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Which sequence represents the acids in order of decreasing acidity?

1. NO2CH2COOH > FCH2COOH > CNCH2COOH > ClCH2COOH
2. FCH2COOH > CNCH2COOH > NO2CH2COOH > ClCH2COOH
3. CNCH2COOH > NO2CH2COOH > FCH2COOH > ClCH2COOH
4. NO2CH2COOH > CNCH2COOH > FCH2COOH > ClCH2COOH

Correct answer: NO2CH2COOH > CNCH2COOH > FCH2COOH > ClCH2COOH

Solution

Acidity tracks the electron-withdrawing power of the substituent. Approximate pKa values: nitroacetic 1.68, cyanoacetic 2.47, fluoroacetic 2.59, chloroacetic 2.87, giving decreasing acidity NO2CH2COOH > CNCH2COOH > FCH2COOH > ClCH2COOH.

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