Absolute configuration
The absolute configuration at the stereogenic carbon is designated as either R or S.
To assign the configuration at a stereogenic carbon as R or S
FIRST Assign Cahn-Ingold-Prelog priorities to the groups at the stereogenic carbon. Priorities are assigned on the basis of the atomic number of atoms in the groups at the stereogenic carbon.
SECOND use the arrangement of the groups to assign an R or S to the stereogenic centre
With the group of lowest priority behind the stereogenic carbon, if the arrangement of the other groups (highest to lowest)
See the examples at the right.
Note: If the group of lowest priority points out, instead of re-orienting the structure, assign the priorities of the groups as above, and then reverse the assignment of R or S.
To assign the configuration at a stereogenic carbon as R or S
FIRST Assign Cahn-Ingold-Prelog priorities to the groups at the stereogenic carbon. Priorities are assigned on the basis of the atomic number of atoms in the groups at the stereogenic carbon.
- The highest priority group (priority 1) has the atom of highest atomic number bonded to the stereogenic carbon.
- If the atoms bonded to the stereogenic centre have the same atomic number compare the atomic number of the next atoms out from the stereogenic centre until a difference is found. Note that doubly-bonded carbons have a higher priority than singly-bonded carbons.
SECOND use the arrangement of the groups to assign an R or S to the stereogenic centre
With the group of lowest priority behind the stereogenic carbon, if the arrangement of the other groups (highest to lowest)
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is clockwise, the assignment is R.
is anti-clockwise, the assignment is S.
is anti-clockwise, the assignment is S.
See the examples at the right.
Note: If the group of lowest priority points out, instead of re-orienting the structure, assign the priorities of the groups as above, and then reverse the assignment of R or S.