Definitions

activating

Activating group

In organic chemistry, a functional group is called an activating 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.

Functional groups are typically divided into three levels of activating ability. Deactivating groups are assigned to similar groupings.

Strongly Activating Groups

-NH2 -NHR -NR2

-OH -O-
Strongly activating substituents favor electrophilic substitution about the ortho and para positions.
This is illustrated by drawing the resonance structure's of aniline:

Moderately Activating Groups

-NHCOCH3 -NHCOR

-OCH3 -OR

Weakly Activating Groups

-CH3 -C2H5 -R

-C6H5

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