The preparation of 4-bromoacetanilide from acetanilide and bromine involves electrophilic aromatic substitution EAS reaction. Here is the step-by-step mechanism:1. Activation of the aromatic ring: The lone pair of electrons on the nitrogen atom in the acetanilide molecule forms a resonance structure with the aromatic ring, activating the ring towards electrophilic substitution. The electron-donating nature of the nitrogen atom makes the ortho and para positions more electron-rich and susceptible to attack by electrophiles.2. Generation of the electrophile: Bromine Br2 reacts with a Lewis acid catalyst, such as aluminum bromide AlBr3 or iron III bromide FeBr3 , to form a highly electrophilic bromonium ion Br+ .3. Electrophilic attack: The electrophilic bromonium ion attacks the electron-rich ortho or para position of the activated aromatic ring, forming a sigma complex also known as an arenium ion .4. Deprotonation: A base, typically the bromide ion Br- generated in step 2, abstracts a proton from the sigma complex, restoring the aromaticity of the ring and yielding the final product, 4-bromoacetanilide.The reaction mechanism affects the yield of the product in several ways:1. Regioselectivity: Since both ortho and para positions are activated, a mixture of ortho and para products 2-bromoacetanilide and 4-bromoacetanilide is formed. However, due to steric hindrance, the para product 4-bromoacetanilide is the major product, while the ortho product 2-bromoacetanilide is the minor product.2. Reaction conditions: The use of a Lewis acid catalyst, such as AlBr3 or FeBr3, is crucial for generating the electrophilic bromonium ion and promoting the reaction. The reaction temperature and concentration of reactants also affect the yield of the product.3. Side reactions: Overbromination can occur, leading to the formation of di- or tri-bromoacetanilide products, which can decrease the yield of the desired 4-bromoacetanilide product. This can be minimized by using a stoichiometric amount of bromine and a controlled reaction temperature.In summary, the reaction mechanism of the preparation of 4-bromoacetanilide from acetanilide and bromine involves electrophilic aromatic substitution. The yield of the product is affected by regioselectivity, reaction conditions, and side reactions.