Acetanilide when treated with bromine in acetic acid mainly gives

• o-Bromoacetanilide
• N-Bromoacetanilide
• p-Bromoacetanilide
• m-Bromoacetanilide

When acetanilide is treated with bromine in acetic acid, the primary product formed is o-Bromoacetanilide. This reaction is an example of electrophilic aromatic substitution, where the bromine acts as an electrophile and substitutes a hydrogen atom on the aromatic ring of acetanilide. Let's break down the process to understand why o-Bromoacetanilide is favored over the other possible products.

Understanding the Reaction Mechanism

Acetanilide, which has an amide functional group attached to a benzene ring, is particularly interesting due to the influence of the nitrogen atom in the amide group. The lone pair of electrons on the nitrogen can donate electron density to the aromatic ring, making it more reactive towards electrophiles like bromine.

Electrophilic Aromatic Substitution

In electrophilic aromatic substitution, the aromatic ring undergoes a two-step process:

• Formation of the Electrophile: Bromine (Br2) can be activated in the presence of acetic acid, leading to the formation of the bromonium ion (Br+), which is a strong electrophile.
• Attack on the Aromatic Ring: The electron-rich aromatic ring then attacks the bromonium ion, resulting in the formation of a sigma complex (also known as an arenium ion).

Position of Substitution

The position at which bromine substitutes on the acetanilide ring is influenced by the electronic effects of the substituents already present. The amide group (-NHCOCH3) is a meta-directing group, but it also has resonance effects that can stabilize ortho and para positions. However, due to steric hindrance and the stability of the intermediates, the ortho position is more favorable in this case.

Product Distribution

When considering the possible products:

• o-Bromoacetanilide: Formed at the ortho position, this product is favored due to less steric hindrance and greater resonance stabilization of the intermediate.
• p-Bromoacetanilide: This product can also form, but it is less favored compared to the ortho product due to the steric factors involved.
• m-Bromoacetanilide: This product is unlikely to form significantly because the amide group does not stabilize the meta position effectively.
• N-Bromoacetanilide: This compound can form through bromination of the nitrogen, but it is not the primary product in this reaction context.

Conclusion on Product Formation

In summary, the reaction of acetanilide with bromine in acetic acid predominantly yields o-Bromoacetanilide due to the favorable electronic and steric factors at play. The resonance stabilization provided by the amide group enhances the reactivity of the ortho position, making it the preferred site for bromination. Thus, while other products can form, o-Bromoacetanilide is the main product of this reaction.