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How to name cis/trans isomers (or geometric isomers)
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Organic ChemistryConfigurations
Let’s compare the conformational analysis we’ve just done with 2-Butene:
Costing 250kJ/mol (64 kcal) to do this, which is far too much
These are different compounds, and they have different configurations which means they can’t
interconvert without breaking a bond(s). Won’t happen under any normal lab conditions. These two
compounds are types of Stereoisomers (same connectivity, different orientation in space)
This type of Stereoisomerism is galled cis/trans isomerism (or geometric isomerism)
How we name them – the old way (has problems)
a. Find the main chain as it runs through the C=C or C=X double bond
b. If that chain makes a C or U shape, it is a cis isomer
c. If that chain makes an S shape, it is a trans isomer
How we name them – official way, create a series of priorities for groups using a CIP (Cahn-Ingold-
Prelog) sequence
a. Assign the highest priority group for atoms attached to the alkene C’s at each end
b. If the two highest priority groups are on the same side = Z-isomer
c. If they’re on opposite sides = E-isomer
How do we decide on CIP priority?
1. The atom with the higher atomic number has priority over the lower atomic number
a. Higher atomic mass proceeds lower atomic mass
b. H atom has higher priority than a lone pair
2. If 1st atom is tied, then move down the chain atom by atom, until a point of difference is found
3. If there’s a branch point in the group, you choose the path leading to the earliest possible
discrimination
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