Aliphatic compound

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]

Acyclic aliphatic/non-aromatic compound (butane)
Cyclic aliphatic/non-aromatic compound (cyclobutane)

Structure

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).

Properties

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.

Examples of aliphatic compounds

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):

FormulaNameStructural formulaChemical classification
CH4MethaneAlkane
C2H2AcetyleneAlkyne
C2H4EthyleneAlkene
C2H6EthaneAlkane
C3H4PropyneAlkyne
C3H6PropeneAlkene
C3H8PropaneAlkane
C4H61,2-ButadieneDiene
C4H61-ButyneAlkyne
C4H81-ButeneAlkene
C4H10ButaneAlkane
C6H10CyclohexeneCycloalkene
C5H12n-pentaneAlkane
C7H14CycloheptaneCycloalkane
C7H14MethylcyclohexaneCyclohexane
C8H8CubanePrismane, Platonic hydrocarbon
C9H20NonaneAlkane
C10H12DicyclopentadieneDiene, Cycloalkene
C10H16PhellandreneTerpene, Diene, Cycloalkene
C10H16α-TerpineneTerpene, Diene, Cycloalkene
C10H16LimoneneTerpene, Diene, Cycloalkene
C11H24UndecaneAlkane
C30H50SqualeneTerpene, Polyene
C2nH4nPolyethyleneAlkane

References
  1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (1995) "aliphatic compounds". doi:10.1351/goldbook.A00217
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