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Which of the following alpha-substituted carboxylic acids has the smallest dissociation constant (i.e., is the weakest acid)?

1. CH3CHFCOOH
2. FCH2CH2COOH
3. BrCH2CH2COOH
4. CH3CHBrCOOH

Correct answer: BrCH2CH2COOH

Solution

F at alpha is more acidifying than Br at alpha; any halogen at alpha is more acidifying than at beta. BrCH2CH2COOH has Br at the beta position (weakest inductive effect among the four), making it the weakest acid with the smallest Ka.

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