Question

Most preferred site for electrophilic substitution in above example?
(Note: The molecule contains a nitrogen-containing ring and a carbonyl-containing ring, labels are U, S, R, P).

• (1) Predominantly at U
• (2) S and R
• (3) Predominantly at R
• (4) R and S

Answer 1

The Correct Option is A

Electrophilic substitution occurs most readily on the most electron-rich aromatic ring. Activation is provided by +M (mesomeric) groups.
1. Analyse the functional groups: The molecule has a nitrogen atom attached to one ring and a carbonyl group ($C=O$) attached to the other.
2. Determine effect: The Nitrogen atom has a lone pair and exerts a +M effect, activating its ring towards electrophiles. The carbonyl group exerts a -M effect, deactivating its ring.
3. Identify preferred ring: Substitution will happen on the ring attached to Nitrogen.
4. Identify preferred position: On the activated ring, substitution occurs at ortho and para positions relative to the activator. Position 'U' is para to the Nitrogen and is less sterically hindered than the ortho positions.

Conclusion: Substitution occurs predominantly at position U. Option (1).