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Assigning Priorities


Determining Stereochemical Relationships


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Assigning Priorities: Double Bonds


Double bonds that have two different groups on both carbons on the double bond can be assigned a spatial orientation. There are two main categories for these orientations.

The first category for double bonds is used when one carbon on the double bond has the same groups as the other carbon on the double bond. This is shown pictorially below.


The structure on the left has two different groups on each carbon (a hydrogen and a methyl group). Furthermore, both the carbon on the left of the double bond and the carbon on the right of the double bond have these groups. Notice that the same holds for the structure on the right.

Assign priorities to the groups in both the structure on the left and the structure on the right.

For each carbon, the methyl group takes priority over the hydrogen.

The double bond in the molecule on the left has a spatial orientation of cis. A double bond is cis when both carbons on the double bond have the same groups, and the first priorities are on the same side of the double bond.

The double bond in the molecule on the right has a spatial orientation of trans. A double bond is trans when both carbons on the double bond have the same groups, and the first priorities are on the opposite side of the double bond.

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