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: :\log \frac{K}{K_0} = \sigma\rho where :{K}_0 = Reference constant :\sigma = Substituent constant :\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. == Comparison with electron-donating groups ==