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See +R and -R effect basically refer to the concentration of negative charge in the delocalised electrons in the benzene ring. If a group is increasing the negative charge, it is said to have +R effect. If it is decreasing, it is said to have -R effect. Atoms like, O, N, X have lone pair of electrons left over after complete bonding has taken place. So these are called electron rich species. These can contribute their lone pair of electrons to the benzene ring. So they are +R groups(Note that for halogens -I effect is stronger than +R effect)
On the other hand, Groups like N(CH3)3 +, -CHO, NO2 have a positive charge(unit positive charge or delta +) on the central atom. So they have a tendency to withdraw electrons from the benzene ring
So they are called -R groups
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Moderation Team
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