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 life.
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Alkanes are extremely simple hydrocarbons: by definition, they are saturated and contain no atoms other than carbon and hydrogen; all the bonds between the atoms are simple single covalent bonds. This very simplicity implies one special set of chemical properties: alkanes are very stable and non-reactive. Alkanes do not readily engage in chemical reactions, allowing them to have storage and transportation properties as fuels. They actually occur from the strong sigma bonds linking the carbon and hydrogen atoms together and, hence are virtually non-reactive under normal conditions.
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
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:
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:
The above mechanism helps us to understand the reason for the formation of ethane as a byproduct during the chlorination of methane.
Nitration is a substitution reaction in which a hydrogen atom of an alkane is replaced by nitro
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.
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:
For example:
However, lower members such as propane, butane, pentane, etc. react with SO3 in vapour phase to form sulphonic acids.
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.
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.
Due to the production of large amount of heat, alkanes are used as fuels.
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:
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.
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.
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
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.
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.
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).
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.
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