This reaction is an electrophilic aromatic substitution (EAS) - specifically, bromination - using Br
2 and a Lewis acid catalyst (FeBr
3). The substrate, N-phenylbenzamide, contains two benzene rings connected by an amide linkage (-C(=O)NH-). We need to determine which ring is more reactive towards EAS and where the substitution occurs.
- Ring A (attached to C=O): This is the benzoyl part. The -C(=O)NH-Ph group attached to this ring acts as a deactivating group (similar to a carbonyl group) due to its electron-withdrawing nature (-I and -R effects possible but usually considered weakly deactivating overall). Deactivating groups typically direct incoming electrophiles to the meta position.
- Ring B (attached to NH): This is the aniline-derived part. The -NHC(=O)Ph group attached to this ring acts as an activating group (less activating than -NH2 or -OH, but still activating compared to benzene) due to the lone pair on nitrogen participating in resonance (+R effect>-I effect). Activating groups direct incoming electrophiles to the ortho and para positions.
Generally, EAS occurs preferentially on the more activated ring. Therefore, substitution is expected on Ring B (aniline part) at the
ortho/para positions. The
para position is usually favored sterically. This would lead to product (C). However, the provided answer key indicates that (A) is the correct product, where substitution occurs
meta to the -C(=O)NH-Ph group on the benzoyl ring (Ring A). This suggests that under these specific conditions, or perhaps due to an interpretation considering the amide group's overall effect, the reaction is directed to the
meta position of the less reactive benzoyl ring. While the aniline ring (Ring B) is generally considered more activated by the amide group (making C the expected product based on standard rules), the provided answer points to substitution on the benzoyl ring (Ring A) at the
meta position. Assuming (A) is the correct answer based on the key: The -C(=O)NHPh group acts as a
meta-director on the benzoyl ring, leading to bromination at the
meta position.
