Functional groups

A functional group is a small arrangement of atoms that behaves much the same whichever molecule it's on. Learn a dozen of them and you can predict how thousands of compounds react. The rest of the molecule, the carbon–hydrogen frame, is often written R.

Alcohols have an –OH (hydroxyl) group: ethanol, CH₃CH₂OH, is the alcohol in drinks; methanol is poisonous. Names end in -ol. Ethers have an oxygen between two carbons (R–O–R). Halides (haloalkanes) have a halogen: chloromethane, CH₃Cl.

The carbonyl group, C=O, appears in several families. An aldehyde has it at the end of the chain (R–CHO; formaldehyde, -al). A ketone has it in the middle (R–CO–R; acetone, the solvent in nail polish remover, -one). A carboxylic acid has C=O and –OH on the same carbon (R–COOH; acetic acid in vinegar, -oic acid).

An ester, R–COO–R, is what forms when a carboxylic acid joins an alcohol and gives off water. Esters smell fruity: pear drops, bananas and pineapple. An amine has nitrogen (R–NH₂) and acts as a weak base; an amide joins C=O to nitrogen (R–CONH₂) and is the link in proteins and nylon.

Functional groups decide the physical properties too. –OH, –COOH and –NH₂ can hydrogen-bond (next lesson), so molecules carrying them boil higher and mix with water better than hydrocarbons of the same size.