What is the general formula for a primary amine?

• A. R-NH2
• B. R-COO-R
• C. R-NH3
• D. R-SH

Answer: (A)

Primary amines feature a nitrogen atom bonded to one carbon-containing group and two hydrogens, giving the general formula R-NH2. Here, R stands for an alkyl or aryl group. This structure is what defines a primary amine: only one substituent on nitrogen and two hydrogens attached.

Why this fits: the nitrogen carries a lone pair and makes the molecule a neutral base, with the -NH2 group attached to the carbon framework. In contrast, an ester would include a carbonyl linkage with -COOR, not -NH2; an ammonium species would have the nitrogen protonated to form -NH3+; and a thiol contains -SH instead of nitrogen. So the neutral, nitrogen-containing amine with two hydrogens is R-NH2. For reference, examples include methylamine (CH3NH2) or aniline (C6H5NH2).