JEE Main · 2025 · 04 Apr 2025 · Shift 2 · Compounds Containing Nitrogen

Basicity here is set by how freely the nitrogen lone pair can be offered to a proton. In the saturated six-membered lactam the nitrogen lone pair is pulled into the adjoining carbonyl by amide resonance, leaving it least available. In the $\alpha ,\beta$-unsaturated lactam the carbon$-$carbon double bond also feeds electron density into that carbonyl, so the carbonyl draws less from nitrogen and the nitrogen retains more of its lone pair, making it more basic than the saturated lactam. In the bridgehead bicyclic lactam the rigid cage stops the nitrogen lone pair and the carbonyl from becoming coplanar, so amide resonance is switched off and the lone pair stays fully available, giving the highest basicity. The order of basicity is the bridgehead lactam greater than the unsaturated lactam greater than the saturated lactam.