Introduction

Organic chemistry is the study of the compounds of carbon. No other element comes close to carbon's ability to build compounds with itself and with hydrogen, oxygen, nitrogen, sulphur and the halogens -- carbon's defining talent is catenation, the tendency of an atom to chain with atoms of its own element, and catenation is possible on this scale only because the C-C bond is unusually strong.

The word 'organic' literally means 'derived from a living organism', and for a long time chemists believed organic compounds could only be produced by living things, never synthesised from inorganic starting materials. That belief -- vitalism -- was broken by Friedrich Wöhler, who synthesised urea (an organic compound) from ammonium cyanate (an inorganic salt). Later, Kolbe's synthesis of acetic acid and Berthelot's synthesis of methane confirmed that organic compounds really can be built in the laboratory, and since then millions of organic compounds have been made and characterised. Living cells are themselves built overwhelmingly from organic matter: DNA (the genetic material), the lipids that build cell membranes, and glycogen (the body's stored energy reserve) are all organic. Beyond biology, the field's principles underpin the food, textile, petrochemical, pharmaceutical, dye, polymer, fertiliser and cosmetics industries -- only a handful of everyday substances (salt, water) are inorganic.

Why carbon behaves this way traces back to its electron configuration: ground-state carbon is 1s2 2s2 2p2, with four valence electrons. To reach a noble-gas configuration it would have to either gain or lose four electrons outright, forming C4- or C4+ -- but that costs far too much energy, so carbon essentially never forms simple ions and instead forms four COVALENT bonds in nearly every compound it makes. Those four bonds are explained by hybridisation: during bonding, one 2s electron is promoted into the empty 2pz orbital, and the resulting mixed orbitals rearrange. Four single (sigma) bonds are explained by sp3 hybridisation (tetrahedral geometry); where carbon forms a double or triple bond, sp2 or sp hybridisation is used instead (trigonal planar or linear geometry respectively) -- the shorter bonds these hybridisations produce let neighbouring unhybridised 2p orbitals overlap sideways, forming one pi bond (sp2) or two pi bonds (sp) in addition to the sigma framework.

Every organic compound, whatever its individual identity, shares a family of properties that follow directly from this covalent, carbon-based structure. They are covalent compounds, so they are generally insoluble in water but dissolve readily in organic solvents such as benzene, toluene, ether and chloroform. Their covalent bonding also means comparatively weak intermolecular forces, so most have low melting and boiling points, and (with the notable exception of CCl4) most are flammable. Organic compounds are also characterised by their functional group -- an atom or specific combination of bonded atoms that reacts in a characteristic way no matter which molecule it sits in; nearly every reaction an organic compound undergoes happens at its functional group, and it is this feature that makes isomerism (the same molecular formula giving rise to genuinely different compounds) possible. Finally, organic compounds sharing a functional group commonly fall into a homologous series: a run of compounds, each carrying that same characteristic functional group, whose successive members differ from one another by exactly one CH2 unit in molecular formula (for example methane, ethane, propane... for the alkanes, or methanol, ethanol, propanol... for the alcohols). Members of a homologous series can be represented by one general formula (alkanes CnH2n+2, alkenes CnH2n, alkynes CnH2n-2), can usually be prepared by the same general methods, show a regular gradation in their physical properties as the chain lengthens, yet behave almost identically in their chemistry.