Which sequence correctly arranges the substituted acetic acid derivatives in decreasing order of their acid strength (Ka)?

Answer and explanation

Correct answer: A. CF3COOH > CCl3COOH > CHCl2COOH > CH2ClCOOH > CH3COOH

  1. Step 1: Electron-withdrawing substituents ( inductive effect) disperse negative charge from the carboxylate anion, stabilizing the conjugate base and increasing acidity.
  2. Step 2: The magnitude of the effect depends on the electronegativity and number of halogens: fluorine is more electronegative than chlorine (), so has a stronger effect than .
  3. Step 3: As the number of electronegative halogens increases, acidity increases: .
  4. Step 4: Therefore, the correct decreasing sequence of acid strength is: .
Answer
CF3COOH > CCl3COOH > CHCl2COOH > CH2ClCOOH > CH3COOH
Why not the others: 'CH3COOH > ...' completely inverts the series; 'CCl3COOH > CF3COOH...' incorrectly ranks chlorine as more electronegative than fluorine; 'CH2ClCOOH > CHCl2COOH...' reverses the effect of increasing halogen substitution.

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