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Which of the following groups can take part in resonance (conjugation) with a suitably placed adjacent unsaturated/charged system?

1. –OH (lone pair on O)
2. –CH2–CH2– (only sigma bonds)
3. –BH2 with no lone pair and no adjacent pi system
4. –CH3 (purely sp3, saturated)

Correct answer: –OH (lone pair on O)

Solution

For a group to participate in resonance it must be able to donate or accept electron density through a p orbital that overlaps with an adjacent pi system. –OH has lone pairs on oxygen that can be delocalised into an attached pi system (as in phenol). Saturated groups such as –CH2–CH2– or –CH3 have only sigma electrons and no p orbital available for conjugation, so they cannot resonate. –BH2 has an empty orbital but, isolated with no adjacent pi system as written, it cannot delocalise.

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