NH2

NH2 refers to the amino group, a functional group consisting of a nitrogen atom bonded to two hydrogen atoms and to the rest of the molecule (R). In organic chemistry, -NH2 is the primary amino substituent; when attached to carbon it designates a primary amine. The group is typically pyramidal, with a lone pair on nitrogen, and nitrogen is sp3-hybridized.

Properties and reactivity: Amino groups are basic and nucleophilic. In water they act as weak bases, accepting a proton to form the ammonium ion (NH3+). The neutral amine can form hydrogen bonds, which often increases the solubility of amines in water; however, aromatic amines (such as aniline) can be less soluble due to the hydrophobic ring. Substituents influence basicity: alkyl groups generally increase it, while certain electron-withdrawing groups decrease it.

Reactions: The amino group participates in many key transformations. It can condense with carbonyl compounds to form imines or Schiff bases, especially with aldehydes or ketones. It can react with carboxylic acids or acid chlorides to give amides. Reductive amination converts carbonyl compounds into amines, allowing the formation of primary, secondary, or tertiary amines. Primary amines can be diazotized to form diazonium salts, a step used in aryl chemistry.

Biology and significance: The NH2 group is central in biology, most notably in amino acids, where it is typically protonated to NH3+ at physiological pH, contributing to peptide bonds and protein structure. Examples of simple amines include methylamine (CH3NH2), ethylamine (C2H5NH2), and aniline (C6H5NH2).

Safety: Many amines are irritants or toxic; handling requires appropriate precautions.