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), also known as non-aromatic hydrocarbons. Aliphatics can be cyclic; however, hydrocarbons with conjugated pi-systems that obey Hückel's rule are instead considered to demonstrate aromaticity.[1] Aliphatic compounds can be saturated, like hexane, or unsaturated, like hexene and hexyne. Open-chain compounds (whether straight or branched) contain no rings of any type, and are thus aliphatic.

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). Besides hydrogen, other elements can be bound to the carbon chain, the most common being oxygen, nitrogen, sulfur, and chlorine.

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 Bunsen burners and as liquefied natural gas (LNG), and ethyne (acetylene) in welding.

Examples of aliphatic compounds / non-aromatic

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
C8H8CubaneOctane
C9H20NonaneAlkane
C10H12DicyclopentadieneDiene, Cycloalkene
C10H16PhellandreneTerpene, Diene Cycloalkene
C10H16α-TerpineneTerpene, Cycloalkene, Diene
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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