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Isomerism in Alkanes

 

Alkanes show structural isomerism.

What is Isomerism?

Isomerism is the existence of two or more compounds with the same molecular formula (i.e. containing the same elements combined in the same proportion by mass), but different molecular structures.

Alkanes exhibit structural isomerism because of the possibility of branching of the carbon chains. The different structures are known as isomers.

There are 2 isomeric butanes, 3 pentanes, 5 hexanes, 9 heptanes, 18 octanes, and 35 nonanes. It has been calculated that the number of possible eicosanes, C20H42, is approximately 366,000.

Note that structural isomerism begins from butane.

Structural isomers of the same homologous series show similar chemical properties but different physical properties - due to structural differences.

Example, the boiling and melting points of the branched isomers of any alkane are lower than their unbranched counterpart.

The branched isomers burn smoothly in air, while the straight chain, i.e., unbranched isomers do not.

However, if they are isomers of different homologous series, they would show difference in both physical and chemical properties.

Example, ethanol (alcohol) and methoxymethane (dimethyl ether) have different physical and chemical properties because, even though they have the same molecular formula, C2H6O, they are of different homologous series.

The table below shows the different isomers of some alkanes.

 Alkanes  Number of Isomers  Isomers
 Butane C4H10           2  normal or n-butane (CH3CH2CH2CH3) and 2-methylpropane (isobutane) (CH3CH(CH3)CH3)
 Pentane C5H12           3  n-pentane (CH3CH2CH2CH2CH3), 2-methyl butane (isopentane - CH3CH2CH(CH3)CH3) and 2,2 - dimethylpropane (neo pentane - CH3C(CH3)2CH3)
 Hexane C6H14           5  n-hexane (CH3CH2CH2CH2CH2CH3), 3-methyl pentane (CH3CH2CH(CH3)CH2CH3), 2-methyl pentane (CH3CH(CH3)CH2CH2CH3), 2,3-dimethyl butane (CH3CH(CH3)CH(CH3)CH3), and 2,2-dimethyl butane, CH3C(CH3)2CH2CH3

Related:

Hydrocarbon and Alkanes
Methane
IUPAC Nomenclature of Alkanes
Petroleum and Petrol

 

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