Question

Why nucleophilic substitution reactions are not very common in phenols?

Answer 1

compounds in which leaving group is directly attached to multiple bonded carbon either don't involve in SN reactions or undergo SN reactions under drastic conditions such as high temperature or pressure.
reasons :-
1. existence of partial double bond character between carbon and leaving group.
2. instability of vinyl or phenyl carbocation
3. delocalized sigma and pi electrons makes the ring electron rich over which the nucleophile cannot attack , therefore any group that makes ring electron deficient can facilitate SN reactions.
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