JEE Main 2022 · 28 Jul · Shift 1 · Chemistry Q74

← JEE Main 2022 · 28 Jul · Shift 1

ChemistrySingle correct+4 / -1Question 74 of 90

Among the following marked proton of which compound shows lowest $\mathrm pK_a$ value ?

\mathrm{H{-}CH_2{-}COOH}

(α-H of acetic acid)

\mathrm{H{-}CH_2{-}CO{-}CH_3}

(α-H of acetone)

Benzylic α-H of phenylacetone,

\mathrm{C_6H_5{-}C\underline{H}H{-}CO{-}CH_3}

Phenylacetone (1-phenylpropan-2-one) with the benzylic CH2 hydrogen marked.

Benzylic α-H of phenylacetic acid,

\mathrm{C_6H_5{-}C\underline{H}H{-}COOH}

Phenylacetic acid with the benzylic CH2 hydrogen marked.

Answer and worked solution

Official answer: C

Most acidic. The benzylic CH₂ of phenylacetone sits between phenyl and C=O, so its carbanion is stabilised by both, and a ketone activates α-H more than a carboxylic acid does.

(C)

Topics:Acidity and basicity Aldol and related condensations

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JEE Main 2022 · 28 Jul · Shift 1, Chemistry question 74