Animated Solution for Chemistry - Organic Chemistry: The major products $A$ and $B$ formed in the following reaction sequence are

The first step involves reacting aniline with acetic anhydride, yielding an intermediate product A. The second step subjects product A to bromination using bromine in acetic acid at room temperature to form the final major product B.

Our objective is to identify the structures of both A and B.

Acetylation Aniline is a highly reactive aromatic compound. The nitrogen atom of the amino group ($-{\text{NH}}_2$) possesses a lone pair of electrons that strongly donates electron density into the benzene ring via the $+R$ effect. This makes the ring exceptionally susceptible to electrophilic aromatic substitution. In fact, if we were to react aniline directly with bromine water, the reaction would be so vigorous that it would immediately yield 2,4,6-tribromoaniline as a white precipitate.

To synthesize a mono-substituted product, we must first tame this reactivity. This is where the first step comes in. Reacting aniline with acetic anhydride results in an acetylation reaction. The nucleophilic nitrogen attacks the electrophilic carbonyl carbon of the anhydride, forming N-phenylacetamide, commonly known as acetanilide.

Therefore, our product A is acetanilide.

Instead, it participates in resonance with the adjacent carbonyl group ($-\text{C}=\text{O}$).

This delocalization pulls electron density away from the nitrogen, significantly reducing its ability to activate the benzene ring. The $-{\text{NHCOCH}}_3$ group is still an ortho/para director, but it is only moderately activating, allowing us to control the subsequent electrophilic substitution.

Bromination With the ring's reactivity successfully moderated, we proceed to the second step: bromination. Acetanilide is treated with bromine ($B{r}_2$) in the presence of acetic acid ($C{H}_{3}COOH$).

The incoming electrophile, the bromonium ion ($B{r}^{+}$), is directed to the ortho and para positions by the $-{\text{NHCOCH}}_3$ group. However, the acetamido group is quite bulky. This steric hindrance makes the ortho positions less accessible, heavily favoring the formation of the para isomer.

Consequently, the major product formed is p-bromoacetanilide. This is our product B.

Comparing this with the given options, we find that it perfectly matches option (b).

As a bonus insight, if the ultimate goal of this synthesis was to obtain p-bromoaniline, we would simply add a third step: acidic or basic hydrolysis of p-bromoacetanilide to remove the acetyl protecting group!