INTOCHEMISTRY / STEREOCHEMISTRY
R/S Configuration & CIP Priorities
Rank substituents, interpret Fischer projections, and assign absolute configuration.
Assign R or S using complete Cahn–Ingold–Prelog priorities and the orientation of the lowest-priority group.
1Recognize a stereogenic center
A tetrahedral atom bonded to four different substituents may be stereogenic. Always inspect the full groups, not just the first drawing.
2Rank by atomic number
Higher atomic number gets higher priority at the first point of difference. Compare outward one layer at a time when directly attached atoms tie. Multiple bonds use duplicate-atom bookkeeping.
3Orient priority 4
With the lowest-priority substituent pointing away, clockwise 1 → 2 → 3 is R and counterclockwise is S.
4Read Fischer projections
Vertical bonds project away; horizontal bonds project toward the viewer. If priority 4 is horizontal, reverse the apparent clockwise/counterclockwise assignment.
5Worked Fischer example
2-Butanol drawn with CH3 at the top, CH2CH3 at the bottom, OH on the left and H on the right is R: OH (1), ethyl (2), methyl (3), H (4). The path 1 → 2 → 3 looks counterclockwise, but H points toward you, so reverse to R.
6Common error
A reflection of a Fischer projection switches configuration. A 180° rotation of an entire Fischer projection in its plane preserves configuration; 90° generally does not.
Concept check
In a Fischer projection, the lowest-ranked group is horizontal. What must you do?
Reveal explanation
Reverse the apparent 1 → 2 → 3 assignment, because a horizontal Fischer bond points toward the observer.