The diazonium salt shown couples with phenol in mildly alkaline solution (pH about 9-10). Which structure is the major azo dye?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: A

The salt shown is benzenediazonium chloride, a weak electrophile. Phenol is converted by the base to the phenoxide ion, a strongly activated nucleophile, and substitution occurs at the para position because of lower steric hindrance. The product is 4-hydroxyazobenzene (p-hydroxyazobenzene).

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