How to assign R and S configuration (and E/Z)

What makes a carbon a stereocentre

A carbon with four different groups attached. Different means different as whole substituents, not different at the first atom — a carbon bearing two ethyls is not a stereocentre, but one bearing an ethyl and a propyl is.

Check this before you start assigning, because the commonest error in a stereochemistry question is assigning a configuration to a carbon that does not have one.

Step 1 — rank the four groups (CIP rules)

Rule 1: atomic number, at the first atom

Higher atomic number wins. So I > Br > Cl > S > F > O > N > C > H. If all four first atoms differ, you are done.

Rule 2: first point of difference

When two substituents start with the same atom, move outward one shell at a time. For each, list the three atoms attached in decreasing order, and compare the lists position by position. The first position where they differ decides it — and it decides it outright, regardless of what comes after.

• –CH₂OH gives (O, H, H); –CH₂CH₃ gives (C, H, H). O beats C at the first position, so –CH₂OH outranks –CH₂CH₃.
• –CH(CH₃)₂ gives (C, C, H); –CH₂CH₂CH₃ gives (C, H, H). They tie at the first position, and (C, C, H) wins at the second, so isopropyl outranks propyl.

Rule 3: double and triple bonds duplicate

Treat a multiple bond as if each π bond were an extra single bond to a phantom copy of the atom at the other end.

• An aldehyde carbon –CH=O counts as (O, O, H) — the real oxygen plus a duplicate.
• A nitrile carbon –C≡N counts as (N, N, N).
• Each benzene ring carbon counts as (C, C, C).

So –CHO outranks –CH₂OH: (O, O, H) beats (O, H, H) at the second position.

Rule 4: isotopes

Same element, higher mass wins. Deuterium outranks ordinary hydrogen. This only appears in questions specifically about isotopic labelling.

Step 2 — orient and read

With the ranking done, put the lowest priority (4) pointing away from you, then trace 1 → 2 → 3 through the remaining three.

• Clockwise = R (from rectus, right)
• Counter-clockwise = S (from sinister, left)

Think of it as a steering wheel with the lowest priority on the steering column, pointing away from the driver.

When the lowest priority is not pointing away

Two reliable ways to handle it, and one to avoid.

1. Read and reverse. If group 4 is on a wedge (toward you), trace 1 → 2 → 3 exactly as drawn and then flip the answer. Clockwise as drawn means S. This is the fastest method and the hardest to get wrong.
2. Swap twice. Swapping any two groups inverts the configuration, so two swaps restore it. Swap group 4 into the back position, assign, then swap once more to undo — or simply remember that one swap inverts, so assign after one swap and flip.
3. Avoid rotating the molecule in your head unless you are confident. It works, but it is where most mistakes happen under time pressure.

Worked example: 2-bromobutane

The stereocentre is C-2, bearing Br, CH₃, CH₂CH₃ and H.

1. Br — highest atomic number, priority 1.
2. H — lowest, priority 4.
3. Ethyl gives (C, H, H) at its first carbon; methyl gives (H, H, H). Ethyl wins, so ethyl is 2 and methyl is 3.

Now orient with H pointing back and read Br → ethyl → methyl. Clockwise gives (R)-2-bromobutane; counter-clockwise gives (S)-2-bromobutane.

E and Z on alkenes

Same priority rules, applied to each end of the double bond separately.

1. Rank the two groups on the left carbon. Rank the two groups on the right carbon.
2. If the two higher-priority groups are on the same side, it is Z (zusammen, together).
3. If they are on opposite sides, it is E (entgegen, opposite).

E/Z is not the same as trans/cis. Cis and trans only work when each carbon carries one hydrogen; E/Z works for every alkene, and on a trisubstituted alkene the two can disagree. Use E/Z unless a question specifically asks for cis/trans.

Enantiomers, diastereomers and meso compounds

• Enantiomers — non-superimposable mirror images. Every stereocentre is inverted. Identical physical properties except for the direction they rotate plane-polarised light.
• Diastereomers — stereoisomers that are not mirror images. Some centres inverted, not all. Different melting points, different solubility, separable by ordinary means.
• Meso — has stereocentres but is achiral overall, because an internal mirror plane makes one half the mirror image of the other. (2R,3S)-tartaric acid is the classic case: two stereocentres, zero optical rotation.

With n stereocentres you can have at most 2ⁿ stereoisomers — fewer when meso forms collapse two possibilities into one.

Where stereochemistry decides a mechanism question

Configuration is not just nomenclature; it is evidence about mechanism.

• SN2 inverts — the nucleophile attacks opposite the leaving group, so the configuration flips. A single clean inversion is proof of a concerted mechanism.
• SN1 racemises — the planar carbocation is attacked from both faces, giving both enantiomers.
• E2 needs anti-periplanar geometry — the β-H and the leaving group must be 180° apart, which on a ring means both axial.
• Additions are syn or anti depending on the intermediate, which fixes the relative configuration of the two new centres.

How Organic Chemistry AI helps here

Stereochemistry questions are easy to get almost right, which is the worst kind of wrong. When a solve names the reaction type and walks the steps, it states what happens at the stereocentre — inversion, racemisation, syn or anti addition — so you can check the part you are least sure about. For the ranking rules themselves, the offline formula reference includes stereochemistry entries you can search without a connection, and the bundled study library has a stereochemistry chapter.

The formula reference holds 522 entries across 11 topics — nomenclature, isomerism and stereochemistry included — and works with no connection.

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