The correct answer is option 4. p-methoxyphenol, p-methylphenol, p-nitrophenol.
To determine the increasing order of acid strength among:
p-methoxyphenol
p-methylphenol
p-nitrophenol
We need to understand how the substituents on the para position affect the acidity of phenol.
Acidity Principle:
The more stable the phenoxide ion (conjugate base), the stronger the acid. Substituents that withdraw electrons stabilize the negative charge and increase acidity; those that donate electrons destabilize it and decrease acidity.
Analyze the Substituents:
p-Nitrophenol
The NO₂ group is a strong electron-withdrawing group via –I and –M (inductive and mesomeric) effects.
It stabilizes the phenoxide ion → strongest acid.
p-Methylphenol (p-cresol)
The CH₃ group is electron-donating via +I effect, but weaker than –OCH₃ in resonance.
It slightly destabilizes the phenoxide ion → moderate acidity.
p-Methoxyphenol
The –OCH₃ group is strong electron-donating via +M (resonance donating), though it also has a –I effect.
Overall, it destabilizes the phenoxide ion more than CH₃ does → weakest acid.
Increasing order of acid strength:
p-methoxyphenol < p-methylphenol < p-nitrophenol
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