Absolute configuration in stereochemistry: R/S nomenclature

Question

I have all the rules of Fischer projections figured out.

But, when the problem asks for finding the configuration of a molecule with 1 chiral carbon and in a Fischer projection form, do we need first to make the manovrations that are allowed ( like fliping 180 degrees, or changing the places of 2-2 substituents ) or do we directly find the priorities ?

Answer 1

Just to remember the rules to assign an R/S configuration:

In order to assign the configuration as R or S:

• Identify each of the chirality centers (most commonly an sp3 C with 4 different groups attached) Then at each chirality center....
• Assign the priority (high = 1 to low = 4) to each group attached to the chirality center based on atomic number.
• Reposition the molecule so that the lowest priority group is away from you as if you were looking along the C-(4) σ bond. If you are using a model, grasp the lowest priority group in your fist.
• Determine the relative direction of the priority order of the three higher priority groups (1 to 2 to 3)
• If this is clockwise then it is the R-stereoisomer (Latin; rectus = right handed)
• If this is counter-clockwise then it is the S-stereoisomer (Latin; sinister = left handed)
• If there is more than one stereocenter, then the location needs to be included with the locant, e.g. (2R)-

You can identify these previous rules on this example:

Source

Answer 2

You can assign the priorities directly. When assigning rotation, keep in mind that the atoms on the horizontal are coming out of the plane of the paper. Since there is usually a hydrogen on the horizontal, usually you will be looking at the stereocenter backwards. If it appears R, it is actually S, and vice versa.