What characterizes a primary alcohol?

A primary alcohol is characterized by the structure of the carbon atom to which the hydroxyl group (-OH) is attached. In a primary alcohol, this hydroxyl group is indeed bonded to a carbon atom that is attached to two hydrogen atoms. This configuration allows for the carbon to be connected to only one other carbon atom, making it a primary carbon.

To clarify this structural distinction, if a carbon with the hydroxyl group were instead bonded to more than one other carbon, it would then be classified as a secondary or tertiary alcohol. The presence of two hydrogen atoms indicates that the carbon is at the end of a carbon chain, which is a hallmark of primary alcohols.

The other options refer to different types of structures or functional groups. For instance, a carbon attached to one hydrogen atom describes a different scenario, which applies to a secondary alcohol. A hydroxyl group at the end of a cyclic structure aligns more with a phenol than a primary alcohol. In contrast, the last option describes a carboxylic acid, which has a different functional group entirely.