Chemical Reactions of Diazonium Salts

Benzenediazonium chloride undergoes two broad classes of reaction. A. REPLACEMENT REACTIONS (involving LOSS of nitrogen, N2 gas): the diazonium group is displaced by a nucleophile such as X-, CN-, H-, or OH-. (1) Replacement by HYDROGEN: mild reducing agents -- hypophosphorous (phosphinic) acid, H3PO2, or ethanol with a cuprous-ion catalyst (CuCl) -- reduce the diazonium salt to benzene, via a free-radical chain mechanism, releasing N2 (+ H3PO3/HCl, or + CH3CHO/HCl for the ethanol variant). (2) Replacement by Cl, Br or CN: the SANDMEYER REACTION mixes freshly prepared benzenediazonium chloride with a cuprous halide (Cu2Cl2 or Cu2Br2) to give the aryl halide (chlorobenzene or bromobenzene) + N2; with cuprous cyanide (CuCN/KCN) it instead gives cyanobenzene + N2. The closely related GATTERMANN REACTION achieves the same chloro/bromo substitution using plain HCl/HBr with copper POWDER (no cuprous salt) -- Sandmeyer generally gives a better yield than Gattermann. (3) Replacement by IODINE: warming aqueous benzenediazonium chloride with KI directly gives iodobenzene + KCl + N2 (no catalyst needed, unlike the Cl/Br/CN cases). (4) Replacement by FLUORINE (the BALZ-SCHIEMANN REACTION): benzenediazonium chloride + fluoroboric acid (HBF4) precipitates the more stable benzenediazonium tetrafluoroborate, which on heating decomposes to fluorobenzene + BF3 + N2. (5) Replacement by HYDROXYL: adding benzenediazonium chloride slowly to a large volume of BOILING water gives phenol + N2 + HCl. (6) Replacement by the NITRO group: benzenediazonium fluoroborate heated with aqueous NaNO2 in the presence of copper replaces -N2+ with -NO2, giving nitrobenzene + N2 + NaBF4. (7) Replacement by an ARYL group (the GOMBERG REACTION): benzenediazonium chloride + benzene, in the presence of NaOH, gives biphenyl + N2 + HCl. (8) Replacement by a CARBOXYLIC ACID group: benzenediazonium fluoroborate heated with acetic acid gives benzoic acid + BF3 -- a route from an aliphatic carboxylic acid's carbon source to an AROMATIC carboxylic acid. These replacement routes matter because aryl fluorides and iodides cannot be made by direct halogenation, and a cyano group cannot be introduced by nucleophilic substitution on chlorobenzene -- so the diazonium salt is the essential intermediate for installing F, I, CN, -OH, -NO2 and more onto an aromatic ring. B. REACTIONS INVOLVING RETENTION OF THE DIAZO GROUP: (9) REDUCTION TO HYDRAZINES: reducing agents such as SnCl2/HCl, zinc dust/acetic acid, or sodium hydrosulphite/sulphite reduce benzenediazonium chloride to phenylhydrazine (C6H5-NH-NH2), keeping both nitrogens. (10) COUPLING REACTIONS: benzenediazonium chloride reacts with electron-rich aromatic compounds -- phenol (at pH 9-10) or aniline (at pH 4-5) -- to form brightly-coloured …