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Arenediazonium salts (ArN2+X-) are among the most versatile intermediates in organic chemistry. Prepared by diazotisation at 0-5°C, they undergo two types of reactions.
Replacement reactions (N2 lost): Sandmeyer (CuCl -> ArCl, CuBr -> ArBr, CuCN -> ArCN). Gattermann (Cu powder/HCl -> ArCl). KI -> ArI (no catalyst). H2O (warm) -> ArOH. H3PO2 -> ArH (deamination). NaBF4 then heat -> ArF (Balz-Schiemann). NaNO2/Cu -> ArNO2.
Coupling reactions (N2 retained): ArN2+ + phenol (alkaline) -> p-hydroxyazobenzene. + aniline (weakly acidic) -> p-aminoazobenzene. + N,N-dimethylaniline -> p-dimethylaminoazobenzene. Coupling occurs at the para position of the activated ring. The azo products are coloured dyes.
Synthetic strategy: Place -NO2 first (meta director), perform substitution, then reduce to -NH2, diazotise, and replace with the desired group. This accesses substitution patterns impossible by direct electrophilic methods. For example: aryl fluorides (Balz-Schiemann), aryl iodides (KI), meta-substituted phenols, and symmetric tribromobenzene (via temporary -NH2 directing group).