Electrophilic Aromatic Nitration of Benzanilide

To determine the major product '$\text{X}$' of the given electrophilic aromatic substitution reaction, we analyze the reactivity of the two aromatic rings present in benzanilide ($\text{C}_6\text{H}_5\text{--NH--CO--C}_6\text{H}_5$):

1. Comparison of Ring Reactivities:

   • Ring A (attached to the nitrogen atom, $-\text{NH--}$): The nitrogen atom possesses a lone pair of electrons that can be delocalized into Ring A via a positive mesomeric effect ($+M$ / $+R$). Although this electron density is partially shared with the adjacent carbonyl group, the group $-\text{NH--CO--Ph}$ remains overall activating towards electrophilic aromatic substitution compared to benzene.
   • Ring B (attached to the carbonyl group, $-\text{C=O}$): The carbonyl carbon is electron-deficient and withdraws electron density from Ring B via a negative mesomeric effect ($-M$ / $-R$) and a negative inductive effect ($-I$). Thus, Ring B is strongly deactivated towards electrophilic aromatic substitution.

2. Site of Electrophilic Attack:

   • The nitronium ion ($\text{NO}_2^+$), which is the electrophile generated by the mixture of concentrated $\text{HNO}_3$ and concentrated $\text{H}_2\text{SO}_4$, attacks the more electron-rich ring, which is Ring A.

3. Regioselectivity (Ortho vs. Para):

   • The $-\text{NH--CO--Ph}$ group is an ortho/para-directing group.
   • Because the $-\text{NH--CO--Ph}$ group is relatively bulky, electrophilic attack at the ortho-position is sterically hindered.
   • Consequently, substitution occurs predominantly at the less hindered para-position of Ring A.

Therefore, the major product is $4\text{-nitrobenzanilide}$ ($p\text{-nitrobenzanilide}$), in which the nitro group ($-\text{NO}_2$) is attached para to the $-\text{NH--}$ group.

This corresponds to the structure shown in option (B).

Correct Answer: (B)