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Diazonium salts enable functional group transformations impossible by direct electrophilic substitution. The general strategy: ArNO2 (nitration) -> ArNH2 (reduction) -> ArN2+ (diazotisation) -> desired product $\frac{replacement}{coupling}$.

Key transformations via diazonium:

• Aryl fluorides: Balz-Schiemann (only route)
• Aryl iodides: KI (direct, no catalyst)
• Aryl cyanides: Sandmeyer/CuCN (then hydrolysable to ArCOOH)
• Aryl halides: Sandmeyer/CuCl or CuBr
• Phenols: warm H2O
• Deamination: H3PO2 -> ArH
• Azo dyes: coupling with activated rings

Synthesis planning tips: (1) Use -NO2 as meta-director, then convert to -NH2 for diazonium route. This is how you get meta-substituted phenols, halides, etc. (2) Use -NH2 as a temporary o/p directing group: aniline -> tribrominate -> diazotise -> deaminate -> 1,3,5-tribromobenzene. (3) For ArCOOH from ArNH2: diazotise -> Sandmeyer (CuCN) -> ArCN -> hydrolysis -> ArCOOH (two steps, not direct).