Organic Lecture 3: Chirality and Determining R/S Configurations

I'll do my best to put these concepts into words but keep in mind that this is generally taught with a molecule kit so students can visually see how this works.

Any molecule that cannot be superimposed on its mirror image is said to be chiral. For the MCAT, it's important that you be able to identify chiral centers. For carbon, a chiral center will have four different groups bonded to it--therefore, it must also be sp3 hybridized with 109 degree bond angles. A chiral carbon can also be referred to as a stereocenter, a stereogenic center, or an asymmetric center.

Chiral centers can be assigned an absolute configuration. This is an arbitrary set of rules known as the Cahn-Ingold-Prelog rules.

Priority is assigned to the four different substituents according to increasing atomic number of the atoms directly attached to the chiral center.

Note on isotopes: MCAT likes to test on these. For example, the isotopes of hydrogen are 1H, 2H=D (deuterium), and 3H=T (tritium). Just assigned on the basis of atomic weight if given isotopes.

If two identical atoms are attached to a stereocenter, then the next atoms in both chains are examined until a difference is found.

A multiple bond is counted as two single bonds for both of the atoms involved. For example, if you see a carbon double bonded to an oxygen, it is treated as a carbon bonded to two oxygens.

Once priorities have been assigned, rotate the molecule so that the lowest priority groups points directly away from the viewer. Then simply trace the path from the highest priority to the lowest priority. If the path is clockwise, the absolute configuration is R. If the path is counterclockwise, the absolute configuration is S.

Note: if the lowest priority is sticking out of the page (pointed toward the viewer), trace your path and then flip it. So if you traced a clockwise path, the configuration will be an S because the lowest priority is not pointed away from the viewer.