Echange d'oxygène lors de la substitution électrophile des phénols

In the bromodesulfonation of sodium 2, 6‐dibromophenol‐4‐sulfonate (I) and similar compounds, the relatively stable quinoloid intermediate II exchanges its carbonyl oxygen with that of H 2 18 O by an addition‐elimination process. The exchange is slower than the formation of the product III; it is roughly comparable to the exchange velocity of quinones of similar structure. By reversibility of the electrophilic additon step, 18 O is incorporated into the starting phenol I. In electrophilic substitutions with shorter life‐time of the quinoloid intermediate, the oxygen exchange is difficult or impossible to observe.