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Topic: Carboxylic acids: acidic strength· Organic Compounds Containing Oxygen

The correct order of decreasing acidity of the following aliphatic acids is :

1. \((\mathrm{CH_3})_3\mathrm{CCOOH} > (\mathrm{CH_3})_2\mathrm{CHCOOH} > \mathrm{CH_3COOH} > \mathrm{HCOOH}\)

2. \(\mathrm{CH_3COOH} > (\mathrm{CH_3})_2\mathrm{CHCOOH} > (\mathrm{CH_3})_3\mathrm{CCOOH} > \mathrm{HCOOH}\)

3. \(\mathrm{HCOOH} > \mathrm{CH_3COOH} > (\mathrm{CH_3})_2\mathrm{CHCOOH} > (\mathrm{CH_3})_3\mathrm{CCOOH}\)

4. \(\mathrm{HCOOH} > (\mathrm{CH_3})_3\mathrm{CCOOH} > (\mathrm{CH_3})_2\mathrm{CHCOOH} > \mathrm{CH_3COOH}\)

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Answer: 3. \(\mathrm{HCOOH} > \mathrm{CH_3COOH} > (\mathrm{CH_3})_2\mathrm{CHCOOH} > (\mathrm{CH_3})_3\mathrm{CCOOH}\)

Key idea: alkyl groups are electron-donating (+I) and destabilise the carboxylate ion, so more alkyl substitution means weaker acid.

HCOOH has no alkyl group (pKa 3.75) and is strongest, then \(\mathrm{CH_3COOH}\) (4.76), \((\mathrm{CH_3})_2\mathrm{CHCOOH}\) (4.86), \((\mathrm{CH_3})_3\mathrm{CCOOH}\) (5.05).

Option (3).

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report
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