Memorize the directing effects of common substituents for electrophilic aromatic substitution:
\textbf{Ortho, Para-directing (Activating):} -NH\(_2\), -OH, -OR, -R (alkyl), -Ph
\textbf{Ortho, Para-directing (Deactivating):} -F, -Cl, -Br, -I (Halogens)
\textbf{Meta-directing (Deactivating):} -NO\(_2\), -CN, -SO\(_3\)H, -CHO, -COR, -COOH, -COOR, -NR\(^+_3\)
Identifying the group attached to the ring is the first step in solving these problems.
Step 1: Understanding the Reaction
The reaction shown is the bromination of 1-phenylbutan-1-one using Br\(_2\) in the presence of a Lewis acid catalyst, AlBr\(_3\). This is an electrophilic aromatic substitution reaction. Step 2: Identifying the Directing Group
The reactant is 1-phenylbutan-1-one. The substituent attached to the benzene ring is an acyl group (-CO-C\(_3\)H\(_7\)). We need to determine the directing effect of this group on the incoming electrophile (Br\(^+\)).
The carbonyl group (C=O) is a deactivating group because it withdraws electron density from the benzene ring through both the inductive effect (-I) and the resonance effect (-R).
The resonance structures show that electron density is reduced at the ortho and para positions, making them less reactive towards electrophiles.
The meta position is less deactivated compared to the ortho and para positions. Step 3: Predicting the Major Product
Because the acyl group is a deactivating and meta-directing group, the incoming electrophile, bromonium ion (Br\(^+\)), generated from Br\(_2\) and AlBr\(_3\), will preferentially attack the meta position of the benzene ring.
The starting material is: (1-phenylbutan-1-one)
The major product 'A' will be: (1-(3-bromophenyl)butan-1-one) Step 4: Final Answer
The major product is 1-(3-bromophenyl)butan-1-one, where the bromine atom is attached to the meta position with respect to the acyl group. This corresponds to option (A).
