Alkanes are saturated hydrocarbon containing carbon and hydrogen with all bonds between carbon and hydrogen are single bonds. They have general formula as Cn
. Marsh gas is a rich source of methane. LPG contains butane as its main constituent. Petrol contains octane as its main constituent. These compounds are examples for alkanes. They are classified as linear, branched and cycloalkanes.1. Straight chain alkanes:
This type of alkane contains linear carbon-carbon chain. N-butane is an example for this type of alkanes.
CH3-CH2-CH2-CH32. Branched alkanes:
This type of alkane contains carbon-carbon chain which has branches in the chain. 2,3 Dimethyl pentane is an example for this type of alkanes.3. Cycloalkanes:
Alkanes having closed ring structure are called as cycloalkanes. Cyclohexane is an example for this type of alkanes.
The nomenclature of alkanes is done according to IUPAC rules. The following are the basic steps in the nomenclature of alkanes.
- The longest chain is selected as root chain and other carbon atoms are treated as substituents.
- The longest chain in numbered in such a way that the substituent gets the least number.
- In case the chain contains more than one substituent, the first coming groups should get least number.
- Then the compound name is arrived as locant + substituent + root name.
- If more than one type of substituent is placed in the ring they are named in the alphabetic order.
- In naming cycloalkanes, the word cyclo is added as prefix with the alkane name. Numbering the ring is done in equal to numbering the chain.
Example (2,3 dimethyl pentane)
Here the longest chain contains five carbon atoms. Hence the root name is pentane. The chain is numbered in such a way that the first substituent gets the least number 2. Hence the alkane is 2,3 dimethyl pentane.
- By Wurtz reaction: When alkyl halides are treated with sodium metal in dry ether, alkanes are formed. For example if we treat methyl chloride with sodium metal we will obtain ethane.
CH3Cl + 2Na + CH3Cl → CH3-CH3 + 2NaCl
- If we take a mixture of alkyl halides we will get a mixture of alkanes owing to several cross reactions. For example if we take a mixture of methyl chloride and ethyl chloride we will get ethane, propane and n-butane.
CH3-Cl + 2Na + CH3-CH2-Cl → CH3-CH3 + CH3-CH2-CH3 + CH3-CH2-CH2-CH3
- By decarboxylation of sodium salts of carboxylic acids: When sodium salts of carboxylic acids like sodium acetate is heated with sodium hydroxide in the presence of calcium oxide, we will get alkanes. For example sodium acetate on decarboxylation gives methane.
CH3COONa + NaOH/CaO → CH4 + Na2CO3
- By reduction of alkenes and alkynes: Alkenes and alkynes on reduction with strong reducing agents like lithium aluminium hydride give alkanes. For example ethylene on reduction gives ethane.
CH2=CH2 + 2[H] → CH3-CH3
- By reduction of alcohols, alkyl halides, carbonyl compounds: Organic compounds containing oxygen like alcohols can be reduced to alkanes by strong reducing agents. For example acetone on reacting with zinc amalgam in concentrated hydrochloric acid gives propane. This reaction is called clemenson reduction.
CH3-CO-CH3 + 2H2 → CH3-CH2-CH3 + H2O
In alkanes the carbon atom is sp3 hybridized with tetrahedral shape. To this four hydrogen atoms or hydrogen and carbon atoms are attached. Hence the structure of all alkanes can be viewed as a continuous chain of tetrahedral units. Here the H/C ratio is high and hence they burn without any smoke. Hence most of them are used as good fuels. They are gases and higher members are liquids.
The melting point and boiling point increases with increase in the number of carbon atoms. However branched alkanes have lesser boiling points than linear alkanes owing to high packing efficiency.
In sufficient oxygen, alkanes burn to give carbon dioxide, water. This is called as complete and efficient combustion.
2C2H6 + 7O2 → 4CO2 + 6H2O
In the limited supply of air they burn to give a mixture carbon monoxide, carbon, carbon dioxide etc. This is called as incomplete combustion and is not efficient.
2CH4 + 3O2 → 2CO + 4H2O
2. Free radical halogenation
In the bright sunlight the hydrogen atoms are substituted by halogen atoms. This is called as free radical halogenation because the reaction proceeds through formation of free radicals. Flourine is highly reactive and the reaction proceeds even in the darkness. Iodine is least reactive and high temperature with pressure is required for the halogenation reaction.
For example methane on chlorination gives methyl chloride, methylene chloride, chloroform and carbon tetrachloride.
CH4 + Cl2 → CH3-Cl + HCl
CH3-Cl + Cl2 → CH2-Cl2 + HCl
CH2-Cl2 + Cl2 → CHCl3 + HCl
CHCl3 + Cl2 → CCl4 + HCl
Alkanes on nitration with fuming nitric acid and concentrated sulfuric acid gives nitro alkanes. This reaction is very slow and the yield is usually very poor.
CH4 + HNO3 → CH3-NO2 + H2O
Alkanes are used as fuels. Combustion of alkanes is a rich source of energy today in fossil fuels and in many other synthetic fuels like water gas etc. They are used to prepare many other important compounds like chloroform, carbon tetrachloride etc. Higher alkanes are used as solvents.
These are saturated hydrocarbons where the chain is closed and forms a ring pattern. They are named by prefixing cyclo to the parent hydrocarbon name. For example the following is the structure of cyclo butane.
Cyclo alkanes are less stable and more reactive than linear alkanes due to ring strain.
These are the saturated hydrocarbons in where the carbon chain is linear or branched. Propane, 2-methyl butane are examples for this type of alkane. Branched alkanes will have less packing efficiency due to lack of symmetry in the structure.
Cyclo alkanes are named by prefixing the cyclo to the parent alkane name. For example the following compound contains five carbon atoms in ring structure. Hence it is named as cyclopentane.The attachments are numbered in the usual way, except the difference that the similar positions are numbered same. So the following compound is named as 1-methyl cyclo pentane.
The nomenclature of alkanes can be done in two ways. Trivial or general name, Here the compound is named by general name which is derived from the source or commercial importance. Isomers are denoted by prefixing iso and neo etc
. For example the following compound is called as isobutane
.IUPAC name: It is the unique way of naming an organic compound. Here the name of alkane is derived using standard rules. We will see about this in detail below.
Alkanes are rich source of energy. As the H/C ratio is very high in alkanes they will undergo complete combustion to give carbon dioxide and water. Many of the fossil fuels like LPG, natural gas, petrol and Kerosene are hydrocarbons only. The combustion can be of two ways.
- Complete combustion: Alkanes undergo complete combustion to give carbon dioxide and water. For example methane on complete combustion gives carbon dioxide and water
CH4+ 2O2 → CO2 + 2H2O
- Incomplete combustion: During incomplete combustion there is only limited supply of oxygen and hence carbon monoxide and carbon are formed. It is not efficient and environmental friendly.