Introduction
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. Alkenes are named as if they were alkanes, but the "-ane" suffix is changed to "-ene". If the alkene contains only one double bond and that double bond is terminal (the double bond is at one end of the molecule or another) then it is not necessary to place any number in front of the name.
butane: C4H10 (CH3CH2CH2CH3)
butene: C4H8 (CH2=CHCH2CH3)
If the double bond is not terminal (if it is on a carbon somewhere in the center of the chain) then the carbons should be numbered in such a way as to give the first of the two double-bonded carbons the lowest possible number, and that number should precede the "ene" suffix with a dash, as shown below.
correct: pent-2-ene (CH3CH=CHCH2CH3)
incorrect: pent-3-ene (CH3CH2CH=CHCH3)
The second one is incorrect because flipping the formula horizontally results in a lower number for the alkene.
If there is more than one double bond in an alkene, all of the bonds should be numbered in the name of the molecule - even terminal double bonds. The numbers should go from lowest to highest, and be separated from one another by a comma. The IUPAC numerical prefixes are used to indicate the number of double bonds.
octa-2,4-diene: CH3CH=CHCH=CHCH2CH2CH3
deca-1,5-diene: CH2=CHCH2CH2CH=CHCH2CH2CH2CH3
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