Four phenols are shown beside the options (the drawn compound is phenol, pKa 10.0). Which structure is the strongest acid?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: B

p-Nitrophenol (pKa 7.15) has a coplanar nitro group, so -M delocalises the phenoxide charge. In 3,5-dimethyl-4-nitrophenol the two flanking methyl groups twist NO2 out of plane (steric inhibition of resonance) and pKa rises to about 8.25; m-nitrophenol is 8.36 and p-cresol 10.26.

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