R/S configuration — Stereochemistry

Calling two mirror images "the left-handed one" and "the right-handed one" is fine in conversation and useless on paper. The Cahn–Ingold–Prelog system assigns each stereocenter an unambiguous label, R or S, derived entirely from the structure — no measurement, no context, no ambiguity. It is the most mechanical skill in the chapter and the one most worth drilling until it is automatic.

The four-step algorithm

Rank the four groups on the stereocenter by CIP priority, 1 (highest) through 4 (lowest).
Orient the molecule so the lowest-priority group — priority 4, usually hydrogen — points directly away from you, into the page.
Trace the path 1 → 2 → 3 among the remaining three groups.
Read it off. Clockwise is R (Latin rectus, right); counterclockwise is S (sinister, left).

The algorithm on the easiest possible case — four different atoms, no ties to break, and the lowest-priority group already pointing away on a dashed bond. Rank, orient, trace, read. The circled numbers are CIP priorities; the arrow follows 1 → 2 → 3 with priority 4 behind the page, and it turns clockwise, so this carbon is R.Clockwise is R (Latin rectus, right). Counterclockwise is S (sinister, left). The whole method is those four steps, and the only hard part is getting priority 4 to the back.

CIP priority rules

Rule 1: compare the atoms directly attached, highest atomic number wins. Bromine beats chlorine beats sulfur beats oxygen beats nitrogen beats carbon beats hydrogen. This settles most comparisons immediately, and note that it is atomic number, not electronegativity or mass — iodine outranks fluorine.

Rule 2: on a tie, move one atom outward. If two attached atoms are the same element, look at the three atoms bonded to each of them, write each set in descending order, and compare them term by term. The first point of difference decides. A carbon carrying (O,H,H) beats one carrying (C,C,H), because oxygen beats carbon at the first term — and note that a single high-ranking atom outranks two lower ones. Keep moving outward, sphere by sphere, until a difference appears.

Rule 3: duplicate multiple bonds. A double bond is treated as two single bonds to duplicate phantom atoms, and a triple bond as three.

The duplication rule is what makes carbonyl carbons rank so high. An aldehyde carbon, C=O plus H, is counted as bonded to (O, O, H) — the real oxygen plus a phantom duplicate. That beats CH₂OH's (O, H, H) at the second term, which is exactly the comparison that sets glyceraldehyde's configuration. Similarly, a carboxylic acid carbon counts as (O, O, O), outranking almost anything else made of carbon.

CIP rule 3, and why carbonyl carbons rank so high. A double bond is counted as two single bonds to a phantom duplicate of the atom at the far end — so an aldehyde carbon, which really has one oxygen, is ranked as though it carried two. This is not a fudge: it is what makes the ranking consistent when you compare a doubly-bonded atom with a singly-bonded one.A COOH carbon counts as (O, O, O), which is why it outranks almost anything else built from carbon.

The flip trick

The one shortcut worth memorising. If the lowest-priority group is pointing at you instead of away, you can still trace 1 → 2 → 3 exactly as drawn — and then simply reverse the answer. You are viewing the centre from the wrong side, so what you see is the mirror image of what the rule asks for. Forgetting this does not give you a random error; it gives you the enantiomer, reliably.

Step 2 asks you to orient the molecule with priority 4 pointing away, which is easy with a model and awkward from a flat drawing. The shortcut: trace the 1 → 2 → 3 path exactly as drawn, then flip your answer if the lowest-priority group is pointing toward you.

The reason it works is simply that you were reading the center from the opposite side, and a mirror-image viewpoint reverses the apparent sense of rotation. It is completely reliable, and it saves redrawing.

Worked example — CHFClBr, no ties

Attached atoms: Br (35), Cl (17), F (9), H (1). Atomic number alone settles it: Br (1) > Cl (2) > F (3) > H (4).

Point H away and trace Br → Cl → F. Clockwise gives R, counterclockwise gives S. No tie-breaking, no duplication, no flip needed if H is already on a dashed bond.

Worked example — butan-2-ol, H on a dashed bond

Groups: OH, CH₂CH₃, CH₃, H. Oxygen beats carbon, so OH is priority 1. H is priority 4.

The two carbons tie at the first sphere, so move out: the ethyl carbon carries (C,H,H); the methyl carbon carries (H,H,H). Carbon beats hydrogen at the first term, so CH₂CH₃ is 2 and CH₃ is 3.

Drawn with OH at the top, CH₂CH₃ lower left, CH₃ lower right and H on a dashed bond pointing away: tracing OH → CH₂CH₃ → CH₃ runs counterclockwise. H already points away, so no flip. The center is S.

Worked example — glyceraldehyde, H on a wedge

Groups: OH, CHO, CH₂OH, H. OH is 1 and H is 4 as before.

The tie between CHO and CH₂OH breaks on duplication: CHO counts as (O,O,H) against CH₂OH's (O,H,H). They tie at the first term and CHO wins at the second. So CHO is 2, CH₂OH is 3.

Drawn with OH at the top, CHO lower left and CH₂OH lower right, the 1 → 2 → 3 path again runs counterclockwise — but here H is on a bold wedge, pointing at you. Flip the answer: the center is R.

Those last two examples trace the same direction on the page and have opposite configurations. The only difference is which bond the hydrogen sits on. This is the single most common way to lose marks in stereochemistry: reading the rotation correctly and forgetting to check where priority 4 points.

Compare sets term by term, never by adding them up. A branch carrying (O,H,H) beats one carrying (C,C,C), because the comparison starts with the highest member of each set and oxygen beats carbon there. Summing atomic numbers — 8+1+1 = 10 against 6+6+6 = 18 — gives the wrong answer. Order each set from high to low, then compare first to first.

R/S is not (+)/(−), and R/S is not D/L

Three labelling systems coexist and they answer different questions. R/S is assigned from structure by the CIP rules. (+)/(−) records a measured direction of optical rotation and cannot be predicted from a drawing. D/L is an older relational system, still standard for sugars and amino acids, that compares a compound's configuration to glyceraldehyde.

There is no general correspondence among them. (S)-glyceraldehyde is levorotatory; (S)-alanine is dextrorotatory. L-amino acids are mostly (S), but L-cysteine is (R) — not because its geometry differs, but because sulfur outranks the carboxyl carbon and changes the priority order. When a problem gives you one label, it has not given you the others.

Naming whole molecules

With several stereocenters, each gets its own descriptor prefixed by its locant: (2R,3S)-3-bromobutan-2-ol. The enantiomer of that compound is (2S,3R); anything else with the same connectivity is a diastereomer. Once you can assign descriptors reliably, the relationship questions from the previous two sections become pure bookkeeping.

What carries forward

R/S notation is how every stereochemical result in the rest of the course is reported. It is how you will state that SN2 proceeds with inversion, that SN1 gives racemization, that an addition is syn or anti. And the CIP priority rules reappear unchanged as the basis of E/Z notation for alkene geometry in Module 7 — the ranking system is the same, applied to a different question.