Discussion
Which of the following group shows –R effect?
Answer: D
The group showing the –R effect is –CN.
A group shows a negative resonance (–R or –M) effect when it withdraws electrons from a conjugated system through a multiple bond to a more electronegative atom. The C≡N group pulls π-electron density towards nitrogen, like –NO2, –CHO and –COOH.
–OH, –OR, –NH2 and –OCOR all have a lone pair on the atom attached to the ring, which they donate into the conjugated system: they show the +R effect.
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