Introductory 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.
  

Functional group is an alkene, therefore suffix = -ene The longest continuous chain is C3 therefore root = prop 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-  2-butene or but-2-ene	CH3CH=CHCH3
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 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-methyl-2-pentene or 4-methylpent-2-ene	(CH3)2CHCH=CHCH3
Functional group is an alkene, therefore suffix = -ene The longest continuous chain is C6 therefore root = hex The C=C is unambiguously between C3 and C4 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