Functional Groups
The characteristic atom groupings in organic molecules that determine their chemical behavior and reactivity.
Functional groups are specific groupings of atoms within organic molecules that confer characteristic chemical behavior. The carbon skeleton provides structure and stability; the functional groups determine how the molecule reacts. A short list covers most of organic chemistry: hydroxyl (−OH, alcohols), carbonyl (C=O, in aldehydes, ketones, carboxylic acids, esters, and amides), amino (−NH₂, amines), carboxyl (−COOH, carboxylic acids), and carbon–halogen bonds (alkyl halides).
Because the same functional group behaves similarly in different molecules, organic chemistry is organized around them rather than around individual compounds. Reaction types are taught by group: nucleophilic substitution on alkyl halides, esterification between carboxylic acids and alcohols, reduction of carbonyl compounds, elimination reactions of alcohols. Roughly twenty core groups suffice to organize most of the discipline's reactivity.
Functional groups also control physical properties. Hydrogen bonding by −OH and −COOH raises boiling points dramatically; ionized carboxylates and ammonium salts dissolve in water; long hydrocarbon chains make molecules nonpolar and insoluble. This is why ethanol mixes with water while its isomer dimethyl ether barely does.
Acidity is tuned by groups too: carboxylic acids have pKa around 4–5, phenols around 10, and alcohols are essentially neutral — differences that matter in the acid–base chemistry of biological and laboratory systems. Electron-withdrawing and electron-donating groups shift reactivity through inductive and resonance effects, the basis of directing effects in aromatic substitution.
Functional-group interconversion is the heart of organic synthesis: chemists plan a target molecule by mapping which known transformations convert available starting materials into the desired arrangement of groups, which is how the discipline scales from simple molecules to pharmaceuticals.
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chemistry functional groups molecules organic chemistry
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