7.4 : Isomérie dans les alcènes

Alkenes differing in the position of the double bond exhibit constitutional isomerism. In addition, those with the same constitution show stereoisomerism when they differ in the spatial arrangement of atoms or groups across the double bond.

The stereoisomer is referred to as cis if the principal chain carbon atoms are on the same side of the double bond and as trans when the opposite is the case.

The cis and trans isomers of alkenes are not interconvertible by internal rotation about the double bond. Hypothetically, such a rotation would twist the p orbitals out of coplanarity, causing the bond to break.   

Alkenes are named according to the Cahn–Ingold–Prelog sequence rules to avoid ambiguity in the cis, trans nomenclature.

Each carbon atom across the double bond is envisioned separately, and the substituent with a higher atomic number connected to the carbon atoms is assigned a higher priority.

When high priority substituents are on the same side of the double bond, the alkene has a Z configuration; otherwise, it has an E configuration.

If the substituents on a carbon atom have identical first atoms, the earliest point of difference is decisive. Therefore, in this example, the substituent having oxygen ranks higher than the isopropyl group; hence, the compound is assigned a Z configuration.  

A substituent containing a double bond is expanded to duplicate the atom at each end.

For instance, the vinyl group takes precedence over ethyl since the carbon is treated as being attached to a hydrogen and two carbon atoms. Since in this example, the two high-priority groups are on the same side of the double bond, the name of the compound includes the location of the double bond along with (Z)-.