Alkanes: Preparation, Properties, and Uses

Alkanes are a group of chemicals that contain single covalent bonds between carbon and hydrogen atoms. This group of chemicals is made up of single covalent bonds between carbon and hydrogen atoms. For example, coal gases which are obtained by destructive distillation of coal, LPG (Liquified Petroleum Gas), and CNG (Compressed Natural Gas), are used as fuels. In addition, kerosene oil is also used as a domestic fuel. All these fuels contain a mixture of hydrocarbons, which are sources of energy. Read the full article for complete details on this topic.

Alkanes – Hydrocarbons

The simplest organic compounds containing carbon and hydrogen only are called hydrocarbons. These are widely distributed in nature as coal, petroleum, and natural gas. Thus, hydrocarbons are considered to be the parent organic compounds. All other compounds, on the other hand, are assumed to have been created by replacing one or more hydrogen atoms with functional groups.

What are Alkanes?

Alkanes are saturated hydrocarbons, and they have a single covalent bond between hydrogen and carbon atoms. This means that all the atoms share one pair of electrons. The general formula of alkanes is \({{\text{C}}_{\text{n}}}{{\text{H}}_{2{\text{n}} + 2}}\). They are categorized into three types: chain alkanes, branched alkanes and cycloalkanes.

Nomenclature of Alkanes

The general formula of alkanes is \({{\text{C}}_{\text{n}}}{{\text{H}}_{2{\text{n}} + 2}}\), where \({\text{n}} = 1,2,3,4……\) According to the IUPAC system, the names of alkanes have a suffix- ane, and the prefix depends on the number of carbon atoms. Some names of the saturated hydrocarbons are given as follows:

Isomerism of Alkanes

The first three alkanes- methane, ethane, and propane- have only one structure since there is only one way to connect \(1,2,\) and \(3\) carbon atoms. However, for butane, two isomers are possible.

Five carbon atoms can be joined in three different ways, and hence pentane has three isomers.

Similarly, hexane has \(5\), heptane has \(9\), and octane has \(18\), nonane has \(35\), while decane has \(75\) isomers.

Preparation of Alkanes

From unsaturated hydrocarbons- The process of the addition of hydrogen to an unsaturated hydrocarbon in the presence of the catalyst is called hydrogenation or reduction.

\( { {\text{C}}_ {\text{2}}}{ {\text{H}}_ {\text{4}}} {\text{ + }} {{\text{H}}_ {\text{2}}} \to {{\text{C}}_{\text{2}}}{{\text{H}}_{\text{6}}}{{\text{C}}_{\text{3}}}{{\text{H}}_{\text{6}}}{\text{ + }}{{\text{H}}_{\text{2}}} \to{{\text{C}}_{\text{3}}}{{\text{H}}_{\text{8}}}\)

1. From alkyl halides- Alkyl halides can be converted into alkanes by any one of the following methods:

i.) Grignard Reagent- Alkyl halides, especially bromides, and iodides react with magnesium metal in the presence of dry ethoxyethane (diethyl ether) to form alkyl magnesium halides. These alkyl magnesium halides further react with water to form alkanes.

ii.) By Wurtz Reaction- When an alkyl halide, especially bromide and iodide, is treated with metallic sodium in the presence of dry ethyl ether, the production of alkanes is higher.

2.From carboxylic acid- Alkanes can be obtained by any of the following methods.

i.) Decarboxylation- It is a laboratory method of preparation of alkanes.

When a carboxylic acid is heated with soda lime \(\left({{\text{NaOH + CaO}}} \right)\) at about \(630\,{\text{k}}\), a molecule of carbon dioxide is lost, and an alkane with a carbon atom less than carboxylic acid is obtained.

ii.)  Kolbe’s electrolytic method- When a concentrated aqueous solution of potassium or sodium

Physical Properties of Alkanes

The physical properties of alkanes depend upon the intermolecular force of attraction. Let us discuss some of the following properties:

1.Boiling point- The boiling points of straight-chain alkanes increase with increases in the molecular mass.
This increase occurs due to a corresponding increase in the molecule’s molecular size and surface area, which increases the magnitude of van der Waals forces of attraction, and hence, boiling point increases.
Among the straight-chain alkanes, the first four members are gases, the next thirteen are liquids, and the higher members are colourless waxy solids.

2. Melting Point- The melting point of alkanes also follows the same trend as the boiling point. The melting point of alkanes increases with an increase in the molecular mass.
The melting point of alkanes depends on the shape and size and how closely the molecules are packed. Therefore, it is observed that even-numbered alkanes have a higher trend at the melting point than the odd-numbered alkane.

3.Solubility- Due to the very small electronegativity between carbon and hydrogen and the covalent property of the \( {\text{C}} – {\text{C}}\) bond and \({\text{C}} – {\text{H}}\) bond, alkanes are non-polar kind of molecules.

Alkanes are insoluble in polar solvents like water, alcohol, etc., but are highly soluble in non-polar solvents like petroleum, ether, benzene, carbon tetrachloride, etc.

4.Density- The densities of alkanes increase with an increase in the molecular masses to a limit value of \(0.8\,{\text{g}}\,{\text{c}}{{\text{m}}^{ – 3}}\) is reached, which means that all alkanes are lighter than water.

Chemical Reaction of Alkanes

1.Substitution reactions- A reaction in which a hydrogen atom in a hydrocarbon is replaced by an atom or a group of atoms is called a substitution reaction. Alkanes only undergo substitution reactions due to \( {\text{C}} – {\text{C}}\) and \({\text{C}} – {\text{H}}\) having a sigma bond. Some of the substitution reactions are as follows:

i) Halogenation- The process of replacing a hydrogen atom with a halogen- \( {\text{F,}}\, {\text{Cl,}}\, {\text{Br,}}\, {\text{l}}\) is halogenation. It can be done by heating the mixture at \(520 – 670\,{\text{k}}\), or in the presence of ultraviolet light.

ii) Nitration- The process of replacing a hydrogen atom with a nitro group is nitration. At ordinary temperatures, alkanes do not react with nitric acid, but when a mixture of an alkane and fuming \( {\text{HN}} {{\text{O}}_3}\) vapors heated at \(423 – 673\,{\text{k}}\) under pressure, and alkanes undergo nitration.

\( {\text{C}} {{\text{H}}_4}{\text{HN}}{{\text{O}}_3} \to {\text{C}}{{\text{H}}_3} – {\text{N}}{{\text{O}}_2} + {{\text{H}}_2}{\text{O}}\)

2.Oxidation- On heating, alkanes burn in the air producing carbon dioxide and water. This process is combustion, which is accompanied by the liberation of a large amount of heat energy.

\({\rm{C}}{{\rm{H}}_4}{\rm{ + 2}}{{\rm{O}}_2} \to 2{\rm{C}}{{\rm{O}}_2}{\rm{ + }}{{\rm{H}}_2}{\rm{O}};{\mkern 1mu} {\mkern 1mu} {\Delta _{\rm{c}}}{\rm{H}}^\circ  = {\mkern 1mu} \, – 80\,{\rm{KJ}}\,{\rm{mo}}{{\rm{l}}^{ – 1}}\)

When alkanes are heated in an insufficient amount of oxygen, it undergoes incomplete combustion and forms carbon monoxide along with unburnt carbon.

\(2 {\text{C}}{{\text{H}}_4} + 3{{\text{O}}_2} \to 2{\text{CO}} + 4{{\text{H}}_2}{\text{O}};{\text{C}}{{\text{H}}_4} + {{\text{O}}_2} \to {\text{C}} + 2{{\text{H}}_2}{\text{O}}\)

Uses of Alkanes

1. Methane is used as natural gas. LPG (mixture of butane and isobutane) is used as a fuel in homes and industries.
2. Alkanes are used in making carbon black which is used in printing inks, painting, and automobile tyres.
3. Catalytic oxidation of alkanes gives alcohols, aldehydes, and carboxylic acids.
4. Higher alkanes in the form of gasoline, kerosene oil, diesel, lubricating oils, and paraffin wax are widely used.
5. Alkanes are used to manufacture certain halogen derivatives like chloroform and carbon tetrachloride, which are used as solvents in industry and laboratory.

Summary

From this article, we can conclude that alkanes are saturated hydrocarbons that is they have a single bond in between the carbon atoms. Also, we learned its preparation and properties like boiling point, melting point, etc., and isomerism. We now know the importance of alkanes in our daily life.

Frequently Asked Questions

Q.1. What are the four alkanes?
Ans: The first four alkanes are methane \(\left({{\text{C}}{{\text{H}}_4}} \right)\), ethane \(\left({{{\text{C}}_2}{{\text{H}}_6}} \right)\), propane \(\left({{{\text{C}}_3} { {\text{H}}_8}} \right)\), butane \(\left({{{\text{C}}_4} {{\text{H}}_ {10}}} \right)\) 

Q.2. What are the ten alkanes?
Ans: The first ten alkanes are methane \(\left({{\text{C}}{{\text{H}}_4}} \right)\), ethane \(\left({{{\text{C}}_2}{{\text{H}}_6}} \right)\), propane \(\left({{{\text{C}}_3}{{\text{H}}_8}}\right)\), butane \(\left({{{\text{C}}_4}{{\text{H}}_{10}}} \right)\), pentane \(\left({{{\text{C}}_4}{{\text{H}}_{10}}} \right)\), hexane \(\left({{{\text{C}}_6}{{\text{H}}_{14}}} \right)\), heptane \(\left({{{\text{C}}_7}{{\text{H}}_{16}}} \right)\), octane \(\left({{{\text{C}}_8}{{\text{H}}_{18}}} \right)\), nonane \(\left({{{\text{C}}_9}{{\text{H}}_{20}}} \right)\), decane \(\left({{{\text{C}}_{10}}{{\text{H}}_{22}}} \right)\).

Q.3. What is the general formula for alkanes?
Ans: The general formula of alkanes is \({{\text{C}}_{\text{n}}}{{\text{H}}_{2{\text{n}} + 2}}\).

Q.4. What are the uses of alkanes
Ans: Some of the uses of alkanes are as follows:
1. Methane is used as natural gas. LPG (mixture of butane and isobutane) is used as a fuel in homes and industries.
2. Alkanes are used in making carbon black which is used in printing inks, painting, and automobile tyres.
3. Catalytic oxidation of alkanes gives alcohols, aldehydes, and carboxylic acids.
4. Higher alkanes in the form of gasoline, kerosene oil, diesel, lubricating oils, and paraffin wax are widely used. Alkanes are used to manufacture certain halogen derivatives like chloroform and carbon tetrachloride, which are used as solvents in industry and laboratory. 

Q.5. What are the properties of alkanes?
Ans: The three physical properties of alkanes are:
It has a high melting point,
It has a high boiling point, and
Alkanes are insoluble in polar solvents but are soluble in non-polar solvents.