Assigning Priorities: Double Bonds
These priority rules can be used to determine the spatial orientation of groups with respect to a double bond or on a stereocenter.
The spatial orientation of groups relative to a double bond can be determined based on whether the first priority groups are on the same or opposite side of the double bond. However, this determination can only be made for internal double bonds. Terminal double bonds, or double bonds on the end of a molecule, have no special orientation.
To see this, assign the priority of the groups attached to both sides of the double bond in the molecule below. (Recall that there should only be priorities 1 and 2 for each carbon of the double bond.)
The missing hydrogens should be drawn in first. Then, using the priority rules, the carbon on the left of the double bond can be assigned with the first priority on the propyl chain and the second priority on the methyl chain. (The extra carbon on the propyl chain has priority over the hydrogens on the methyl chain.) However, no priorities can be assigned for the carbon on the right of the double bond, because both hydrogens are equal in priority. Because of this, no spatial orientation can be determined.
No spatial orientation can be assigned for an internal
double bond that has two identical groups on one carbon, as in the example below.
Notice that the carbon on the right of the double bond has two identical methyl groups, so no priorities can be assigned.
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