The correct order of basicity for the following molecules is : [figure]
- R > P > Q
- P > Q > R
- R > Q > P
- Q > P > R
Answer
(3) R > Q > P
Solution
In an amide the lone pair of N is delocalised onto the C=O group: - N-C=O - N=C-O , so amides are very weak bases.
R: N is at the bridgehead of a small bicyclic ring. The resonance form N=C would put a double bond at the bridgehead, which is not possible (Bredt's rule; the N lone pair cannot become parallel to the C=O π orbital).
The lone pair of N stays localised R behaves like an amine most basic.
Q: the C=O group is also conjugated with the ring C=C (cross-conjugation). The carbonyl group is shared by the C=C and the N lone pair, so the N lone pair is less delocalised than in P.
Q is more basic than P.
P: a simple lactam; the N lone pair is fully involved in amide resonance least basic.
R > Q > P
Answer: (3)