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Among the following acids, which has the lowest pKa value (i.e., is the strongest acid)? (1) CH3CH2COOH (2) (CH3)2CHCOOH (3) HCOOH (4) CH3COOH

1. (1) CH3CH2COOH
2. (2) (CH3)2CHCOOH
3. (3) HCOOH
4. (4) CH3COOH

Correct answer: (3) HCOOH

Solution

Formic acid (HCOOH) lacks any alkyl group and is the strongest of the four, with pKa ~3.75. Acetic acid (pKa ~4.76), propionic acid (pKa ~4.87), and isobutyric acid (pKa ~4.86) are all weaker due to +I effect of alkyl groups.

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