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Electrophilic aromatic directing groups
In organic chemistry, an electron donating group (EDG) or electron releasing group (ERG) is an atom or functional group that donates some of its electron density into a conjugated π system via resonance or inductive electron withdrawal, thus making the π system more nucleophilic. When attached to a benzene molecule, an electron donating group makes it more likely to participate 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. EDGs are often known as activating groups.
An electron withdrawing group (EWG) will have the opposite effect on nucleophilicity as an EDG, as it removes electron density from a π system, making the π system more electrophilic.〔 When attached to a benzene molecule an electron withdrawing group makes electrophilic aromatic substitution reactions slower and more complex, and EWGs are often called deactivating groups. Depending on their relative strengths, EWGs also determine the positions (relative to themselves) on the benzene ring where substitutions must take place; this property is therefore important in processes of organic synthesis.
Electron donating groups are generally ortho/para directors for electrophilic aromatic substitutions, while electron withdrawing groups are generally meta directors with the exception of the halogens which are also ortho/para directors as they have lone pairs of electrons that are shared with the aromatic ring.
==Categories==
Electron donating groups are typically divided into three levels of activating ability. Electron withdrawing groups are assigned to similar groupings. Activating substituents favor electrophilic substitution about the ortho and para positions. Weakly deactivating groups direct electrophiles to attack the benzene molecule at the ortho- and para- positions, while strongly and moderately deactivating groups direct attacks to the meta- position.〔(【引用サイトリンク】title=Substituent Effects )〕 This is not a case of favoring the meta- position like para- and ortho- directing functional groups, but rather disfavoring the para- and ortho- positions more than they disfavor the meta- position.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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