Achiral molecules MCQs With Answer — This concise introduction helps B. Pharm students master achiral molecules through focused, exam-oriented content. Learn core stereochemistry concepts such as achirality, meso compounds, mirror planes, centers of inversion, racemic mixtures, and reasons for optical inactivity with pharmaceutical relevance. Questions emphasize identification of achiral structures, real drug examples, stereocenter vs stereogenic center distinctions, and how symmetry elements determine chirality in drug molecules. Ideal for revision, test prep, and strengthening applied knowledge in pharmaceutical chemistry and stereochemistry. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the best definition of an achiral molecule?
- A molecule that rotates plane-polarized light
- A molecule that is non-superimposable on its mirror image
- A molecule that is superimposable on its mirror image
- A molecule that has more than one stereocenter
Correct Answer: A molecule that is superimposable on its mirror image
Q2. Why are achiral molecules optically inactive?
- They lack electrons to interact with light
- They have no stereocenters at all
- Their mirror images are superimposable, canceling optical rotation
- They always exist as racemic mixtures
Correct Answer: Their mirror images are superimposable, canceling optical rotation
Q3. What is a meso compound?
- A compound with no stereocenters
- A racemic mixture of enantiomers
- A molecule with stereocenters that is overall achiral due to internal symmetry
- A compound showing optical activity due to conformational chirality
Correct Answer: A molecule with stereocenters that is overall achiral due to internal symmetry
Q4. Which of the following is a classic example of a meso compound?
- Glycine
- meso-tartaric acid
- 2-butanol
Correct Answer: meso-tartaric acid
Q5. Which symmetry element most directly causes achirality when present internally?
- Proper rotation axis (Cn)
- Mirror plane (σ)
- Translation symmetry
- None of the above
Correct Answer: Mirror plane (σ)
Q6. Which amino acid is achiral and why?
- Alanine, because it has two chiral centers
- Glycine, because its α-carbon has two identical substituents
- Proline, because it is cyclic
- Valine, because it racemizes rapidly
Correct Answer: Glycine, because its α-carbon has two identical substituents
Q7. A racemic mixture is overall achiral because:
- It contains only achiral molecules
- It contains equal amounts of enantiomers whose optical rotations cancel
- It lacks stereocenters entirely
- It has an internal mirror plane
Correct Answer: It contains equal amounts of enantiomers whose optical rotations cancel
Q8. Can a molecule with stereocenters be achiral?
- No — stereocenters always make a molecule chiral
- Yes — if it is a meso compound with internal symmetry
- Only if it is part of a racemic mixture
- Only when heated above its melting point
Correct Answer: Yes — if it is a meso compound with internal symmetry
Q9. Which of these molecules is achiral?
- Lactic acid
- 2-butanol
- Benzene
- Ibuprofen
Correct Answer: Benzene
Q10. Which statement about optical activity and achirality is TRUE?
- All achiral molecules are optically active
- Achiral molecules do not rotate plane-polarized light
- Achiral molecules always have no stereocenters
- Achiral molecules are always racemic mixtures
Correct Answer: Achiral molecules do not rotate plane-polarized light
Q11. Which common analgesic drug is achiral?
- Ibuprofen
- Aspirin
- Paracetamol (acetaminophen)
- Thalidomide
Correct Answer: Paracetamol (acetaminophen)
Q12. Thalidomide’s clinical issue is related to chirality because:
- It is achiral and therefore inactive
- Its enantiomers interconvert in vivo, so isolated enantiomers are not stable
- Only the R-enantiomer causes teratogenicity
- It has no stereocenters
Correct Answer: Its enantiomers interconvert in vivo, so isolated enantiomers are not stable
Q13. Which symmetry element, if present, always makes a molecule achiral?
- Proper rotation axis (Cn) alone
- Improper rotation axis (Sn) or mirror plane or inversion center
- Only translation
- None; symmetry never affects chirality
Correct Answer: Improper rotation axis (Sn) or mirror plane or inversion center
Q14. Which of the following is an achiral stereoisomer of 2,3-butanediol?
- (R,R)-2,3-butanediol
- (S,S)-2,3-butanediol
- meso-2,3-butanediol
- racemic (R,S) mixture only
Correct Answer: meso-2,3-butanediol
Q15. When identifying chirality centers, which condition is required for a carbon atom to be stereogenic?
- It must bear two identical substituents
- It must be sp-hybridized
- It must have four different substituents
- It must be part of an aromatic ring
Correct Answer: It must have four different substituents
Q16. Which pair correctly describes enantiomers?
- Non-superimposable mirror images with identical physical properties except optical rotation
- Superimposable mirror images with different melting points
- Isomers that differ only in connectivity
- Compounds related by rotation around a single bond
Correct Answer: Non-superimposable mirror images with identical physical properties except optical rotation
Q17. A molecule has a center of inversion (i). What does this imply about chirality?
- The molecule must be chiral
- The molecule is achiral
- The molecule has no stereocenters
- It guarantees optical activity
Correct Answer: The molecule is achiral
Q18. Which of these is an achiral form of 2,3-dibromobutane?
- meso-2,3-dibromobutane
- racemic (R,R)/(S,S)
- (R,R)-2,3-dibromobutane only
- (S,S)-2,3-dibromobutane only
Correct Answer: meso-2,3-dibromobutane
Q19. In pharmaceutical chemistry, why is distinguishing achiral from chiral important?
- Chirality affects drug solubility only
- Chirality influences pharmacodynamics and pharmacokinetics, hence efficacy and safety
- Achiral drugs are always more potent
- Regulatory agencies do not consider chirality
Correct Answer: Chirality influences pharmacodynamics and pharmacokinetics, hence efficacy and safety
Q20. Which statement about meso compounds is FALSE?
- They contain stereogenic centers
- They have an internal plane of symmetry
- They are optically active
- They are achiral despite stereocenters
Correct Answer: They are optically active
Q21. Which of these determines that two molecules are enantiomers?
- They have different connectivity of atoms
- One is a mirror image of the other and they are non-superimposable
- They rotate plane-polarized light by the same amount in the same direction
- They are identical compounds
Correct Answer: One is a mirror image of the other and they are non-superimposable
Q22. Which is a correct example of an achiral drug molecule?
- R-ibuprofen
- S-ibuprofen
- A racemic mixture of ibuprofen (as supplied industrially)
- Acetaminophen (paracetamol)
Correct Answer: Acetaminophen (paracetamol)
Q23. What is the optical rotation of a pure achiral compound measured in a polarimeter?
- Positive value
- Negative value
- Zero
- Undefined
Correct Answer: Zero
Q24. Which of the following symmetry elements can still allow a molecule to be chiral?
- Mirror plane (σ)
- Center of inversion (i)
- Proper rotation axis (Cn) without improper elements
- Improper rotation axis (Sn)
Correct Answer: Proper rotation axis (Cn) without improper elements
Q25. A molecule with an Sn axis (improper rotation) is typically:
- Chiral
- Achiral
- Always a racemic mixture
- Optically active in achiral solvent only
Correct Answer: Achiral
Q26. Which of the following best explains why meso-tartaric acid is achiral?
- It lacks any stereogenic carbons
- It is a racemic mixture of enantiomers
- It has stereogenic centers but an internal mirror plane makes it superimposable on its mirror image
- It does not contain carbon atoms
Correct Answer: It has stereogenic centers but an internal mirror plane makes it superimposable on its mirror image
Q27. When a chiral drug is marketed as a racemate, the formulation is:
- Always more potent
- Composed of equal amounts of both enantiomers, often achiral overall
- Only composed of the active enantiomer
- Chemically unstable
Correct Answer: Composed of equal amounts of both enantiomers, often achiral overall
Q28. Which of these indicates a molecule is definitely chiral?
- Presence of a plane of symmetry
- Presence of an inversion center
- Presence of a stereogenic carbon with four different substituents and no symmetry elements rendering it superimposable
- Being aromatic
Correct Answer: Presence of a stereogenic carbon with four different substituents and no symmetry elements rendering it superimposable
Q29. Which stereochemical feature can make a molecule achiral despite multiple stereocenters?
- Asymmetric carbon atoms only
- Internal plane of symmetry or center of inversion
- Presence of chlorine atoms
- High molecular weight
Correct Answer: Internal plane of symmetry or center of inversion
Q30. Which of these statements about enantiomers and interactions with biological systems is CORRECT?
- Enantiomers always have identical pharmacological effects
- Achiral drugs always bind chiral receptors equally
- Enantiomers can have different biological activities due to chiral receptor environments
- Enantiomers cannot be separated
Correct Answer: Enantiomers can have different biological activities due to chiral receptor environments
Q31. Which of the following molecules is achiral due to an internal plane of symmetry?
- trans-1,2-dichlorocyclohexane (when substituents are axial and equivalent)
- All enantiomers
- R-lactic acid
- Chiral atropisomer
Correct Answer: trans-1,2-dichlorocyclohexane (when substituents are axial and equivalent)
Q32. How does conformational analysis affect apparent chirality?
- Conformation never affects chirality
- A chiral conformation can interconvert to an achiral conformation, making the molecule effectively achiral if interconversion is rapid
- Achiral molecules become chiral at all temperatures
- Only ionic bonds influence chirality
Correct Answer: A chiral conformation can interconvert to an achiral conformation, making the molecule effectively achiral if interconversion is rapid
Q33. Which of the following is TRUE about meso compounds in drug synthesis?
- Meso compounds are always pharmacologically inactive
- Meso compounds cannot be separated into enantiomers
- Meso compounds are easily resolved into enantiomers
- Meso compounds lack stereocenters
Correct Answer: Meso compounds cannot be separated into enantiomers
Q34. Which term best describes two molecules that are non-superimposable but are not mirror images?
- Enantiomers
- Constitutional isomers
- Atropisomers
- Diastereomers
Correct Answer: Diastereomers
Q35. A molecule with a symmetric substitution pattern such that its mirror image can be rotated to match it is called:
- Chiral
- Achiral
- Enantiomeric
- Polymeric
Correct Answer: Achiral
Q36. For a chiral drug that racemizes under physiological conditions, the practical implication is:
- Administering one enantiomer guarantees safety
- Isolating one enantiomer is unnecessary if it rapidly converts to the racemate in vivo
- The drug is achiral and inactive
- Chirality has no effect on toxicity
Correct Answer: Isolating one enantiomer is unnecessary if it rapidly converts to the racemate in vivo
Q37. Which of the following is NOT a reason a molecule with stereocenters might be achiral?
- Internal mirror plane
- Center of inversion
- Improper rotation axis (Sn)
- Having more than one chiral center always ensures chirality
Correct Answer: Having more than one chiral center always ensures chirality
Q38. Which statement about optical rotation measurements in mixtures is correct?
- Optical rotation can distinguish meso compounds from racemates easily
- A racemic mixture shows no net optical rotation
- A racemate shows strong positive rotation
- Only achiral compounds show rotation
Correct Answer: A racemic mixture shows no net optical rotation
Q39. Which is the best description of axial chirality compared to stereogenic carbon-based chirality?
- Axial chirality arises from restricted rotation around a bond and can be chiral even without stereocenters
- Axial chirality always makes a molecule achiral
- Axial chirality depends solely on the presence of a plane of symmetry
- Axial chirality is unrelated to stereochemistry
Correct Answer: Axial chirality arises from restricted rotation around a bond and can be chiral even without stereocenters
Q40. Which experimental observation indicates a pure compound is achiral?
- It shows optical rotation that changes with solvent
- It has constant non-zero rotation at all concentrations
- It gives zero optical rotation under polarimetry
- It crystallizes in a chiral space group
Correct Answer: It gives zero optical rotation under polarimetry
Q41. Which molecule is achiral: 2-butanol, meso-2,3-dibromobutane, propanol, or (R)-2-butanol?
- 2-butanol
- meso-2,3-dibromobutane
- (R)-2-butanol
- (S)-2-butanol
Correct Answer: meso-2,3-dibromobutane
Q42. In a crystalline sample, which observation suggests the molecules are achiral?
- The crystal belongs to a chiral space group
- The crystal is optically active in bulk
- The crystal structure shows a center of symmetry or mirror plane
- The crystal contains only one enantiomer
Correct Answer: The crystal structure shows a center of symmetry or mirror plane
Q43. Which of the following does NOT automatically render a compound achiral?
- Internal mirror plane
- Center of inversion
- Presence of a stereogenic carbon
- Improper rotation axis
Correct Answer: Presence of a stereogenic carbon
Q44. How can two stereocenters lead to an achiral molecule?
- If they are both R configuration only
- If they are enantiomeric centers in different molecules
- If configurations are opposite and the molecule has an internal symmetry plane (meso)
- If one center is sp2 hybridized
Correct Answer: If configurations are opposite and the molecule has an internal symmetry plane (meso)
Q45. Which of these pharmaceutical considerations is linked to molecular achirality?
- Achiral drugs always require chiral synthesis
- Achiral molecules have no stereoselective interactions with chiral targets
- Achiral drugs are always less toxic
- Achiral molecules cannot be metabolized
Correct Answer: Achiral molecules have no stereoselective interactions with chiral targets
Q46. Which of the following pairs represents identical molecules rather than stereoisomers?
- Two enantiomers
- R-2-butanol and S-2-butanol
- Two conformations of an achiral molecule that are superimposable
- cis- and trans- isomers of 1,2-dichloroethene
Correct Answer: Two conformations of an achiral molecule that are superimposable
Q47. What feature allows a meso compound to be separated into enantiomers?
- It can be separated easily because it is a racemate
- Meso compounds cannot be separated into enantiomers because they are identical to their mirror image
- Using simple distillation
- By changing the temperature only
Correct Answer: Meso compounds cannot be separated into enantiomers because they are identical to their mirror image
Q48. Which technique can distinguish enantiomers but not achiral compounds?
- NMR in achiral solvent only
- Polarimetry or chiral chromatography
- Simple UV-Vis without chiral reagents
- Elemental analysis
Correct Answer: Polarimetry or chiral chromatography
Q49. If a substitution pattern creates an internal mirror plane in a molecule with two stereocenters, that molecule will be:
- Chiral and optically active
- A racemic mixture by definition
- Achiral and optically inactive as a single stereoisomer (meso)
- Impossible to synthesize
Correct Answer: Achiral and optically inactive as a single stereoisomer (meso)
Q50. Which approach is practical for a B. Pharm student to determine if a drug molecule is achiral?
- Assume all drugs are chiral unless proven otherwise
- Analyze molecular structure for stereocenters and internal symmetry elements (σ, i, Sn)
- Only rely on the drug’s common name
- Measure melting point only
Correct Answer: Analyze molecular structure for stereocenters and internal symmetry elements (σ, i, Sn)
