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Alcohols, Phenols and Ethers
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Which one of the following is most reactive towards electrophilic reagent?All four compounds have a methyl group on the ring; they differ only in the second substituent, so reactivity depends on how strongly that group donates electrons to the ring.
$-OH$ donates its lone pair to the ring by a strong +R (resonance) effect and is a very powerful activating group.
$-OCH_3$ also shows a +R effect, but it is weaker than that of $-OH$ (+R effect: $-OH > -OCH_3$).
In $-NHCOCH_3$ the lone pair on nitrogen is also delocalised onto the carbonyl group, so its +R effect on the ring is reduced.
$-CH_2OH$ has no lone pair attached directly to the ring and cannot activate it by resonance.
Hence o-cresol (the compound with $-OH$) is the most reactive towards electrophilic reagents.