Cis-trans isomerism is observed in some alkenes but not others. Explain why 1,2-dichloroethene can exist as cis and trans isomers, and predict whether 1,1-dichloroethene can exist as cis and trans isomers, giving a reason for your answer.

1,2-dichloroethene has a C=C double bond that cannot rotate, so the groups attached to each carbon are fixed in space. Each carbon of the double bond carries two different groups (Cl and H), giving two distinct spatial arrangements: the Cl atoms can be on the same side (cis) or on opposite sides (trans). Therefore 1,2-dichloroethene can exist as both cis and trans isomers.

In 1,1-dichloroethene one carbon of the double bond has two identical groups (both Cl). Because the two groups on that carbon are identical, there is only one spatial arrangement; cis and trans forms cannot be distinguished. Hence 1,1-dichloroethene cannot exist as cis and trans isomers.