3.3 : Nomenclature of Alkanes

The IUPAC names of unbranched alkanes include a stem to signify the length of the carbon chain and the suffix “ane” to identify the compounds as alkanes.

For example, the five-carbon alkane with the stem 'pent' and the suffix 'ane' acquires the name — pentane.

A substituent formed from an alkane is called an alkyl group. For instance, removing one of the terminal hydrogen atoms from pentane gives the pentyl group.

The IUPAC names of branched alkanes have three parts: parent, substituent, and locant. The parent includes a stem and the suffix, and the locant indicates the substituent’s location on the parent.

The parent is identified as the longest continuous chain of carbon atoms. If two or more chains have the same number of carbon atoms, the chain with the greater number of branching points becomes the parent.

Locants are assigned by numbering the carbon atoms of the parent chain.

In the case of a single substituent, numbering begins from the end of the chain that assigns the lower value to the branching point. Thus, the correct name is 3-methylheptane.

A hyphen separates the number and the substituent.

If a molecule has multiple identical substituents, the numbers are assigned such that the first branching point gets the lower value. The locants in the name are separated using a comma. Furthermore, a multiplier prefix like di- precedes the substituent’s name, indicating two identical substituents.

Non-identical substituents are listed alphabetically, irrespective of their position on the parent chain.

All carbon atoms in branched alkanes can be classified based on the number of carbon atoms directly connected to them.

Primary carbons are attached to one other carbon. A secondary carbon connects to two neighboring carbons. A tertiary carbon is linked to three other carbons, while a quaternary carbon is bonded to four carbons.

Hydrogens are also classified as primary, secondary, and tertiary depending on which carbon atom they are linked to.