JEE Main 2021 · 26 February · morning · Single correct · +4 / −1
A →(Hydrolysis) B
B reacts with Hydroxyl amine but does not give Tollen's test. Identify A and B.
- (a) 1, 1-Dichlorobutane and Butanal
- (b) 2, 2-Dichlorobutane and Butan-2-one
- (c) 1, 1-Dichlorobutane and 2-Butanone
- (d) 2, 2-Dichlorobutane and Butanal
Answer and solution
Answer (b) 2, 2-Dichlorobutane and Butan-2-one
Solution
Chemistry ma’am · 4 steps
Reacts with hydroxylamine but fails Tollens' test: that is a ketone, so both chlorines sit on C2.
- Gem-dihalide hydrolysis. When both chlorines are on the same carbon, hydrolysis puts two OH groups on that carbon. Such a gem-diol is unstable and loses water to give C=O at that same carbon.
CH₃CCl₂CH₂CH₃ →(H₂O, 373 K) CH₃C(OH)₂CH₂CH₃ →(-H₂O) CH₃COCH₂CH₃
- Read the tests. Reaction with hydroxylamine (an oxime forms) shows that B has a C=O group. No silver mirror with Tollens' reagent rules out an aldehyde, so B is a ketone.
- Match the position. The only C₄H₈O ketone is butan-2-one, with C=O on C2. So the two chlorines in A must also be on C2: A is 2,2-dichlorobutane.
- So. A is 2,2-dichlorobutane and B is butan-2-one, which is option (b).
The idea to keep. A gem-dihalide on hydrolysis gives a carbonyl at the same carbon. Terminal CCl₂ gives an aldehyde, internal CCl₂ gives a ketone. Use Tollens' test to tell which one you have.
Answer from the NTA final answer key. Solution by PrepWiser.