MCAT Organic Chemistry Question 107: Answer and Explanation
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Question: 107
2. Carboxylic acids have higher boiling points than their corresponding alcohols primarily because:
- A. molecular weight is increased by the additional carboxyl group.
- B. the pH of the compound is lower.
- C. acid salts are soluble in water.
- D. hydrogen bonding is much stronger than in alcohols.
Correct Answer: D
Explanation:
The boiling points of compounds depend on the strength of the attractive forces between molecules. In both alcohols and carboxylic acids, the major form of intermolecular attraction is hydrogen bonding; however, hydrogen bonding is much stronger in carboxylic acids as compared to alcohols because carboxylic acids are more polar and the carbonyl also contributes to hydrogen bonding in addition to the hydroxyl group. The stronger hydrogen bonds elevate the boiling points of carboxylic acids compared to alcohols. Boiling points also depend on molecular weight, choice (A), but in this case, the difference in molecular weight is insignificant compared to the effect of hydrogen bonding. Choices (B) and (C) are both true but do not explain the difference in boiling points.