Sequence Rules

Assign the priority order to the susbtituents on the asymmetric carbon according to the Cahn-Ingold-Prelog rules.

1. For every substituent, look at the first atom linked to the asymmetric carbon: the atom having the highest atomic number has the highest priority
2. If it is not possible to define all the priorities in this way, look at the second atom away form the double bond carbon atom, and so on, until a point of difference can be found.
3. The atoms having multiple bonds are equivalents to the same number of atoms linked by single bonds.

Correct Orientation of the Molecule

Orient the molecule with the lowest priority group (4) back, away from the observer.

Configuration Assignment

Observe the sequence 1 ---> 2 ---> 3. If, in order to follow the correct order the rotation is:

       1) clockwise, the configuration is R
       2) counterclockwise, the configuration is S