In organic chemistry, aliphatic compounds (pronounced /ˌælɨˈfætɨk/; G. aleiphar, fat, oil) are acyclic or cyclic, not aromatic carbon compounds. Thus, aliphatic compounds are opposite to aromatic compounds.
In aliphatic compounds, carbon atoms can be joined together in straight chains, branched chains, or non-aromatic rings (in which case they are called alicyclic). 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 simplest aliphatic compound is methane (CH4). Aliphatics include alkanes (e.g. paraffin hydrocarbons), alkenes (e.g. ethylene) and alkynes (e.g. acetylene). Fatty acids consist of an unbranched aliphatic tail attached to a carboxyl group.
Most aliphatic compounds are flammable, allowing the use of hydrocarbons as fuel, such as methane in Bunsen burners and as Liquified Natural Gas (LNG), and acetylene in welding.
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