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Question

Which one of the following has maximum acid strength?


A
o-nitrobenzoic acid
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B
m-nitrobenzoic acid
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C
p-nitrobenzoic acid
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D
p-nitrophenol
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Solution

The correct option is A o-nitrobenzoic acid
The acidity increase of ortho-nitrobenzoic acid compared with para-nitrobenzoic acid is believed to be a result of the so called 'ortho effect'. The explanation is relying on the steric inhibition of the resonance of the $$COOH$$ group. The ortho substituent may force the carboxyl group out of the plane of the benzene ring, thus decreasing the resonance of the molecule. At the same time, carboxylate anion $$COO^-$$ is believed to be less sterically hindered and more apt to be in the energetically favorable planar configuration. This makes an additional reason to the ortho substituted acid to pass to the anion form, thereby increasing its acidity as compared with the meta and para substituted acids. 
Phenol groups are less acidic than the benzoic acid group.

Chemistry
NCERT
Standard XII

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