R/S Configuration Calculator

Determine R or S configuration at chiral centers from substituent priority rules.

Substituent 1 priority

Substituent 2 priority

Substituent 3 priority

Lowest priority (4) group position

Rotation direction 1→2→3

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The CIP Rules in Order

Assigning R or S means ranking the four substituents, then viewing the centre with the lowest priority pointing away. The ranking follows Cahn–Ingold–Prelog rules applied strictly in sequence.

Rule	Compare	Example
1	Atomic number at the first atom	Br > Cl > O > N > C > H
2	If tied, move outward to the next atoms	CH₂Cl > CH₂OH > CH₃
3	Duplicate atoms for double and triple bonds	C=O counts as (O, O, C)
4	Higher mass isotope wins if still tied	D > H

The first point of difference decides it. Once two branches differ at any level, you stop — a single higher-priority atom outranks any number of lower ones further along. CH₂Br beats C(CH₃)₃, because bromine appears at the first point of comparison.

When the Lowest Priority Points Toward You

The rules assume the lowest priority group points away. If it points toward you, determine the rotation as seen and then reverse the answer. This is the single most common source of error, and the reason many students prefer to redraw the molecule rather than reverse mentally.

Position of lowest priority	Observed rotation	Assignment
Away (dashed)	Clockwise	R
Away (dashed)	Counterclockwise	S
Toward you (wedge)	Clockwise	S — reversed
Toward you (wedge)	Counterclockwise	R — reversed

Note that R/S describes three-dimensional arrangement while (+) and (−) describe measured optical rotation. There is no reliable correlation between them — an R compound may rotate light either way, and the direction can even change with wavelength or solvent.

Worked Examples

Example 1: (R)-2-bromobutane: Br>CH2CH3>CH3>H

1→2→3 clockwise, H behind

Result: R configuration

1-Br highest AN, then chain length

Example 2: (S)-lactic acid: OH>COOH>CH3>H

1→2→3 counterclockwise, H behind

Result: S configuration

L-lactic acid in muscle: S configuration

Example 3: Comparing branches

CH₂Br versus C(CH₃)₃

Result: CH₂Br has higher priority

At the first carbon both are C. Moving out, one has (Br, H, H) and the other (C, C, C). Bromine's atomic number wins immediately.

Example 4: Wedge reversal

Priorities 1→2→3 clockwise, lowest priority on a wedge

Result: Assignment is S

Clockwise would be R with the lowest priority pointing away. Because it points toward the viewer, the answer reverses.

Common Mistakes

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Reversing the assignment incorrectly

When the lowest priority group points toward the viewer, determine the rotation then reverse it. Forgetting this inverts every assignment.

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Ranking by size or number of atoms

Priority is decided by atomic number at the first point of difference. CH₂Br outranks C(CH₃)₃ despite being smaller.

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Forgetting duplicate atoms for multiple bonds

A double bond counts the far atom twice. An aldehyde carbon is treated as bonded to (O, O, H), which outranks an alcohol carbon.

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Assuming R means dextrorotatory

R/S and (+)/(−) are unrelated. Configuration describes geometry; rotation is a measured optical property that must be determined experimentally.

Frequently Asked Questions

CIP priority rules?

1. Higher atomic number = higher priority (Cl>O>N>C>H). 2. Tie: go to next atom out. 3. Double/triple bond: expand to single bonds (C=O counts as C bonded to 2 O). 4. Isotopes: higher mass = higher priority.

R/S vs +/-?

R/S = absolute configuration (structure). (+)/(-) = optical rotation direction (measured). They don't necessarily correlate. (R)-glyceraldehyde = (+). (R)-lactic acid = (-). Must determine [α] experimentally or from literature.

What are the CIP priority rules?

Rank by atomic number at the first point of difference, working outward atom by atom. Double bonds count duplicate atoms, and isotope mass breaks remaining ties.

What if the lowest priority group points toward me?

Determine the rotation as drawn, then reverse the answer. Clockwise becomes S and counterclockwise becomes R.

Does R always mean the compound rotates light clockwise?

No. R/S describes spatial arrangement while (+)/(−) describes measured rotation. There is no reliable relationship between them.

How do double bonds affect priority?

The doubly bonded atom is counted twice. A carbonyl carbon is treated as bonded to two oxygens, which raises its priority substantially.

Formula Explorer connections

Interpretation: This relationship uses electronic structure, bonding or molecular geometry to predict a chemical property or structural descriptor. Assumption: The model may be an approximation; resonance, solvent, coordination environment, conformation and experimental conditions can affect real molecules.

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