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In organic chemistry, hydrocarbons (compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (/ˌælɪˈfætɪk/; G. aleiphar, fat, oil). Aliphatic compounds can be saturated (in which all the C-C bonds are single requiring the structure to be completed, or 'saturated', by hydrogen) like hexane, or unsaturated, like hexene and hexyne. 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 not aromatic.^[1]

Aliphatic compounds can be saturated, joined by single bonds (alkanes), or unsaturated, with double bonds (alkenes) or triple bonds (alkynes). If other elements (heteroatoms) are bound to the carbon chain, the most common being oxygen, nitrogen, sulfur, and chlorine, 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 from aromatic amines.

The least complex aliphatic compound is methane (CH₄).

Most aliphatic compounds are flammable, allowing the use of hydrocarbons as fuel, such as methane in natural gas for stoves or heating; butane in torches and lighters; various aliphatic (as well as aromatic) hydrocarbons in liquid transportation fuels like petrol/gasoline, diesel, and jet fuel; and other uses such as ethyne (acetylene) in welding.

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):

Formula	Name	Structural formula	Chemical classification
CH4	Methane		Alkane
C2H2	Acetylene		Alkyne
C2H4	Ethylene		Alkene
C2H6	Ethane		Alkane
C3H4	Propyne		Alkyne
C3H6	Propene		Alkene
C3H8	Propane		Alkane
C4H6	1,2-Butadiene		Diene
C4H6	1-Butyne		Alkyne
C4H8	1-Butene		Alkene
C4H10	Butane		Alkane
C6H10	Cyclohexene		Cycloalkene
C5H12	n-pentane		Alkane
C7H14	Cycloheptane		Cycloalkane
C7H14	Methylcyclohexane		Cyclohexane
C8H8	Cubane		Prismane, Platonic hydrocarbon
C9H20	Nonane		Alkane
C10H12	Dicyclopentadiene		Diene, Cycloalkene
C10H16	Phellandrene		Terpene, Diene, Cycloalkene
C10H16	α-Terpinene		Terpene, Diene, Cycloalkene
C10H16	Limonene		Terpene, Diene, Cycloalkene
C11H24	Undecane		Alkane
C30H50	Squalene		Terpene, Polyene
C2nH4n	Polyethylene		Alkane

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