One stereocenter: R or S?

a tetrahedral carbon with four different groups  ·  ideal bond lengths, all angles 109.47° ⌂ Home

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Enantiomer:

Wedge-and-dash drawing

Solid wedge = toward you, hashed = away. The drawing is generated from the 3D coordinates and its own R/S is re-derived and compared with the model’s, so the two can never disagree.

Step 1 · Rank the four groups (Cahn–Ingold–Prelog rules)

The CIP rules in four lines
  1. Atomic number first. Look only at the atom bonded to the stereocenter: higher atomic number = higher priority (Br > Cl > O > N > C > H).
  2. Tie? Step outward. If two attached atoms are the same, list what each one carries, highest first, and compare list against list at the first point of difference. Ethyl (C,H,H) beats methyl (H,H,H); CH2OH (O,H,H) beats CH2CH3 (C,H,H).
  3. Double and triple bonds count twice or three times. C=O is treated as (O,O); a C≡C carbon as (C,C,C). That is why COOH (O,O,O) outranks any alkyl group.
  4. Look with group 4 away, then trace 1 → 2 → 3. Clockwise is R (rectus, right), counterclockwise is S (sinister, left). If group 4 points toward you, the answer is reversed.
Named for Robert Cahn, Christopher Ingold, and Vladimir Prelog (1956–1966). Only rules 1 and 2 are needed for the molecules on this page; rule 3 matters for alanine and lactic acid, where COOH outranks CH3.

Step 2 · Look with group 4 pointing away

The model opens with group 4 pointing straight into the screen and an orange arrow tracing 1 → 2 → 3 around the stereocenter. Clockwise = R, counterclockwise = S. Press Trace to watch it drawn again. Rotate the model freely: the arrow follows, and it turns red with a warning whenever group 4 comes toward you, because then the sense you see is the reverse of the real answer. Lowest priority away restores the correct view.

Step 3 · Why the mirror image is a different molecule

Mirror image builds the enantiomer next to the original across a mirror plane. Superimpose then slides the mirror image onto the original so that groups 1 and 2 coincide: groups 3 and 4 land on each other’s positions. No rotation can fix that, which is what “non-superimposable” means.

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