In organic chemistry, a functional group is called an activating group (or electron donating group) if a benzene molecule to which it is attached more readily participates in electrophilic substitution reactions. Benzene itself will normally undergo substitutions by electrophiles, but additional substituents can alter the reaction rate or products by electronically or sterically affecting the interaction of the two reactants.
Activating groups are generally ortho/para directing for electrophilic aromatic substitution (though the deactivating halogens are also ortho/para directing as they have unshared lone pairs of electrons that are shared with the aromatic ring)
Functional groups are typically divided into three levels of activating ability. Deactivating groups are assigned to similar groupings.
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R = alkyl
Strongly activating substituents favor electrophilic substitution about the ortho and para positions.
This is illustrated by drawing the resonance structures of aniline: [1]
![[1]](http://s60.photobucket.com/albums/h6/adamboygenius/?action=view¤t=scan0001.jpg){kind=link}