What defines a primary alcohol?

A primary alcohol is defined by the presence of a hydroxyl group (-OH) attached to a carbon atom that is bonded to only one other carbon atom. This structural characteristic indicates that the carbon to which the hydroxyl group is connected has a two-bond relationship — one with the hydroxyl group itself and another with a single carbon chain or fragment. Such structural positioning is crucial for distinguishing primary alcohols from secondary or tertiary alcohols, where the bonding environment around the carbon with the hydroxyl group differs significantly.

In secondary alcohols, the hydroxyl group would be on a carbon that is bonded to two other carbons, while in tertiary alcohols, that carbon is bonded to three other carbons. An aromatic carbon situation, as described in the last option, refers to a carbon within an aromatic ring which is distinct from the definitions of primary, secondary, or tertiary alcohols.

Understanding this classification helps in predicting the reactivity and properties of different alcohols in organic chemistry, making it essential to grasp the concept of primary, secondary, and tertiary alcohols.