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Stereoisomers

C6H14
Constitutional isomers are compounds that have the same molecular formula but a different atom-to-atom bonding sequence. This can be due to the isomers having

a different arrangement of carbons (carbon skeleton) and/or  a functional group at a different position in the chain.
C4H8





cis-but-2-ene

trans-but-2-ene
Stereoisomers are compounds that have the same numbers and types of atoms and have the same atom-to-atom bonding sequence, but they have a different arrangement of the atoms in space. 

Some, but not all, alkene constitutional isomers can exist as two different stereoisomers.

To exist as two stereoisomers, each carbon of the double bond must be bonded to two different groups.
 

But-2-ene exists two stereoisomers because both doubly bonded carbons are bonded to two different groups (H and CH3).

But-1-ene does not exist as stereoisomers because only one double-bonded carbon is bonded to two different groups.


Roll your mouse over the left one of the two images of but-1-ene below to see that, after flipping about the x-axis, the two structures are the same.
 
    
but-1-ene

Stereoisomers of this type where one (or both) of the groups at the doubly-bonded carbons are the same were historically differentiated using either cis or trans in their name.
cis isomers have the same groups on the same side of the double bond.

trans isomers have the same groups on opposite sides of the double bond.

Stereoisomers of this type may also be referred to as cis-trans isomers or geometric isomers.
Dr Sheila Woodgate
© The University of Auckland 2002