MCAT Organic Chemistry Question 83: Answer and Explanation
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Question: 83
8. Hemiacetals and hemiketals usually keep reacting to form acetals and ketals. Why is it difficult to isolate hemiacetals and hemiketals?
I. These molecules are unstable.
II. The hydroxyl group is rapidly protonated and lost as water under acidic conditions, leaving behind a reactive carbocation.
III. The molecules are extremely basic and react rapidly with one another.
- A. I only
- B. I and II only
- C. II and III only
- D. I, II, and III
Correct Answer: B
Explanation:
Hemiacetals and hemiketals are usually short-lived because the –OH group will rapidly become protonated in acidic conditions and is lost as water, leaving behind a carbocation that is very susceptible to attack by an alcohol. Once the alcohol has been added, the acetal or ketal becomes more stable because the newly added group is less likely to become protonated and leave as compared to –OH.