Which of the following has the largest acid equilibrium constant, $K_{a}$ ?

Benzoic acid

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p-methyl benzoic acid

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o-nitro benzoic acid

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p-methoxy benzoic acid

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Solution

The correct option is D o-nitro benzoic acid
In aromatic acids, electron-withdrawing groups like nitro increase the acidity whereas electron releasing groups like methyl decrease the acidity.
Here ortho-nitro benzoic acid has largest acid equilibrium value.
Further, since electron-withdrawing groups increase the acid strength, therefore, nitrobenzoic acid is stronger.
Stronger the acid higher is the $K_{a}$ value.

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nitrobenzoic acid (II),

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- methoxy benzoic acid (IV) is:

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Benzoic acid, 4 - nitrobenzoic acid, 3, 4 - dinitro benzoic acid, 4 - methoxy benzoic acid (acid strength).

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Which of the following has the largest acid equilibrium constant, Ka?

Which of the following has the largest acid equilibrium constant, $K_{a}$ ?