Section 16.3 The Effect of Substituents on Reactivity 633These substituents are less effective at donating electrons into the ring by resonance
because, unlike the strongly activating substituents that donate electrons by resonance
only intothe ring, the moderately activating substituents can donate electrons by reso-
nance in two competing directions:intothe ring and away fromthe ring. The fact that
these substituents are activators indicates that, despite their diminished resonance
electron donation into the ring, overall they donate electrons by resonance more
strongly than they withdraw electrons inductively.
Alkyl, aryl, and groups are weakly activating substituents. We have
seen that an alkyl substituent, compared with a hydrogen, donates electrons induc-
tively. Aryl and groups can donate electrons into the ring by resonance
and can withdraw electrons from the ring by resonance. The fact that they are weak
activators indicates that they are slightly more electron donating than they are elec-
tron withdrawing.
The halogens are weakly deactivating substituents; they donate electrons into the
ring by resonance and withdraw electrons from the ring inductively. Because the halo-
gens have been found experimentally to be deactivators, we can conclude that they
withdraw electrons inductively more strongly than they donate electrons by resonance.
weakly deactivating substituents
F Cl Br IR CH CHRweakly activating substituents
CH“CHRCH“CHROOHN CCH 3 HN CCH 3
substituent donates
electrons by resonance
away from the benzene
ring+−OOmoderately activating substituents
NH CR O CROO O OOHNCCH 3 HNCCH 3 HNCCH 3 HNCCH 3 HNCCH 3
substituent donates
electrons by resonance
into the benzene ring−−−+ + +