Ch 7: Stereoisomers
Nov 3, Specifically, the relationship between the two molecules above is that We apply this to two stereoisomers which are (non-superimposable) . To nail down the specific molecule, we need to specify cis– or trans– 3-hexene. Further down the page, you will find a link to a second page which describes the The cis-trans system of naming is still widely used - especially for the sort of . are CH3 groups attached to the carbon-carbon double bond, as in cis-butene. Stereoiosmers are a type of isomer where the molecules have the same connectivity, If yes and they are superimposable mirror images, then these are the same cishexene, transhexene, cishexene and Zmethyl pentene.
This leads to a special kind of isomerism. These two compounds are cis-trans isomers or geometric isomerscompounds that have different configurations groups permanently in different places in space because of the presence of a rigid structure in their molecule.
Cis-trans isomers have different physical, chemical, and physiological properties. Cisbutene has both methyl groups on the same side of the molecule. Transbutene has the methyl groups on opposite sides of the molecule. Their structural formulas are as follows: Models of left CisButene and right TransButene.
We can draw two seemingly different propenes: Different views of the propene molecule flip vertically. These are not isomers. However, these two structures are not really different from each other. If you could pick up either molecule from the page and flip it over top to bottom, you would see that the two formulas are identical.
Thus there are two requirements for cis-trans isomerism: Rotation must be restricted in the molecule. There must be two nonidentical groups on each doubly bonded carbon atom.
And we call that trans. So this is trans isomer. I'm going to write trans here in italics, attempt to anyway. So we have cisbutene and transbutene. These are different molecules with different properties. So let's look at these next two examples here and figure out which one is cis and which one is trans. We're looking for identical groups. So over here we have an ethyl group attached to our double bond and on the right we have an ethyl group to our double bond.
Those two ethyl groups are on the same side of our double bond so this must be the cis isomer. On the right we have this ethyl group and this ethyl group on opposite sides of our double bond.
So that must be the trans isomer. All right, let's do some more examples. I'll go down to here. On the left we have cinnamaldehyde molecule. We're looking for two identical groups so we can use cis or trans.
Alkene Nomenclature: Cis and Trans and E and Z
You can also use hydrogens, right. You don't have to use a methyl group or an ethyl group so if we look at our double bond we know there's a hydrogen attached to this carbon and we know there's a hydrogen attached to this carbon. And those two hydrogens are on opposite sides of our double bonds. And I'm drawing a line here to make it easier to see. Right, these two hydrogens are on opposite side so we're talking about trans here. Those hydrogens are across from each other.
What about the tetra-substituted alkene on the right? All right over here we have a methyl group and an isopropyl group. But the two methyl groups are on the same side of our double bond.
So I draw a line in here and we see that these two groups are on the same side, therefore we're talking about cis here. So this double bond has a cis configuration. Let's compare the drawing on the left to the drawing on the right.
2-Hexene, (2E)- | C6H12 - PubChem
However, you can't because these are just two ways to represent the same molecule. If you picked up this molecule on the left and you flipped it up, you would get the drawing on the right. So they're not isomers of each other. This is the same molecule.