Optical activity MCQs With Answer

Optical activity MCQs With Answer is a focused study guide for B. Pharm students covering optical isomerism, polarimetry, specific rotation, and chiral drug implications. This concise, keyword-rich introduction emphasizes stereochemistry, enantiomers, racemic mixtures, specific rotation calculations, polarimeter use, and pharmacological relevance of chiral centers. Ideal for exam prep and practical understanding, the questions explore definitions, mathematical problems, experimental variables (wavelength, temperature, solvent), optical purity, and analytical techniques like optical rotatory dispersion and circular dichroism. Clear explanations and answer choices help reinforce core concepts essential to pharmaceutical analysis and drug development. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is optical activity?

• The ability of a substance to rotate the plane of polarized light
• The ability to absorb ultraviolet light
• The property of molecules to fluoresce
• The capacity of a substance to undergo racemization

Correct Answer: The ability of a substance to rotate the plane of polarized light

Q2. Which of the following is rotated clockwise (to the right) when viewed towards the light source?

• Dextrorotatory (d- or +)
• Levorotatory (l- or -)
• Optically inactive
• Racemate

Correct Answer: Dextrorotatory (d- or +)

Q3. Specific rotation [α] is defined by which equation?

• [α] = αobs / (l × c)
• [α] = αobs × l × c
• [α] = αobs / (c − l)
• [α] = (l × c) / αobs

Correct Answer: [α] = αobs / (l × c)

Q4. In the specific rotation formula [α] = αobs / (l × c), what are the standard units for l and c in most pharmaceutical calculations?

• l in decimeters and c in g/mL (or g·mL⁻¹)
• l in centimeters and c in mol/L
• l in meters and c in g/L
• l in millimeters and c in mg/mL

Correct Answer: l in decimeters and c in g/mL (or g·mL⁻¹)

Q5. A 1.00 g/mL solution in a 10 cm (1.0 dm) tube shows an observed rotation of +5.0°. What is the specific rotation?

• +5.0°
• +50.0°
• +0.50°
• +0.005°

Correct Answer: +5.0°

Q6. Which sample is optically inactive?

• A racemic mixture of enantiomers
• A pure enantiomer
• A chiral compound with one stereocenter
• An enantiomerically enriched sample

Correct Answer: A racemic mixture of enantiomers

Q7. What does an enantiomeric excess (ee) of 100% indicate?

• Sample is a single pure enantiomer
• Sample is a racemic mixture
• Sample contains no chiral molecules
• Sample has equal amounts of diastereomers

Correct Answer: Sample is a single pure enantiomer

Q8. How is enantiomeric excess (ee) calculated from optical rotation?

• ee = (observed [α] / [α] of pure enantiomer) × 100%
• ee = observed [α] × [α] of pure enantiomer
• ee = (pure [α] − observed [α]) / 100
• ee = observed [α] / 100

Correct Answer: ee = (observed [α] / [α] of pure enantiomer) × 100%

Q9. Which statement about the relationship between absolute configuration (R/S) and sign of rotation (+/−) is correct?

• No direct correlation; R may be + or −
• R always corresponds to +, S always to −
• R always corresponds to −, S always to +
• Sign of rotation determines R/S configuration uniquely

Correct Answer: No direct correlation; R may be + or −

Q10. What is a meso compound?

• A molecule with stereocenters but overall achiral due to internal compensation
• A racemic mixture of two enantiomers
• A compound with no stereocenters but optically active
• A chiral molecule with multiple enantiomers

Correct Answer: A molecule with stereocenters but overall achiral due to internal compensation

Q11. Which instrument measures optical rotation?

• Polarimeter
• Spectrophotometer
• NMR spectrometer
• Gas chromatograph

Correct Answer: Polarimeter

Q12. Which factor does NOT typically affect measured optical rotation?

• Color of the solution
• Wavelength of light used
• Temperature
• Solvent

Correct Answer: Color of the solution

Q13. Optical rotatory dispersion (ORD) refers to:

• Variation of optical rotation with wavelength
• Absorption of circularly polarized light
• Conversion of a racemate to enantiomers
• Measurement of mass-specific rotation

Correct Answer: Variation of optical rotation with wavelength

Q14. Circular dichroism (CD) is particularly useful because it measures:

• Difference in absorption of left- and right-circularly polarized light
• Overall optical rotation at a single wavelength
• Mass of chiral molecules
• Heat released during chiral interactions

Correct Answer: Difference in absorption of left- and right-circularly polarized light

Q15. If the observed rotation of a 2.0 g/100 mL solution in a 0.5 dm cell is −2.0°, what is the specific rotation?

• −20°
• −2.0°
• −0.20°
• −200°

Correct Answer: −20°

Q16. A substance with two enantiomers in unequal amounts is called:

• An enantiomeric mixture
• A racemate
• A meso compound
• A constitutional isomer

Correct Answer: An enantiomeric mixture

Q17. Which term describes the rotation observed in solution compared to the pure compound’s rotation?

• Observed rotation vs. specific rotation
• Absolute configuration vs. relative configuration
• Optical density vs. optical clarity
• Chiral index vs. achiral index

Correct Answer: Observed rotation vs. specific rotation

Q18. What is the observed rotation when a pure enantiomer with [α] = +25° (c = 1 g/100 mL, l = 1 dm) is measured?

• +25°
• +2.5°
• −25°
• +0.25°

Correct Answer: +25°

Q19. Which of the following is true for racemic mixtures?

• They are optically inactive because enantiomer rotations cancel
• They are highly optically active with double rotation
• They always have positive rotation
• They only occur for molecules without stereocenters

Correct Answer: They are optically inactive because enantiomer rotations cancel

Q20. Diastereomers are:

• Isomers that are not mirror images and have different physical properties
• Mirror images with identical physical properties
• Compounds with identical optical activity always
• Always enantiomerically pure

Correct Answer: Isomers that are not mirror images and have different physical properties

Q21. Which pharmaceutical issue is most associated with chirality?

• Differences in biological activity and toxicity between enantiomers
• Change in melting point of salts
• Non-specific UV absorbance
• Inability to crystallize

Correct Answer: Differences in biological activity and toxicity between enantiomers

Q22. The unit commonly used for specific rotation is:

• Degrees·mL·g⁻¹·dm⁻¹
• Degrees per second
• Radians per meter
• moles per liter

Correct Answer: Degrees·mL·g⁻¹·dm⁻¹

Q23. What happens to optical rotation when concentration is doubled, assuming linear behavior and no aggregation?

• Observed rotation doubles
• Observed rotation halves
• Observed rotation remains the same
• Specific rotation doubles

Correct Answer: Observed rotation doubles

Q24. In polarimetry, the plane-polarized light is produced by:

• A polarizer (analyzer before the sample)
• A monochromator alone
• A detector
• A temperature controller

Correct Answer: A polarizer (analyzer before the sample)

Q25. Cotton effect is observed in which technique?

• Circular dichroism and optical rotatory dispersion near absorption bands
• Infrared spectroscopy
• Mass spectrometry
• Thin-layer chromatography

Correct Answer: Circular dichroism and optical rotatory dispersion near absorption bands

Q26. A sugar sample shows [α]D = +52° for the pure enantiomer. A solution has observed rotation +26° under same conditions. What is the enantiomeric excess?

• 50%
• 25%
• 100%
• 0%

Correct Answer: 50%

Q27. Which solvent effect is commonly observed for optical rotation?

• Solvent can change magnitude and sometimes sign of rotation
• Solvent never affects rotation
• Solvent only affects diastereomer ratios
• Solvent only changes melting point

Correct Answer: Solvent can change magnitude and sometimes sign of rotation

Q28. Resolution of enantiomers refers to:

• Separation of enantiomers from a racemic mixture
• Establishing absolute configuration only
• Converting enantiomers into achiral compounds
• Measuring specific rotation

Correct Answer: Separation of enantiomers from a racemic mixture

Q29. Which statement about optical activity of molecules with no chiral center is true?

• They can be optically active if they have axial or planar chirality
• They are always optically inactive
• They must be meso compounds
• They cannot interact with polarized light

Correct Answer: They can be optically active if they have axial or planar chirality

Q30. If a pure enantiomer rotates plane-polarized light by −40°, its mirror image will rotate by:

• +40°
• −40°
• 0°
• +20°

Correct Answer: +40°

Q31. Optical purity is equivalent to:

• Enantiomeric excess (ee)
• Molecular weight
• Melting point depression
• Stereoisomer count

Correct Answer: Enantiomeric excess (ee)

Q32. For pharmaceutical QC, polarimetry is commonly used to:

• Assess identity and purity of chiral drugs and intermediates
• Measure particle size distribution
• Determine log P values
• Analyze heavy metals

Correct Answer: Assess identity and purity of chiral drugs and intermediates

Q33. Which of these is NOT a cause of optical inactivity?

• A single enantiomer in pure form
• A racemic mixture
• A meso compound
• Symmetrical achiral molecule

Correct Answer: A single enantiomer in pure form

Q34. How many stereoisomers are possible for a molecule with two non-equivalent stereocenters and no meso form?

• 4
• 2
• 1
• 8

Correct Answer: 4

Q35. Which of the following best describes a racemic modification (racemate) in crystalline form?

• Both enantiomers are present in the same crystal lattice in a 1:1 ratio
• Only one enantiomer crystallizes while the other remains in solution
• Enantiomers are separated into different crystals always
• The crystal is optically active due to packing

Correct Answer: Both enantiomers are present in the same crystal lattice in a 1:1 ratio

Q36. The sign convention for rotation is based on:

• Direction of rotation when viewed along the light source toward the observer
• Observer standing at the light source facing the sample
• Direction of chemical synthesis
• Absolute configuration R or S only

Correct Answer: Direction of rotation when viewed along the light source toward the observer

Q37. Optical rotation is an example of which type of physical property?

• Chiroptical property
• Thermochemical property
• Magnetic susceptibility
• Electrical conductivity

Correct Answer: Chiroptical property

Q38. Which example is a chiral pharmacologically important molecule?

• Thalidomide
• Sodium chloride
• Acetone
• Benzene

Correct Answer: Thalidomide

Q39. What is the effect of temperature on specific rotation generally?

• It can change the magnitude and sometimes the sign; specific rotation is temperature dependent
• It never changes specific rotation
• It only affects achiral compounds
• It converts enantiomers to diastereomers

Correct Answer: It can change the magnitude and sometimes the sign; specific rotation is temperature dependent

Q40. Which measurement would directly give you the enantiomeric composition if you know pure enantiomer rotation?

• Observed rotation of the sample
• UV-Vis absorption maximum
• NMR chemical shift of solvent peaks
• Mass spectrum fragmentation pattern

Correct Answer: Observed rotation of the sample

Q41. A mixture containing 70% of (+)-enantiomer and 30% of (−)-enantiomer has an enantiomeric excess of:

• 40%
• 70%
• 30%
• 100%

Correct Answer: 40%

Q42. If the specific rotation of pure compound is −80° and observed rotation of a sample is −40°, what is the percent composition of the (−)-enantiomer?

• 50%
• 25%
• 75%
• 100%

Correct Answer: 50%

Q43. Which analytical method can separate enantiomers without derivatization using chiral stationary phases?

• Chiral HPLC
• Normal-phase TLC with achiral adsorbent
• Ion-exchange chromatography for achiral molecules
• Size-exclusion chromatography

Correct Answer: Chiral HPLC

Q44. Which term describes two molecules that are non-superimposable mirror images?

• Enantiomers
• Conformers
• Isotopes
• Polymorphs

Correct Answer: Enantiomers

Q45. Optical rotation is zero for which of the following?

• A racemic mixture
• A pure dextrorotatory enantiomer
• A concentrated solution of chiral drug with only one enantiomer
• An enantiomerically pure compound with high specific rotation

Correct Answer: A racemic mixture

Q46. Epimers are:

• Stereoisomers that differ in configuration at only one stereocenter
• Isomers differing in connectivity only
• Mirror images of each other
• Compounds with no stereocenters

Correct Answer: Stereoisomers that differ in configuration at only one stereocenter

Q47. Which chiral phenomenon can be directly related to molecular electronic transitions and used in structural studies?

• Circular dichroism (CD)
• IR stretching frequency
• Mass fragmentation
• Chromatographic retention time

Correct Answer: Circular dichroism (CD)

Q48. Which of the following is true about the optical rotation of a compound measured at sodium D-line (589 nm) versus another wavelength?

• The reported specific rotation must include wavelength since [α] is wavelength-dependent
• Specific rotation is independent of wavelength
• Wavelength only matters in UV-active compounds
• Sodium D-line is never used in polarimetry

Correct Answer: The reported specific rotation must include wavelength since [α] is wavelength-dependent

Q49. A sample gives zero rotation but contains chiral molecules. Which explanation is plausible?

• The sample is a meso compound or racemate
• The polarimeter is broken and cannot detect any rotation ever
• Chiral molecules cannot give zero rotation
• Sample concentration must be infinite

Correct Answer: The sample is a meso compound or racemate

Q50. In quality control, why is knowledge of specific rotation important for B. Pharm students?

• It helps identify stereochemical purity, which affects drug safety and efficacy
• It determines tablet hardness only
• It replaces the need for any chromatographic analysis
• It directly measures dissolution rate

Correct Answer: It helps identify stereochemical purity, which affects drug safety and efficacy

Download