A statement of Assertion is given followed by a corresponding statement of Reason (R). Choose the correct answer from the given options.
Assertion: Addition reaction of water to but-1-ene in acidic medium yields butan-1-ol.
Reason: Addition of water in acidic medium proceeds through the formation of primary carbocation.
Answer & explanation
Correct answer: option 4
The correct answer is option 4. Both Assertion and Reason are incorrect.
To understand why both statements fail, we need to look at the mechanism of acid-catalyzed hydration of alkenes, which follows Markovnikov's Rule.
Assertion: "Addition reaction of water to but-1-ene in acidic medium yields butan-1-ol."
- The Reality: When but-1-ene ($CH_{3}-CH_{2}-CH=CH_{2}$) reacts with water in the presence of an acid catalyst ($H^{+}$), the hydroxyl group ($-OH$) attaches to the carbon with the fewer hydrogen atoms.
- The Product: This results in the formation of butan-2-ol, not butan-1-ol.
Reason: "Addition of water in acidic medium proceeds through the formation of primary carbocation."
- The Reality: The reaction proceeds via the formation of the most stable carbocation.
- The Comparison: For but-1-ene, the proton ($H^{+}$) adds to the terminal carbon ($C_{1}$), creating a secondary carbocation at $C_{2}$. A secondary carbocation is much more stable than a primary carbocation due to inductive effects and hyperconjugation.
- Conclusion: The reaction specifically avoids the formation of a primary carbocation whenever a more stable secondary or tertiary intermediate is possible.