1. Define chirality and explain its importance.
Answer:
Chirality is the property of a molecule whose mirror image is non-superimposable on it. Such molecules are called chiral molecules and usually contain a carbon atom attached to four different groups.
Importance:
- Responsible for optical activity.
- Important in biological systems.
- Different enantiomers of a drug may have different biological effects.
2. Assign R/S configuration to a given chiral molecule.
Answer (Procedure):
- Assign priorities (1–4) using CIP rules.
- Place the lowest priority group (4) away from the observer.
- Trace the path from 1 → 2 → 3.
- Clockwise rotation = R configuration.
- Counterclockwise rotation = S configuration.
Example:
For lactic acid, CH₃–CH(OH)–COOH:
Priority order:
OH > COOH > CH₃ > H
If the sequence 1 → 2 → 3 is clockwise, the configuration is R; otherwise S.
3. Draw Newman projections of ethane and butane.
Answer:
Ethane
Staggered Conformation (Most Stable)
- H atoms on the rear carbon lie between those on the front carbon.
- Dihedral angle = 60°.
Eclipsed Conformation (Least Stable)
- H atoms overlap with each other.
- Dihedral angle = 0°.
Butane
Anti Conformation
- Two CH₃ groups are 180° apart.
- Most stable conformation.
Gauche Conformation
- Two CH₃ groups are 60° apart.
- Less stable due to steric hindrance.
4. Explain why meso compounds are optically inactive.
Answer:
Meso compounds contain two or more chiral centers but possess an internal plane of symmetry. The optical rotations produced by one half of the molecule are canceled by the other half, making the compound optically inactive despite having chiral centers.
Example: Meso-tartaric acid.
5. Differentiate between enantiomers and racemic mixtures.
| Enantiomers | Racemic Mixture |
|---|---|
| Non-superimposable mirror images | Equal mixture of two enantiomers |
| Rotate plane-polarized light | No net optical rotation |
| Exist as separate compounds | Exists as a mixture |
| Optically active | Optically inactive |
6. Explain the relationship between molecular symmetry and optical activity.
Answer:
A molecule with a plane of symmetry or center of symmetry is generally achiral and optically inactive. Chiral molecules lack such symmetry elements and can rotate plane-polarized light.
Rule:
- Presence of symmetry → Usually optically inactive.
- Absence of symmetry → Usually optically active.
7. Discuss the stability of different conformations of butane.
Answer:
Stability Order
Anti > Gauche > Eclipsed (CH₃-H) > Fully Eclipsed (CH₃-CH₃)
Reason:
- Anti conformation has minimum steric repulsion.
- Gauche conformation has slight methyl–methyl interaction.
- Eclipsed conformations experience torsional strain.
- Fully eclipsed conformation has maximum torsional and steric strain.
8. Explain the significance of stereochemistry in drug action.
Answer:
Biological receptors are chiral and can distinguish between different stereoisomers.
Importance:
- One enantiomer may show therapeutic activity.
- The other enantiomer may be less active, inactive, or toxic.
- Stereochemistry affects absorption, distribution, metabolism, and excretion (ADME).
Example:
Thalidomide enantiomers exhibit different biological effects.
9. Assign E/Z configuration to alkenes using CIP rules.
Answer (Procedure):
- Assign priorities on each carbon of the double bond using CIP rules.
- Compare the higher-priority groups.
- Z (Zusammen): Higher-priority groups are on the same side.
- E (Entgegen): Higher-priority groups are on opposite sides.
Example:
CH₃CH=CHCl
Priorities:
- Left carbon: CH₃ > H
- Right carbon: Cl > H
If CH₃ and Cl are on the same side → Z
If opposite sides → E
10. Explain optical activity and its measurement using a polarimeter.
Answer:
Optical activity is the ability of a chiral substance to rotate the plane of plane-polarized light.
Measurement Using a Polarimeter
- Plane-polarized light is passed through a sample tube containing the optically active substance.
- The analyzer is rotated until maximum brightness is observed.
- The angle of rotation (α) is recorded.
Types of Rotation
- Dextrorotatory (+): Rotates light clockwise.
- Levorotatory (−): Rotates light counterclockwise.
Specific Rotation
[α]=lcα
where:
- α = observed rotation
- l = path length (dm)
- c = concentration (g mL⁻¹)
Conclusion: The polarimeter is used to determine the optical activity and purity of chiral compounds.
