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 1000× stronger than chloroacetic 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 lower

This effect of EWG has been quantified in many of ways. The Tolman electronic parameter is determined by the frequency of a C-O vibrational mode (ν(CO)) of the coordination 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 to

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

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.^[7]

• Electron-donating group