Methods for configuration determination of geometrical isomers MCQs With Answer

Methods for configuration determination of geometrical isomers MCQs With Answer

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Introduction

Understanding methods for configuration determination of geometrical isomers is essential for B. Pharm students studying stereochemistry and drug action. This introduction covers practical approaches—Cahn–Ingold–Prelog (CIP) E/Z assignment, cis–trans nomenclature for rings, NMR coupling constants, NOE experiments, IR out-of-plane vibrations, chromatographic separation, X‑ray crystallography and chemical correlation—that help identify and distinguish geometrical isomers. Emphasis on interpretation of spectral data, priority rules, and separation techniques prepares students for formulation, stability and activity assessment of stereoisomeric pharmaceuticals. Clear examples and method selection based on molecular features reinforce lab and exam readiness. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary rule used to assign E/Z configuration to a double bond?

  • Cahn–Ingold–Prelog priority rules
  • Markovnikov’s rule
  • Saytzeff rule
  • Hückel rule

Correct Answer: Cahn–Ingold–Prelog priority rules

Q2. According to CIP rules, which factor determines higher priority between two atoms directly attached to the double bond?

  • Atomic number of the attached atom
  • Molecular mass of the whole substituent
  • Bond length to the double bond
  • Electronegativity of remote atoms

Correct Answer: Atomic number of the attached atom

Q3. For an alkene with substituents on each carbon, when is the configuration called E?

  • When highest-priority groups are on opposite sides of the double bond
  • When highest-priority groups are on the same side of the double bond
  • When both highest-priority groups are hydrogen
  • When substituents are identical

Correct Answer: When highest-priority groups are on opposite sides of the double bond

Q4. Which experimental technique most directly gives the three-dimensional arrangement of atoms and unambiguous geometry of a crystalline isomer?

  • X‑ray crystallography
  • Infrared spectroscopy
  • Ultraviolet-visible spectroscopy
  • Thin-layer chromatography

Correct Answer: X‑ray crystallography

Q5. In 1,2-dichloroethene, the isomer with chlorines on the same side is called:

  • cis-1,2-dichloroethene
  • trans-1,2-dichloroethene
  • E-1,2-dichloroethene
  • meso-1,2-dichloroethene

Correct Answer: cis-1,2-dichloroethene

Q6. In proton NMR, which vicinal coupling constant (3J) is typically larger for alkenic protons?

  • Trans coupling (Jtrans) is larger than cis coupling (Jcis)
  • Cis coupling (Jcis) is larger than trans coupling (Jtrans)
  • Geminal coupling is largest
  • All vicinal couplings are equal

Correct Answer: Trans coupling (Jtrans) is larger than cis coupling (Jcis)

Q7. Typical approximate ranges: what is the usual Jtrans value (Hz) across an alkene?

  • 12–18 Hz
  • 0–3 Hz
  • 5–8 Hz
  • 20–30 Hz

Correct Answer: 12–18 Hz

Q8. Which NMR experiment can provide information on spatial proximity (through-space interactions) useful to distinguish geometrical isomers?

  • NOE (Nuclear Overhauser Effect) experiments
  • DEPT experiment
  • T1 relaxation only
  • IR spectroscopy

Correct Answer: NOE (Nuclear Overhauser Effect) experiments

Q9. Which IR spectral feature can help distinguish cis and trans disubstituted alkenes?

  • Out-of-plane C–H bending vibrations
  • C=O stretching frequency
  • N–H stretching frequency
  • Fingerprint region peak at 5000 cm⁻¹

Correct Answer: Out-of-plane C–H bending vibrations

Q10. Which property often differs between E and Z isomers and can be used for separation by chromatography?

  • Polarity and retention time in HPLC
  • Isotopic composition
  • Atomic number of carbon
  • Covalent bond order

Correct Answer: Polarity and retention time in HPLC

Q11. When applying CIP rules to a double bond, how are multiple bonds treated?

  • Multiple-bonded atoms are treated as bonded to duplicate atoms (equivalent to single-bonded duplicates)
  • Multiple bonds are ignored
  • Only the highest bond order is considered, others are removed
  • Double bonds always get lower priority

Correct Answer: Multiple-bonded atoms are treated as bonded to duplicate atoms (equivalent to single-bonded duplicates)

Q12. Which descriptor applies to a substituted cyclohexane where two substituents are on the same face of the ring?

  • Cis
  • Trans
  • E
  • Z

Correct Answer: Cis

Q13. For an alkene where identical substituents are present on one carbon, geometrical isomerism is possible when:

  • Each carbon of the double bond has two different substituents
  • At least one carbon bears two identical substituents
  • Both carbons are quaternary
  • There is conjugation with an aromatic ring

Correct Answer: Each carbon of the double bond has two different substituents

Q14. Which chemical correlation method helps assign geometry by converting unknown isomer to a known stereoisomer?

  • Derivatization to a product with known stereochemistry
  • Measuring boiling point only
  • Counting degrees of unsaturation
  • Adding molecular oxygen

Correct Answer: Derivatization to a product with known stereochemistry

Q15. Which computational approach can support experimental assignment of E/Z geometry?

  • Density Functional Theory (DFT) geometry optimization and energy comparison
  • Empirical formula calculation only
  • IR peak matching without geometry optimization
  • Simple molecular weight computation

Correct Answer: Density Functional Theory (DFT) geometry optimization and energy comparison

Q16. In oximes, the stereoisomers formed around the C=N bond are commonly called:

  • syn and anti
  • E and Z exclusively
  • R and S
  • Cis and racemic

Correct Answer: syn and anti

Q17. Which of the following is a reliable spectroscopic sign that two protons are trans across a double bond?

  • Large vicinal coupling constant (~15 Hz)
  • Chemical shift identical to water
  • No splitting observed
  • Broad singlet only

Correct Answer: Large vicinal coupling constant (~15 Hz)

Q18. If the highest priority groups on each alkene carbon are on the same side, the configuration is labeled as:

  • Z
  • E
  • cis only
  • rectus

Correct Answer: Z

Q19. In practice, why might HPLC separate E and Z isomers differently?

  • Different polarity and steric interactions with stationary phase
  • Different atomic numbers of carbons
  • Different isotopic ratios
  • Different total number of electrons

Correct Answer: Different polarity and steric interactions with stationary phase

Q20. Which NMR observation would support a cis arrangement in a substituted cyclohexane?

  • NOE enhancement between the two substituent protons indicating spatial proximity
  • Absence of any peaks
  • Large trans alkenic coupling constant
  • Only IR data without NMR signals

Correct Answer: NOE enhancement between the two substituent protons indicating spatial proximity

Q21. For E/Z assignment, when two atoms attached to the double bond are the same element, what is the next criterion to decide priority?

  • Compare the set of atoms bonded to those identical atoms in order of decreasing atomic number
  • Compare molecular weights of the substituents only
  • Use stereochemical descriptors R/S immediately
  • Consider bond angles only

Correct Answer: Compare the set of atoms bonded to those identical atoms in order of decreasing atomic number

Q22. Which spectral method is least useful by itself to assign cis/trans for simple alkenes?

  • UV-Vis spectroscopy
  • Proton NMR
  • IR out-of-plane C–H bending
  • X‑ray crystallography

Correct Answer: UV-Vis spectroscopy

Q23. Which statement about meso compounds and geometrical isomerism is true?

  • Meso compounds are achiral despite stereogenic centers and may still have cis/trans isomerism in rings
  • Meso compounds are always chiral and optically active
  • Meso compounds cannot show cis/trans isomerism
  • Meso compounds lack any stereogenic elements

Correct Answer: Meso compounds are achiral despite stereogenic centers and may still have cis/trans isomerism in rings

Q24. Which of the following conversions helps determine geometry by forming a diastereomeric derivative?

  • Reaction of an alkene to give a chiral diastereomeric adduct that can be separated
  • Simple dilution in water
  • Ionization to a radical only
  • Measuring theoretical pKa

Correct Answer: Reaction of an alkene to give a chiral diastereomeric adduct that can be separated

Q25. Which factor most influences the chemical shift differences of vinylic protons in E versus Z isomers?

  • Spatial proximity to electron-withdrawing or donating groups
  • Atomic mass of hydrogen
  • Color of the compound
  • Melting point only

Correct Answer: Spatial proximity to electron-withdrawing or donating groups

Q26. For a disubstituted alkene bearing halogen and methyl on each carbon, what determines cis/trans nomenclature (not E/Z)?

  • Whether substituents are on same or opposite sides (cis = same side)
  • Only CIP priorities
  • Absolute configuration R/S
  • Boiling point

Correct Answer: Whether substituents are on same or opposite sides (cis = same side)

Q27. Which physical property difference is commonly observed: E isomers often are more stable than Z isomers because:

  • Less steric hindrance between bulky substituents
  • Lower atomic numbers
  • Higher number of hydrogen bonds
  • Greater isotopic enrichment

Correct Answer: Less steric hindrance between bulky substituents

Q28. When assigning priority on an alkene carbon with substituents Br and CH2CH3 versus Cl and CH3, which group gets highest priority?

  • The atom with the highest atomic number directly attached, so Br-containing substituent
  • CH3 because it is smaller
  • Cl-containing substituent because chlorine is lighter
  • Use molecular weight of whole substituent only

Correct Answer: The atom with the highest atomic number directly attached, so Br-containing substituent

Q29. Which analytical technique combined with derivatization can convert E/Z mixtures into separable diastereomers?

  • Formation of diastereomeric esters followed by HPLC
  • Direct UV-Vis measurement
  • Simple evaporation
  • Calculating empirical formula

Correct Answer: Formation of diastereomeric esters followed by HPLC

Q30. What is a practical laboratory sign that a reaction mixture contains a cis and trans isomer mixture?

  • Two distinct spots on TLC or two peaks on HPLC
  • Single melting point matching literature
  • No signals in any spectroscopy
  • Equal electronegativity readings

Correct Answer: Two distinct spots on TLC or two peaks on HPLC

Q31. In 1H NMR, geminal coupling (2J) between two hydrogens on the same sp2 carbon is typically:

  • Small or negligible (often 0–3 Hz) in many alkenes
  • Always 15 Hz
  • Always larger than trans coupling
  • Negative and unmeasurable

Correct Answer: Small or negligible (often 0–3 Hz) in many alkenes

Q32. Which molecular feature can prevent E/Z isomerism even if a C=C exists?

  • If one alkene carbon has two identical substituents
  • If the alkene is conjugated
  • If the alkene is terminal
  • If hydrogen is present

Correct Answer: If one alkene carbon has two identical substituents

Q33. Which experimental evidence would strongly indicate Z configuration in a disubstituted alkene where two polar groups are present?

  • Higher dipole moment measured for Z isomer compared to E
  • Lower boiling point than E always
  • No difference in polarity between isomers
  • Mass spectrometry showing different m/z for each isomer

Correct Answer: Higher dipole moment measured for Z isomer compared to E

Q34. For cyclic alkenes, why is cis/trans nomenclature often used instead of E/Z?

  • Cis/trans reflects relative positions on ring faces more simply for small rings
  • E/Z rules do not apply to rings at all
  • CIP rules are banned for rings
  • Cyclic compounds cannot be stereoisomeric

Correct Answer: Cis/trans reflects relative positions on ring faces more simply for small rings

Q35. Which of the following is true about NOE enhancements when comparing E and Z isomers?

  • NOE can show proximity of protons that are spatially near in Z but not in E
  • NOE measures bond orders only
  • NOE identifies isotopes exclusively
  • NOE is irrelevant for stereochemical assignments

Correct Answer: NOE can show proximity of protons that are spatially near in Z but not in E

Q36. When two substituents on a double bond are hydrogen and methyl on one carbon and ethyl and propyl on the other, which comparison matters for E/Z?

  • Priority between hydrogen vs methyl and ethyl vs propyl at each carbon, using CIP rules
  • Only total carbon count
  • Only mass spectrometry fragmentation
  • Only IR carbonyl signals

Correct Answer: Priority between hydrogen vs methyl and ethyl vs propyl at each carbon, using CIP rules

Q37. Which method can distinguish geometrical isomers in the gas phase quickly and is useful for volatile drug impurities?

  • Gas chromatography (GC)
  • Paper chromatography
  • Simple titration
  • Flame test only

Correct Answer: Gas chromatography (GC)

Q38. In assigning E/Z, a phosphorus atom attached to the alkene carbon would be prioritized over which of these atoms?

  • Carbon and hydrogen, because phosphorus has higher atomic number than carbon and hydrogen
  • Oxygen automatically
  • Any halogen always
  • Only hydrogen is higher priority

Correct Answer: Carbon and hydrogen, because phosphorus has higher atomic number than carbon and hydrogen

Q39. Which experimental combination gives the most confident assignment of alkene geometry in a non-crystalline small molecule?

  • 1H NMR (coupling constants), NOE experiments and chromatographic separation with standards
  • Only UV-Vis
  • Only melting point
  • Only elemental analysis

Correct Answer: 1H NMR (coupling constants), NOE experiments and chromatographic separation with standards

Q40. For an alkene with substituents F and H on one carbon and Cl and Br on the other, which substituent pairing determines E/Z using CIP?

  • Compare F vs H and Cl vs Br by atomic number at each carbon
  • Compare total mass of both sides only
  • Choose alphabetically
  • Use boiling points exclusively

Correct Answer: Compare F vs H and Cl vs Br by atomic number at each carbon

Q41. Which of the following ring systems commonly show restricted rotation leading to isolable geometrical isomers?

  • Cycloalkenes and substituted olefins in small rings
  • Simple alkanes with free rotation
  • Monatomic gases
  • Fully conjugated aromatic benzene rings only

Correct Answer: Cycloalkenes and substituted olefins in small rings

Q42. How does steric bulk influence the E/Z equilibrium position?

  • Bulky groups favor the isomer with less steric clash, often the E isomer
  • Bulky groups always favor Z regardless
  • Bulky groups convert double bonds into single bonds
  • Steric bulk has no effect on isomer stability

Correct Answer: Bulky groups favor the isomer with less steric clash, often the E isomer

Q43. Which technique can be used to measure dipole moment differences between E and Z isomers experimentally?

  • Dielectric constant measurements or solvent-dependent UV/IR shifts
  • Mass spectrometry only
  • pH titration only
  • Paper chromatography

Correct Answer: Dielectric constant measurements or solvent-dependent UV/IR shifts

Q44. When using CIP rules, which isotopic substitution affects priority?

  • Higher mass isotopes have higher priority when atoms are the same element
  • Isotopes are always ignored
  • Only charge, not mass, matters
  • Isotopes decrease priority

Correct Answer: Higher mass isotopes have higher priority when atoms are the same element

Q45. Which of the following is a limitation of using 1H NMR J-values alone to assign geometry?

  • Overlapping signals, coupling to other spins, or solvent effects can complicate interpretation
  • J-values are always identical for E and Z
  • NMR cannot detect protons on alkenes
  • NMR destroys sample geometry

Correct Answer: Overlapping signals, coupling to other spins, or solvent effects can complicate interpretation

Q46. Which synthetic transformation preserves the stereochemistry of an alkene and thus can be used to correlate configuration?

  • Stereospecific hydrogenation under syn or anti addition conditions may or may not preserve double-bond stereochemistry depending on mechanism
  • Complete radical isomerization under uncontrolled conditions
  • Any oxidation that breaks the double bond
  • Molten salt treatment that fragments molecule

Correct Answer: Stereospecific hydrogenation under syn or anti addition conditions may or may not preserve double-bond stereochemistry depending on mechanism

Q47. For a pharmaceutical impurity, why is correct geometrical assignment critical?

  • Different geometrical isomers can have different biological activity, potency, or toxicity
  • Isomers always have identical activity so assignment is unnecessary
  • Only melting point matters for impurities
  • CIP rules change chemical composition

Correct Answer: Different geometrical isomers can have different biological activity, potency, or toxicity

Q48. Which is true about stability trends in substituted alkenes?

  • Alkenes stabilized by conjugation or hyperconjugation are lower in energy and more stable
  • Conjugation always destabilizes alkenes
  • Hyperconjugation is irrelevant to stability
  • Only ring strain dictates stability

Correct Answer: Alkenes stabilized by conjugation or hyperconjugation are lower in energy and more stable

Q49. What should a B. Pharm student do first when confronted with an unknown geometrical isomer in the lab?

  • Collect 1H NMR, compare coupling constants, run NOE if needed, and compare to standards or literature
  • Immediately discard the sample
  • Assume it is E without data
  • Only check color visually

Correct Answer: Collect 1H NMR, compare coupling constants, run NOE if needed, and compare to standards or literature

Q50. Which educational concept best helps students predict whether two substituents on a double bond will give distinct geometrical isomers?

  • Check if each carbon of the double bond has two different substituents (necessary condition for stereoisomerism)
  • Count total electrons only
  • Measure the refractive index only
  • Verify the presence of aromaticity

Correct Answer: Check if each carbon of the double bond has two different substituents (necessary condition for stereoisomerism)

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