Which alkanes are characterized by reactions

Each class of chemical compounds is capable of exhibiting properties due to their electronic structure. The alkanes are characterized by reactions of substitution, cleavage or oxidation of molecules. All chemical processes have their own percolation characteristics, which will be discussed later.

What are alkanes

These are saturated hydrocarbon compounds, which are called paraffins. Their molecules consist only of carbon and hydrogen atoms, have a linear or branched acyclic chain, in which there are only single compounds. Given the characteristics of the class, it is possible to calculate which reactions are characteristic of alkanes. They obey the formula for the whole class: H _{2n + 2} C _n .

Chemical structure

The paraffin molecule includes carbon atoms that exhibit sp ³ -hybridization. They have all valence four orbitals with the same shape, energy and direction in space. The angle between the energy levels is 109 ° and 28 '.

The presence of single bonds in molecules determines which reactions are characteristic for alkanes. They contain σ-compounds. The bond between the carbons is nonpolar and weakly polarizable, it is slightly longer than in C-H. There is also a shift in the electron density to the carbon atom, as the most electronegative one. As a result, the C-H compound is characterized by a low polarity.

Substitution reactions

Substances of the class of paraffins have weak chemical activity. This can be explained by the strength of the bonds between C-C and C-H, which are difficult to break due to non-polarity. Their destruction is based on the homolytic mechanism, in which radicals of a free type participate. This is why alkanes are characterized by substitution reactions. Such substances are not able to interact with water molecules or charge-carrying ions.

They are considered to be a free-radical substitution, in which the hydrogen atoms are replaced by halogen elements or other active groups. Such reactions include processes associated with halogenation, sulfochlorination and nitration. Their result is the production of alkane derivatives. At the core of the mechanism of substitution reactions over the free radical type, there are three main stages:

1. The process begins with the initiation or nucleation of a chain, as a result of which free radicals are formed. The catalysts are ultraviolet light sources and heating.
2. Then a chain develops, in which successive interactions of active particles with inactive molecules are carried out. Their transformation into molecules and radicals, respectively.
3. The final stage will be a break in the chain. Recombination or disappearance of active particles is observed. Thus the development of the chain reaction ceases.

The halogenation process

It is based on a radical-type mechanism. The reaction of halogenation of alkanes takes place when irradiated with ultraviolet light and the mixture is heated from halogens and hydrocarbons.

All the stages of the process are subject to the rule that Markovnikov expressed. It indicates that the substitution of halogen, first of all, the hydrogen atom, which belongs to the hydrogenated carbon itself. Halogenation proceeds in this order: from the tertiary atom to the primary carbon.

The process passes better for molecules of alkanes with a long main carbon chain. This is due to the decrease in ionizing energy in a given direction, the electron is more easily separated from the substance.

An example is the chlorination of a methane molecule. The action of ultraviolet leads to the splitting of chlorine into radical particles, which carry out an attack on the alkane. There is a detachment of atomic hydrogen and the formation of H ₃ C · or methyl radical. Such a particle, in turn, attacks molecular chlorine, leading to the destruction of its structure and the formation of a new chemical reagent.

At each stage of the process, only one hydrogen atom is replaced. The reaction of halogenation of alkanes leads to the gradual formation of a chloromethane, dichloromethane, trichloromethane and tetrachloromethane molecule.

Schematically the process is as follows:

H ₄ C + Cl: Cl → H ₃ CCl + HCl,

H ₃ CCl + Cl: Cl → H ₂ CCl ₂ + HCl,

H ₂ CCl ₂ + Cl: Cl → HCCl ₃ + HCl,

HCCl ₃ + Cl: Cl → CCl ₄ + HCl.

In contrast to the chlorination of the methane molecule, carrying out such a process with other alkanes is characterized by obtaining substances in which the substitution of hydrogen occurs not in one carbon atom, but in several. Their quantitative relationship is related to temperature. In cold conditions, the rate of formation of derivatives with a tertiary, secondary and primary structure is reduced.

With an increase in the temperature index, the speed of formation of such compounds is equalized. The halogenation process has the influence of a static factor, which indicates a different probability of a radical colliding with a carbon atom.

The process of halogenation with iodine does not occur under normal conditions. It is necessary to create special conditions. When exposed to methane with this halogen, hydrogen iodide occurs. It is affected by iodide methyl, as a result, the original reagents are released: methane and iodine. Such a reaction is considered reversible.

Wurz reaction for alkanes

It is a method of obtaining saturated hydrocarbons with a symmetrical structure. Reactive substances are sodium metal, alkyl bromides or alkyl chlorides. When they interact, sodium halide and an increased hydrocarbon chain are obtained, which is the sum of two hydrocarbon radicals. Schematically, the synthesis is as follows: R-Cl + Cl-R + 2Na → R-R + 2NaCl.

The Wurz reaction for alkanes is possible only if in their molecules the halogens are located at the primary carbon atom. For example, CH ₃ -CH ₂ -CH ₂ Br.

If a halogen-hydrocarbon mixture of two compounds participates in the process, three different products are formed during the condensation of their chains. An example of such an alkane reaction is the interaction of sodium with chloromethane and chloroethane. The output is a mixture containing butane, propane and ethane.

In addition to sodium, you can use other alkali metals, which include lithium or potassium.

Process of sulfochlorination

It is also called Reed's reaction. It flows according to the principle of free radical replacement. This is a typical type of reaction of alkanes to the action of a mixture of sulfur dioxide and molecular chlorine in the presence of ultraviolet radiation.

The process begins with the initiation of a chain mechanism, in which two radicals are obtained from chlorine. One of them attacks the alkane, which leads to the appearance of an alkyl particle and a hydrogen chloride molecule. Sulfur dioxide is attached to the hydrocarbon radical to form a complex particle. For stabilization, one chlorine atom is captured from another molecule. The final substance is alkane sulfonyl chloride, it is used in the synthesis of surface-active compounds.

Schematically, the process looks like this:

ClCl → ^Hv ∙ Cl + ∙ Cl,

HR + ∙ Cl → R ∙ + HCl,

R ∙ + OSO → ∙ RSO ₂ ,

∙ RSO ₂ + ClCl → RSO ₂ Cl + ∙ Cl.

The processes associated with nitration

The alkanes react with a nitric acid in the form of a solution of 10%, and also with the nitrogen of a tetravalent oxide in the gaseous state. Conditions for its passage are high temperature values (about 140 ° C) and low pressure values. At the output, nitroalkanes are produced.

This free-radical process was named after the scientist Konovalov, who discovered the synthesis of nitration: CH ₄ + HNO ₃ → CH ₃ NO ₂ + H ₂ O.

Cleavage mechanism

For alkanes, dehydrogenation and cracking reactions are characteristic. The methane molecule undergoes complete thermal decomposition.

The main mechanism of the above reactions is the elimination of atoms from alkanes.

The process of dehydrogenation

When separating hydrogen atoms from the carbon skeleton of paraffins, with the exception of methane, unsaturated compounds are obtained. Such chemical reactions of alkanes take place under high temperature conditions (from 400 to 600 ° C) and under the action of accelerators in the form of platinum, nickel, chromium and aluminum oxides.

If the reaction involves the molecules of propane or ethane, then its products will be propene or ethen with one double bond.

When dehydrogenating a four or five-carbon skeleton, diene compounds are obtained. Butane is formed from butane-1,3 and butadiene-1,2.

If the reaction contains substances with 6 or more carbons, then benzene is formed. It has an aromatic core with three bonds double.

The process associated with the decomposition

Under high temperature conditions, the reactions of alkanes can proceed with the rupture of carbon bonds and the formation of active radical-type particles. Such processes are called cracking or pyrolysis.

Heating the reactants to temperatures exceeding 500 ° C leads to the decomposition of their molecules, during which complex mixtures of alkyl radicals are formed.

Carrying out strong pyrolysis of alkanes with long carbon chains is associated with obtaining limiting and unsaturated compounds. It is called thermal cracking. This process was used until the middle of the 20th century.

The disadvantage was the production of hydrocarbons with a low octane number (no more than 65), so it was replaced by catalytic cracking. The process takes place under temperature conditions that are below 440 ° C, and pressures of less than 15 atmospheres, in the presence of an aluminosilicate accelerator, with the release of alkanes having a branched structure. An example is methane pyrolysis: 2CH ₄ → ^{t °} C ₂ H ₂ + 3H ₂ . During this reaction, acetylene and molecular hydrogen are formed.

A methane molecule can undergo conversion. For such a reaction, water and a nickel catalyst are necessary. The output is a mixture of carbon monoxide and hydrogen.

Oxidation processes

The chemical reactions characteristic of alkanes are related to the release of electrons.

There is an auto-oxidation of paraffins. It involves a free radical mechanism of oxidation of saturated hydrocarbons. During the reaction, hydroperoxides are obtained from the liquid phase of the alkanes. At the initial stage, the paraffin molecule interacts with oxygen, resulting in the release of active radicals. Further with an alkyl particle, another O ₂ molecule interacts, we obtain ∙ ROO. With the peroxy fatty acid radical, an alkane molecule is contacted, after which hydroperoxide is released. An example is the auto-oxidation of ethane:

C ₂ H ₆ + O ₂ → ∙ C ₂ H ₅ + HOO ,

∙ C ₂ H ₅ + O ₂ → ∙ OOC ₂ H ₅ ,

∙ OOC ₂ H ₅ + C ₂ H ₆ → HOOC ₂ H ₅ + ∙ C ₂ H ₅ .

For alkanes, combustion reactions are characteristic, which relate to the main chemical properties when they are determined in the composition of the fuel. They have oxidative character with heat release: 2C ₂ H ₆ + 7O ₂ → 4CO ₂ + 6H ₂ O.

If a small amount of oxygen is observed in the process, the end product may be coal or carbon bivalent oxide, which is determined by the concentration of O ₂ .

In the oxidation of alkanes under the influence of catalytic substances and heating to 200 ° C, molecules of alcohol, aldehyde or carboxylic acid are obtained.

Example with ethane:

C ₂ H ₆ + O ₂ → C ₂ H ₅ OH (ethanol),

C ₂ H ₆ + O ₂ → CH ₃ CHO + H ₂ O (ethanal and water),

2C ₂ H ₆ + 3 O ₂ → 2CH ₃ COOH + 2H ₂ O (ethanoic acid and water).

Alkanes can be oxidized by the action of three-membered cyclic peroxides on them. They include dimethyldioxirane. The result of the oxidation of paraffins is an alcohol molecule.

Representatives of paraffins do not react to KMnO ₄ or potassium permanganate, as well as to bromine water.

Isomerization

For alkanes, the type of reaction is characterized by a substitution with an electrophilic mechanism. This is the isomerization of the carbon chain. Catalyzes this process of aluminum chloride, which interacts with saturated paraffin. An example is the isomerization of a butane molecule, which becomes 2-methylpropane: C ₄ H ₁₀ → C ₃ H ₇ CH ₃ .

The process of aromatization

Saturated substances, in which the main chain of carbon contains six or more carbon atoms, are capable of performing dehydrocyclization. For short molecules, this reaction is not typical. The result is always a six-membered cycle in the form of cyclohexane and its derivatives.

In the presence of reaction accelerators, further dehydrogenation and conversion to a more stable benzene ring takes place. There is a transformation of acyclic hydrocarbons into aromatic compounds or arenes. An example is dehydrocyclization of hexane:

H ₃ C-CH ₂ -CH ₂ -CH ₂ -CH ₂ -CH ₃ → C ₆ H ₁₂ (cyclohexane),

C ₆ H ₁₂ → C ₆ H ₆ + 3H ₂ (benzene).