Question

What are \(X\) and \(Y\) respectively in the following reaction sequence?

• A. \(X=\) 3-Phenylpropanoic acid,\(Y=\) \(p\)-Bromobenzoic acid
• B. \(X=\) 2-Phenylpropanoic acid,\(Y=\) \(p\)-Bromobenzoic acid
• C. \(X=\) 2-(4-Bromophenyl)propanoic acid,\(Y=\) Benzoic acid
• D. \(X=\) 3-Phenylpropanoic acid,\(Y=\) Benzoic acid

Hint:

Remember: \[ \boxed{ \mathrm{ArCH_2Br} \xrightarrow{\mathrm{Mg}} \mathrm{ArCH_2MgBr} \xrightarrow{\mathrm{CO_2/H_3O^+}} \mathrm{ArCH_2COOH} } \] Benzylic bromination occurs with \[ \boxed{\mathrm{Br_2/h\nu}.} \]

Answer 1

The Correct Option is B

Step 1: Identify \(Y\). Ethylbenzene undergoes bromination with \[ \mathrm{Br_2/Fe} \] by electrophilic substitution to give mainly \[ p\text{-bromoethylbenzene}. \] Oxidation of the side chain using \[ \mathrm{KMnO_4/OH^-} \] followed by acidification converts the ethyl group into \[ \boxed{p\text{-bromobenzoic acid}.} \] Thus, \[ Y= \boxed{p\text{-bromobenzoic acid}.} \]

Step 2: Identify \(X\). Ethylbenzene reacts with \[ \mathrm{Br_2/h\nu} \] to give \[ \mathrm{PhCHBrCH_3} \] (benzylic bromination). Treatment with \[ \mathrm{Mg/dry\ ether} \] forms the corresponding Grignard reagent, which on reaction with \[ \mathrm{CO_2} \] followed by hydrolysis gives \[ \boxed{\mathrm{PhCH(CH_3)COOH}} \] (2-phenylpropanoic acid). Hence, \[ X= \boxed{\text{2-Phenylpropanoic acid}.} \] Therefore, \[ \boxed{(B)} \] is the correct answer.