Sostituenti

Sostituenti (Italian for "substituents") are atoms or groups of atoms that replace hydrogen atoms on a parent molecule, most commonly in organic compounds. By altering electronic distribution, steric environment and polarity, substituents modify the physical, chemical and biological properties of the parent structure.

Common classes of substituents include alkyl and aryl groups, halogens, hydroxyl, amino, nitro and carbonyl-containing groups. They are often described by their electronic character: electron-donating groups (EDGs) increase electron density through inductive or resonance effects, while electron-withdrawing groups (EWGs) decrease electron density. Inductive effects operate through sigma bonds, whereas resonance (mesomeric) effects involve pi-electron delocalization.

Substituent effects influence reactivity, acidity/basicity, stability and spectroscopic properties. In aromatic chemistry, substituents direct electrophilic and nucleophilic substitution to particular ring positions (ortho, meta, para) according to their electronic and steric properties. Steric bulk can hinder reactions or change conformational preferences. Nomenclature uses specific prefixes (methyl-, chloro-, nitro-) or locants to indicate position and identity of substituents on the parent framework.

In synthesis and molecular design, strategic placement of substituents tunes selectivity, potency and pharmacokinetics in pharmaceuticals and optimizes electronic and mechanical properties in materials. Quantitative measures such as Hammett constants and Taft parameters are used to correlate substituent-induced changes with reactivity and selectivity in systematic studies.