Organic Chemistry: Structure of Alkanes: Nomenclature and Isomerism

Isomerism in Branched Alkanes

In n-alkanes, no carbon is bonded to more than two other carbons, giving rise to a linear chain. When a carbon is bonded to more than two other carbons, a branch is formed. The smallest branched alkane is isobutane. Notice that isobutane has the same molecular formula, C₄H₁0, as n-butane but has a different structural formula. Two different molecules which have the same molecular formula are isomers. Isomers which differ in the connectivity of bonds are constitutional isomers, or structural isomers. Isobutane is a constitutional isomer of n-butane. The prefix "iso" indicates that branches off of the central carbon are equivalent.

Figure %: The constitutional isomers butane and isobutane.

n-butane and isobutane are the only constitutional isomers of C₄H₁0. Pentane, C₅H₁2, has three while hexane, C₆H₁4, has five.

Figure %: Constitutional isomers of pentane and hexane.

Nomenclature of Alkanes

Isobutane, neopentane, etc. are trivial names that arise from common usage. As you can see, the number of isomers increases rapidly for larger alkanes. It would be impractical to give trivial names to every isomer. What is needed is a systematic, easy-to-use method of naming that works for even the most complex of molecules. Such a name should unambiguously identify the structural formula of the named molecule. This system is IUPAC nomenclature, devised by the International Union of Pure and Applied Chemists.

The IUPAC system considers molecules in terms of a parent hydrocarbon chain with substituents attached to it. The parent is the longest continuous carbon chain in the compound, and the base name of the compound is the alkane that corresponds to the parent chain. Then, consecutively number the carbons of the parent chain in such a way that the substituents are attached to carbons with lower numbers. The name of the compound is the parent alkane prefixed by its substituents and their position numberings.

Figure %: Correct and incorrect IUPAC names for isopentane.

The -CH3 group is called a methyl group. In general, alkyl substituents are derived from the corresponding alkanes by replacing the -ane suffix with -yl. These substituents are used so frequently that they are given abbreviated names. For instance, methyl groups can be abbreviated as -Me. In some instances, the exact nature of the substituent is unimportant. In such cases the notation -R can be used to denote a radical group, a general substituent that can be any organic component.