3.2: Constitutional Isomers of Alkanes
Organic compounds of the same molecular formula can have different structural formulas called constitutional isomers, and the phenomenon is known as constitutional isomerism. Alkanes with four or more carbons showing multiple structures with the same molecular formula thereby exhibit constitutional isomerism.
The linear isomer of an alkane is prefixed by the term “n”; hence a linear isomer of pentane is known as n-pentane. Based on the type of branching, some of the branched isomers are given the prefixes “iso” and “neo”. For example, pentane has two-branched isomers, iso-pentane and neo-pentane. Iso-pentane has a four-carbon straight chain with a single methyl substituent, whereas neo-pentane has a three-carbon straight chain with two methyl groups as substituents.
The possibility of branching increases with the growing number of carbon atoms in the chain across the homologue, leading to the exponential rise in the number of constitutional isomers. For example, the six-carbon alkane hexane has five isomers, whereas the number of isomers increases to 75 for the 10-carbon alkane decane.
Despite having the same molecular formula, isomers exhibit significant variation in their physical properties, such as melting points and boiling points.