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Organic Chemistry Text Book (CHEM 3401 and 3402)

9.2 Nomenclature of Alkynes

Alkenes and Alkynes

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.
 2.   The longest chain chosen for the root name must include both carbon atoms of the triple bond. 
 3.   The root chain must be numbered from the end nearest a triple bond carbon atom. If the triple bond is in the center of the chain, the nearest substituent rule is used to determine the end where numbering starts.
 4.   The smaller of the two numbers designating the carbon atoms of the triple bond is used as the triple bond locator.
 5.   If several multiple bonds are present, each must be assigned a locator number. Double bonds precede triple bonds in the IUPAC name, but the chain is numbered from the end nearest a multiple bond, regardless of its nature.
 6.   Because the triple bond is linear, it can only be accommodated in rings larger than ten carbons. In simple cycloalkynes the triple bond carbons are assigned ring locations #1 and #2. Which of the two is #1 may be determined by the nearest substituent rule.
 7.   Substituent groups containing triple bonds are:
            HC≡C–   Ethynyl group
            HC≡C–CH2–   Propargyl group