The common nomenclature for the alkenes uses the radical name representing the total number of carbons present and the suffix "-ene" which indicates the presence of a double bond. This type of nomenclature becomes awkward for branched chain alkenes, and the official IUPAC nomenclature become useful.
The functional group is the center of reactivity in a molecule. In an alkene, the double bond is the functional group. The IUPAC system uses a suffix to denote certain functional groups. The systematic name of an alkene for example, is obtained by replacing the ane at the end of the parent hydrocarbon name with the suffix ene.
Nomenclature for alkenes uses the prefix to indicate the number of carbon atoms along with a suffix to indicate the functional group. A suffix is assigned to each functional group and a number is used to designate the position of the functional group in the chain or ring. For those hydrocarbon molecules containing a double bond (an alkene) the suffix is taken from the class name for an alkene (-ene).
Cis Trans IsomersBack to Top
Alkenes having identical substituents at either end of the double bond can only exist as one molecule. However, alkenes having different substituents at both ends of the double bond can exist as two possible isomers. For example, 1-butene has two hydrogens at one end of the double bond and there is only one way of constructing it. On the other hand 2-butene has different substituents at both ends of the double bond and can be constructed in two ways.
The methyl groups can be on the same side of the double bond or on opposite sides. The cis and trans isomers of an alkene are configurational isomers also called geometric isomers because they have different shapes and cannot inter convert since the double bond of an alkene cannot rotate. Therefore, the substituents are fixed in space relative to each other. The structures are different compounds with different chemical and physical properties.
E & Z NomenclatureBack to Top
The cis and trans nomenclature for alkenes is an old method of classifying the configurational isomers of alkenes and is still commonly used. However it is only suitable for simple 1,2-disubstituted alkenes where one can compare the relative position of the two substituents with respect to each other. When it comes to tri substituted and tetra substituted alkenes, a different nomenclature is required.
The E and Z nomenclature allows a clear, unambiguous definition of the configuration of alkenes. The method in which alkenes are classified as Z or E is called E/Z nomenclature. The next stage is to compare the two atoms at each end of the alkene. The one with the highest atomic number takes priority over the other. At the left hand side, oxygen has a higher atomic number than hydrogen and takes priority. At the right hand side, both atoms are the same and we cannot choose between them.
Therefore the atom with the highest atomic number has to be identified which is attached to each of these identical carbons. These corresponds to a hydrogen for the methyl substituent and a carbon for the ethyl substitutent. Having identified which groups have priority we can see whether the priority groups are on the same side of the double bond or on the opposite sides. If the two priority groups are on the same side of the double bond the alkenes is designated as Z. If the two priority groups are on opposite sides of the double bond the alkene is designated as E.