Electron-withdrawing group

An electron-withdrawing group (EWG) is a

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

The inductive effect is cumulative: trichloroacetic acid is 1000x stronger than chloroacetic acid.

The impact of the EWG group 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

Effect on Lewis acidity

EWGs enhance the

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 to

Oxidants with EWGs are stronger than the parent compound. Acetylferrocenium is 300 mV more oxidizing than ferrocene.^{[citation needed]}

Comparison with electron-donating groups

Electron-withdrawing groups are the opposite effect of electron-donating groups (EDGs). Both describe functional groups, however, electron-withdrawing groups pull electron density away from a molecule, whereas EDGs push electron density onto a substituent.^[6]

See also

• Electron-donating group

References