Basic IUPAC Organic Nomenclature

Epoxides

Nomenclature	Formula	3D structure
Functional class name = alkene oxides   e.g. ethene oxide Substituent suffix = -ene oxide e.g. ethene oxide Substituent prefix = epoxy-   e.g. epoxyethane Note: The term oxirane is also used to describe epoxides.

• Epoxides are cyclic ethers, a 3 membered ring (see above diagram). Their reactivity is such that they are essentially a separate functional group.
  
• There are two methods for naming epoxides:
• as the oxide of the corresponding alkene (this relates to a method of synthesising them).
• using the prefix epoxy- to indicate the epoxide as a substituent.
  

• The root name is for the corresponding alkene (think of removing the oxygen and adding a C=C at that location).
• Add the suffix oxide. 
• This is common for very simple epoxides.
  

Epoxy-

• The root name is based on the longest chain with the two C-O bonds attached.
• The chain is numbered so as to give the epoxide unit the lowest possible locant (again like alkenes)
  
• The epoxide prefix is inserted prior to the root name along with both locants e.g. 1,2-epoxypropane.
• Both locants are included since this method is also used for naming other cyclic ethers.
  

Alkene oxide style: Functional group is an epoxide, therefore suffix = -ene oxide The longest continuous chain is C3 therefore root = prop Location of "alkene" is unambiguous, so no locant needed. propene oxide Epoxy style: The longest continuous chain is C3 therefore root = prop The epoxide is a substituent therefore prefix = epoxy Number to give epoxide (only group present) the lowest locants = 1,2- 1,2-epoxypropane
Alkene oxide style: Functional group is an epoxide, therefore suffix = -ene oxide The longest continuous chain is C6 therefore root = hex The system is cyclic therefore prefix = cyclo Location of "alkene" is unambiguous, so no locant needed. cyclohexene oxide Epoxy style: The longest continuous chain is C6 therefore root = hex The root system is cyclic therefore prefix = cyclo The epoxide is a substituent therefore prefix = epoxy Number to give epoxide (only group present) the lowest locants = 1,2- 1,2-epoxycyclohexane
Alkene oxide style: Functional group is an epoxide, therefore suffix = -ene oxide The longest continuous chain is C6 therefore root = hex There is a C1 alkyl substituent = methyl The first point of difference rule requires numbering from the right as drawn to make the "alkene" locant = 2- Hence the methyl group locant = 5- 5-methyl-2-hexene oxide Epoxy style: The longest continuous chain is C6 therefore root = hex There is a C1 alkyl substituent = methyl The first point of difference rule requires numbering from the right as drawn The epoxide is a substituent therefore prefix = epoxy Number to give epoxide (only group present) the lowest locants = 2,3- 2,3-epoxy-5-methylhexane

©Dr. Ian Hunt, Department of Chemistry