Stereoisomers
C4H8
C6H14
cis-but-2-ene
trans-but-2-ene
- a different arrangement of carbons (carbon skeleton) and/or
- a functional group at a different position in the chain.
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.
A group is an atom or collection of atoms.
In but-2-ene at the right and but-1-ene below CH3 is a group and H is a group.
But-2-ene exists two stereoisomers because each of the doubly-bonded carbons are bonded to two different groups (H and CH3).
In but-2-ene at the right and but-1-ene below CH3 is a group and H is a group.
But-2-ene exists two stereoisomers because each of the doubly-bonded carbons are bonded to two different groups (H and CH3).
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.
Cis-trans isomers may also be referred to as geometric isomers.
But-1-ene does not exist as stereoisomers because only one doubly-bonded carbon is bonded to two different groups (H and CH3CH2), BUT the other doubly-bonded carbon is bonded to two groups that are the same (H).
but-1-ene
but-1-ene
Roll your mouse over the left image to see that
after flipping about the x-axis, the two structures are the same.
after flipping about the x-axis, the two structures are the same.