Ammonia derivatives and reduction

Ammonia and its derivatives, H₂N–Z, add to C=O in a reversible, acid-catalysed reaction. The intermediate loses water quickly to form >C=N–Z, which pulls the equilibrium towards product.

Table 8.2 (Z → reagent → product): –H, ammonia → imine; –R, amine → substituted imine (Schiff's base); –OH, hydroxylamine → oxime; –NH₂, hydrazine → hydrazone; –NHC₆H₅, phenylhydrazine → phenylhydrazone; –NHCONH₂, semicarbazide → semicarbazone; 2,4-dinitrophenylhydrazine → 2,4-dinitrophenylhydrazone.

With 2,4-DNP an aldehyde or ketone gives a coloured solid (yellow, orange or red), which chemists use to identify it.

Semicarbazide has two –NH₂ groups, but only the one joined to the other nitrogen forms semicarbazones; the –NH₂ next to C=O has its lone pair shared with the carbonyl by resonance.

NaBH₄, LiAlH₄ or catalytic hydrogenation reduce aldehydes to primary alcohols and ketones to secondary alcohols (Unit 7).

Clemmensen reduction: zinc amalgam and concentrated hydrochloric acid turn C=O into CH₂.

Wolff-Kishner reduction: the hydrazone formed with hydrazine is heated with sodium or potassium hydroxide in a high-boiling solvent such as ethylene glycol, and C=O again becomes CH₂.