Basic IUPAC Organic Nomenclature

Alkenes

Nomenclature	Formula	3D structure
Functional group suffix = -ene Substituent name = alkenyl Structural unit :  alkenes contain C=C bonds.

• The root name is based on the longest chain containing both ends of the alkene unit, the C=C.
• The chain is numbered so as to give the alkene unit the lowest possible numbers.
• The locant for the first carbon of the alkene is used in the name.
• If the position of the alkene is unambiguous, the locant is not required, see examples below.
• As we will see later, in certain cases we will also need to specify the stereochemistry. We are going to ignore stereochemistry on this page to deal with the basics first.
  

Functional group is an alkene, therefore suffix = -ene The longest continuous chain is C3 therefore root = prop In order to give the alkene the lowest number, number from the right as drawn The C=C is unambiguously between C1 and C2 therefore the locant isn't required  propene	CH3CH=CH2
Functional group is an alkene, therefore suffix = -ene The longest continuous chain is C4 therefore root = but The C=C is between C2 and C3 so the locant is 2-  but-2-ene or 2-butene	CH3CH=CHCH3
Functional group is an alkene, therefore suffix = -ene The longest continuous chain is C4 therefore root = but In order to give the alkene the lowest number, number from the right as drawn The C=C is between C1 and C2 so the locant is 1-  but-1-ene or 1-butene	CH3CH2CH=CH2
Functional group is an alkene, therefore suffix = -ene The longest continuous chain is C6 therefore root = hex The system is cyclic there prefix = cyclo The C=C is unambiguously between C1 and C2 therefore the locant isn't required. cyclohexene
Functional group is an alkene, therefore suffix = -ene The longest continuous chain is C5 therefore root = pent In order to give the alkene the lowest number, number from the right as drawn The C=C is between C2 and C3 so the locant is  2- The substituent is a C1 alkyl group i.e. a methyl group The methyl group locant is 4- 4-methylpent-2-ene or 4-methyl-2-pentene
Functional group is an alkene, therefore suffix = -ene The longest continuous chain is C6 therefore root = hex The C=C is between C3 and C4 so the locant is  3- The substituent is a C1 alkyl group i.e. a methyl group The first point of difference rule requires numbering from the right as drawn to make the methyl group locant 3-  3-methylhex-3-ene or 3-methyl-3-hexene

©Dr. Ian Hunt, Department of Chemistry