Chemical Properties of Alkanes

Think about vast fields filled with natural Gas , long trunk lines to bring the fuel to the site of generation, or rail cars shuttling the fuel between homes and industry. These and many other forms of energy resources are based on alkanes, the simplest class of hydrocarbons. They hold an essential place in our daily life. These are the components of natural gas that we use as heating at home, as well as the gasoline in our cars. They are not only a fuel, but in combination with other ones, the basic material for vast amounts of synthetic products and chemicals that are important to our lives.

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This Story also Contains

1. Main Idea of Alkanes
2. Halogenation
3. Nitration
4. Sulphonation
5. Types and Kinds of Alkanes
6. Combustion
7. Catalytic oxidation
8. Isomerization
9. Aromatization
10. Pyrolysis
11. Uses
12. Academic Industrial Interest
13. Some Solved Examples
14. Summary

Main Idea of Alkanes

The Alkanes are the class of Saturated Hydrocarbons containing a simple structural formula of carbon and hydrogens having connected with each other only single covalent bounds. The general formula of alkanes is CnH2n+2 where n is the number of C atoms Connected with each other. These are some of the properties in this class of Organic compounds that they are comparatively stable and shows less reactivity with other organic compounds. The single bonds in alkanes are of the sigma type. Since these are strong in nature, and since they are nonpolar, they are expected to be inert. Their boiling and melting points increase with molecular weight as a result of the increased van der Waal's force in larger molecules. Alkanes are non-polar and therefore do not dissolve in water, but they dissolve in all organic solvents. They have low reactivity because the dissociation energy between the C−C,C−H bond is very high. The high stability makes them unreactive; their unreactivity is, in fact, a major feature of their chemical behavior.

Halogenation

When alkanes are treated with halogens in the presence of light or at elevated temperatures, the hydrogen atoms of alkanes are successively replaced by halogen atoms. This process is known as halogenation. The rate of reaction of alkanes with halogen follows the following order:
F₂>C_l2>Br₂>I₂
This reaction is carried out with chlorine as fluorine is too violent to be controlled and iodine is too slow and reversible.
The mechanism of this reaction occurs by a free radical mechanism and it is done in three successive steps as follows:

1. Chain initiation step: The reaction is initiated by homolysis of chlorine molecules in the presence of light or heat. The Cl−Clbond is weaker than theC−C and C−Hbond and hence, is easiest to break.
   
2. Chain propagation step: Chlorine-free radical attacks the methane molecule and takes the reaction in the forward direction by breaking the C-H bond to generate methyl free radical with the formation of H-Cl.
   
3. Chain termination step: The reaction stops after some time due to the consumption of reactants and/or due to the following side reactions.
   The possible chain-terminating steps are:

The above mechanism helps us to understand the reason for the formation of ethane as a byproduct during the chlorination of methane.

Nitration

Nitration is a substitution reaction in which a hydrogen atom of an alkane is replaced by nitro(−NO₂)group. The reaction occurs as follows:

$\mathrm{R}-\mathrm{H}+\mathrm{HONO}_2 \xrightarrow[\text { Temperature }]{\text { Hign }} \mathrm{R}-\mathrm{NO}_2+\mathrm{H}_2 \mathrm{O}$
Lower members do not react with concentrated nitric acid at ordinary temperatures but long-chain members on heating with fuming nitric acid yield nitroalkanes. However, when a mixture of vapours of an alkane and nitric acid is heated at 673-773K, nitroalkane is formed readily. This is known as vapour phase nitration. By this process, lower, as well as higher alkanes, can be converted into nitroalkanes.

$\mathrm{CH}_3+\mathrm{HONO}_2 \xrightarrow{723 \mathrm{~K}} \mathrm{CH}_3-\mathrm{NO}_2+\mathrm{H}_2 \mathrm{C}$ CH₃+HONO₂→ KCH₃−NO₂+H₂C

Sulphonation

The replacement of hydrogen atom by sulphonic cid group(-SO₃H) is known as sulphonation. Lower alkanes do not undergo sulphonation but higher members (from hexane onwards) are sulphonated slowly when treated with fuming sulphuric acid. at about 673K. The reaction occurs as follows:

$\mathrm{R}-\mathrm{H}+\mathrm{HOSO}_3 \mathrm{H} \xrightarrow[\text { Prolonged heating }]{\mathrm{SO}_3} \mathrm{R}-\mathrm{SO}_3 \mathrm{H}+\mathrm{H}_2 \mathrm{O}$
For example:

$\mathrm{C}_6 \mathrm{H}_{13}+\mathrm{HOSO}_3 \mathrm{H} \xrightarrow{\mathrm{SO}_3} \mathrm{C}_6 \mathrm{H}_{12} \mathrm{SO}_3 \mathrm{H}+\mathrm{H}_2 \mathrm{O}$
However, lower members such as propane, butane, pentane, etc. react with SO₃ in vapour phase to form sulphonic acids.

Types and Kinds of Alkanes

Three types of alkanes have been identified based on the types of carbon chains that they have, such as straight-chain (normal alkanes), branched-chain (iso alkanes), and cycloalkanes. Normal alkanes hold those carbon atoms that are linked in with a consecutive chain and branched alkanes hold one or many carbon atoms that branch from the main chain. Alkanes whose atoms create a ring of their own self, like that of cyclohexane, are termed cycloalkanes.
Alkanes show mainly substitution and combustion reactions. In substitution reactions, for instance, a hydrogen atom in the alkane may be replaced by another atom or group, especially a halogen, in the presence of light or heat. Combustion reactions involve an alkane reacting with an oxygen molecule to produce carbon dioxide and water, besides the liberation of energy: alkanes are hence important fuels. Another principal reaction of alkanes is cracking, the process of breaking larger alkanes into smaller ones, significantly increasing the yield of readily useful hydrocarbons such as gasoline.

Combustion

Alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of large amount of heat.

$\mathrm{CH}_4(\mathrm{~g})+2 \mathrm{O}_2(\mathrm{~g}) \rightarrow \mathrm{CO}_2(\mathrm{~g})+2 \mathrm{H}_2 \mathrm{O}(1), \Delta_{\mathrm{c}} \mathrm{H}^{\circ}=-890 \mathrm{~kJ} / \mathrm{mol}$
Due to the production of large amount of heat, alkanes are used as fuels.

Catalytic oxidation

Alkanes on heating with a regulated supply of dioxygen or air at high pressure and in the presence of suitable catalysts give a variety of oxidation products. The reactions occur as follows:

$\begin{aligned} & 2 \mathrm{CH}_4+\mathrm{O}_2 \xrightarrow{\mathrm{Cu} \xrightarrow{3} 3 \mathrm{~K}} 2 \mathrm{CH}_3 \mathrm{OH} \\ & 2 \mathrm{CH}_3 \mathrm{CH}_3+3 \mathrm{O}_2 \xrightarrow{\left(\mathrm{CH}_3 \mathrm{COOO}_2 \mathrm{Mn}\right.} 2 \mathrm{CH}_3 \mathrm{COOH}+2 \mathrm{H}_2 \mathrm{O}\end{aligned}$

Isomerization

n-Alkanes on heating in the presence of anhydrous aluminum chloride and hydrogen chloride gas isomerize to branched-chain alkanes. Major products are given below. Some minor products are also possible which you can think over. Minor products are generally not reported in organic reactions.

Aromatization

n-Alkanes having six or more carbon atoms on heating to 773K at 10-20 atmospheric pressure in the presence of oxides of vanadium, molybdenum or chromium supported over alumina get dehydrogenated and cyclized to benzene and its homologs. This reaction is known as aromatization or reforming.

Pyrolysis

The higher alkanes split into lower alkanes when heated strongly at a high temperature in the absence of air. During pyrolysis, C-C bond breaks rather than C-H bonds as bond energy of $\mathrm{C}-\mathrm{H}>\mathrm{C}-\mathrm{C}$. Here product formation depends upon the structure of alkane, the extent of temperature and pressure and the presence/absence of catalysts like$\mathrm{SiO}_2-\mathrm{Al}_2 \mathrm{O}_3$ etc. tc. Pyrolysis of alkanes is believed to be a free radical reaction. Preparation of oil gas or petrol gas from kerosene oil or petrol involves the principle of pyrolysis. For example:

$\underset{\text { Dodecane }}{\mathrm{C}_{12} \mathrm{H}_{26} \xrightarrow[\text { atak }]{\text { Ptedane }} \mathrm{C}_7 \mathrm{H}_{16}+\mathrm{C}_5 \mathrm{H}_{10}+\text { other products }}$

Uses

Alkanes are very useful in our daily lives and in a myriad of industries. Natural gas consisting mostly of methane is used to heat buildings and cook food, as well as to generate electricity. Propane and butane are marketed as liquefied petroleum gas (LPG) for domestic use and are also valuable for many industrial applications. Gasoline, primarily a mixture of alkanes, is the major fuel used for transportation. The alkanes are also raw materials for a very wide range of important chemicals, including plastics, synthetic fibers, and detergents.

Academic Industrial Interest

Academically, much support is extended through the study of alkanes in organic chemistry, and the study takes an in-depth position toward the knowledge of how behavior and reactivity in hydrocarbons might be, which will help in initiating new structures of chemical processing and material. On the other hand, with regard to industry, the refining of crude oil is a very important process for the generation of different fractions of alkanes starting from the raw material. The improvement of its efficiency due to the development in the field of catalytic reactions has increased cracking and reforming.

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Some Solved Examples

Example 1

Question:
The major product obtained in the photo-catalyzed bromination of 2-methylbutane is:

1) 1-bromo-2-methylbutane
2) 1-bromo-3-methylbutane
3) 2-bromo-3-methylbutane
4) 2-bromo-2-methylbutane (correct)

Solution:
In the photo catalyzed bromination of 2-methylbutane, the bromine atom preferentially substitutes at the tertiary carbon due to stability considerations. This gives us 2-bromo-2-methylbutane as the major product. Hence, the correct answer is option (4).

Example 2

Question:
How many chiral compounds are possible on monochlorination of 2-methylbutane? (Report the number of enantiomeric pairs as the answer)

1) 8
2) 2 (correct)
3) 4
4) 6

Solution:
Monochlorination of 2-methylbutane leads to the formation of two chiral compounds, each with its enantiomer. Therefore, the total number of chiral compounds (enantiomeric pairs) is 2. Hence, the correct answer is option (2)

Example 3

Question:
How many isomers are obtained on monochlorination of isopentane? (excluding stereoisomers)

1) 2
2) 3
3) 4 (correct)
4) 5

Solution:
Monochlorination of isopentane results in four different positional isomers, excluding stereoisomers. These positional isomers arise due to the chlorine substituting at different carbon atoms within the molecule. Hence, the correct answer is option (3).

Summary

Hence, these are the simplest types of hydrocarbons, significant in an individual's basic energy needs and daily survival. Furthermore, they have the distinction of being stable; their reactivity is generally low while their diversity is extremely high since they can exist in three different forms: straight chain, branched chain, and cycloalkanes. The main reactions that they undergo are essentially only substitution and combustion, although the applications used for the alkanes span from household fuel to industrial raw materials. The understanding of alkanes augments the study of organic chemistry and helps usher in a scientific revolution in energy and material sciences.