Amines are characterized by a nitrogen atom bonded to one or more carbon groups with a lone pair (R-NH₂). They are classified as primary (1°), secondary (2°), or tertiary (3°) by the number of carbon groups attached to nitrogen—a convention distinct from that used for alcohols, which counts carbons attached to the carbinol carbon. The nitrogen lone pair accepts a proton readily, making amines basic. Aniline is far less basic than alkylamines because its nitrogen lone pair is delocalized into the aromatic ring by resonance, reducing its availability. IR spectra of primary amines show two N–H stretch bands near 3300–3500 cm\(^{-1}\), secondary amines show one, and tertiary amines show none. Reaction of an amine with an acid chloride produces an amide through nucleophilic acyl substitution, and the Hinsberg test uses benzenesulfonyl chloride to distinguish 1°, 2°, and 3° amines based on their differing solubilities and reactivities. A persistent problem with primary amines is over-alkylation: excess alkyl halide progressively alkylates them all the way to a quaternary ammonium salt. The Gabriel synthesis avoids this by alkylating the phthalimide anion with an alkyl halide and then cleaving the phthalimide group to release a pure primary amine.
Nitriles contain a carbon triple-bonded to nitrogen (-C≡N) and are named with the suffix -nitrile (ethanenitrile, also called acetonitrile), counting the nitrile carbon in the parent chain. Their IR spectra show a sharp, medium-intensity C≡N stretch near 2210–2260 cm\(^{-1}\). Under acidic or basic aqueous conditions, a nitrile is hydrolyzed to a carboxylic acid (or carboxylate salt) via an amide intermediate, and reduction with \(LiAlH_4\) gives a primary amine by full reduction of the triple bond. A Grignard reagent adds once to the nitrile carbon, and acidic aqueous workup then hydrolyzes the resulting imine to a ketone—an important method for installing a ketone one carbon away from an alkyl halide precursor. Nitriles can be synthesized by \(S_N2\) displacement on an alkyl halide with cyanide ion (CN⁻), which simultaneously extends the carbon chain by one carbon.
Thiols are the sulfur analogs of alcohols, bearing a sulfhydryl group (-SH), and are named with the suffix -thiol (ethanethiol). Sulfur's larger size and greater polarizability make the S–H bond weaker than O–H, so thiols are more acidic than alcohols and form more stable thiolate conjugate bases. Their IR spectra show a weak S–H stretch near 2550–2600 cm\(^{-1}\)—much weaker and at lower frequency than the O–H stretch of alcohols. Many thiols have a strong, often unpleasant odor, a property exploited when trace thiols are added to natural gas so leaks can be detected by smell. Oxidation of two thiols together joins them through a new S–S bond to form a disulfide (R-S-S-R), a linkage of great biological importance: cysteine residues in proteins form disulfide bridges that stabilize tertiary and quaternary structure.