Which functional group is associated with the formula NH2?

The functional group associated with the formula NH2 is known as an amine. Amines are organic compounds derived from ammonia by replacing one or more hydrogen atoms with alkyl or aryl groups. Specifically, when we refer to NH2, it denotes a primary amine, where one hydrogen atom of ammonia (NH3) is replaced with a carbon-containing group.

Among the provided options, amides, which are characterized by the structure RCONH2, contain the NH2 group as part of their overall structure. Thus, while the NH2 functional group is part of the amide, amides also possess a carbonyl group (C=O) that further defines their structure.

In contrast, the other choices do not include the NH2 functional group. The phenyl group is a cyclic structure derived from benzene, aldehydes contain a carbonyl group at the end of a carbon chain, and thiols feature a sulfur atom bonded to a carbon atom and have the general formula R-SH. Therefore, the identification of NH2 with amines and its specific recognition within the context of amides makes it the most fitting choice here.