Substituents

In chemistry, a substituent is an atom or group that replaces a hydrogen atom on a parent molecule. Substituents are the parts of a molecule that distinguish one compound from another and can be as simple as a halogen atom or as complex as a bulky functional group. Examples include methyl (−CH3), fluoro (−F), nitro (−NO2), hydroxyl (−OH), and amino (−NH2) groups. Substituents influence a compound’s physical properties, reactivity, and overall behavior in chemical reactions.

Substituents are categorized by their chemical nature and effects. They can be alkyl, aryl, halogen, nitro, carbonyl, or other functional groups. Substituents may be neutral or charged and can donate or withdraw electrons through inductive or resonance effects, affecting acidity and basicity. In systematic naming, substituents are indicated as prefixes with locants (for example, 3-chlorobutane). In aromatic chemistry, substituent type and placement influence directing patterns in electrophilic substitution; electron-donating groups tend to direct ortho/para, while electron-withdrawing groups often direct meta.

Beyond reactivity, substituents alter physical properties such as boiling point, solubility, and lipophilicity, and can stabilize reactive intermediates or charges. They are central to structure–activity relationships in medicinal chemistry and to the design of materials. Substitution patterns can be manipulated through synthetic methods to achieve desired properties, enabling the creation of diverse molecules from a common scaffold.