hydrocarbon
/ HY-droh-kar-bun /
Strip an organic molecule down to its simplest possible form — only carbon and hydrogen, nothing else — and you have a hydrocarbon. Natural gas, gasoline, candle wax, the propane in a barbecue tank: all hydrocarbons. They are the plain canvas of organic chemistry, the carbon-and-hydrogen framework onto which everything more interesting gets attached.
A hydrocarbon is a compound made only of carbon and hydrogen atoms. They divide into families by the kind of carbon-carbon bonds they contain: alkanes have only single bonds (saturated, like methane CH4 and propane C3H8); alkenes contain at least one carbon-carbon double bond (like ethene CH2=CH2); alkynes contain a triple bond (like acetylene HC≡CH); and arenes, or aromatic hydrocarbons, contain a special flat, fully-conjugated ring such as benzene. Because carbon and hydrogen have similar electronegativities, the C-H and C-C bonds are nearly nonpolar, so simple hydrocarbons are greasy, water-insoluble, and — apart from combustion — fairly unreactive.
Hydrocarbons matter both as the world's dominant fuels and as the structural backbone of nearly every organic molecule. Petroleum is essentially a soup of hydrocarbons that refineries separate and reshape. In the bigger picture, you can think of almost any organic compound as a hydrocarbon skeleton with one or more functional groups bolted on — which is exactly why the hydrocarbon families form the opening chapters of organic chemistry.
Octane (C8H18), a component of gasoline, is a pure hydrocarbon: an eight-carbon chain saturated with hydrogens, nothing else attached. Add an OH and it becomes octanol — no longer a hydrocarbon, but an alcohol.
Pure carbon-and-hydrogen is a hydrocarbon; add any other element and you have left the family.
Hydrocarbon means carbon and hydrogen ONLY. A molecule with even one oxygen, nitrogen, or halogen is no longer a hydrocarbon — it belongs to a functional-group class instead.