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 correct option is (D): b > d > c > a. 

The order of dehydration of alcohols depends on the stability of the carbocation intermediate formed during the reaction. The more stable the carbocation intermediate, the easier it is for the alcohol to undergo dehydration. Therefore, we analyze the structures of the alcohols to determine the stability of the carbocation intermediates:

• a > d > b > c: This order suggests that alcohol a is the most easily dehydrated, followed by alcohol d, then alcohol b, and finally alcohol c. This implies:
  • Alcohol a: Tertiary
  • Alcohol d: Secondary
  • Alcohol b: Primary
  • Alcohol c: Primary
• d > b > c > a: This order suggests that alcohol d is the most easily dehydrated, followed by alcohol b, then alcohol c, and finally alcohol a. This implies:
  • Alcohol d: Tertiary
  • Alcohol b: Secondary
  • Alcohol c: Primary
  • Alcohol a: Primary
• b > a > d > c: This order suggests that alcohol b is the most easily dehydrated, followed by alcohol a, then alcohol d, and finally alcohol c. This implies:
  • Alcohol b: Tertiary
  • Alcohol a: Secondary
  • Alcohol d: Primary
  • Alcohol c: Primary
• b > d > c > a: This order suggests that alcohol b is the most easily dehydrated, followed by alcohol d, then alcohol c, and finally alcohol a. This implies:
  • Alcohol b: Tertiary
  • Alcohol d: Secondary
  • Alcohol c: Primary
  • Alcohol a: Primary

Summary: Based on the given orders of dehydration, we conclude that alcohol b is the most easily dehydrated in all cases, as it is tertiary. The order of the remaining alcohols varies depending on the specific order given.

Answer 2

Dehydration of Alcohols and Carbocation Stability: 

The dehydration of alcohols follows the stability of the carbocations formed during the reaction. The stability of the carbocation depends on the degree of substitution:

• Alcohol b: Forms a tertiary carbocation, which is the most stable due to maximum alkyl group stabilization.
• Alcohol d: Forms a secondary carbocation, which is moderately stable.
• Alcohol c: Forms another secondary carbocation, similar to d, but slightly less stable due to fewer substituents.
• Alcohol a: Forms a primary carbocation, which is the least stable.

Conclusion: The order of dehydration follows the carbocation stability: b > d > c > a.