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Arrange the following compounds in order of increasing acid strength: (a) CH3CO2H (acetic acid) (b) MeO-CH2-CO2H (methoxyacetic acid) (c) CF3CO2H (trifluoroacetic acid) (d) (CH3)2CH-CO2H (isobutyric acid)

1. d < a < c < b
2. d < a < b < c
3. a < d < c < b
4. b < d < a < c

Correct answer: d < a < b < c

Solution

Acid strength increases as the conjugate base (carboxylate) is better stabilised. The isopropyl group (d) is electron-donating (weakest acid). Acetic acid (a) is next. Methoxyacetic acid (b) has a weak -I effect from OMe, making it stronger than acetic. Trifluoroacetic acid (c) has three highly electronegative F atoms giving the strongest inductive withdrawal, making it the strongest acid.

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