Calculate enantiomeric excess from enantiomer amounts, known % ee, or optical rotation and determine major/minor composition and ratio.
Enantiomeric Excess Formula
The calculator uses one of three formulas depending on the mode you select.
From amounts or percentages:
From a known ee (composition):
From optical rotation:
- A, B — measured amount of each enantiomer (same basis: mol, mass, response-corrected resolved peak area, or %)
- ee — enantiomeric excess, expressed as a percentage
- major / minor — percentage of each enantiomer in the mixture
- observed rotation — specific rotation measured for the sample
- pure enantiomer rotation — literature specific rotation of the pure enantiomer at matching solvent, concentration, temperature, and wavelength
Both A and B must be measured in the same units. The optical rotation method assumes the sample contains only the two enantiomers (no impurities, no other chiral species) that conditions match the reference value, and that rotation is linear with composition. Rotation sign alone does not assign R/S configuration. A racemic mixture has 0% ee; a single enantiomer has 100% ee.
Reference Tables
Use these tables to convert between ee and binary composition. The ranges are arithmetic descriptions, not chemical-purity or pharmaceutical acceptance criteria.
| % ee | Major : Minor | Major % | Minor % |
|---|---|---|---|
| 0 | 1 : 1 | 50 | 50 |
| 20 | 1.5 : 1 | 60 | 40 |
| 50 | 3 : 1 | 75 | 25 |
| 80 | 9 : 1 | 90 | 10 |
| 90 | 19 : 1 | 95 | 5 |
| 95 | 39 : 1 | 97.5 | 2.5 |
| 98 | 99 : 1 | 99 | 1 |
| 99 | 199 : 1 | 99.5 | 0.5 |
| % ee Range | Binary composition |
|---|---|
| 0 – 5 | Major fraction: 50% to 52.5% |
| 5 – 50 | Major fraction: 52.5% to 75% |
| 50 – 90 | Major fraction: 75% to 95% |
| 90 – 99 | Major fraction: 95% to 99.5% |
| ≥ 99 | Major fraction: 99.5% to 100% (ee must not exceed 100%) |
Worked Example
A resolved HPLC trace gives response-corrected peak areas of 85 for the (R) enantiomer and 15 for the (S) enantiomer.
ee = |85 − 15| / (85 + 15) × 100 = 70 / 100 × 100 = 70% ee
Composition: major = (100 + 70) / 2 = 85%; minor = (100 − 70) / 2 = 15%. Ratio = 85 : 15, or about 5.67 : 1.
FAQ
Is ee the same as enantiomeric ratio (er)? No. ee is the absolute difference between the two enantiomers as a percentage of the total. er is the direct ratio, often written as 85:15. Convert with er = (100 + ee)/2 : (100 − ee)/2.
Why does my optical-rotation ee read above 100%? An estimate above 100% is incompatible with the binary linear model. Check measurement and preparation errors, reference conditions (solvent, concentration, temperature, wavelength), sample composition and possible nonlinear behavior.
Can I use HPLC peak areas directly? Use resolved peaks that represent only the two enantiomers, with equal validated detector responses or appropriate response corrections; exclude coeluting impurities.
What if A equals B? The mixture is racemic and ee is 0%.
