question_answer 84)

  A solution contains 1 g mol. each of p-toluenediazonium chloride and p-nitrophenyldiazonium chloride. To this 1 g mol. of alkaline solution of phenol is added. Predict the major product Explain your answer.

Answer:

                  In alkaline medium, phenol forms phenoxide ion which is more electron rich and hence more reactive than phenol. Coupling reactions between diazonium salts is an example of electrophilic aromatic substitution. In this reaction, diazonium salt acts as the electrophile and phenoxide ion as the nucleophile. Evidently stronger the electrophile faster is the reaction. Now due to electro-withdrawing effect of the  group, p-nitrophenyldiazonium cation is a stronger electrophile than p-toluenediazonium cation (+I-effect of . group reduces its electrophilicity) and hence couples preferentially with phenol to form 4-hydroxy-4'-nitroazobenzene as the major product.