Inorganic chemistry,hydrocarbons (compounds composed solely ofcarbon andhydrogen) are divided into two classes:aromatic compounds andaliphatic compounds (/ˌælɪˈfætɪk/; G.aleiphar, fat, oil). Aliphatic compounds can besaturated (in which all the C-C bonds are single requiring the structure to be completed, or 'saturated', by hydrogen) likehexane, orunsaturated, likehexene andhexyne.Open-chain compounds, whether straight or branched, and which contain no rings of any type, are always aliphatic.Cyclic compounds can be aliphatic if they are notaromatic.^[1]

Aliphatics compounds can besaturated, joined by single bonds (alkanes), or unsaturated, with double bonds (alkenes) or triple bonds (alkynes). If other elements (heteroatoms) are bound to thecarbon chain, the most common beingoxygen,nitrogen,sulfur, andchlorine, it is no longer a hydrocarbon, and therefore no longer an aliphatic compound. However, such compounds may still be referred to as aliphatic if the hydrocarbon portion of the molecule is aliphatic, e.g.aliphatic amines, to differentiate them fromaromatic amines.

The least complex aliphatic compound ismethane (CH₄).

Most aliphatic compounds areflammable, allowing the use ofhydrocarbons asfuel, such as methane innatural gas for stoves or heating;butane intorches andlighters; various aliphatic (as well as aromatic) hydrocarbons in liquid transportation fuels likepetrol/gasoline,diesel, andjet fuel; and other uses such asethyne (acetylene) inwelding.

The most important aliphatic compounds are:

• n-, iso- and cyclo-alkanes (saturated hydrocarbons)
• n-, iso- and cyclo-alkenes and -alkynes (unsaturated hydrocarbons).

Important examples of low-molecular aliphatic compounds can be found in the list below (sorted by the number of carbon-atoms):