Cis–Trans Isomerism of Monocyclic Compounds
Although rings of four carbons and larger are not generally planar,they will be treated as such in this section, since the correct number of isomers can be determined when this is done and the principles are easier to visualize. The presence of a ring, like that of a double bond, prevents rotation. Cis and trans isomers are possible whenever there are two carbons on a ring, each of which is substituted by two different groups. The two carbons need not be adjacent. Examples are