Classification of organic compounds and reaction types

Organic compounds fall into three large groups by composition: hydrocarbons (only C and H), oxygen-containing compounds (C, H, O) and nitrogen-containing compounds (C, H, N, often O too). Hydrocarbons are saturated (alkanes, cycloalkanes) or unsaturated (alkenes, alkadienes, alkynes, arenes) depending on the C–C bonds, and open-chain or closed-chain by skeleton. The class is set by the functional group: –OH alcohols and phenols, –CHO aldehydes, >C=O ketones, –COOH carboxylic acids, –NO₂ nitro compounds, –NH₂ amines. Reactions are of three main types: substitution (an atom or group is replaced, e.g. CH₄ + Cl₂ → CH₃Cl + HCl), addition (a multiple bond opens, CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br) and elimination (a small molecule is split off, C₂H₅OH → CH₂=CH₂ + H₂O). Reactions peculiar to organic substances are polymerization and polycondensation.