1. Why is chirality important in biological systems?
Answer:
Biological systems are chiral (enzymes, receptors). Therefore, only one enantiomer of a drug may fit properly into a receptor site, leading to different biological effects. The other enantiomer may be less active or even toxic.
2. A molecule has two chiral centers but is optically inactive. Why?
Answer:
Such a molecule is a meso compound. It has an internal plane of symmetry, so optical rotations of both halves cancel each other, making it optically inactive.
3. Why do enantiomers have identical physical properties but different biological effects?
Answer:
Enantiomers have identical physical properties in an achiral environment because their structures are mirror images. However, biological systems are chiral, so they interact differently with enzymes and receptors.
4. Why does R/S configuration not determine (+)/(−) optical rotation?
Answer:
R/S describes absolute spatial arrangement, while (+)/(−) is an experimental property. There is no direct relationship between configuration and direction of optical rotation.
5. Why is rotation around a C=C double bond restricted?
Answer:
A double bond consists of one σ bond and one π bond. Rotation would break the π bond, which requires high energy, so rotation is restricted.
6. Why are meso compounds optically inactive despite having stereocenters?
Answer:
Because they contain an internal plane of symmetry, the optical effects of one half cancel the other, resulting in zero net rotation.
7. Why do diastereomers have different physical properties?
Answer:
Diastereomers are not mirror images, so their spatial arrangement differs, leading to different melting points, boiling points, solubility, and reactivity.
8. Why is anti conformation of butane more stable than gauche?
Answer:
In anti conformation, bulky CH₃ groups are far apart (180°), minimizing steric repulsion. In gauche, they are closer (60°), increasing repulsion.
9. Why are racemic mixtures optically inactive?
Answer:
A racemic mixture contains equal amounts of two enantiomers, and their opposite rotations cancel each other out.
10. Why can SN1 reactions cause racemization?
Answer:
SN1 reactions form a planar carbocation intermediate, allowing nucleophilic attack from both sides, producing a mixture of enantiomers (racemization).
