Consider the following statements about the dehydration of ethanol to ethene by heating it with concentrated sulphuric acid.
(A) In the dehydration of ethanol, the first step is the formation of a protonated alcohol.
(B) In the dehydration of ethanol, the formation of the carbocation is the fastest step of the reaction.
(C) In the dehydration of ethanol, the acid used in the first step is released in the last step.
(D) In the dehydration of ethanol, ethene is removed as it is formed to drive the equilibrium to the right.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (C) and (D) only
The acid-catalysed dehydration of ethanol takes place in three steps:
Step 1: \( \displaystyle \ce{CH3CH2-OH + H+ <=> CH3CH2-OH2+} \)
Step 2 (slow): \( \displaystyle \ce{CH3CH2-OH2+ <=> CH3CH2+ + H2O} \)
Step 3: \( \displaystyle \ce{CH3CH2+ -> CH2=CH2 + H+} \)
(A) In the dehydration of ethanol, the first step is the formation of a protonated alcohol. — True. The lone pair on oxygen takes up a proton, giving the protonated alcohol, CH3CH2OH2+.
(B) In the dehydration of ethanol, the formation of the carbocation is the fastest step of the reaction. — False. The loss of water to give the carbocation is the slowest step, so it is the rate determining step.
(C) In the dehydration of ethanol, the acid used in the first step is released in the last step. — True. The H+ taken up in step 1 is given back when the carbocation loses a proton in step 3, so the acid acts as a catalyst.
(D) In the dehydration of ethanol, ethene is removed as it is formed to drive the equilibrium to the right. — True. Taking away the product shifts the equilibrium towards more ethene.
NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “The mechanism of dehydration of ethanol involves the following steps: Step 1: Formation of protonated alcohol.” … “Step 2: Formation of carbocation: It is the slowest step and hence, the rate determining step of the reaction.” … “Step 3: Formation of ethene by elimination of a proton.”
“The acid used in step 1 is released in step 3. To drive the equilibrium to the right, ethene is removed as it is formed.”