Electron-withdrawing group

[1] 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.

For example, fluorine is a stronger electron-withdrawing substituent than methyl, resulting in an increased Lewis acidity of boron trifluoride relative to trimethylborane.

[4] 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 attachment to four cyano substituents, which are electron-withdrawing groups.