Alkenes and alkynes are hydrocarbons which respectively have carbon-carbon double bond and carbon-carbon triple bond functional groups. The molecular formulas of these unsaturated hydrocarbons reflect the multiple bonding of the functional groups:
|Alkane||R–CH2–CH2–R||CnH2n+2||This is the maximum H/C ratio for a given number of carbon atoms.|
|Alkene||R–CH=CH–R||CnH2n||Each double bond reduces the number of hydrogen atoms by 2.|
|Alkyne||R–C≡C–R||CnH2n-2||Each triple bond reduces the number of hydrogen atoms by 4.|
As noted earlier in the Analysis of Molecular Formulas section, the molecular formula of a hydrocarbon provides information about the possible structural types it may represent. For example, consider compounds having the formula C5H8. The formula of the five-carbon alkane pentane is C5H12 so the difference in hydrogen content is 4. This difference suggests such compounds may have a triple bond, two double bonds, a ring plus a double bond, or two rings. Some examples are shown here, and there are at least fourteen others!
IUPAC Rules for Alkyne Nomenclature
1. The yne suffix (ending) indicates an alkyne or cycloalkyne.