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What is the correct order of pKa values of the following acids?

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in Chemistry – Class 12 by (188k points)
What is the correct order of pKa values of the following acids?

(a) Benzoic acid > m-Nitrobenzoic acid > p-Nitrobenzoic acid

(b) Benzoic acid > p-Nitrobenzoic acid > m-Nitrobenzoic acid

(c) p-Nitrobenzoic acid > m-Nitrobenzoic acid > Benzoic acid

(d) m-Nitrobenzoic acid > p-Nitrobenzoic acid > Benzoic acid

This question was addressed to me during a job interview.

My question is based upon Carboxylic Acids Chemical Reactions in portion Aldehydes, Ketones and Carboxylic Acids of Chemistry – Class 12

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Right choice is (a) Benzoic acid > m-Nitrobenzoic acid > p-Nitrobenzoic acid

Easy explanation: Nitro group is electron withdrawing in nature and increases the acidic strength of acids. The electron withdrawing effect of NO2 is pronounced at para position more than meta position, hence the p-isomer is more acidic and has a lower pKa value than m-isomer.

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