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Which one is most reactive towards electrophilic reagent?
A
2-Methylanisole (benzene ring with $-CH_3$ and ortho $-OCH_3$)
B
o-Cresol (benzene ring with $-CH_3$ and ortho $-OH$)
C
2-Methylbenzyl alcohol (benzene ring with $-CH_3$ and ortho $-CH_2OH$)
D
N-(2-methylphenyl)acetamide (benzene ring with $-CH_3$ and ortho $-NHCOCH_3$)
Detailed Solution
All four compounds have a methyl group on the ring and differ only in the second substituent, so the reactivity depends on how strongly that group releases electrons into the ring.
$-OH$ releases its lone pair into the ring by a strong +R effect; it is one of the most powerful activating groups.
$-OCH_3$ also has a +R effect, but slightly weaker than that of $-OH$.
In $-NHCOCH_3$ the nitrogen lone pair is shared with the adjacent C=O group, which reduces its donation to the ring.
$-CH_2OH$ has no lone pair directly attached to the ring, so it cannot activate the ring by resonance.
Hence o-cresol is the most reactive towards an electrophilic reagent.
$-OH$ releases its lone pair into the ring by a strong +R effect; it is one of the most powerful activating groups.
$-OCH_3$ also has a +R effect, but slightly weaker than that of $-OH$.
In $-NHCOCH_3$ the nitrogen lone pair is shared with the adjacent C=O group, which reduces its donation to the ring.
$-CH_2OH$ has no lone pair directly attached to the ring, so it cannot activate the ring by resonance.
Hence o-cresol is the most reactive towards an electrophilic reagent.