JEE Advanced 2019 · Paper 1 · Chemistry Q4 · Dronacharya Exams

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ChemistrySingle correct+3 / -1Question 22 of 54

The correct order of acid strength of the following carboxylic acids is

I propiolic acid (HC≡C-CO₂H); II acrylic acid (CH₂=CH-CO₂H); III 4-methoxybenzoic acid; IV propanoic acid (CH₃CH₂CO₂H).

A

III > II > I > IV

B

I > II > III > IV

C

I > III > II > IV

D

II > I > IV > III

Answer and worked solution

Official answer: B

Order of acidity.

I, propiolic acid (pKa ≈ 1.9): the sp carbon next to $\ce{COOH}$ is strongly electron-withdrawing.
II, acrylic acid (≈ 4.25): an sp² carbon is less withdrawing.
III, 4-methoxybenzoic acid (≈ 4.5): the $+R$ effect of OMe reduces the acidity of benzoic acid.
IV, propanoic acid (≈ 4.9): the $+I$ effect of ethyl makes it the weakest.

I > II > III > IV. (B)

Topics:Acidity of carboxylic acids Electronic effects (inductive, resonance, hyperconjugation)

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