MCAT Organic Chemistry Question 94: Answer and Explanation
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Question: 94
4. Why does the equilibrium between keto and enol tautomers lie far to the keto side?
I. The keto form is more thermodynamically stable.
II. The enol form is lower energy.
III. The enol form is more thermodynamically stable.
- A. I only
- B. III only
- C. I and II only
- D. II and III only
Correct Answer: A
Explanation:
The keto–enol equilibrium lies far to the keto side because the keto form is significantly more thermodynamically stable than the enol form. This thermodynamic stability stems from the fact that the oxygen is more electronegative than the carbon, and the keto tautomer puts more electron density around the oxygen than the enol tautomer. If the enol tautomer is less thermodynamically stable, it is also higher energy than the keto tautomer.