Electron-withdrawing group

Consequences of EWGs

Effects on Brønsted–Lowry acidity

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.

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

Effect on Lewis acidity

Electron-withdrawing groups tend to lower Lewis basicity. EWGs enhance the Lewis acidity, making compounds more reactive as Lewis acids. For example, fluorine is a stronger electron-withdrawing substituent than methyl, resulting in an increased Lewis acidity of boron trifluoride relative to trimethylborane.

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).

Effect on a aromatic substitution reactions

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

When it comes to nucleophilic substitution reactions, electron-withdrawing groups are more prone to nucleophilic substitution. For example, chlorodinitrobenzene is far more susceptible to reactions displacing chloride compared to chlorobenzene.

Effects on redox potential

In the context of electron transfer, these groups enhance the oxidizing power tendency of the attached species. For example, tetracyanoethylene serves as an oxidant due to its four cyano substituents, which are electron-withdrawing.

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