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Assigning Priorities


Determining Stereochemical Relationships


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Assigning Priorities: Double Bonds


The second category for double bonds is used when one carbon on the double bond has different groups from the other carbon on the double bond. All four groups on the double bond do not have to be different, but there must be at least three different groups. This is shown pictorally below.


The structure on the left has two different groups on each carbon. The carbon on the left of the double bond has a propyl group and a hydrogen, and the carbon on the right of the double bond has an ethyl group and a hydrogen. Overall, there are three different groups on the double bond. 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 the carbon on the left side of the double bond, the propyl group takes priority over the hydrogen. For the carbon on the right side of the double bond, the ethyl group takes priority over the hydrogen. This is true for both structures.

The double bond in the molecule on the left has a spatial orientation of Z. A double bond is Z when both carbons on the double bond have different groups, and the first priorities are on the same side of the double bond. Z is an abbreviation for the German word zusammen, which means "together." To help remember this, Z is a consonant, and "same" starts with a consonant.

The double bond in the molecule on the right has a spatial orientation of E. A double bond is E when both carbons on the double bond have different groups, and the first priorities are on the opposite side of the double bond. E is an abbreviation for the German word entgegen, which means "against," or "opposite." To help remember this, E is a vowel, and "opposite" starts with a vowel.

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