1. A chiral molecule is optically active — it rotates plane-polarized light.
2. One enantiomer is dextrorotatory (+) and its mirror image is levorotatory (−) by an equal amount.
3. The magnitude of rotation is the specific rotation [α], a physical constant of each pure enantiomer.
4. A racemic mixture (exactly 50:50) is optically inactive because the opposite rotations cancel.
5. Enantiomeric excess measures how one-handed a sample is: ee = |%major − %minor| = (observed [α] / pure [α]) × 100%.
6. Note (+)/(−) is measured in the lab, while (R)/(S) is assigned from structure — the two are unrelated.
Summary: Chiral = optically active · (+) dextro vs (−) levo, equal & opposite · [α] is a constant · racemic (50:50) and meso = inactive · ee = |%major − %minor| = observed/pure × 100% · (+)/(−) measured, (R)/(S) assigned.
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It contains equal amounts of both enantiomers, whose (+) and (−) rotations are equal and opposite, so they cancel exactly.
ee = |75 − 25| = 50% enantiomeric excess.
No. (R)/(S) is assigned from structure by priority rules; (+)/(−) is a measured physical property. There is no fixed correspondence between them.
ee = (20 / 40) × 100% = 50% ee (a 75:25 ratio favoring the (+) enantiomer).
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