Cis–Trans Isomerism Resulting from Double Bonds
It has been mentioned (p. 10) that the two carbon atoms of a C__C double bond and the four atoms directly attached to them are all in the same plane and that rotation around the double bond is prevented. This means that in the case of a molecule WXC=CYZ, stereoisomerism exists when W X and Y Z. There are two and only two isomers (62 and 63), each superimposable on its mirror image unless one of the groups happens to carry a stereogenic center. Note that 62 and 63 are diastereomers, by the definition given on p. 155. There are two ways to name