Question 25 of 39intermediate💡 UnderstandLong Answer3 marks

Write the mechanism of acid dehydration of ethanol to yield ethene.

Correct Answer

Ethanol undergoes dehydration when heated with concentrated H₂SO₄ at 443 K to yield ethene (CH₂=CH₂) and water. The mechanism involves the following steps:

Step 1: Formation of protonated alcohol – Ethanol reacts rapidly with a proton (H⁺) to form protonated alcohol (ethyl oxonium ion).

Step 2: Formation of carbocation – The protonated alcohol loses a water molecule to form a carbocation. This is the slowest step and hence the rate-determining step of the reaction.

Step 3: Formation of ethene – The carbocation loses a proton (H⁺) to form ethene. 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.

Exercise: EXERCISES | Q: 7.19 | (Page Page 31)
For More Understanding

Explanation

The textbook provides a clear three-step mechanism for acid-catalyzed dehydration of ethanol. Step 1 involves protonation of the alcohol oxygen. Step 2 is the rate-determining step where water leaves, forming a carbocation intermediate. Step 3 involves deprotonation to yield the alkene. The reaction conditions (conc. H₂SO₄, 443 K) are essential to mention for completeness.

Solution Steps

  1. Step 1: Protonation of ethanol to form ethyl oxonium ion (fast)

  2. Step 2: Loss of water molecule to form carbocation (slow, rate-determining)

  3. Step 3: Elimination of proton to form ethene

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