One of my homework problems is asking whether cis-but-2-ene and methylpropene are cis-trans isomers or constitutional isomers. But I'm not sure about whether cis-trans isomers include those which the two functional groups exist on the same carbon in the double bond. So does it?

I've searched on Google and looked in my textbook, but they doesn't seem to have anything.


1 Answer 1


(2​Z)-But-2-ene and 2-methylprop-1-ene are constitutional isomers as they have different connectivities, i.e. the sequence in which the atoms are bonded to each other is not the same. For example, the carbons highlighted here are different: in (2​Z)-but-2-ene it is only bonded to two other carbons (through a single bond and a double bond), but in 2-methylprop-1-ene it is bonded to three other carbons:

Constitutional isomers

Cis/trans isomers are a subset of stereoisomers, which need to have the same constitution, or molecular connectivity. The two isomers of but-2-ene satisfy this:

Configurational isomers

Note how both highlighted carbons are now bonded to the same partners, but with a different spatial arrangement.


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