R and S label the two mirror-image arrangements — the handedness — of a stereocenter, using the Cahn–Ingold–Prelog (CIP) rules: rank four groups, read the rotation.
The two enantiomers of 2-bromobutane — same connectivity, mirror-image handedness, opposite CIP labels.
1. Assign a Priority (1–4) to the Four Groups by Atomic Number (CIP)
Rank the four directly bonded atoms by atomic number alone (Br > Cl > F > O > N > C > H); in 2-bromobutane Br is 1, H is 4, and the two carbons stay tied.
2. Break Ties by Exploring Outward, Atom by Atom, to the First Point of Difference
When first atoms tie, compare the sets one bond out until they differ: ethyl's (C,H,H) beats methyl's (H,H,H), giving Br > CH₂CH₃ > CH₃ > H.
3. Treat Every Double and Triple Bond as Duplicated Atoms
Each multiple bond adds a phantom duplicate atom, so a carboxyl carbon reads as (O,O,O) — outranking a CH₂OH carbon's (O,H,H), as in lactic acid.
4. Orient the Molecule So the Lowest Priority (4) Points Away From You
Aim priority 4 into the page; when it already sits on a dashed bond — as with H in bromochlorofluoromethane (Br > Cl > F > H) — read the front three straight off.
5. Trace 1 → 2 → 3: Clockwise = R, Counterclockwise = S
With #4 away, a clockwise 1→2→3 sweep is R (rectus) and counterclockwise is S (sinister) — trace both glyceraldehyde enantiomers below.
6. If the Lowest Priority Points Toward You, Determine the Rotation and Flip It
When #4 is on a wedge, read the 1→2→3 rotation as drawn and reverse it — clockwise then becomes S, as in 1-bromo-1-chloroethane (Br > Cl > CH₃ > H).
7. Remember: R/S Is Independent of (+)/(−) Optical Rotation
R/S is pure geometry; (+)/(−) is the measured direction of optical rotation, with no fixed link — (S)-alanine (NH₂ > COOH > CH₃ > H) happens to be natural L-alanine, a fact you memorize.
8. Summary
Rank by atomic number · break ties one bond out · count multiple bonds as duplicates · put #4 away · trace 1→2→3 (CW = R, CCW = S) · flip if #4 faces you · R/S ≠ (+)/(−).
Quiz yourself
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Both start with carbon, so it's a tie at the first atom. Exploring one bond out, the ethyl carbon is bonded to (C, H, H) while the methyl carbon is bonded to (H, H, H). C beats H at the first point of difference, so ethyl (CH₂CH₃) has higher priority than methyl (CH₃).
The double bond is treated as duplicated atoms. A carboxyl carbon (–COOH) is seen as bonded to (O, O, O) — the real –OH oxygen, the carbonyl oxygen, and a duplicate of that carbonyl oxygen. That's why –COOH outranks a –CH₂OH carbon, which is only bonded to (O, H, H).
S. The clockwise reading would mean R only if priority 4 pointed away. Because #4 points toward you, you flip the result — so a clockwise-as-drawn rotation is actually S.
No. R/S describes 3-D geometry from the CIP rules; (+)/(−) describes the measured direction of optical rotation. There is no fixed relationship between them — an (R) compound may be (+) or (−), and the sign must be determined experimentally, not deduced from the configuration.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.