Cis-Trans diastereomers: Alkenes (& Meso Compounds)

alkenes can have cis–trans forms

Alkenes (C=C double bonds) cannot freely rotate because the double bond is “locked.”

That means:

• Groups attached to the double bond get stuck in place

• So different spatial arrangements are possible

What cis & trans mean for Alkenes

Split your molecule down the middle.

• Cis: If you have both high priority groups in the top half, and both lower priority groups in bottom half (Z)

• Trans: If you have one high priority and a low priority in the top half, and a high priority and low priority in the bottom half (E)

How to Assign Cis or Trans for Alkenes

1. Look at sp² carbons separately (so each carbon on the ends of the double bond)

2. Assign high & low priority based on Atomic # (you are comparing the elements that are on the same side (so compare both on the left up and down, and both on the right up and down)). You can also look at the next element attached for priority ranking, like for Fischer projections.

3. Look at double bond (split down horizontally across) and see where your groups are.

4. If 2 high priority groups are on the same side (Z) cis.

5. If both up and down for the same side are the same priority, it is neither cis nor trans.

For isomers of alkenes…

You can pick either side to switch
(see image)

Meso Compounds

Mirror images but not enantiomers, because it has a chiral center but is not chiral overall, i.e., Its mirror image can be superimposed exactly (you can line them up perfectly).