A-level Chemistry/OCR (Salters)/Isomerism
Geometric isomerism, also known as cis-trans isomerism, is commonly observed in alkenes.
Cis is Latin for "on this side of" and refers to the isomer that has both substituents on the same side of the carbon-carbon double bond.
Trans is Latin for "across" and refers to the isomer that has the substituents on opposite sides of the carbon-carbon double bond.
Rotation around an alkene double bond is restricted
In alkenes, there is no free rotation about the double bond, unlike in alkanes. Consequently, groups attached to a double bonded carbon cannot exchange places. Therefore, the cis and trans isomers are separable compounds because they cannot interconvert.
Alkenes such as ethene (shown to the right) have a carbon-carbon double bond. In such a double bond, there is significant electron density above and below the plane of the bond. Rotation around the bond is greatly restricted by a large energy barrier. For rotation to occur, groups attached to one of the double-bonded carbons would have to temporarily occupy this high electron density region, resulting in very high repulsion between the electrons in the group and the electrons in the double bond.
Sometimes cis and trans isomers can interconvert, but this usually requires UV light to provide the energy to overcome the high rotational energy barrier.
Rotation around an alkane single bond is free
In alkanes, the situation is different. There is free rotation about the carbon-carbon bond because there are no regions of electron density above and below the bond that repel rotating groups. Whereas alkenes have cis and trans isomers, alkanes have eclipsed and anti conformers. Unlike geometric isomers, conformers can interconvert with ease and cannot normally be isolated from each other — they are both molecules of the same compound, just in different rotational orientations.
For compounds to exhibit optical isomerism, they need to have two non-superimposable mirror images.
For the purposes of this A-level, that means tetrahedral carbon with four different groups attached.