In organic nomenclature, what does stem refer to?

• A. The parent carbon chain
• B. The substituent group
• C. The functional group suffix
• D. The side chain with the highest priority

Answer: (A)

Stem refers to the parent carbon chain—the main backbone of the molecule used to form the base name in organic nomenclature. It is usually the longest continuous chain that contains the principal functional group, and it determines the root of the name. For example, if the molecule’s main chain has eight carbons, the stem gives the base name octane, and all substituents are added as prefixes while the functional group is indicated by the appropriate suffix attached to that stem. The substituent group is a side branch, not the base, and the functional group suffix describes the type of functional group but is not the backbone itself. A term like a “side chain with highest priority” isn’t used to define the stem—the stem is simply the main chain.