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In which of the following molecules is the indicated lone pair NOT able to take part in resonance (conjugation)?

1. CH2 = CH - NH - CH3 (N lone pair)
2. CH2 = CH - CH = O (O lone pair)
3. CH2 = CH - O - CH = CH2 (O lone pair)
4. CH2 = CH - C(triple bond)N: (N lone pair)

Correct answer: CH2 = CH - C(triple bond)N: (N lone pair)

Solution

For the amine, ether and enal, the heteroatom lone pair sits in a p-type orbital that overlaps with the adjacent C=C or C=O pi system and so delocalises. In the nitrile, nitrogen is sp hybridised; its lone pair is in an sp orbital perpendicular to the pi bonds and cannot conjugate.

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