JEE Main 2022 · 26 July · morning · Single correct · +4 / −1
Which reactant will give the following alcohol on reaction with one mole of phenyl magnesium bromide (PhMgBr) followed by acidic hydrolysis?
Ph; |; Ph, -, C, -, OH; |; CH₃
- (a) CH₃ - C ≡ N
- (b) Ph - C ≡ N
- (c) CH₃-C(∥( O))-O-Ph
- (d)
Answer and solution
Answer (d)
Solution
Chemistry ma’am · 3 steps
Work backwards: one Ph on the carbinol carbon came from PhMgBr, so the other two groups sat on a C=O.
- Read the product. The alcohol is a tertiary alcohol. Its carbinol carbon (the one carrying OH) holds three groups: Ph, Ph and CH₃.
- Undo the Grignard. PhMgBr adds one Ph to the carbon of a C=O group, and acidic hydrolysis turns the O into OH. Remove one Ph from the carbinol carbon and put back the C=O. That leaves Ph and CH₃ on a carbonyl carbon, which is acetophenone, PhCOCH₃.
Ph-CO-CH₃ →(1. PhMgBr) Ph₂C(OH)CH₃
Option Reactant With 1 mole PhMgBr, then H₃O⁺ (a) CH₃CN ketone PhCOCH₃, not an alcohol (b) PhCN ketone PhCOPh, not an alcohol (c) CH₃COOPh ketone PhCOCH₃; a second mole is needed for the alcohol (d) PhCOCH₃ Ph₂C(OH)CH₃, the target - So. Only acetophenone gives the tertiary alcohol with a single mole of PhMgBr. The answer is (d).
The idea to keep. For a tertiary alcohol from one mole of Grignard, start from a ketone. Strip one R group (the Grignard's) off the carbinol carbon, and the other two groups tell you which ketone you need.
Answer from the NTA final answer key. Solution by PrepWiser.