Anelectron-withdrawing group (EWG) is agroup or atom that has the ability to draw electron density toward itself and away from other adjacent atoms.^[1] This electron density transfer is often achieved by resonance or inductive effects. Electron-withdrawing groups have significant impacts on fundamental chemical processes such asacid-base reactions,redox potentials, and substitution reactions.^[1]

Electron-withdrawing groups exert an "inductive" or "electron-pulling" effect oncovalent bonds. The strength of the electron-withdrawing group is inversely proportional to thepKa of thecarboxylic acid.^[2]

The inductive effect is cumulative:trichloroacetic acid is 1000× stronger thanchloroacetic acid.

The impact of the EWG on pKa decreases with distances from the carboxylic group.

For benzoic acids, the effect is quantified by the Hammett equation:

${\displaystyle \log \frac{K}{K_0}=\sigma \rho }$

where

${\displaystyle K_0}$ = Reference constant

${\displaystyle \sigma }$ = Substituent constant

${\displaystyle \rho }$ =Reaction rate constant

Electron-withdrawing groups tend to lowerLewis basicity.^[3] EWGs enhance theLewis acidity, making compounds more reactive asLewis acids. For example, fluorine is a stronger electron-withdrawing substituent thanmethyl, resulting in an increasedLewis acidity ofboron trifluoride relative totrimethylborane.

This effect of EWG has been quantified inmany of ways. TheTolman electronic parameter is determined by the frequency of a C-O vibrational mode (ν(CO)) of thecoordination complexes [LNi(CO)₃] (L = Lewis base).^[4]

Electrophilic aromatic substitution is famously affected by EWGs. The effect is transmitted by inductive and resonance effects.^[1] Benzene with an EWG typically undergoes electrophilic substitution at meta positions. Overall the rates are diminished. thus EWGs are called deactivating.^{[citation needed]}

When it comes tonucleophilic substitution reactions, electron-withdrawing groups are more prone tonucleophilic substitution. For example,chlorodinitrobenzene is far more susceptible to reactions displacing chloride compared tochlorobenzene.^[5]

In the context ofelectron transfer, these groups enhance theoxidizing power tendency of the attached species. For example,  Tetracyanoethylene serves as an oxidant due to its fourcyano substituents, which are electron-withdrawing.^[6]

Oxidants with EWGs are stronger than the parent compound.Acetylferrocenium is 300 mV more oxidizing thanferrocene.^[6]

Electron-withdrawing groups are the opposite effect of electron-donating groups (EDGs). Both describefunctional groups, however, electron-withdrawing groups pullelectron density away from a molecule, whereas EDGs push electron density onto a substituent.^[7]

• Electron-donating group

• Physical organic chemistry
• Chemical bonding

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