RNH2

RNH2 denotes a primary amine, in which a single substituent R is attached to nitrogen that bears two hydrogens. The R group can be alkyl or aryl. Primary amines are typically pyramidal at the nitrogen, with a lone pair that makes them both basic and nucleophilic. The chemistry of RNH2 is governed by the nitrogen center, whose basicity and reactivity depend on the nature of R.

Common examples include methylamine (CH3NH2), ethylamine (C2H5NH2), and aniline (C6H5NH2). Aliphatic primary amines are generally more basic in water than aryl primary amines, which are weakened by conjugation with the aromatic ring. The conjugate acid pKa values typically fall in the range of about 9–11 for aliphatic amines and around 4–5 for aryl amines, reflecting this difference. Solubility in water also tends to be higher for smaller R groups and decreases as the hydrocarbon portion grows.

Reactivity of RNH2 centers on the lone pair on nitrogen. They act as nucleophiles in substitution reactions and readily condense with aldehydes or ketones to form imines (Schiff bases), which can be reduced to secondary or primary amines. Reaction with acyl chlorides delivers amides (RCONH2). Primary amines are also versatile intermediates in reductive amination, the synthesis of pharmaceuticals, and the production of dyes and polymers. Aromatic primary amines can be diazotized with nitrous acid to give diazonium salts, enabling further ring-functionalization.

Synthesis of primary amines commonly proceeds by reduction of nitro compounds or nitriles, or by reductive amination of carbonyl compounds with ammonia or a primary amine. As ligands, RNH2 donate the lone pair to metal centers, forming coordination compounds. Safety considerations include irritant and toxic potential; many primary amines are flammable and require proper handling.