By Team Aakash Byju's | 15th December 2022

A

B

C

D

Elimination of water

Formation of an ester

Formation of carbocation

Protonation of an alcohol molecule

During the dehydration of alcohols to alkenes by heating with concentrated H2SO4, the initiation step is:

?

Detailed Explanation

The correct answer is  D) Protonation of an alcohol molecule.

Arrow

Dehydration of alcohol is an E1 elimination reaction.

Dehydration of Alcohol

 C

C

R

R

R

X

H

 C

C

R

R

R

+

H

R

R

B

 C

C

R

R

R

R

Rate  determining

The rate of the reaction is dependent on the stability of the carbocation formed.

Dehydration of Alcohol

R

(Protonation)

Conc . H  SO

4

H

+

2

2

OH

(Carbocation)

R

O

H

R

+

Step 1:  Protonation of an alcohol molecule.

The H  SO  will protonate the alcohol group to form  OH  , this step occurs very fast.

2

2

4

+

 C

C

OH

H

+

 C

C

H

H

OH

+

2

Alcohol

Protonated Alcohol

 C

C

OH

H

-

 C

C

H

H

+

2

Protonated Alcohol

Step 2:  Loss of leaving group (Slowest step).

The formed  OH  leaves as  H  O leaving behind a carbocation.

2

2

+

2

O

+

Carbonium Ion

 C

C

OH

H

Step 3:  Loss of proton to give an alkene.

Hence, the initial step is the protonation of an alcohol molecule.

H

+

 C

C

OH

H

+

2

H

2

O

 C

C

H

+

H

+

-

-

 C

C

Alcohol

Protonated Alcohol

Carbonium Ion

Alkene

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