Question

How many isomers of [insert straight-chained alkane here] exist?

Answer 1

In the above question, we are asked to find out the number of isomers, an alkane has. An isomer can be defined as the element which has the same molecular formula but different structural orientation. Isomers are categorised as straight chained, branched and cyclic isomers.

Complete step by step solution:
Alkanes are the compounds which are structurally connected with each other by single bond. Alkane having more than three carbon atoms can be arranged in various ways, resulting in formation of structural isomers. The simplest isomer of any alkane is the one in which the carbon atoms are arranged in a single chain with no branches. This isomer is sometimes called the n-isomer (where n stands for "normal"). However, the chain of carbon atoms may also be branched at one or more points. The number of possible isomers increases rapidly with the number of carbon atoms. For example, for acyclic alkanes:
${{\text{C}}_{\text{1}}}$ : methane only
${{\text{C}}_{\text{2}}}$ : ethane only
${{\text{C}}_{\text{3}}}$ : propane only
${{\text{C}}_{\text{4}}}$ : 2 isomers: butane and isobutane
${{\text{C}}_{\text{5}}}$ : 3 isomers: pentane, isopentane and neopentane.
${{\text{C}}_{\text{6}}}$ : 5 isomers: hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane.
${{\text{C}}_{\text{7}}}$ : 9 isomers: heptane, methylhexane(2 isomers), 4 isomers of dimethylpentane, 3-ethylpentane, 2,2,3-trimethylbutane.
${{\text{C}}_{{\text{12}}}}$ : 355 isomers
Branched alkanes can be chiral. For example, 3-methylhexane and its higher homologues are chiral due to their stereogenic center at carbon atom number 3.

Note:
In addition to the alkane isomers, the chain of carbon atoms may form one or more rings. Such compounds are called cycloalkanes. Cyclobutane and methylcyclopropane are isomers of each other, but are not isomers of butane.