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Enantiomers are non-superimposable mirror images, identical in every property that involves nothing chiral; they rotate light equally and oppositely, a racemic mixture shows no rotation, and the enantiomeric excess sets the rotation a sample shows.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will calculate enantiomeric excess, predict a sample's rotation from it and work it back, sort properties that differ between enantiomers from those that do not, and explain how the body tells them apart.
You can find a stereocentre and name it R or S. A molecule with one stereocentre exists as two forms, R and S: a pair of enantiomers.
Enantiomers are non-superimposable mirror images. A racemic mixture is a 50:50 mixture of them. Specific rotation is how far a pure enantiomer rotates plane-polarized light under standard conditions. Enantiomeric excess (ee) is the major enantiomer's percentage less the minor's.
Enantiomers have the same connections and the same energy, so every property that does not involve anything chiral is identical: melting point, boiling point, density, solubility in water.
They differ only when they meet something chiral:
A racemic mixture rotates light by zero, because each rotation is cancelled by its mirror image.
Another way: steps
For a partly pure sample:
Only the excess of one enantiomer rotates light; the rest cancels as a racemic mixture. A sample that is 80% R and 20% S has an ee of 60%, and if pure R rotates +30°, the sample rotates +18°. Chemists run this backwards to check how pure a product is. It matters because a drug's two enantiomers can do different things in the body, and regulators ask for each to be tested.
All molecules with a mirror image are enantiomers. Only chiral ones: an achiral molecule's mirror image is itself.
Enantiomers have different melting points. Only chiral surroundings tell them apart.
A racemic mixture is achiral. Every molecule is chiral; the rotations cancel.
ee is the major percentage. It is major minus minor.
Minor enantiomer: 10% (R).
Find both percentages.
ee = 90 − 10 = 80% (S).
Major less minor.
If pure (S) rotates −50°, the sample rotates −50 × 0.8 = −40°.
Only the excess rotates.
What is the ee?
70 − 30 = 40%.
What rotation does the sample show?
+40 × 0.4 = +16°.
A sample contains $70$% of the (R) enantiomer and $30$% of the (S). What is its enantiomeric excess, in per cent?
Answer:
A pure enantiomer has a specific rotation of $+33$°. A sample of it has an enantiomeric excess of $60$%. What rotation, in degrees, does the sample show under the same conditions?
Answer:
(R)- and (S)-carvone are enantiomers. Match each property to how the two compare.
| identical | equal size, opposite sign | different, because the receptors are chiral | |
|---|---|---|---|
| boiling point | |||
| solubility in water | |||
| optical rotation | |||
| smell |
A reaction makes butan-2-ol as an exact 50:50 mixture of its (R) and (S) forms. What optical rotation does the product show?
The two enantiomers of the drug thalidomide have identical melting points and solubilities, yet in the body they behave very differently. Why can the body tell them apart when a thermometer cannot?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
A pure enantiomer has a specific rotation of $+80$°. A sample of it, under the same conditions, shows $+40$°. What is the sample's enantiomeric excess, in per cent?
The sample's enantiomeric excess is a per cent.
You can say how enantiomers differ and measure a sample's purity. Tell someone why a racemic mixture does not rotate light. Next: stereoisomers that are not mirror images — diastereomers.
8. Your turn: a sample that is 70% (R), pure (R) rotating +40°, step 3