Each class of chemical compounds is capable of exhibiting properties due to their electronic structure. Alkanes are characterized by substitution, elimination or oxidation reactions of molecules. All chemical processes have their own characteristics of the flow, which will be discussed further.
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 class-wide formula: H2n+2C.
Chemical structure
The paraffin molecule includes carbon atoms showing sp3-hybridization. They have all four valence orbitals have the same shape, energy and direction in space. The size of the angle between energy levels is 109° and 28'.
The presence of single bonds in molecules determines which reactionscharacteristic of alkanes. They contain σ-compounds. The bond between carbons is nonpolar and weakly polarizable, and is slightly longer than in C−H. There is also a shift in the electron density to the carbon atom, as the most electronegative. As a result, the C−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–C and C–H, which are difficult to break due to nonpolarity. Their destruction is based on a homolytic mechanism, in which free-type radicals participate. That is why alkanes are characterized by substitution reactions. Such substances are not able to interact with water molecules or charge-carrying ions.
They include free radical substitution, in which hydrogen atoms are replaced by halogen elements or other active groups. These reactions include processes associated with halogenation, sulfochlorination and nitration. Their result is the preparation of alkane derivatives.
The mechanism of free radical substitution reactions is based on the main three stages:
- The process begins with the initiation or origin of the chain, as a result of which free radicals are formed. The catalysts are ultraviolet light sources and heat.
- 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 final step is to break the chain. Recombination or disappearance of active particles is observed. This stops the development of a chain reaction.
Halogenation process
It is based on a mechanism of radical type. The halogenation reaction of alkanes takes place by ultraviolet irradiation and heating of a mixture of halogens and hydrocarbons.
All stages of the process are subject to the rule stated by Markovnikov. It states that, first of all, the hydrogen atom, which belongs to the most hydrogenated carbon, is subjected to replacement by a halogen. Halogenation proceeds in the following sequence: from the tertiary atom to the primary carbon.
The process is 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 action of ultraviolet leads to the splitting of chlorine into radical particles that attack the alkane. There is a detachment of atomic hydrogen and the formation of H3C· or a 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.
Only one hydrogen atom is replaced at each stage of the process. The halogenation reaction of alkanes leads to the gradual formation of chloromethane, dichloromethane, trichloromethane and carbon tetrachloride molecules.
Schematically, the process looks like this:
H4C + Cl:Cl → H3CCl + HCl, H3CCl + Cl:Cl → H2CCl2 + HCl, H2CCl2 + Cl:Cl → HCCl3 + HCl, HCCl3 + Cl:Cl → CCl4 + HCl.
Unlike the chlorination of a methane molecule, carrying out such a process with other alkanes is characterized by obtaining substances in which the replacement of hydrogen occurs not at one carbon atom, but at several. Their quantitative ratio is associated with temperature indicators. Under cold conditions, there is a decrease in the rate of formation of derivatives with a tertiary, secondary and primary structure.
With an increase in temperature, the rate of formation of such compounds levels off. The halogenation process is influenced by the static factor, which indicates a different probability of a radical colliding with a carbon atom.
The process of halogenation with iodine does not proceed under normal conditions. It is necessary to create special conditions. When methane is exposed to this halogen, hydrogen iodide is formed. It is affected by methyl iodide, as a result, the initial reagents are released: methane and iodine. Such a reaction is considered reversible.
Wurtz reaction for alkanes
Is a method for obtaining saturated hydrocarbons with a symmetrical structure. Sodium metal, alkyl bromides or alkyl chlorides are used as reactants. Attheir interaction produces sodium halide and an extended hydrocarbon chain, which is the sum of two hydrocarbon radicals. Schematically, the synthesis is as follows: R−Cl + Cl−R + 2Na → R−R + 2NaCl.
The Wurtz reaction for alkanes is possible only if the halogens in their molecules are at the primary carbon atom. For example, CH3−CH2−CH2Br.
If a halocarbon mixture of two compounds is involved in the process, then three different products are formed during the condensation of their chains. An example of such a reaction of alkanes is the interaction of sodium with chloromethane and chloroethane. The output is a mixture containing butane, propane and ethane.
In addition to sodium, other alkali metals can be used, which include lithium or potassium.
Sulfochlorination process
It is also called the Reed reaction. It proceeds according to the principle of free radical substitution. This is a characteristic 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, resulting in an alkyl species 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 → ∙RSO2, ∙RSO2 + ClCl → RSO2Cl + ∙Cl.
Processes related to nitration
Alkanes react with nitric acid in the form of a 10% solution, as well as with tetravalent nitrogen oxide in a gaseous state. The conditions for its flow are high temperature values (about 140 ° C) and low pressure indicators. Nitroalkanes are produced at the output.
This free radical process was named after the scientist Konovalov, who discovered the synthesis of nitration: CH4 + HNO3 → CH 3NO2 + H2O.
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 elimination 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 conditions of high temperature (from 400 to 600 ° C) and under the influence of accelerators in the form of platinum, nickel, chromium and aluminum oxides.
If propane or ethane molecules are involved in the reaction, then its products will be propene or ethene with one double bond.
When dehydrogenating a four or five carbon skeleton, dieneconnections. Butane is formed from butadiene-1, 3 and butadiene-1, 2.
If substances with 6 or more carbon atoms are present in the reaction, then benzene is formed. It has an aromatic core with three double bonds.
Decomposition process
Under conditions of high temperature, reactions of alkanes can take place with the breaking of carbon bonds and the formation of active particles of the radical type. 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-type radicals are formed.
Carrying out the pyrolysis of alkanes with long carbon chains under strong heating is associated with obtaining saturated 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 below 15 atmospheres, in the presence of an aluminosilicate accelerator with the release of alkanes having a branched structure. An example is methane pyrolysis: 2CH4 →t°C2 H2+ 3H2. During this reaction, acetylene and molecular hydrogen are formed.
Methane molecule can undergo conversion. This reaction requires water and a nickel catalyst. On thethe output is a mixture of carbon monoxide and hydrogen.
Oxidation processes
The chemical reactions characteristic of alkanes involve the donation of electrons.
There is auto-oxidation of paraffins. It involves a free-radical mechanism for the 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. Further, another molecule O2 interacts with the alkyl particle, resulting in ∙ROO. An alkane molecule contacts the fatty acid peroxide radical, after which hydroperoxide is released. An example is the autoxidation of ethane:
C2H6 + O2 → ∙C2 H5 + HOO∙, ∙C2H5 + O2 → ∙OOC 2H5, ∙OOC2H5 + C2H6→ HOOC2H5 + ∙C2H5.
Alkanes are characterized by combustion reactions, which are among the main chemical properties when they are determined in the composition of the fuel. They have an oxidative character with heat release: 2C2H6 + 7O2 → 4CO 2 + 6H2O.
If there is a small amount of oxygen in the process, then the final product may be coal or carbon divalent oxide, which is determined by the concentration of O2.
When alkanes are oxidized under the influence of catalytic substances and heated to 200 ° C, molecules of alcohol, aldehyde orcarboxylic acid.
Ethane example:
C2H6 + O2 → C2 H5OH (ethanol),
C2H6 + O2 → CH3 CHO + H2O (ethanal and water), 2C2H6 + 3O2 → 2CH3 COOH + 2H2O (ethanoic acid and water).
Alkanes can be oxidized when exposed to 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 KMnO4 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. This process is catalyzed by aluminum chloride, which interacts with saturated paraffin. An example is the isomerization of a butane molecule, which becomes 2-methylpropane: C4H10 → C3H 7CH3.
Fragrance process
Saturates with six or more carbon atoms in the main carbon chain are capable of dehydrocyclization. Such a reaction is not typical for short molecules. The result is always a six-membered ring in the form of cyclohexane and its derivatives.
In the presence of reaction accelerators, further dehydrogenation takes place andtransformation into a more stable benzene ring. Acyclic hydrocarbons are converted to aromatic compounds or arenes. An example is the dehydrocyclization of hexane:
H3C−CH2− CH2− CH 2− CH2−CH3 → C6H 12 (cyclohexane), C6H12 → C6H6+ 3H2 (benzene).
