Practice Problems on Isomers

1. Optical Activity

• Problem: Identify which option does not show optical activity.
• Key Point: Racemic mixtures (equal concentrations of two enantiomers) are not optically active as their rotations cancel out.
• Answer Choices:
  • (A) R-2-butanol
  • (B) S-2-butanol
  • (C) 1 Molar R and 2 Molar S
  • (D) 2 Molar R and 2 Molar S ➡️ Correct answer: D

2. Stereoisomers Count

• Problem: Calculate the number of stereoisomers for an aldehyde.
• Key Point:
  • Maximum number of stereoisomers = 2^x, where x = number of chiral centers.
  • Identify chiral centers in the molecule.
• Example: If there are 3 chiral centers, the number is 2^3 = 8.
• Note: If there’s an internal plane of reflection, check for meso compounds (reduces the count of stereoisomers).
• Correct answer: B (8 Stereoisomers)

3. Optical Inactivity

• Problem: Identify which compound is optically inactive.
• Key Point: Meso compounds (contain chiral centers but have internal plane of symmetry) are optically inactive.
• Example: Compound with two chiral centers but having an internal plane of symmetry is a meso compound.
• Correct answer: C

4. Chiral Centers in Cholesterol

• Problem: Identify the number of chiral centers in cholesterol.
• Key Point: A chiral center is a carbon with four different groups attached to it.
• Answer: Cholesterol has 8 chiral centers.
• Correct answer: C

5. Stability of Isomer Conformations

• Problem: Identify the most stable isomer conformation.
• Key Point: Chair conformation of cyclohexane is more stable than boat conformation.
  • Further stability comes from having bulky groups in equatorial positions.
• Answer: B (with both methyl groups in equatorial positions)

6. Reaction Result

• Problem: Determine the changes in configuration following a reaction.
• Key Point: Relative configuration is retained if bonds on chiral centers are not broken.
• Example: Forming hydrochloric acid without affecting chiral center bonds retains both relative and absolute configurations.
• Correct answer: C (Retention of both relative and absolute configurations)

7. Enantiomers

• Problem: Determine whether two given molecules are enantiomers.
• Key Point: Enantiomers are non-superimposable mirror images with opposite configurations at all chiral centers.
• Method: Assign R/S configurations and compare.
• Correct answer: A

8. Optical Activity of Enantiomers

• Problem: Compare optical activities of one molar Omeprazole and esomeprazole (S-enantiomer).
• Key Point: Racemic mixtures (like omeprazole) are not optically active; a single enantiomer (like esomeprazole) is optically active.
• Correct answer: B (Esomeprazole exhibits optical activity)

9. E/Z Isomerism and Diastereomers

• Problem: Identify the relationship between E- and Z-2-butene.
• Key Point: E/Z (cis/trans) isomers are a type of diastereomers.
• Correct answer: C

10. Constitutional Isomers

• Problem: Relationship between 3-methylpentane and hexane.
• Key Point: Same molecular formula but different atomic connectivity ➡️ Constitutional isomers.
• Correct answer: A

11. Diastereomers

• Problem: Relationship between R-2-chloro-S-3-bromobutane and S-2-chloro-S-3-bromobutane.
• Key Point: Configurations at chiral centers differ: Opposite at one or more but not all configurations.
• Correct answer: B

12. Specific Rotation

• Problem: Calculate specific rotation given concentration and observed rotation.
• Formula: [α] = α_obs / (c × l), where l (path length) = 1 cm = 0.1 dm.
• Example Calculation: (12 degrees) / (0.5 M × 0.1 dm) = 240 degrees.
• Correct answer: D

13. Meso Compounds

• Problem: Determine if 2R,3S-2,3-dihydroxybutanedioic acid and 2S,3R-2,3-dihydroxybutanedioic acid are meso compounds or the same molecules.
• Key Point: Meso compounds have internal plane of symmetry and are superimposable mirror images.
• Correct answer: C

14. Staggered and Eclipsed Butane

• Problem: Identify the conformation of butane when methyl groups are 120 degrees apart.
• Key Point: 120-degree apart is an eclipsed conformation, but not totally eclipsed.
• Correct answer: C