How to Assign Re and Si Faces of Alkenes

Re and Si describe the two faces of a trigonal planar unit, such as a carbon atom in an alkene. They are useful when a reaction approaches a planar molecule from one side and may create a stereocenter. The main point is to assign the label at a specific alkene carbon: an alkene has two trigonal carbons, so a bare phrase such as “the Re face of the alkene” may not say which end is meant.

The Re/Si rule

At the carbon you are analyzing, rank its three attached groups using the Cahn–Ingold–Prelog (CIP) priority rules. Look at that carbon from one face of the alkene and trace the order from priority 1 to priority 2 to priority 3.

• If the sequence runs clockwise, that face is Re.
• If it runs counterclockwise, that face is Si.

When you look from the opposite face, the apparent clockwise direction reverses. Therefore, the opposite face has the opposite label, provided the three groups have different priorities. If two groups are identical, the center does not have a Re/Si descriptor under this method.

Re and Si are not the same as R and S. Re/Si label a face of a planar unit; R/S describe the configuration of a tetrahedral stereocenter. A reaction at an alkene may turn a planar carbon into a tetrahedral center, but the resulting R or S configuration depends on the new bond and the priorities in the product.

Assigning a face at an alkene carbon

Use this sequence:

1. Choose the carbon. Decide which end of the C=C bond matters for the reaction or stereochemical question.
2. Identify its three ligands. These are the other alkene carbon and the two groups attached to the chosen carbon.
3. Rank the ligands by CIP rules. Compare atomic numbers at the first point of difference. Treat multiple bonds using the CIP duplicate-atom convention.
4. View from the face in question. Follow priorities 1, 2, and 3 as they appear from that side.
5. Assign the label. Clockwise gives Re; counterclockwise gives Si.

For a double bond where both ends can become stereogenic, repeat the assignment at each end. Depending on the substituents and their arrangement, the same physical side of the alkene can receive the same label at both carbons or different labels. State the carbon you used rather than assuming that one label describes both ends.

Worked example: propene

Consider propene, CH3–CH=CH2, and assign the face at the middle carbon, the one bonded to CH3 and H.

Its three ligands are the terminal alkene carbon, the methyl group, and hydrogen. The terminal alkene carbon has higher priority than the methyl carbon: under the multiple-bond convention, its attached-atom list includes a duplicate carbon and two hydrogens, whereas the methyl carbon is attached to three hydrogens. Hydrogen is lowest. The priorities are therefore:

1. Terminal alkene carbon
2. Methyl group
3. Hydrogen

Now draw the alkene in the plane with the terminal =CH2 carbon to the right, the methyl group upper-left of the middle carbon, and hydrogen lower-left. Viewed from above, priority 1 is on the right, priority 2 is upper-left, and priority 3 is lower-left. Tracing 1 → 2 → 3 goes counterclockwise, so the face viewed from above is Si at the middle carbon. Viewed from below, the order appears clockwise, so that face is Re.

The assignment is tied to this drawing and viewpoint. If you rotate the molecule in its plane, the label does not change; if you view it from the opposite face, the label reverses.

Why the alkene end matters

Each carbon of a C=C bond is approximately trigonal planar and has three ligands. Those ligands need not have the same priority order at the two ends. In a substituted alkene, the other alkene carbon may outrank a directly attached alkyl group because the CIP multiple-bond convention accounts for atoms attached across the double bond.

For that reason, describing an approach as “from the Re face” is clearest when the relevant trigonal carbon is understood from the context or stated explicitly. If a reaction forms bonds at both alkene carbons, assign the face at each center separately. A reaction drawing can also make the chosen viewing side clear.

Common mistakes

• Confusing Re/Si with R/S. Re/Si are face labels for a planar group; R/S are descriptors for a stereogenic center in the product.
• Using E/Z as the face label. E/Z describes the relative arrangement of higher-priority substituents across a double bond. It does not by itself tell you whether a particular face is Re or Si.
• Ignoring the alkene’s second end. Specify the carbon where you are applying the face rule, especially when both ends react.
• Forgetting the multiple-bond convention. The alkene partner may have higher CIP priority than an attached carbon group; do not rank ligands by apparent size or distance.
• Assigning a label when priorities tie. If two ligands are identical, the required three-way priority order is absent, so the Re/Si assignment is not defined at that carbon.