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JEE Main levelSingle correct optionDifficulty 2 of 3nc134

Benzenediazonium chloride couples with phenol most efficiently in a medium of

  1. ApH 9-10, where phenol is present as the more strongly activated phenoxide ion
  2. BpH 4-5, where phenol is protonated and more reactive
  3. CpH below 1, where the diazonium ion concentration is highest
  4. DpH above 13, where the diazonium ion is converted into the more electrophilic diazotate

Answer: A

Phenol is converted into phenoxide at pH 9-10, and phenoxide is a far stronger nucleophile in electrophilic substitution, giving 4-hydroxyazobenzene. In strongly alkaline solution the diazonium ion is converted into the unreactive diazotate, and in strongly acidic solution phenol is not ionised, so coupling is slow.
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