Designating Configuration of Enantiomers

The following steps are used to assign the configuration of a chiral compound  

View the molecule so that the bond from the chirality center to group number 4 is pointed directly away from you. Now determine whether the direction of the cycle proceeding from group I to 2 to 3 and back to I is clockwise or counterclockwise. If this rotation is clockwise, the configuration is R (from the Latin word for right, rectus). If the rotation is counterclockwise, the configuration is S (from the Latin word for left, sinister). 

The group priorities are chlorine = I, ethyl = 2, methyl = 3, and hydrogen = 4. Viewing from the right side, so that the bond from the carbon to the hydrogen is pointed away from the observer, I — 2 — 3 — I is counterclockwise, so the configuration is S. This is easier to see if the structure is rotated. 

Designating CONFIGURATIONS. Q (S)-2-ChlorobutaNE AND O (R)-2-CYCLOHEXENOL. Test yourself on the concepts in this figure at OrganicChemistryNow. 

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There are two systems to designate configuration of enantiomers the D and L system, and the (R) and (S) system (also known as the Cahn-Ingold-Prelog system). 

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