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In organic chemistry, an electron withdrawing group (EWG) (deactivating group) is an atom or functional group that removes electron density from a conjugated π system via resonance or inductive electron withdrawal, thus making the π system more electrophilic. They have the opposite effect on electrophilicity as an electron donating groups (EDGs). When attached to a benzene molecule an electron withdrawing group makes electrophilic aromatic substitution reactions slower and more complex. 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 withdrawing groups are generally sorted into three categories. 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. 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.
Strongly deactivating groups
-NO2, nitro groups
-CF3, CCl3 trihalides
Moderately deactivating groups
-CN, cyano groups
-COOH, carboxylic acid
Halides are ortho- para- directing groups but unlike most ortho para directors halides tend to deactivate benzene. This unusual behavior can be explained by two properties: 1.) Since the halogens are very electronegative they cause inductive withdrawal ( withdrawal of electrons from the carbon atom of benzene) 2.) Since the halogens have non-bonding electrons they can donate electron density through pi bonding. ( resonance donation) The inductive and resonance properties compete with each other but overall the inductive wins and halogens as a result become deactivating ortho para directors. Fluorine directs strongly to the para position (86%) while Iodine directs to ortho and para (45% and 54% respectively).
While steric effects are a consideration, the major contribution of EWGs is achieved by utilizing the nature of conjugated systems (specifically the ease through which mesomeric effects travel through such systems) to create regions of positive charge within the resonance contributors. For example, in nitrobenzene the resonance structures have positive charges around the ring system:
The resulting resonance hybrid, now possessing δ+ charges in the ortho- and para- positions repels approaching electrophiles increasing the relative success of attack in the meta position.
- "Electron withdrawing group". Illustrated Glossary of Organic Chemistry. UCLA Department of Chemistry. Retrieved 16 November 2012.
- Hunt, Ian. "Substituent Effects". University of Calgary Department of Chemistry. Retrieved 16 November 2012.
- A. Crum Brown and J. Gibson, J. Chem. Soc. 61, 367 (1892)
- Crum Brown–Gibson rule @ chempensoftware.com Link