Question

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Treatment of bromine water with propene yields 1-bromopropan-2-ol.
Reason (R) : Attack of water on bromonium ion follows Markovnikov rule and results in 1-bromopropan-2-ol.
In the light of the above statements, choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

In halohydrin formation, the halogen always goes to the less substituted carbon, and the hydroxyl group goes to the more substituted carbon.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The reaction of an alkene with bromine water (\( Br_2 / H_2O \)) is a halohydrin formation. It involves the electrophilic addition of bromine followed by the nucleophilic attack of water.
Step 2: Detailed Explanation:
1. Mechanism: In the first step, \( Br_2 \) acts as an electrophile. Propene (\( CH_3-CH=CH_2 \)) reacts with \( Br_2 \) to form a cyclic bromonium ion intermediate.
2. Regioselectivity: In the second step, water (the solvent and nucleophile) attacks the more substituted carbon atom of the bromonium ion. This is because the more substituted carbon bears more partial positive charge in the transition state (Markovnikov's logic).
3. Product Formation: The attack of \( H_2O \) on the 2nd carbon of propene (after \( Br^+ \) adds to the 1st carbon) followed by deprotonation yields 1-bromopropan-2-ol (\( CH_3-CH(OH)-CH_2Br \)).
4. Evaluating Statements: Assertion (A) is true as it describes the correct product. Reason (R) is true as it explains the regioselectivity of the water attack based on the Markovnikov rule logic applied to the bromonium ion.
Step 3: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A).