9701 · 29.4
Isomerism: optical — FAQ
Frequently asked questions for 9701 Isomerism: optical. Direct answers first, then deeper explanation — then practise with marking.
Can a molecule have more than one chiral centre?
Yes, many complex molecules, especially in biology like sugars, can have multiple chiral centres. The maximum number of possible stereoisomers is 2ⁿ, where n is the number of chiral centres (though this can be complicated by molecules with internal symmetry, known as meso compounds).
Why is chirality so important in medicine?
Biological systems, like enzymes and cell receptors, are themselves chiral. They can distinguish between enantiomers of a drug. For example, the drug thalidomide was sold as a racemate; one enantiomer was an effective sedative, while the other tragically caused birth defects. This highlights the need for stereospecific synthesis in pharmaceuticals.
How do you know if an enantiomer is (+) or (-) just by looking at its structure?
You can't. The direction of rotation ((+) or (-)) is an experimentally determined property and cannot be predicted simply from the 3D structure drawing. A separate convention (R/S system) is used to label the absolute configuration at a chiral centre, but this does not directly correlate to the direction of light rotation.