When a molecule contains multiple potential nucleophilic sites, the reaction will occur at the most nucleophilic site. The order of nucleophilicity for common nitrogen-containing functional groups is generally:
Alkyl amine > Aromatic amine >> Amide
The strong resonance in amides makes their nitrogen atoms poor nucleophiles. This principle is key to predicting regioselectivity in such reactions.
Step 1: Understanding the Reaction The reaction is the acylation (specifically, acetylation) of 3-aminobenzamide with acetic anhydride ((CH\(_3\)CO)\(_2\)O). We need to determine which of the two nitrogen atoms in the molecule will react to form the major product. Step 2: Comparing the Nucleophilicity of the Nitrogen Atoms The starting material, 3-aminobenzamide, has two nitrogen atoms, each with a lone pair of electrons: 1. The aromatic amine nitrogen (-NH\(_2\)): The lone pair on this nitrogen is delocalized into the benzene ring through resonance. This reduces its availability and nucleophilicity compared to a simple alkyl amine, but it is still quite nucleophilic. 2. The amide nitrogen (-CONH\(_2\)): The lone pair on this nitrogen is strongly delocalized onto the adjacent carbonyl oxygen atom. This resonance is very significant, making the amide nitrogen much less basic and significantly less nucleophilic than the aromatic amine nitrogen. \[ \text{Resonance in amide: } \text{O=C-NH}_2 \leftrightarrow \text{O}^-\text{-C=NH}_2^+ \] Step 3: Predicting the Site of Acylation Acetic anhydride is an electrophile. The reaction is a nucleophilic acyl substitution where the nucleophile is one of the nitrogen atoms. Since the aromatic amine nitrogen is significantly more nucleophilic than the amide nitrogen, it will preferentially attack the electrophilic carbonyl carbon of the acetic anhydride. The reaction is: The amino group (-NH\(_2\)) on the ring is converted to an acetamido group (-NHCOCH\(_3\)). Step 4: Final Answer The major product 'P' is 3-(acetylamino)benzamide, where acylation has occurred at the more nucleophilic aromatic amino group. This corresponds to the structure given in option (A).
