Electron-withdrawing

An 'electron-withdrawing' group or substituent is a functional group that decreases the electron density in the molecule to which it is attached, pulling electron density away from the parent molecule through resonance or inductive effects. This can significantly alter the chemical and physical properties of the molecule. The strength of an electron-withdrawing group is often quantified using parameters like the Hammett sigma constant. These groups typically contain electronegative atoms (e.g., oxygen, nitrogen, halogens) or have a positive formal charge. They influence reactivity by making other parts of the molecule more electrophilic, thereby impacting reaction rates and products.

Electron-withdrawing meaning with examples

• Trifluoromethyl (CF3) is a classic example of an electron-withdrawing group. Its strong electronegativity and the presence of three fluorine atoms efficiently pull electron density away from the adjacent carbon atom. This makes molecules containing CF3 less electron-rich and more susceptible to nucleophilic attack. Consequently, the presence of CF3 affects reaction kinetics significantly.
• In aromatic systems, nitro groups (NO2) act as electron-withdrawing substituents. The nitrogen atom is bonded to two highly electronegative oxygen atoms, effectively reducing the electron density of the benzene ring. This can make the ring more susceptible to electrophilic attack, making the benzene ring less reactive in electrophilic aromatic substitution.
• Halogens like chlorine (Cl) are moderately electron-withdrawing due to their high electronegativity, although their ability to donate through resonance can sometimes counteract this effect. Chlorinated molecules often show decreased electron density on carbons directly bonded to the chlorine, influencing the reaction mechanisms and selectivity.
• Carbonyl groups (C=O) attached to an alkyl group behave as electron-withdrawing substituents. The oxygen atom's high electronegativity attracts electron density from the carbon atom, making the carbonyl carbon more electrophilic. This is crucial in reactions like nucleophilic additions, where the electrophilic carbon is targeted.
• The cyano group (C≡N) is a strong electron-withdrawing group. The electronegative nitrogen atom and the triple bond significantly reduce the electron density of the carbon atom to which it is attached. This makes molecules containing this group react with nucleophiles and can be useful to influence various chemical transformations.