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Which of the following groups can act both as an electron-pair donor (+R) and as an electron-pair acceptor (-R) in resonance, depending on the situation?
- -NO2
- -CH=CH2 (vinyl)
- -CHO
- -NH2
Correct answer: -CH=CH2 (vinyl)
Solution
A vinyl group (-CH=CH2) and similar unsaturated/conjugating groups can either donate or withdraw electrons through resonance depending on what they are attached to: attached to an electron-deficient site they donate pi electrons, while attached to an electron-rich site they can withdraw. Pure +R groups like -NH2 only donate, and pure -R groups like -NO2 and -CHO only withdraw.
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