The correct order of increasing acidic strength is
(A) Ethanol
(B) Phenol
(C) Chloroacetic acid
(D) Acetic acid
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (B), (D), (C)
An acid is stronger when its conjugate base (the anion left after losing \( \ce{H+} \)) is more stable. Two factors decide this for the four compounds:
Resonance: in a carboxylate ion the negative charge is shared equally by two oxygen atoms; in a phenoxide ion it is spread into the ring but mostly onto less electronegative carbon atoms; in an alkoxide ion it stays on one oxygen. So carboxylic acids > phenol > alcohols.
Inductive effect: an electron withdrawing Cl on the α-carbon (–I effect) pulls charge away from the carboxylate and stabilises it further, so chloroacetic acid is stronger than acetic acid.
Combining the two factors:
\( \displaystyle \text{alcohol} \lt \text{phenol} \lt \text{carboxylic acid} \lt \text{halogen-substituted carboxylic acid} \)
\( \displaystyle \ce{C2H5OH} \lt \ce{C6H5OH} \lt \ce{CH3COOH} \lt \ce{ClCH2COOH} \)
(A) \( \ce{C2H5OH} \): ethanol (alcohol, pKa about 16)
(B) \( \ce{C6H5OH} \): phenol (pKa 10.0)
(D) \( \ce{CH3COOH} \): acetic acid (pKa 4.76)
(C) \( \ce{ClCH2COOH} \): chloroacetic acid (pKa 2.86)
Shortcut: Ethanol is the weakest and chloroacetic acid the strongest, so the order starts with (A) and ends with (C): Options (1) and (2). Phenol is weaker than acetic acid, so (B) comes before (D), which leaves Option (1).