If electrophilic aromatic substitution of a monosubstituted benzene is faster than that of benzene under identical conditions, the substituent in the monosubstituted benzene is called an activating group.

eg:

Under identical conditions, Reaction 2 is faster than Reaction 1.  Thus, the methyl group is an activating group.

All activating groups are electron-donating groups.

Common activating groups:

See also deactivating group, ortho,para directing group, and meta directing group.

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