Each class of chemical compounds is capable of exhibiting properties due to their electronic structure. Alkanes are characterized by reactions of substitution, cleavage, or oxidation of molecules. All chemical processes have their own flow characteristics, which will be discussed later.
What are alkanes?
These are saturated hydrocarbon compounds 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 exhibiting sp 3 hybridization. They all valence four orbitals have the same shape, energy and direction in space. The angle between the energy levels is 109 Β° and 28 '.
The presence of single bonds in the molecules determines which reactions are characteristic of alkanes. They contain Ο compounds. The bond between the carbons is nonpolar and weakly polarizable; it is slightly longer than in C β H. A shift in the electron density to the carbon atom, as the most electronegative, is also observed. As a result, the C β H compound is characterized by low polarity.
Substitution Reactions
Substances of the paraffin class have weak chemical activity. This can be explained by the strength of the bonds between C β C and C β H, which are difficult to break due to non-polarity. Their destruction is based on a homolytic mechanism in which free-type radicals participate. That is why substitution reactions are characteristic of alkanes. Such substances are not able to interact with water molecules or charge-carrying ions.
They consider free radical substitution in which 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.
The basis of the mechanism of substitution reactions according to the free radical type are the main three stages:
- 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.
- Then a chain develops in which successive interactions of active particles with inactive molecules take place. They are converted into molecules and radicals, respectively.
- The end stage will be an open chain. Recombination or disappearance of active particles is observed. So the development of a chain reaction stops.
Halogenation process
It is based on a mechanism of a radical type. The alkane halogenation reaction takes place by irradiation with ultraviolet light and heating a mixture of halogens and hydrocarbons.
All stages of the process obey the rule expressed by Markovnikov. It indicates that undergoes halogen substitution, primarily a hydrogen atom, which belongs to the hydrogenated carbon itself. Halogenation proceeds in the following sequence: from a tertiary atom to primary carbon.
The process goes better for alkane molecules with a long main carbon chain. This is due to a decrease in ionizing energy in this direction, an electron is more easily split off from the substance.
An example is the chlorination of a methane molecule. The effect of ultraviolet radiation leads to the splitting of chlorine into radical particles that carry out an attack on alkane. The separation of atomic hydrogen and the formation of H 3 C Β· 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 alkane halogenation reaction leads to the gradual formation of a chloromethane, dichloromethane, trichloromethane and tetrachloromethane molecules.
Schematically, the process is as follows:
H 4 C + Cl: Cl β H 3 CCl + HCl,
H 3 CCl + Cl: Cl β H 2 CCl 2 + HCl,
H 2 CCl 2 + Cl: Cl β HCCl 3 + HCl,
HCCl 3 + Cl: Cl β CCl 4 + HCl.
Unlike the chlorination of a methane molecule, carrying out such a process with other alkanes is characterized by the production of substances in which hydrogen is replaced not by one carbon atom, but by several. Their quantitative ratio is related to temperature indicators. In cold conditions, a decrease in the rate of formation of derivatives with a tertiary, secondary, and primary structure is observed.
With an increase in temperature, the rate of formation of such compounds is leveled off. The halogenation process is influenced by a static factor, which indicates a different probability of collision of a radical with a carbon atom.
The process of halogenation with iodine under normal conditions does not proceed. It is necessary to create special conditions. When methane is exposed to this halogen, hydrogen iodide occurs. Methyl iodide acts on it, as a result, the initial reagents are released: methane and iodine. Such a reaction is considered reversible.
WΓΌrz reaction for alkanes
It is a method for producing saturated hydrocarbons with a symmetrical structure. Sodium metal, alkyl bromides or alkyl chlorides are used as reactants. During their interaction, sodium halide and an increased hydrocarbon chain are obtained, which is the sum of two hydrocarbon radicals. Schematically, the synthesis is as follows: R β Cl + Cl β R + 2Na β R β R + 2NaCl.
The Wurz reaction for alkanes is possible only if the halogens in their molecules are located at the primary carbon atom. For example, CH 3 βCH 2 βCH 2 Br.
If a halogen-hydrocarbon mixture of two compounds is involved in the process, then upon the condensation of their chains three different products are formed. An example of such an alkane reaction is the interaction of sodium with chloromethane and chloroethane. The result is a mixture containing butane, propane and ethane.
In addition to sodium, you can use other alkali metals, which include lithium or potassium.
Sulfochlorination process
It is also called Reed's reaction. It proceeds according to the principle of free radical substitution. This is a typical type of alkane reaction 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 formed from chlorine. One of them attacks the alkane, which leads to the appearance of an alkyl particle and a molecule of hydrogen chloride. Sulfur dioxide is attached to the hydrocarbon radical to form a complex particle. To stabilize, 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 β hv β Cl + β Cl,
HR + β Cl β R β + HCl,
R β + OSO β β RSO 2 ,
β RSO 2 + ClCl β RSO 2 Cl + β Cl.
Nitration processes
Alkanes react with nitric acid in the form of a 10% solution, as well as with tetravalent nitric oxide in a gaseous state. The conditions of its course are high temperature values ββ(about 140 Β° C) and low pressure values. Nitroalkanes are produced at the output.
This free radical type process was named after the scientist Konovalov, who discovered the synthesis of nitration: CH 4 + HNO 3 β CH 3 NO 2 + H 2 O.
Cleavage mechanism
Alkanes are characterized by dehydrogenation and cracking reactions. The methane molecule undergoes complete thermal decomposition.
The main mechanism of the above reactions is the removal of atoms from alkanes.
Dehydrogenation process
When hydrogen atoms are separated 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 Β° C) and under the action of accelerators in the form of platinum, nickel, chromium and aluminum oxides.
If propane or ethane molecules participate in the reaction, then its products will be propene or ethene with one double bond.
When dehydrogenating a four or five-carbon skeleton, diene compounds are obtained. Butadiene-1,3 and butadiene-1,2 are formed from butane.
If substances with 6 or more carbon atoms are present in the reaction, benzene is formed. It has an aromatic core with three double bonds.
Decomposition process
Under high temperature conditions, alkane reactions can proceed with the breaking of carbon bonds and the formation of radical-type active particles. Such processes are called cracking or pyrolysis.
Heating the reactants to temperatures exceeding 500 Β° C leads to the decomposition of their molecules, during which complex mixtures of radicals of the alkyl type are formed.
Carrying out with strong heating the pyrolysis of alkanes with long carbon chains is associated with the preparation of limit and unsaturated compounds. It is called thermal cracking. Such a process was used until the middle of the 20th century.
The disadvantage was the production of hydrocarbons with a low octane number (not more than 65), so it was replaced by catalytic cracking. The process takes place under temperature conditions below 440 Β° C and pressure values ββof less than 15 atmospheres in the presence of an aluminosilicate accelerator with the release of alkanes with a branched structure. An example is methane pyrolysis: 2CH 4 β t Β° C 2 H 2 + 3H 2 . During this reaction, acetylene and molecular hydrogen are formed.
The methane molecule may undergo conversion. Such a reaction requires water and a nickel catalyst. The output is a mixture of carbon monoxide and hydrogen.
Oxidative processes
Chemical reactions characteristic of alkanes are associated with the release of electrons.
There is an autooxidation of paraffins. It uses the free-radical mechanism of oxidation of saturated hydrocarbons. During the reaction, hydroperoxides are obtained from the liquid phase of alkanes. At the initial stage, the paraffin molecule interacts with oxygen, as a result, active radicals are released. Next, another O 2 molecule interacts with the alkyl particle, and ROO is obtained. An alkane molecule contacts the fatty acid peroxide radical, after which hydroperoxide is released. An example is auto-oxidation of ethane:
C 2 H 6 + O 2 β β C 2 H 5 + HOO β,
β C 2 H 5 + O 2 β β OOC 2 H 5 ,
β OOC 2 H 5 + C 2 H 6 β HOOC 2 H 5 + β C 2 H 5 .
Alkanes are characterized by combustion reactions, which relate to the main chemical properties, when determined in the composition of the fuel. They have an oxidizing character with heat emission: 2C 2 H 6 + 7O 2 β 4CO 2 + 6H 2 O.
If a small amount of oxygen is observed in the process, then the final product may be coal or carbon divalent oxide, which is determined by the concentration of O 2 .
When alkanes are oxidized under the influence of catalytic substances and heated to 200 Β° C, alcohol, aldehyde, or carboxylic acid molecules are obtained.
Example with ethane:
C 2 H 6 + O 2 β C 2 H 5 OH (ethanol),
C 2 H 6 + O 2 β CH 3 CHO + H 2 O (ethanal and water),
2C 2 H 6 + 3O 2 β 2CH 3 COOH + 2H 2 O (ethanoic acid and water).
Alkanes can be oxidized by the action of three-membered cyclic peroxides. These 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
On alkanes, the type of reaction is characterized by substitution with an electrophilic mechanism. This includes the isomerization of the carbon chain. Catalyzes this process aluminum chloride, which interacts with saturated paraffin. An example is the isomerization of a butane molecule, which becomes 2-methylpropane: C 4 H 10 β C 3 H 7 CH 3 .
Flavoring process
Saturated substances in which the carbon backbone contains six or more carbon atoms are capable of dehydrocyclizing. Short molecules are not characterized by such a reaction. 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. Acyclic hydrocarbons are converted into aromatic compounds or arenas. An example is hexane dehydrocyclization:
H 3 C β CH 2 - CH 2 - CH 2 - CH 2 βCH 3 β C 6 H 12 (cyclohexane),
C 6 H 12 β C 6 H 6 + 3H 2 (benzene).