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Electrophilic aromatic directing groups
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<p> In <a href="/facts/Electrophilic_aromatic_substitution/trwuWp1L">electrophilic aromatic substitution</a> reactions, existing <a href="/facts/Substituent/6y9IE6G0">substituent</a> groups on the <a href="/facts/Aromatic_ring/wK0DzRY6">aromatic ring</a> influence the overall <a href="/facts/Reaction_rate/Prwvxq0y">reaction rate</a> or have a <a href="/facts/Directing_group/a8a018Vw">directing</a> effect on <a href="/facts/Positional_isomer/vEQEBbWQ">positional isomer</a> of the <a href="/facts/Product_(chemistry)/8uCoDUdv">products</a> that are formed. </p><p>An electron donating group (EDG) or electron releasing group (ERG, Z in structural formulas) is an atom or <a href="/facts/Functional_group/GXkSPsjM">functional group</a> that donates some of its <a href="/facts/Electron_density/L5yDsk6j">electron density</a> into a <a href="/facts/Conjugated_system/Ddx9qzox">conjugated π system</a> via <a href="/facts/Resonance_(chemistry)/5tymwoIP">resonance</a> (mesomerism) or <a href="/facts/Inductive_effect/CN4c8VRt">inductive effects</a> (or induction)—called +M or +I effects, respectively—thus making the π system more <a href="/facts/Nucleophile/xYccoaJQ">nucleophilic</a>. As a result of these <a href="/facts/Electronic_effect/dGKl290x">electronic effects</a>, an <a href="/facts/Aromatic_ring/wK0DzRY6">aromatic ring</a> to which such a group is attached is more likely to participate in <a href="/facts/Electrophilic_substitution/Scs6w1W9">electrophilic substitution</a> reaction. EDGs are therefore often known as activating groups, though <a href="/facts/Steric_effect/TnX5c8HG">steric effects</a> can interfere with the reaction. </p><p>An electron withdrawing group (EWG) will have the opposite effect on the nucleophilicity of the ring. The EWG removes electron density from a π system, making it less reactive in this type of reaction, and therefore called deactivating groups. </p><p>EDGs and EWGs also determine the positions (relative to themselves) on the aromatic ring where substitution reactions are most likely to take place. Electron donating groups are generally <a href="/facts/Electrophilic_aromatic_substitution/trwuWp1L">ortho/para directors</a> for <a href="/facts/Electrophilic_aromatic_substitution/trwuWp1L">electrophilic aromatic substitutions</a>, while electron withdrawing groups (except the <a href="/facts/Halogen/YeFgWHeA">halogens</a>) are generally <a href="/facts/Electrophilic_aromatic_substitution/trwuWp1L">meta directors</a>. The selectivities observed with EDGs and EWGs were first described in 1892 and have been known as the Crum Brown–Gibson rule. </p>