Orbitals of all carbon atoms overlap with each other, i.e. not only between the first and second, third and fourth atoms, but also between the second and third. This shows that the bond between the second and third carbon atoms is not a simple s-bond, but has a certain density of p-electrons, i.e. weak double bond. This means that s-electrons do not belong to strictly defined pairs of carbon atoms. In the molecule, there are no single and double bonds in the classical sense, and delocalization of p-electrons is observed, i.e. uniform distribution of p-electron density throughout the molecule with the formation of a single p-electron cloud.
The interaction of two or more neighboring p-bonds with the formation of a single p-electron cloud, resulting in the transfer of the interaction of atoms in this system, is called conjugation effect.
Thus, the -1,3 butadiene molecule is characterized by a system of conjugated double bonds.
This feature in the structure of diene hydrocarbons makes them capable of adding various reagents not only to neighboring carbon atoms (1,2-addition), but also to the two ends of the conjugated system (1,4-addition) with the formation of a double bond between the second and third carbon atoms. . Note that the 1,4-addition product is very often the main product.
Consider the reactions of halogenation and hydrohalogenation of conjugated dienes.

Polymerization of diene compounds

In a simplified form, the polymerization reaction of -1,3 butadiene according to the addition scheme 1,4 can be represented as follows:

Both double bonds of the diene are involved in the polymerization. During the reaction, they break, the pairs of electrons that form s-bonds are separated, after which each unpaired electron participates in the formation of new bonds: the electrons of the second and third carbon atoms, as a result of generalization, give a double bond, and the electrons of the extreme carbon atoms in the chain, when generalized with electrons the corresponding atoms of another monomer molecule link the monomers into a polymer chain.

The elemental cell of polybutadiene is represented as follows:

As can be seen, the resulting polymer is characterized by trance- the configuration of the elemental cell of the polymer. However, the most valuable products in practical terms are obtained by stereoregular (in other words, spatially ordered) polymerization of diene hydrocarbons according to the 1,4-addition scheme with the formation cis- configuration of the polymer chain. For example, cis- polybutadiene

Natural and synthetic rubbers

Natural rubber is obtained from the milky sap (latex) of the Hevea rubber tree, which grows in the rainforests of Brazil.

When heated without access to air, rubber decomposes to form a diene hydrocarbon - 2-methylbutadiene-1,3 or isoprene. Rubber is a stereoregular polymer in which isoprene molecules are connected to each other in a 1,4-addition scheme with cis- polymer chain configuration:

The molecular weight of natural rubber ranges from 7 . 10 4 to 2.5 . 10 6 .

trance- Isoprene polymer also occurs naturally in the form of gutta-percha.

Natural rubber has a unique set of properties: high fluidity, wear resistance, adhesiveness, water and gas impermeability. To give rubber the necessary physical and mechanical properties: strength, elasticity, resistance to solvents and aggressive chemical environments, rubber is vulcanized by heating up to 130-140 ° C with sulfur. In a simplified form, the rubber vulcanization process can be represented as follows:

Sulfur atoms are attached at the point of breaking some double bonds and the linear rubber molecules are "crosslinked" into larger three-dimensional molecules - rubber is obtained, which is much stronger than unvulcanized rubber. Rubbers filled with active carbon black are used in the manufacture of car tires and other rubber products.

In 1932, S.V. Lebedev developed a method for the synthesis of synthetic rubber based on butadiene obtained from alcohol. And only in the fifties, domestic scientists carried out catalytic stereopolymerization of diene hydrocarbons and obtained stereoregular rubber, similar in properties to natural rubber. At present, rubber is produced in the industry,

Characteristic chemical properties of hydrocarbons: alkanes, alkenes, dienes, alkynes, aromatic hydrocarbons

Alkanes

Alkanes are hydrocarbons in whose molecules the atoms are linked by single bonds and which correspond to the general formula $C_(n)H_(2n+2)$.

Homologous series of methane

As you already know, homologues are substances that are similar in structure and properties and differ by one or more $CH_2$ groups.

Limit hydrocarbons make up the homologous series of methane.

Isomerism and nomenclature

Alkanes are characterized by the so-called structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. As you already know, the simplest alkane, which is characterized by structural isomers, is butane:

Let us consider in more detail for alkanes the basics of the IUPAC nomenclature:

1. Choice of the main circuit.

The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in the molecule, which is, as it were, its basis.

2.

The atoms of the main chain are assigned numbers. The numbering of atoms of the main chain starts from the end closest to the substituent (structures A, B). If the substituents are at an equal distance from the end of the chain, then the numbering starts from the end at which there are more of them (structure B). If different substituents are at an equal distance from the ends of the chain, then the numbering starts from the end to which the older one is closer (structure D). The seniority of hydrocarbon substituents is determined by the order in which the letter with which their name begins follows in the alphabet: methyl (—$CH_3$), then propyl ($—CH_2—CH_2—CH_3$), ethyl ($—CH_2—CH_3$ ) etc.

Note that the name of the substitute is formed by replacing the suffix -en to suffix -silt in the name of the corresponding alkane.

3. Name formation.

Numbers are indicated at the beginning of the name - the numbers of carbon atoms at which the substituents are located. If there are several substituents at a given atom, then the corresponding number in the name is repeated twice, separated by commas ($2.2-$). After the number, a hyphen indicates the number of substituents ( di- two, three- three, tetra- four, penta- five) and the name of the deputy ( methyl, ethyl, propyl). Then without spaces and hyphens - the name of the main chain. The main chain is called as a hydrocarbon - a member of the homologous series of methane ( methane, ethane, propane, etc.).

The names of the substances whose structural formulas are given above are as follows:

- structure A: $2$ -methylpropane;

- Structure B: $3$ -ethylhexane;

- Structure B: $2,2,4$ -trimethylpentane;

- structure Г: $2$ -methyl$4$-ethylhexane.

Physical and chemical properties of alkanes

physical properties. The first four representatives of the homologous series of methane are gases. The simplest of them is methane - a colorless, tasteless and odorless gas (the smell of gas, upon smelling which you need to call $104$, is determined by the smell of mercaptans - sulfur-containing compounds specially added to methane used in household and industrial gas appliances so that people those near them could smell the leak).

Hydrocarbons of composition from $С_5Н_(12)$ to $С_(15)Н_(32)$ are liquids; heavier hydrocarbons are solids.

The boiling and melting points of alkanes gradually increase with increasing carbon chain length. All hydrocarbons are poorly soluble in water; liquid hydrocarbons are common organic solvents.

Chemical properties.

1. substitution reactions. The most characteristic of alkanes are free radical substitution reactions, during which a hydrogen atom is replaced by a halogen atom or some group.

Let us present the equations of the most characteristic reactions.

Halogenation:

$CH_4+Cl_2→CH_3Cl+HCl$.

In the case of an excess of halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms by chlorine:

$CH_3Cl+Cl_2→HCl+(CH_2Cl_2)↙(\text"dichloromethane(methylene chloride)")$,

$CH_2Cl_2+Cl_2→HCl+(CHСl_3)↙(\text"trichloromethane(chloroform)")$,

$CHCl_3+Cl_2→HCl+(CCl_4)↙(\text"tetrachloromethane(carbon tetrachloride)")$.

The resulting substances are widely used as solvents and starting materials in organic synthesis.

2. Dehydrogenation (elimination of hydrogen). During the passage of alkanes over the catalyst ($Pt, Ni, Al_2O_3, Cr_2O_3$) at a high temperature ($400-600°C$), a hydrogen molecule is split off and an alkene is formed:

$CH_3—CH_3→CH_2=CH_2+H_2$

3. Reactions accompanied by the destruction of the carbon chain. All saturated hydrocarbons are burning with the formation of carbon dioxide and water. Gaseous hydrocarbons mixed with air in certain proportions can explode. The combustion of saturated hydrocarbons is a free radical exothermic reaction, which is of great importance when using alkanes as a fuel:

$CH_4+2O_2→CO_2+2H_2O+880 kJ.$

In general, the combustion reaction of alkanes can be written as follows:

$C_(n)H_(2n+2)+((3n+1)/(2))O_2→nCO_2+(n+1)H_2O$

Thermal breakdown of hydrocarbons:

$C_(n)H_(2n+2)(→)↖(400-500°C)C_(n-k)H_(2(n-k)+2)+C_(k)H_(2k)$

The process proceeds according to the free radical mechanism. An increase in temperature leads to a homolytic rupture of the carbon-carbon bond and the formation of free radicals:

$R—CH_2CH_2:CH_2—R→R—CH_2CH_2+CH_2—R$.

These radicals interact with each other, exchanging a hydrogen atom, with the formation of an alkane molecule and an alkene molecule:

$R—CH_2CH_2+CH_2—R→R—CH=CH_2+CH_3—R$.

Thermal splitting reactions underlie the industrial process - hydrocarbon cracking. This process is the most important stage of oil refining.

When methane is heated to a temperature of $1000°C$, pyrolysis of methane begins - decomposition into simple substances:

$CH_4(→)↖(1000°C)C+2H_2$

When heated to a temperature of $1500°C$, the formation of acetylene is possible:

$2CH_4(→)↖(1500°C)CH=CH+3H_2$

4. Isomerization. When linear hydrocarbons are heated with an isomerization catalyst (aluminum chloride), substances with a branched carbon skeleton are formed:

5. Aromatization. Alkanes with six or more carbon atoms in the chain in the presence of a catalyst are cyclized to form benzene and its derivatives:

What is the reason that alkanes enter into reactions proceeding according to the free radical mechanism? All carbon atoms in alkane molecules are in the $sp^3$ hybridization state. The molecules of these substances are built using covalent nonpolar $C—C$ (carbon—carbon) bonds and weakly polar $C—H$ (carbon—hydrogen) bonds. They do not contain areas with high and low electron density, easily polarizable bonds, i.e. such bonds, the electron density in which can be shifted under the influence of external factors (electrostatic fields of ions). Therefore, alkanes will not react with charged particles, because bonds in alkane molecules are not broken by a heterolytic mechanism.

Alkenes

Unsaturated hydrocarbons include hydrocarbons containing multiple bonds between carbon atoms in molecules. Unlimited are alkenes, alkadienes (polyenes), alkynes. Cyclic hydrocarbons containing a double bond in the cycle (cycloalkenes), as well as cycloalkanes with a small number of carbon atoms in the cycle (three or four atoms) also have an unsaturated character. The property of unsaturation is associated with the ability of these substances to enter into addition reactions, primarily hydrogen, with the formation of saturated, or saturated, hydrocarbons - alkanes.

Alkenes are acyclic hydrocarbons containing in the molecule, in addition to single bonds, one double bond between carbon atoms and corresponding to the general formula $C_(n)H_(2n)$.

Its second name olefins- alkenes were obtained by analogy with unsaturated fatty acids (oleic, linoleic), the remains of which are part of liquid fats - oils (from lat. oleum- oil).

Homologous series of ethene

Unbranched alkenes make up the homologous series of ethene (ethylene):

$C_2H_4$ is ethene, $C_3H_6$ is propene, $C_4H_8$ is butene, $C_5H_(10)$ is pentene, $C_6H_(12)$ is hexene, etc.

Isomerism and nomenclature

For alkenes, as well as for alkanes, structural isomerism is characteristic. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkene, which is characterized by structural isomers, is butene:

A special type of structural isomerism is the double bond position isomerism:

$CH_3—(CH_2)↙(butene-1)—CH=CH_2$ $CH_3—(CH=CH)↙(butene-2)—CH_3$

Almost free rotation of carbon atoms is possible around a single carbon-carbon bond, so alkane molecules can take on a wide variety of shapes. Rotation around the double bond is impossible, which leads to the appearance of another type of isomerism in alkenes - geometric, or cis-trans isomerism.

cis- isomers are different from trance- isomers by the spatial arrangement of fragments of the molecule (in this case, methyl groups) relative to the $π$-bond plane, and, consequently, by properties.

Alkenes are isomeric to cycloalkanes (interclass isomerism), for example:

The nomenclature of alkenes developed by IUPAC is similar to the nomenclature of alkanes.

1. Choice of the main circuit.

The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in a molecule. In the case of alkenes, the main chain must contain a double bond.

2. Atom numbering of the main chain.

The numbering of the atoms of the main chain starts from the end to which the double bond is closest. For example, the correct connection name is:

$5$-methylhexene-$2$, not $2$-methylhexene-$4$, as might be expected.

If it is impossible to determine the beginning of the numbering of atoms in the chain by the position of the double bond, then it is determined by the position of the substituents, just as for saturated hydrocarbons.

3. Name formation.

The names of alkenes are formed in the same way as the names of alkanes. At the end of the name indicate the number of the carbon atom at which the double bond begins, and the suffix indicating that the compound belongs to the class of alkenes - -en.

For example:

Physical and chemical properties of alkenes

physical properties. The first three representatives of the homologous series of alkenes are gases; substances of the composition $C_5H_(10)$ - $C_(16)H_(32)$ are liquids; higher alkenes are solids.

The boiling and melting points naturally increase with an increase in the molecular weight of the compounds.

Chemical properties.

Addition reactions. Recall that a distinctive feature of the representatives of unsaturated hydrocarbons - alkenes is the ability to enter into addition reactions. Most of these reactions proceed by the mechanism

1. hydrogenation of alkenes. Alkenes are able to add hydrogen in the presence of hydrogenation catalysts, metals - platinum, palladium, nickel:

$CH_3—CH_2—CH=CH_2+H_2(→)↖(Pt)CH_3—CH_2—CH_2—CH_3$.

This reaction proceeds at atmospheric and elevated pressure and does not require high temperature, because is exothermic. With an increase in temperature on the same catalysts, the reverse reaction, dehydrogenation, can occur.

2. Halogenation (addition of halogens). The interaction of an alkene with bromine water or a solution of bromine in an organic solvent ($CCl_4$) leads to a rapid discoloration of these solutions as a result of the addition of a halogen molecule to the alkene and the formation of dihalogen alkanes:

$CH_2=CH_2+Br_2→CH_2Br—CH_2Br$.

3.

$CH_3-(CH)↙(propene)=CH_2+HBr→CH_3-(CHBr)↙(2-bromopropene)-CH_3$

This reaction is subject to Markovnikov's rule:

When a hydrogen halide is added to an alkene, hydrogen is attached to a more hydrogenated carbon atom, i.e. the atom at which there are more hydrogen atoms, and the halogen - to the less hydrogenated one.

Hydration of alkenes leads to the formation of alcohols. For example, the addition of water to ethene underlies one of the industrial methods for producing ethyl alcohol:

$(CH_2)↙(ethene)=CH_2+H_2O(→)↖(t,H_3PO_4)CH_3-(CH_2OH)↙(ethanol)$

Note that a primary alcohol (with a hydroxo group at the primary carbon) is formed only when ethene is hydrated. When propene or other alkenes are hydrated, secondary alcohols are formed.

This reaction also proceeds in accordance with Markovnikov's rule - the hydrogen cation is attached to the more hydrogenated carbon atom, and the hydroxo group to the less hydrogenated one.

5. Polymerization. A special case of addition is the polymerization reaction of alkenes:

$nCH_2(=)↙(ethene)CH_2(→)↖(UV light,R)(...(-CH_2-CH_2-)↙(polyethylene)...)_n$

This addition reaction proceeds by a free radical mechanism.

6. Oxidation reaction.

Like any organic compounds, alkenes burn in oxygen to form $CO_2$ and $H_2O$:

$CH_2=CH_2+3O_2→2CO_2+2H_2O$.

In general:

$C_(n)H_(2n)+(3n)/(2)O_2→nCO_2+nH_2O$

Unlike alkanes, which are resistant to oxidation in solutions, alkenes are easily oxidized by the action of potassium permanganate solutions. In neutral or alkaline solutions, alkenes are oxidized to diols (dihydric alcohols), and hydroxyl groups are attached to those atoms between which a double bond existed before oxidation:

Alkadienes (diene hydrocarbons)

Alkadienes are acyclic hydrocarbons containing in the molecule, in addition to single bonds, two double bonds between carbon atoms and corresponding to the general formula $C_(n)H_(2n-2)$.

Depending on the mutual arrangement of double bonds, there are three types of dienes:

- alkadienes with cumulated arrangement of double bonds:

- alkadienes with conjugated double bonds;

$CH_2=CH—CH=CH_2$;

- alkadienes with isolated double bonds

$CH_2=CH—CH_2—CH=CH_2$.

All three types of alkadienes differ significantly from each other in structure and properties. The central carbon atom (an atom that forms two double bonds) in alkadienes with cumulated bonds is in the $sp$-hybridization state. It forms two $σ$-bonds lying on the same straight line and directed in opposite directions, and two $π$-bonds lying in perpendicular planes. $π$-bonds are formed due to unhybridized p-orbitals of each carbon atom. The properties of alkadienes with isolated double bonds are very specific, because conjugated $π$-bonds significantly affect each other.

p-Orbitals forming conjugated $π$-bonds make up practically a single system (it is called a $π$-system), because p-orbitals of neighboring $π$-bonds partially overlap.

Isomerism and nomenclature

Alkadienes are characterized by both structural isomerism and cis- and trans-isomerism.

Structural isomerism.

— isomerism of the carbon skeleton:

— isomerism of the position of multiple bonds:

$(CH_2=CH—CH=CH_2)↙(butadiene-1,3)$ $(CH_2=C=CH—CH_3)↙(butadiene-1,2)$

cis-, trans- isomerism (spatial and geometric)

For example:

Alkadienes are isomeric compounds of the classes of alkynes and cycloalkenes.

When forming the name of the alkadiene, the numbers of double bonds are indicated. The main chain must necessarily contain two multiple bonds.

For example:

Physical and chemical properties of alkadienes

physical properties.

Under normal conditions, propandien-1,2, butadiene-1,3 are gases, 2-methylbutadiene-1,3 is a volatile liquid. Alkadienes with isolated double bonds (the simplest of them is pentadiene-1,4) are liquids. Higher dienes are solids.

Chemical properties.

The chemical properties of alkadienes with isolated double bonds differ little from those of alkenes. Alkadienes with conjugated bonds have some special features.

1. Addition reactions. Alkadienes are capable of adding hydrogen, halogens, and hydrogen halides.

A feature of addition to alkadienes with conjugated bonds is the ability to attach molecules both in positions 1 and 2, and in positions 1 and 4.

The ratio of the products depends on the conditions and method of carrying out the corresponding reactions.

2.polymerization reaction. The most important property of dienes is the ability to polymerize under the influence of cations or free radicals. The polymerization of these compounds is the basis of synthetic rubbers:

$nCH_2=(CH—CH=CH_2)↙(butadiene-1,3)→((... —CH_2—CH=CH—CH_2— ...)_n)↙(\text"synthetic butadiene rubber")$ .

The polymerization of conjugated dienes proceeds as 1,4-addition.

In this case, the double bond turns out to be central in the link, and the elementary link, in turn, can take both cis-, and trance- configuration.

Alkynes

Alkynes are acyclic hydrocarbons containing in the molecule, in addition to single bonds, one triple bond between carbon atoms and corresponding to the general formula $C_(n)H_(2n-2)$.

Homologous series of ethine

Unbranched alkynes make up the homologous series of ethyne (acetylene):

$C_2H_2$ - ethyne, $C_3H_4$ - propyne, $C_4H_6$ - butyne, $C_5H_8$ - pentine, $C_6H_(10)$ - hexine, etc.

Isomerism and nomenclature

For alkynes, as well as for alkenes, structural isomerism is characteristic: isomerism of the carbon skeleton and isomerism of the position of the multiple bond. The simplest alkyne, which is characterized by structural isomers of the multiple bond position of the alkyne class, is butyne:

$CH_3—(CH_2)↙(butyn-1)—C≡CH$ $CH_3—(C≡C)↙(butyn-2)—CH_3$

The isomerism of the carbon skeleton in alkynes is possible, starting from pentyn:

Since the triple bond assumes a linear structure of the carbon chain, the geometric ( cis-, trans-) isomerism is not possible for alkynes.

The presence of a triple bond in hydrocarbon molecules of this class is reflected by the suffix -in, and its position in the chain - the number of the carbon atom.

For example:

Alkynes are isomeric compounds of some other classes. So, hexine (alkyne), hexadiene (alkadiene) and cyclohexene (cycloalkene) have the chemical formula $С_6Н_(10)$:

Physical and chemical properties of alkynes

physical properties. The boiling and melting points of alkynes, as well as alkenes, naturally increase with an increase in the molecular weight of the compounds.

Alkynes have a specific smell. They are more soluble in water than alkanes and alkenes.

Chemical properties.

Addition reactions. Alkynes are unsaturated compounds and enter into addition reactions. Basically, these are reactions. electrophilic addition.

1. Halogenation (addition of a halogen molecule). Alkyne is able to attach two halogen molecules (chlorine, bromine):

$CH≡CH+Br_2→(CHBr=CHBr)↙(1,2-dibromoethane),$

$CHBr=CHBr+Br_2→(CHBr_2-CHBr_2)↙(1,1,2,2-tetrabromoethane)$

2. Hydrohalogenation (addition of hydrogen halide). The addition reaction of hydrogen halide, proceeding according to the electrophilic mechanism, also proceeds in two stages, and at both stages the Markovnikov rule is fulfilled:

$CH_3-C≡CH+Br→(CH_3-CBr=CH_2)↙(2-bromopropene),$

$CH_3-CBr=CH_2+HBr→(CH_3-CHBr_2-CH_3)↙(2,2-dibromopropane)$

3. Hydration (addition of water). Of great importance for the industrial synthesis of ketones and aldehydes is the water addition reaction (hydration), which is called Kucherov's reaction:

4. hydrogenation of alkynes. Alkynes add hydrogen in the presence of metal catalysts ($Pt, Pd, Ni$):

$R-C≡C-R+H_2(→)↖(Pt)R-CH=CH-R,$

$R-CH=CH-R+H_2(→)↖(Pt)R-CH_2-CH_2-R$

Since the triple bond contains two reactive $π$ bonds, alkanes add hydrogen in steps:

1) trimerization.

When ethyne is passed over activated carbon, a mixture of products is formed, one of which is benzene:

2) dimerization.

In addition to trimerization of acetylene, its dimerization is also possible. Under the action of monovalent copper salts, vinylacetylene is formed:

$2HC≡CH→(HC≡C-CH=CH_2)↙(\text"butene-1-yn-3(vinylacetylene)")$

This substance is used to produce chloroprene:

$HC≡C-CH=CH_2+HCl(→)↖(CaCl)H_2C=(CCl-CH)↙(chloroprene)=CH_2$

polymerization of which produces chloroprene rubber:

$nH_2C=CCl-CH=CH_2→(...-H_2C-CCl=CH-CH_2-...)_n$

Alkyne oxidation.

Ethine (acetylene) burns in oxygen with the release of a very large amount of heat:

$2C_2H_2+5O_2→4CO_2+2H_2O+2600kJ$ This reaction is based on the action of an oxy-acetylene torch, the flame of which has a very high temperature (more than $3000°C$), which makes it possible to use it for cutting and welding metals.

In air, acetylene burns with a smoky flame, because. the carbon content in its molecule is higher than in the molecules of ethane and ethene.

Alkynes, like alkenes, decolorize acidified solutions of potassium permanganate; in this case, the destruction of the multiple bond occurs.

Ionic (V.V. Markovnikov's rule) and radical reaction mechanisms in organic chemistry

Types of chemical reactions in organic chemistry

The reactions of organic substances can be formally divided into four main types: substitution, addition, elimination (elimination) and rearrangement (isomerization). Obviously, the whole variety of reactions of organic compounds cannot be reduced to the proposed classification (for example, combustion reactions). However, such a classification will help to establish analogies with the reactions that take place between inorganic substances already familiar to you from the course of inorganic chemistry.

As a rule, the main organic compound participating in the reaction is called the substrate, and the other component of the reaction is conditionally considered as a reagent.

Substitution reactions

Reactions that result in the replacement of one atom or group of atoms in the original molecule (substrate) with other atoms or groups of atoms are called substitution reactions.

Substitution reactions involve saturated and aromatic compounds such as alkanes, cycloalkanes or arenes.

Let us give examples of such reactions.

Under the action of light, hydrogen atoms in a methane molecule can be replaced by halogen atoms, for example, by chlorine atoms:

$CH_4+Cl_2→CH_3Cl+HCl$

Another example of replacing hydrogen with halogen is the conversion of benzene to bromobenzene:

The equation for this reaction can be written differently:

With this form of writing, the reagents, catalyst, reaction conditions are written above the arrow, and the inorganic reaction products are written below it.

Addition reactions

Reactions, as a result of which two or more molecules of reactants combine into one, are called addition reactions.

Unsaturated compounds, such as alkenes or alkynes, enter into addition reactions.

Depending on which molecule acts as a reagent, hydrogenation (or reduction), halogenation, hydrohalogenation, hydration, and other addition reactions are distinguished. Each of them requires certain conditions.

1. hydrogenation- the reaction of the addition of a hydrogen molecule to a multiple bond:

$CH_3(-CH=)↙(\text"propene")CH_2+H_2(→)↖(Pt)CH_3(-CH_2-)↙(\text"propane")-CH_3$

2.Hydrohalogenation— hydrogen halide addition reaction (hydrochlorination):

$(CH_2=)↙(\text"ethene")CH_2+HCl→CH_3(-CH_2-)↙(\text"chloroethane")-Cl$

3.Halogenation- halogen addition reaction:

$(CH_2=)↙(\text"ethene")CH_2+Cl_2→(CH_2Cl-CH_2Cl)↙(\text"1.2-dichloroethane")$

4. Polymerization- a special type of addition reactions, during which molecules of a substance with a small molecular weight are combined with each other to form molecules of a substance with a very high molecular weight - macromolecules.

Polymerization reactions are the processes of combining many molecules of a low molecular weight substance (monomer) into large molecules (macromolecules) of a polymer.

An example of a polymerization reaction is the production of polyethylene from ethylene (ethene) under the action of ultraviolet radiation and a radical polymerization initiator $R:$

$(nCH_2=)↙(\text"ethene")CH_2(→)↖(\text"UV light,R")((...-CH_2-CH_2-...)_n)↙(\text" polyethylene")$

The covalent bond most characteristic of organic compounds is formed when atomic orbitals overlap and the formation of common electron pairs. As a result of this, an orbital common to two atoms is formed, on which a common electron pair is located. When the bond is broken, the fate of these common electrons can be different.

Types of reactive particles in organic chemistry

An orbital with an unpaired electron belonging to one atom can overlap with an orbital of another atom that also contains an unpaired electron. This results in the formation of a covalent bond exchange mechanism:

$H + H→H:H,$ or $H-H$

exchange mechanism The formation of a covalent bond is realized if a common electron pair is formed from unpaired electrons belonging to different atoms.

The process opposite to the formation of a covalent bond by the exchange mechanism is bond breaking, in which one electron goes to each atom. As a result, two uncharged particles with unpaired electrons are formed:

Such particles are called free radicals.

free radicals- atoms or groups of atoms that have unpaired electrons.

Reactions that take place under the action and with the participation of free radicals are called free radical reactions.

In the course of inorganic chemistry, these are reactions of interaction of hydrogen with oxygen, halogens, combustion reactions. Please note that reactions of this type are characterized by high speed, release of a large amount of heat.

A covalent bond can also be formed by the donor-acceptor mechanism. One of the orbitals of an atom (or anion), on which an unshared electron pair is located, is overlapped by an unfilled orbital of another atom (or cation), which has an unfilled orbital, and a covalent bond is formed, for example:

$H^(+)+(:O-H^(-))↙(\text"acceptor")→(H-O-H)↙(\text"donor")$

The rupture of a covalent bond leads to the formation of positively and negatively charged particles; since in this case both electrons from the common electron pair remain with one of the atoms, the second atom has an unfilled orbital:

$R:|R=R:^(-)+R^(+)$

Consider the electrolytic dissociation of acids:

$H:|Cl=H^(+)+Cl^(-)$

One can easily guess that a particle having an unshared electron pair $R:^(-)$, i.e. a negatively charged ion, will be attracted to positively charged atoms or to atoms on which there is at least a partial or effective positive charge. Particles with unshared electron pairs are called nucleophilic agents (nucleus- the nucleus, the positively charged part of the atom), that is, the "friends" of the nucleus, the positive charge.

Nucleophiles ($Nu$)- anions or molecules that have a lone pair of electrons that interact with parts of the molecules on which an effective positive charge is concentrated.

Examples of nucleophiles: $Cl^(-)$ (chloride ion), $OH^(-)$ (hydroxide anion), $CH_3O^(-)$ (methoxide anion), $CH_3COO^(-)$ ( acetate anion).

Particles that have an unfilled orbital, on the contrary, will tend to fill it and, therefore, will be attracted to the regions of the molecules that have an increased electron density, a negative charge, and an unshared electron pair. They are electrophiles, "friends" of an electron, a negative charge, or particles with an increased electron density.

electrophiles- cations or molecules that have an unfilled electron orbital, tending to fill it with electrons, as this leads to a more favorable electronic configuration of the atom.

Examples of electrophiles: $NO_2$ (nitro group), -$COOH$ (carboxyl), -$CN$ (nitrile group), -$COH$ (aldehyde group).

Not every particle with an empty orbital is an electrophile. So, for example, alkali metal cations have the configuration of inert gases and do not tend to acquire electrons, since they have a low electron affinity. From this we can conclude that, despite the presence of an unfilled orbital, such particles will not be electrophiles.

Main reaction mechanisms

We have identified three main types of reacting particles - free radicals, electrophiles, nucleophiles - and three types of reaction mechanism corresponding to them:

- free radical;

- electrophilic;

- nucleophilic.

In addition to classifying reactions according to the type of reacting particles, organic chemistry distinguishes four types of reactions according to the principle of changing the composition of molecules: addition, substitution, elimination, or elimination (from lat. elimination- delete, split off) and rearrangement. Since addition and substitution can occur under the action of all three types of reactive species, several main reaction mechanisms can be distinguished.

1.Free radical substitution:

$(CH_4)↙(\text"methane")+Br_2(→)↖(\text"UV light")(CH_3Br)↙(\text"bromomethane")+HBr$

2. Free radical addition:

$nCH_2=CH_2(→)↖(\text"UV light,R")(...-CH_2-CH_2-...)_n$

3. Electrophilic substitution:

4. Electrophilic connection:

$CH_3-(CH=)↙(\text"propene")CH_2+HBr(→)↖(\text"solution")(CH_3-CHBr-CH_3)↙(\text"2-bromopropane")$

$CH_3(-C≡)↙(\text"propyne")CH+Cl_2(→)↖(\text"solution")(CH_3-CCl=CHCl)↙(\text"1,2-dichloropropene")$

5. Nucleophilic addition:

In addition, we will consider the cleavage or elimination reactions that take place under the influence of nucleophilic particles - bases.

6. Elimination:

$CH_3-CHBr-CH_3+NaOH(→)↖(\text"alcohol solution")CH_3-CH=CH_2+NaBr+H_2O$

Rule of V. V. Markovnikov

A distinctive feature of alkenes (unsaturated hydrocarbons) is the ability to enter into addition reactions. Most of these reactions proceed by the mechanism electrophilic addition.

Hydrohalogenation (addition of hydrogen halide):

$CH_3(-CH-)↙(\text"propene")CH_2+HBr→CH_3(-CHBr-CH_3)↙(\text"2-bromopropane")$

This reaction is subject to V. V. Markovnikov's rule: when a hydrogen halide is added to an alkene, hydrogen is added to a more hydrogenated carbon atom, i.e. the atom at which there are more hydrogen atoms, and the halogen - to the less hydrogenated one.

Hydrocarbons, in the molecules of which the atoms are connected by single bonds and which correspond to the general formula C n H 2 n +2.
In alkane molecules, all carbon atoms are in a state of sp 3 hybridization. This means that all four hybrid orbitals of the carbon atom are identical in shape, energy and are directed to the corners of an equilateral triangular pyramid - a tetrahedron. The angles between the orbitals are 109° 28'.

Almost free rotation is possible around a single carbon-carbon bond, and alkane molecules can take on a wide variety of shapes with angles at carbon atoms close to tetrahedral (109 ° 28 ′), for example, in a molecule n-pentane.

It is especially worth recalling the bonds in the molecules of alkanes. All bonds in the molecules of saturated hydrocarbons are single. Overlapping occurs along the axis,
connecting the nuclei of atoms, i.e., these are σ-bonds. Carbon-carbon bonds are non-polar and poorly polarizable. The length of the C-C bond in alkanes is 0.154 nm (1.54 10 - 10 m). C-H bonds are somewhat shorter. The electron density is slightly shifted towards the more electronegative carbon atom, i.e., the C-H bond is weakly polar.

The absence of polar bonds in the molecules of saturated hydrocarbons leads to the fact that they are poorly soluble in water and do not interact with charged particles (ions). The most characteristic of alkanes are reactions that involve free radicals.

Homologous series of methane

homologues- substances similar in structure and properties and differing by one or more CH 2 groups.

Isomerism and nomenclature

Alkanes are characterized by the so-called structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkane, which is characterized by structural isomers, is butane.

Fundamentals of nomenclature

1. Selecting the main circuit. The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in a molecule, which is, as it were, its basis.
2. Numbering of atoms of the main chain. The atoms of the main chain are assigned numbers. The numbering of atoms of the main chain starts from the end closest to the substituent (structures A, B). If the substituents are at an equal distance from the end of the chain, then the numbering starts from the end at which there are more of them (structure B). If different substituents are at an equal distance from the ends of the chain, then the numbering starts from the end to which the older one is closer (structure D). The seniority of hydrocarbon substituents is determined by the order in which the letter with which their name begins follows in the alphabet: methyl (-CH 3), then ethyl (-CH 2 -CH 3), propyl (-CH 2 -CH 2 -CH 3 ) etc.
Note that the name of the substitute is formed by replacing the suffix -an with the suffix - silt in the name of the corresponding alkane.
3. Name formation. Numbers are indicated at the beginning of the name - the numbers of carbon atoms at which the substituents are located. If there are several substituents at a given atom, then the corresponding number in the name is repeated twice separated by a comma (2,2-). After the number, a hyphen indicates the number of substituents ( di- two, three- three, tetra- four, penta- five) and the name of the substituent (methyl, ethyl, propyl). Then without spaces and hyphens - the name of the main chain. The main chain is referred to as a hydrocarbon - a member of the methane homologous series ( methane CH 4, ethane C 2 H 6, propane C 3 H 8, C 4 H 10, pentane C 5 H 12, hexane C 6 H 14, heptane C 7 H 16, octane C 8 H 18, nonan C 9 H 20, dean C 10 H 22).

Physical properties of alkanes

The first four representatives of the homologous series of methane are gases. The simplest of them is methane - a colorless, tasteless and odorless gas (the smell of "gas", having felt which, you need to call 04, is determined by the smell of mercaptans - sulfur-containing compounds specially added to methane used in household and industrial gas appliances so that people those near them could smell the leak).
Hydrocarbons of composition from C 4 H 12 to C 15 H 32 - liquids; heavier hydrocarbons are solids. The boiling and melting points of alkanes gradually increase with increasing carbon chain length. All hydrocarbons are poorly soluble in water; liquid hydrocarbons are common organic solvents.

Chemical properties of alkanes

substitution reactions.
The most characteristic of alkanes are free radical substitution reactions, during which a hydrogen atom is replaced by a halogen atom or some group. Let us present the equations of characteristic reactions halogenation:


In the case of an excess of halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms by chlorine:

The resulting substances are widely used as solvents and starting materials in organic synthesis.
Dehydrogenation reaction(hydrogen splitting off).
During the passage of alkanes over the catalyst (Pt, Ni, Al 2 0 3, Cr 2 0 3) at a high temperature (400-600 ° C), a hydrogen molecule is split off and an alkene is formed:


Reactions accompanied by the destruction of the carbon chain.
All saturated hydrocarbons burn with the formation of carbon dioxide and water. Gaseous hydrocarbons mixed with air in certain proportions can explode.
1. Combustion of saturated hydrocarbons is a free radical exothermic reaction, which is very important when using alkanes as a fuel:

In general, the combustion reaction of alkanes can be written as follows:

2. Thermal splitting of hydrocarbons.

The process proceeds according to the free radical mechanism. An increase in temperature leads to a homolytic rupture of the carbon-carbon bond and the formation of free radicals.

These radicals interact with each other, exchanging a hydrogen atom, with the formation of an alkane molecule and an alkene molecule:

Thermal splitting reactions underlie the industrial process - hydrocarbon cracking. This process is the most important stage of oil refining.

3. Pyrolysis. When methane is heated to a temperature of 1000 ° C, pyrolysis of methane begins - decomposition into simple substances:

When heated to a temperature of 1500 ° C, the formation of acetylene is possible:

4. Isomerization. When linear hydrocarbons are heated with an isomerization catalyst (aluminum chloride), substances with a branched carbon skeleton are formed:

5. Aromatization. Alkanes with six or more carbon atoms in the chain in the presence of a catalyst are cyclized to form benzene and its derivatives:

Alkanes enter into reactions that proceed according to the free radical mechanism, since all carbon atoms in alkane molecules are in a state of sp 3 hybridization. The molecules of these substances are built using covalent non-polar C-C (carbon - carbon) bonds and weakly polar C-H (carbon - hydrogen) bonds. They do not have areas with high and low electron density, easily polarizable bonds, i.e., such bonds, the electron density in which can be shifted under the influence of external factors (electrostatic fields of ions). Consequently, alkanes will not react with charged particles, since bonds in alkane molecules are not broken by a heterolytic mechanism.