Reactions of cyclopropane MCQs With Answer

Reactions of cyclopropane MCQs With Answer is a focused study tool for B. Pharm students exploring cyclopropane chemistry in pharmaceutical chemistry. This introduction emphasizes ring strain, bent bonds, and how cyclopropane undergoes ring-opening, electrophilic addition, nucleophilic attack, hydrogenation, oxidation, and metal-catalyzed cleavage. Understanding mechanisms, regioselectivity, and stereochemical outcomes is vital for drug synthesis and metabolite prediction. These MCQs cover key concepts like Walsh orbitals, strain energy, reaction conditions, reagents, and typical products, helping you master both theory and practical applications. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which property primarily accounts for the high reactivity of cyclopropane compared to cyclohexane?

• Ring strain due to 60° bond angles
• Higher molecular weight
• Increased polarity
• Presence of aromaticity

Correct Answer: Ring strain due to 60° bond angles

Q2. Which term describes the unusual bonding in cyclopropane where C–C bonds have increased s-character and are bent?

• Walsh orbitals
• Sigma conjugation
• Bent bonds
• Pi delocalization

Correct Answer: Bent bonds

Q3. What is the approximate ring strain energy of cyclopropane?

• 1–5 kcal/mol
• 10–15 kcal/mol
• 25–30 kcal/mol
• 50–60 kcal/mol

Correct Answer: 25–30 kcal/mol

Q4. Acid-catalyzed ring opening of protonated cyclopropane typically yields which type of intermediate?

• Carbanion
• Carbocation
• Carbene
• Radical

Correct Answer: Carbocation

Q5. Cyclopropane behaves chemically similar to which functional group due to its ability to open and form substituted products?

• Alkyne
• Alkene
• Alcohol
• Aldehyde

Correct Answer: Alkene

Q6. Which reagent is commonly used for the hydrogenation of cyclopropane to propane?

• H2 with Pd/C
• O3
• mCPBA
• KMnO4

Correct Answer: H2 with Pd/C

Q7. Nucleophilic ring opening of donor-substituted cyclopropanes often proceeds via which mechanism?

• SN2 at sp2 carbon
• SN1 via stabilized carbocation
• Pericyclic rearrangement
• Radical substitution

Correct Answer: SN1 via stabilized carbocation

Q8. Which oxidizing agent cleaves cyclopropane rings oxidatively to give carbonyl compounds?

• NaBH4
• KMnO4
• LiAlH4
• TBAF

Correct Answer: KMnO4

Q9. Simmons–Smith reaction is used to synthesize cyclopropanes. What is the primary reagent in this reaction?

• Dichlorocarbene
• Carbene from CH2I2 with Zn(Cu)
• CH2N2 alone
• Potassium tert-butoxide

Correct Answer: Carbene from CH2I2 with Zn(Cu)

Q10. Which spectroscopic feature is characteristic for cyclopropane protons in 1H NMR compared to typical aliphatic protons?

• Significantly downfield (~7–8 ppm)
• Typical around 0.9 ppm only
• Slightly deshielded/upfield shifts due to ring current effects
• Broad singlet at 5 ppm

Correct Answer: Slightly deshielded/upfield shifts due to ring current effects

Q11. Which bond cleavage in cyclopropane is favored under radical conditions?

• Heterolytic C–C cleavage
• Homolytic C–C cleavage
• H–C bond cleavage only
• No cleavage under radical conditions

Correct Answer: Homolytic C–C cleavage

Q12. Cyclopropylcarbinyl cations show unusual stability due to what interaction?

• Hyperconjugation with adjacent methyl groups
• Resonance stabilization with aromatic ring
• Interaction between cyclopropane C–C bonds and vacant p-orbital (homoconjugation)
• Hydrogen bonding

Correct Answer: Interaction between cyclopropane C–C bonds and vacant p-orbital (homoconjugation)

Q13. Which of the following reactions converts cyclopropane to a substituted alkane via addition of halogen across a bond?

• Electrophilic aromatic substitution
• Halogenation via radical addition
• Hydroboration–oxidation
• Friedel–Crafts acylation

Correct Answer: Halogenation via radical addition

Q14. What product results from acid-catalyzed opening of a 1,2-disubstituted cyclopropane with water?

• Epoxide
• 1,3-diol
• Alcohol from ring-opened carbocation (Markovnikov type)
• Alkene

Correct Answer: Alcohol from ring-opened carbocation (Markovnikov type)

Q15. In medicinal chemistry, why are cyclopropane rings incorporated into drug molecules?

• To increase molecular weight only
• To introduce conformational rigidity and modulate metabolic stability
• To make compounds highly polar
• To add aromaticity

Correct Answer: To introduce conformational rigidity and modulate metabolic stability

Q16. Which reagent generates dibromocarbene for cyclopropanation of alkenes?

• CH2I2 with Zn(Cu)
• CHBr3 with strong base (e.g., KOH)
• Diazomethane alone
• BH3·THF

Correct Answer: CHBr3 with strong base (e.g., KOH)

Q17. Which statement about cyclopropane aromaticity is correct?

• Cyclopropane is fully aromatic like benzene
• Cyclopropane is antiaromatic
• Cyclopropane exhibits σ-aromatic character due to ring currents in bent bonds
• Cyclopropane has no special electronic features

Correct Answer: Cyclopropane exhibits σ-aromatic character due to ring currents in bent bonds

Q18. Ozonolysis of cyclopropane would most likely produce which type of products?

• Carboxylic acids only
• Carbonyl compounds (aldehydes/ketones) via oxidative cleavage
• No reaction
• Alkenes

Correct Answer: Carbonyl compounds (aldehydes/ketones) via oxidative cleavage

Q19. Which catalytic system can mediate transition-metal-catalyzed ring opening of cyclopropanes to form nucleophilic organometallic intermediates?

• Pd(0) complexes
• H2 with Pd/C only
• KMnO4
• Concentrated HCl

Correct Answer: Pd(0) complexes

Q20. What is the expected stereochemical outcome when a cis-disubstituted cyclopropane undergoes concerted addition of a carbene to an alkene precursor?

• Retention of relative stereochemistry (cis remain cis)
• Complete racemization
• Trans products only
• Formation of radicals exclusively

Correct Answer: Retention of relative stereochemistry (cis remain cis)

Q21. Which is a common method to introduce a cyclopropyl group in a drug candidate?

• Aldol condensation
• Simmons–Smith cyclopropanation of alkenes
• Wittig reaction
• Fischer esterification

Correct Answer: Simmons–Smith cyclopropanation of alkenes

Q22. Which intermediate is characteristic of radical-promoted cyclopropane ring opening?

• Allylic cation
• Cyclopropyl radical
• Carbanion stabilized by oxygen
• Concerted pericyclic transition state only

Correct Answer: Cyclopropyl radical

Q23. In the context of cyclopropane chemistry, the term “homoaromaticity” refers to:

• Classical aromatic rings fused to cyclopropane
• Delocalization across a nonconjugated bridge involving cyclopropane
• Antiaromatic behavior in cyclopropane
• No electronic delocalization

Correct Answer: Delocalization across a nonconjugated bridge involving cyclopropane

Q24. Which condition favors heterolytic cleavage of a cyclopropane bond to form ionic products?

• Strongly reducing radical conditions
• Strong acid and polar solvent
• Photochemical UV light only
• Inert gas at low temperature

Correct Answer: Strong acid and polar solvent

Q25. Which reagent would convert a cyclopropane to a cyclopropanol via nucleophilic attack at a protonated ring?

• NaBH4
• Water under acidic conditions
• Ozone
• tert-Butoxide

Correct Answer: Water under acidic conditions

Q26. Which substituent on cyclopropane stabilizes a developing positive charge during ring opening?

• Electron-withdrawing group (e.g., NO2)
• Electron-donating group (e.g., OMe)
• Fluorine only
• Nitrile only

Correct Answer: Electron-donating group (e.g., OMe)

Q27. Which reaction demonstrates that cyclopropane can act as an alkene equivalent in synthesis?

• Acid-catalyzed ring opening to give substituted alkanes resembling addition to alkenes
• Direct electrophilic aromatic substitution
• Nucleophilic acyl substitution
• Favors elimination to aromatic products

Correct Answer: Acid-catalyzed ring opening to give substituted alkanes resembling addition to alkenes

Q28. Which product arises from reaction of cyclopropane with bromine under radical conditions?

• Brominated open-chain dibromoalkane
• Epoxide formation
• No reaction
• Formation of an alcohol

Correct Answer: Brominated open-chain dibromoalkane

Q29. How does substitution at the cyclopropane ring affect ring-opening regioselectivity under SN1-like conditions?

• Opening occurs preferentially to form the less-stabilized carbocation
• Opening occurs to form the more-stabilized carbocation (adjacent electron-donating substituents)
• Substitution has no effect
• Ring enlarges instead of opening

Correct Answer: Opening occurs to form the more-stabilized carbocation (adjacent electron-donating substituents)

Q30. Which spectroscopic technique best distinguishes cyclopropane C–C stretching modes?

• UV-Vis spectroscopy
• IR spectroscopy
• Mass spectrometry only
• Polarimetry

Correct Answer: IR spectroscopy

Q31. What is the typical outcome when a cyclopropane adjacent to an aromatic ring is ionized to form a carbocation?

• No stabilization by the aromatic ring
• Delocalization of positive charge into aromatic system via homoaromatic interaction
• Immediate reduction to an alkane
• Formation of an epoxide

Correct Answer: Delocalization of positive charge into aromatic system via homoaromatic interaction

Q32. Which mechanism is involved when cyclopropylcarbinyl systems undergo rearrangement to homoallylic systems?

• Sigmatropic rearrangement
• Ring-opening via carbocation followed by resonance stabilization
• Direct nucleophilic substitution at sp2 carbon
• Photochemical dehydrogenation

Correct Answer: Ring-opening via carbocation followed by resonance stabilization

Q33. Which reagent is used for electrophilic addition across a cyclopropane in acidic media to yield haloalkanes?

• HBr in presence of acid
• Base-catalyzed dehydration
• LiAlH4
• Ozone

Correct Answer: HBr in presence of acid

Q34. Which is TRUE about substituted cyclopropanes in drug molecules regarding metabolic stability?

• Cyclopropane insertion always increases metabolism rate
• It can block sites of oxidative metabolism and increase bioavailability
• Cyclopropane always causes toxicity
• It makes compounds highly water-soluble

Correct Answer: It can block sites of oxidative metabolism and increase bioavailability

Q35. A cyclopropane with an adjacent carbonyl group is more prone to which transformation?

• Reduction to ether
• Ring opening stabilized by resonance with carbonyl (enolate-like behavior)
• Pericyclic dimerization
• Polymerization to polyethylene

Correct Answer: Ring opening stabilized by resonance with carbonyl (enolate-like behavior)

Q36. Which product is expected when a vinylcyclopropane undergoes thermal rearrangement (vinylcyclopropane rearrangement)?

• Alkane only
• 1,3-diene via ring opening and rearrangement
• Epoxide
• Carbocation that collapses to ester

Correct Answer: 1,3-diene via ring opening and rearrangement

Q37. In enzymatic metabolism, cyclopropane rings in drugs are often cleaved by which type of process?

• Hydrolytic cleavage by esterases
• Oxidative metabolism by cytochrome P450 leading to ring opening
• Direct methylation
• Photochemical cleavage in vivo

Correct Answer: Oxidative metabolism by cytochrome P450 leading to ring opening

Q38. Which is a synthetic advantage of using cyclopropane intermediates in multi-step synthesis?

• They are fully inert and never react
• They enable masked alkene chemistry and controlled ring-opening to install substituents
• They always reduce yields
• They prevent any further functionalization

Correct Answer: They enable masked alkene chemistry and controlled ring-opening to install substituents

Q39. Which reagent is suitable for generating carbenoids in Simmons–Smith cyclopropanation?

• CH2I2 with Zn(Cu)
• HCl
• KMnO4
• Sulfuric acid

Correct Answer: CH2I2 with Zn(Cu)

Q40. Which statement best describes regiochemistry in nucleophilic opening of donor-substituted cyclopropanes?

• Nucleophile attacks the most substituted carbon only
• Nucleophile attacks to give the most stabilized carbocation, often adjacent to electron-donating groups
• Regiochemistry is unpredictable and random
• Only the least hindered carbon is attacked always

Correct Answer: Nucleophile attacks to give the most stabilized carbocation, often adjacent to electron-donating groups

Q41. Which technique is commonly used to monitor cyclopropane ring-opening reactions in real time in the lab?

• NMR spectroscopy
• Thin-layer chromatography cannot be used
• Polarimetry exclusively
• Melting point determination

Correct Answer: NMR spectroscopy

Q42. Which of the following increases cyclopropane ring-opening tendency?

• Electron-withdrawing substituents adjacent to ring
• Electron-donating substituents adjacent to ring
• Presence of only hydrogen substituents
• Steric hindrance alone prevents opening always

Correct Answer: Electron-donating substituents adjacent to ring

Q43. Which transformation converts a substituted cyclopropane into a γ-lactone under oxidative conditions?

• Hydrogenation with H2
• Oxidative ring opening followed by intramolecular cyclization to lactone
• UV irradiation without oxygen
• Base-catalyzed elimination only

Correct Answer: Oxidative ring opening followed by intramolecular cyclization to lactone

Q44. Which is a common side reaction during metal-catalyzed cyclopropane opening that must be controlled?

• Competitive hydrogenation of other functional groups
• Complete polymerization to polyethylene
• Immediate combustion
• No side reactions occur

Correct Answer: Competitive hydrogenation of other functional groups

Q45. Cyclopropyl methyl ethers under acidic conditions typically undergo which transformation?

• Stable with no reaction
• Ring opening to form stabilized carbocation and cleavage of ether linkage
• Oxidation to carboxylic acid directly
• Photochemical cycloaddition

Correct Answer: Ring opening to form stabilized carbocation and cleavage of ether linkage

Q46. Which factor does NOT significantly influence the rate of cyclopropane ring opening?

• Substituent electronic effects
• Solvent polarity
• Temperature
• Color of the reaction flask

Correct Answer: Color of the reaction flask

Q47. What role do metal catalysts often play in cyclopropane ring-opening reactions?

• They always prevent reactions
• They facilitate bond cleavage and formation of organometallic intermediates for further transformations
• They only act as inert supports
• They convert cyclopropane to benzene

Correct Answer: They facilitate bond cleavage and formation of organometallic intermediates for further transformations

Q48. The reactivity of bicyclic systems containing cyclopropane rings is often altered due to:

• Reduced ring strain compared to monocyclic cyclopropane only
• Additional strain and conformational constraints influencing reaction pathways
• No change in reactivity
• Conversion to aromatic systems spontaneously

Correct Answer: Additional strain and conformational constraints influencing reaction pathways

Q49. Which reagent is typically avoided when trying to preserve a cyclopropane ring in a multifunctional molecule?

• Neutral nonreactive solvents
• Strong acids or oxidizing agents
• Low temperature inert conditions
• Protected atmosphere with dry nitrogen

Correct Answer: Strong acids or oxidizing agents

Q50. Predict the major product when a donor-substituted cyclopropane reacts with a soft nucleophile under Lewis-acid catalysis.

• Retention of cyclopropane with no substitution
• Ring-opened product with nucleophile attached at the more substituted carbon (stabilized carbocation pathway)
• Polymer formation
• Formation of an epoxide

Correct Answer: Ring-opened product with nucleophile attached at the more substituted carbon (stabilized carbocation pathway)

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

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