determination+of+stereochemistry

There are four steps to use in order to determine whether a chiral carbon has an "R" or an "S" configuration.

(In order to determine the configuration, the chiral center must have a stereo bond (wedge or dash) present.)

Use butan-2-ol as an example:



Step 1: number the carbon chain for identification purposes (follow IUPAC rules if you are naming it).



Step 2: Determine the order of priority for all four attached atoms, based on their atomic number. In case of a tie, use the first point of difference rule.

__C#2 priority list:__ 1) O (oxygen has an atomic # of 8) 2) C3 (higher than C2, even though both are C, because one more atom out is C4, not an H) 3) C1 (carbon's atomic # is 6) 4) H (atomic #1)

Step 3: Draw a Fischer Projection (known as a 'bow tie'). Below is a generic bow tie which you will use for every chiral center:



To draw the Fischer projection for a chiral carbon, remember these two hints:
 * if the attached atom has a "wedge" bond in the molecule, it will be in the same direction (position) in the Fischer projection. In other words, if it is pointing up like in the above molecule, put it on the top of the bow tie.
 * if the attached atom has a "dashed" bond in the molecule, it will be in the opposite position in the bow tie.
 * the backbone of the molecule will be the horizontal wedges in the bow tie.

Here is the Fischer (bow tie) for butan-2-ol:



Notice where the OH goes, and where the backbone carbons (C1 and C3) are. They should be in the same positions around C2 as they are in the molecule.

Step 4: Draw an arrow from the highest-priority atom to the second-highest priority atom:



The direction of the arrow is counter-clockwise, so therefore the stereo designation is "S".

The IUPAC name for the molecule is (2S)-butan-2-ol.

Go here for another look at how to determine R and S.