Diastereoisomerism MCQs With Answer provides B.Pharm students a focused review of stereochemistry concepts essential in pharmaceutical chemistry. This introduction covers diastereomers, differences from enantiomers, meso compounds, epimers, conformational effects, and their influence on drug properties such as solubility, melting point, and biological activity. Practicing these Diastereoisomerism MCQs will improve your ability to predict stereoisomer counts, assign relative/configurational descriptors, and interpret analytical data (NMR, polarimetry, chromatography) used for separation and identification. Each question targets exam-relevant scenarios and problem-solving skills required in medicinal chemistry and drug development. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the defining feature of diastereoisomers?
- They are nonsuperimposable mirror images
- They are stereoisomers that are not mirror images of each other
- They have identical physical properties in all solvents
- They differ only at one stereocenter
Correct Answer: They are stereoisomers that are not mirror images of each other
Q2. How do diastereomers differ from enantiomers?
- Enantiomers are not mirror images; diastereomers are
- Enantiomers have different physical properties; diastereomers do not
- Enantiomers are mirror images; diastereomers are not mirror images
- Enantiomers always have different melting points; diastereomers never do
Correct Answer: Enantiomers are mirror images; diastereomers are not mirror images
Q3. For a molecule with two stereocenters and no internal plane of symmetry, what is the maximum number of stereoisomers?
- 2
- 3
- 4
- 8
Correct Answer: 4
Q4. Which pair are diastereomers?
- (R,R) and (S,S)
- (R,R) and (R,R)
- (R,S) and (S,R)
- (R,S) and (R,R)
Correct Answer: (R,S) and (R,R)
Q5. What is a meso compound?
- A compound with a chiral center but no stereoisomers
- An achiral compound that contains stereocenters due to an internal plane of symmetry
- A pair of enantiomers in equal amounts
- A diastereomer with opposite optical rotation
Correct Answer: An achiral compound that contains stereocenters due to an internal plane of symmetry
Q6. Which analytical method readily differentiates diastereomers?
- Polarimetry alone
- NMR spectroscopy
- Visual inspection
- Infrared spectroscopy only
Correct Answer: NMR spectroscopy
Q7. Two stereoisomers have different melting points and solubilities. They are most likely:
- Enantiomers
- Conformers
- Diastereomers
- Identical compounds
Correct Answer: Diastereomers
Q8. Epimers are:
- Stereoisomers differing in configuration at all stereocenters
- Enantiomers of each other
- Diastereomers that differ at only one stereogenic center
- Conformational isomers only
Correct Answer: Diastereomers that differ at only one stereogenic center
Q9. Which statement about optical rotation is true for diastereomers?
- They always have equal and opposite rotations
- They are optically inactive by definition
- They can have different magnitudes and signs of rotation
- Optical rotation cannot distinguish diastereomers
Correct Answer: They can have different magnitudes and signs of rotation
Q10. A molecule with three stereocenters and no symmetry elements will have a maximum of:
- 4 stereoisomers
- 6 stereoisomers
- 8 stereoisomers
- 16 stereoisomers
Correct Answer: 8 stereoisomers
Q11. Which is an example of diastereomeric relationship in sugars?
- Anomers
- Conformers
- Enantiomers only
- Identical molecules
Correct Answer: Anomers
Q12. Erythro and threo nomenclature is used to describe:
- Absolute configuration (R/S)
- Relative configuration in vicinal diols or amino alcohols
- Optical rotation sign
- Constitutional isomerism
Correct Answer: Relative configuration in vicinal diols or amino alcohols
Q13. A racemic mixture of two enantiomers has identical physical properties except:
- Boiling point
- Optical rotation
- Melting point
- Density
Correct Answer: Optical rotation
Q14. Which technique is commonly used to separate diastereomers?
- Chiral HPLC only
- Ordinary column chromatography or recrystallization
- Mass spectrometry alone
- Polarimetry
Correct Answer: Ordinary column chromatography or recrystallization
Q15. If two stereoisomers have different chemical reactivity under achiral conditions, they are most likely:
- Enantiomers
- Constitutional isomers
- Diastereomers
- Isotopomers
Correct Answer: Diastereomers
Q16. What happens to the number of stereoisomers when a molecule has an internal plane of symmetry?
- Number doubles
- Number is unaffected
- Number decreases due to formation of meso form
- All stereoisomers become enantiomers
Correct Answer: Number decreases due to formation of meso form
Q17. Which of the following pairs are enantiomers, not diastereomers?
- (R,S) and (R,R)
- (R,R) and (S,S)
- (R,S) and (S,R)
- (R,S) and (R,S)
Correct Answer: (R,R) and (S,S)
Q18. Diastereomeric excess (de) is analogous to which concept for enantiomers?
- Optical purity (enantiomeric excess, ee)
- Specific rotation
- Boiling point elevation
- Molar mass
Correct Answer: Optical purity (enantiomeric excess, ee)
Q19. Which descriptor indicates relative configuration rather than absolute?
- R/S
- E/Z
- cis/trans for a cyclic system (when used relationally)
- Specific rotation value
Correct Answer: cis/trans for a cyclic system (when used relationally)
Q20. In a molecule with two stereocenters, what is the relationship between (R,S) and (S,R)?
- They are identical
- They are enantiomers
- They are diastereomers
- One is meso and the other is racemic
Correct Answer: They are enantiomers
Q21. Which physical property is most likely to differ between diastereomers?
- Optical rotation only
- Refractive index identically
- Melting point and solubility
- Isotopic composition
Correct Answer: Melting point and solubility
Q22. Which statement about conformers and diastereomers is correct?
- Conformers are usually interconvertible by rotation and are not diastereomers
- Conformers are always diastereomers
- Diastereomers interconvert rapidly at room temperature
- Conformers have different atomic connectivity
Correct Answer: Conformers are usually interconvertible by rotation and are not diastereomers
Q23. How many optically active stereoisomers exist for a molecule with two stereocenters including a meso form?
- 1
- 2
- 3
- 4
Correct Answer: 3
Q24. Diastereomers in drug molecules can lead to:
- Identical pharmacokinetics
- Different biological activity and side effects
- Always toxic products
- No change in receptor binding
Correct Answer: Different biological activity and side effects
Q25. Which of the following is true about resolution of racemates versus separation of diastereomers?
- Resolution requires forming diastereomeric salts; diastereomers can often be separated directly
- Resolution and diastereomer separation are identical processes
- Resolution only applies to diastereomers
- Diastereomers cannot be separated by chromatography
Correct Answer: Resolution requires forming diastereomeric salts; diastereomers can often be separated directly
Q26. Which molecular feature can convert potential stereoisomers into identical compounds lowering the count?
- Presence of stereogenic centers only
- Internal mirror plane or centre of symmetry
- High boiling point
- Presence of a chiral solvent
Correct Answer: Internal mirror plane or centre of symmetry
Q27. An epimeric pair differs at which position?
- At two or more stereocenters
- At exactly one stereocenter
- Only in conformation
- Only in connectivity
Correct Answer: At exactly one stereocenter
Q28. For cycloalkenes, diastereomerism often arises from:
- Rotation about single bonds only
- Restricted rotation leading to cis/trans stereochemistry
- Lack of stereocenters entirely
- Isotopic substitution
Correct Answer: Restricted rotation leading to cis/trans stereochemistry
Q29. Which property makes diastereomers separable by ordinary chromatography?
- Identical polarity and solubility
- Different polarity and intermolecular interactions
- Same retention times always
- They have identical UV spectra always
Correct Answer: Different polarity and intermolecular interactions
Q30. In stereochemical terms, what is an anomer?
- A diastereomer differing at the anomeric carbon in cyclic sugars
- An enantiomer of a sugar
- A conformer of a linear sugar
- A meso sugar
Correct Answer: A diastereomer differing at the anomeric carbon in cyclic sugars
Q31. Which notation is used for double-bond stereochemistry that can produce diastereomers?
- R/S
- E/Z
- α/β only
- cis/trans never
Correct Answer: E/Z
Q32. Diastereomers typically have different:
- Mass spectra fragmentation patterns only
- Boiling points, melting points and NMR spectra
- Number of atoms
- Connectivity of atoms
Correct Answer: Boiling points, melting points and NMR spectra
Q33. Which situation reduces the theoretical 2^n stereoisomer count?
- All stereocenters being equivalent by symmetry (meso)
- Increasing molecular weight
- Presence of enantiomers only
- Using chiral reagents
Correct Answer: All stereocenters being equivalent by symmetry (meso)
Q34. Which statement about diastereomers and biological receptors is correct?
- Diastereomers always bind identically to chiral receptors
- Diastereomers cannot interact with chiral receptors
- They may have markedly different receptor affinities and activities
- Receptor binding depends only on molecular weight
Correct Answer: They may have markedly different receptor affinities and activities
Q35. When two stereoisomers have different 1H NMR chemical shifts for many protons, they are likely:
- Enantiomers in an achiral solvent
- Diastereomers
- Identical compounds
- Isotopic variants
Correct Answer: Diastereomers
Q36. Which synthetic strategy can preferentially form one diastereomer over another?
- Use of a chiral auxiliary or chiral catalyst
- Heating without catalyst only
- Using achiral solvent only
- Performing reaction in the dark
Correct Answer: Use of a chiral auxiliary or chiral catalyst
Q37. Two diastereomers in a mixture show different retention times on a GC column. What does that indicate?
- They have identical volatility
- They interact differently with the stationary phase
- They are enantiomers and cannot be separated by GC
- They are not separable by any method
Correct Answer: They interact differently with the stationary phase
Q38. A compound shows no optical rotation despite having stereocenters. This is most likely because it is:
- An enantiomeric excess sample
- A meso compound
- Pure enantiomer
- A diastereomeric mixture with ee = 100%
Correct Answer: A meso compound
Q39. In carbohydrate chemistry, C-2 epimerization of D-glucose yields:
- D-fructose
- D-galactose
- D-mannose
- L-glucose
Correct Answer: D-mannose
Q40. Which description best fits diastereomeric salts used in resolution?
- They are achiral salts formed from enantiomers
- They are salts of enantiomers with an enantiopure counterion producing diastereomeric species
- They are identical crystalline forms
- They convert enantiomers into constitutional isomers
Correct Answer: They are salts of enantiomers with an enantiopure counterion producing diastereomeric species
Q41. Which factor often causes diastereomers to have different 13C NMR chemical shifts?
- Different connectivity of carbon atoms
- Different magnetic environments due to stereochemistry
- Isotopic substitution at carbons
- Presence of identical electronic environments
Correct Answer: Different magnetic environments due to stereochemistry
Q42. If a molecule has n stereocenters but is locked by symmetry into a meso form, total stereoisomers will be:
- Always 2^n
- Less than 2^n
- More than 2^n
- Exactly n
Correct Answer: Less than 2^n
Q43. Stereochemical inversion at one center in a multi-center molecule typically produces:
- An enantiomer if all centers are inverted
- A diastereomer if only one center is inverted
- Identical compound always
- Constitutional isomer
Correct Answer: A diastereomer if only one center is inverted
Q44. Which is the best reason enantiomers have identical physical properties in achiral environments but diastereomers do not?
- Enantiomers are constitutional isomers
- Enantiomers are mirror images and differ only in interaction with chiral entities
- Diastereomers are identical molecules
- Enantiomers have different molecular formulas
Correct Answer: Enantiomers are mirror images and differ only in interaction with chiral entities
Q45. Which of the following is a diastereomeric relationship specific to cyclic systems?
- Optical isomerism only
- cis/trans stereoisomerism around ring substituents
- Constitutional isomerism only
- Isotopic stereoisomerism
Correct Answer: cis/trans stereoisomerism around ring substituents
Q46. Which process can convert an enantiomer into its diastereomer?
- Inverting all stereocenters
- Changing configuration at only one stereocenter in a multi-center molecule
- Rotating a single bond freely
- Changing solvent polarity only
Correct Answer: Changing configuration at only one stereocenter in a multi-center molecule
Q47. In medicinal chemistry, why is controlling diastereomer formation important?
- Because all diastereomers have identical toxicity
- Because different diastereomers can have different therapeutic indices and metabolism
- Because diastereomers cannot be synthesized selectively
- It isn’t important; only enantiomers matter
Correct Answer: Because different diastereomers can have different therapeutic indices and metabolism
Q48. Which is true about chiral HPLC when separating stereoisomers?
- It separates enantiomers and diastereomers using achiral stationary phases only
- It can separate enantiomers when using chiral stationary phases, and can also separate diastereomers on normal phases
- It never separates diastereomers
- It is only used for isotopic separation
Correct Answer: It can separate enantiomers when using chiral stationary phases, and can also separate diastereomers on normal phases
Q49. A drug exists as two diastereomers: one is active, the other inactive. Which approach is most practical during development?
- Use racemic mixture and hope for best
- Isolate or synthesize the active diastereomer selectively
- Always use the inactive diastereomer
- Mix both at equimolar amounts to ensure activity
Correct Answer: Isolate or synthesize the active diastereomer selectively
Q50. Which statement correctly links diastereoisomerism and NMR coupling constants?
- All diastereomers show identical coupling constants
- Vicinal coupling constants may differ between diastereomers due to different dihedral angles
- NMR cannot provide any stereochemical information
- Coupling constants depend only on atomic number
Correct Answer: Vicinal coupling constants may differ between diastereomers due to different dihedral angles
