IIT-JEE 2009 · Paper 1 · Chemistry Q6

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ChemistrySingle correct+3 / -1Question 6 of 60

The correct acidity order of the following is

Four compounds: (I) phenol; (II) 4-chlorophenol; (III) benzoic acid; (IV) 4-methylbenzoic acid.

(III) > (IV) > (II) > (I)

(IV) > (III) > (I) > (II)

(III) > (II) > (I) > (IV)

(II) > (III) > (IV) > (I)

Answer and worked solution

Official answer: A

Carboxylic acids before phenols. Benzoic acid (pK $_a$ 4.2) is a stronger acid than p-toluic acid (pK $_a$ 4.4), whose methyl group donates electrons and destabilises the carboxylate.

Phenols. The −I effect of Cl makes p-chlorophenol (pK $_a$ ≈ 9.4) more acidic than phenol (10.0).

Order: III > IV > II > I.

(A)

Topics:Acidity and basicity Phenols Acidity of carboxylic acids

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IIT-JEE 2009 · Paper 1, Chemistry question 6