Final answer:
An electron withdrawing group (EWG) is a substituent that pulls electron density away from a molecule, altering its reactivity and stability. The effect varies with the substituent's nature and number, impacting phenomena like nucleophilic aromatic substitutions and stabilization of intermediates in electrophilic additions. EWGs play a crucial role in the reactivity patterns of organic molecules.
Step-by-step explanation:
An electron withdrawing group (EWG) refers to a substituent that pulls electron density away from other parts of a molecule through a sigma bond. This phenomenon is known as the inductive effect, which is particularly noted in organic molecules with electronegative substituents. The strength of this effect can alter the reactivity of the molecule, as seen in nucleophilic aromatic substitutions where the presence of an EWG allows the aromatic ring to accommodate a carbanion. Electron withdrawing groups are essential in electrophilic addition reactions since they stabilize the negative charge that forms on the intermediate.
The magnitude of the electron-withdrawing effect is influenced by the nature and number of these substituents, as demonstrated by the varying pKa values of acetic acid derivatives. Similarly, the electron affinity (EA) of an atom depends on its position in the periodic table, generally becoming more negative across a row and less negative down a column.
Electrophiles are species that are electron-deficient and can receive an electron pair to form a covalent bond. The presence of an EWG can transform a neutral molecule into an electrophile by reducing its electron density, thus making it more prone to attack by nucleophiles.