(Sect. 19.7): For a substituted aromatic ring, the effect of an electron-donating substituent that increases the rate of electrophilic aromatic substitution.
Activating groups are said in reference to aromatic substitution reactions where certain substitutents on an aromatic ring increae the rate (or speed) or the reaction.
Let’s take the nitration of toluene for example:
It is said that toluene undergoes nitration 25 times faster than benzene. In other words, the methyl group on toluene “activates” the aromatic ring. Why is that? This is due to the fact that alkyl groups are generally electron donating.