Discussion
Which of the following functional groups will show −R effect?
Answer: D
The group showing a -R effect is \(\mathrm{-NO_{2}}\).
The resonance (mesomeric) effect is -R when a group withdraws pi electrons from the conjugated system it is attached to. The nitro group has a N=O double bond conjugated with the ring and pulls electron density towards its oxygen atoms, so it is a -R group (as are -CHO, -COOH and -CN).
\(\mathrm{-OH}\), \(\mathrm{-OCH_{3}}\), \(\mathrm{-NH_{2}}\) and \(\mathrm{-NHCOCH_{3}}\) all have a lone pair on the atom joined to the ring, which they donate into the pi system. That is a +R effect, which is why these groups activate a benzene ring towards electrophilic substitution, while the nitro group deactivates it.
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