Chapter 2 : Stereochemistry

[admin]

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[minichibixD]

Could someone help me on Q-A2 #16? When I tried to find the conformation for molecule Z, I kept getting R, and not S. Plus, I am mighty confused on the explanation, where it says "molecule Z has an s-cis conformation". I thought you were only supposed to use cis and trans for molecules with double bonds. Can you use cis and trans notation for single bonds, too?

[admin]

Q & A - 2 #16: There is no error here. It's important to recognize that R and S refer to the 3D orientation (optical isomers). Cis/trans often refer to orientation around permanent 3D shapes of a molecule (e.g. double bonds or the plane of a cyclic compound) but they are generic terms which also apply to the orientation around a single bond.
You may wonder: Is that really important for the DAT? Actually, it is a necessary concept to be able to understand the Diels Alder reaction which is explored in Chapter 4.
So back to question 16: "conformation" refers to the temporary orientation due to free rotation around the single bond (this refers to orientations like eclipsed, staggered, anti, etc.). Another, easier, way to imagine the conformer is the orientation around the single bond. Structure Z, A and C are all s-cis or cis with respect to the single bond.