Question

Decreasing order of dehydration of the following alcohols is

• A. $a>d>b>c$
• B. $d > b > c > a$
• C. $b > a > d > c$
• D. $b>d>c>a$

Answer 1

The Correct Option is D

The question is about determining the order of dehydration of the given alcohols. Dehydration of alcohols is often influenced by the stability of the carbocation formed during the reaction.

Explanation:

The dehydration reaction in alcohols generally follows the mechanism where the stability of the resulting carbocation plays a crucial role. More stable carbocations lead to a faster dehydration process. Let's analyze the stability of the carbocations that would result from the dehydration of each alcohol:

1. Alcohol (a): Phenol
   • Phenol does not form a stable carbocation when dehydrated due to its resonance stabilization of the oxygen lone pair with the benzene ring.
2. Alcohol (b): Cyclohexen-1-ol
   • This compound can form a stable allylic carbocation, which is stabilised by resonance.
3. Alcohol (c): Cyclopentenol
   • This also forms an allylic carbocation, but it is less stable than the one formed from alcohol (b) due to a smaller ring structure.
4. Alcohol (d): Cyclohexanol
   • A secondary carbocation is formed here, which is relatively stable but not as stable as allylic carbocations.

Order of Dehydration:

Based on the stability of the carbocations, the order of dehydration from fastest to slowest is:

• b > d > c > a

Thus, the alcohol with the most stable resulting carbocation (cyclohexen-1-ol, b) will dehydrate the fastest, followed by cyclohexanol (d), cyclopentenol (c), and then phenol (a).

The correct answer is b > d > c > a.