Allylic rearrangement MCQs With Answer are essential for B. Pharm students to master reaction mechanisms that influence drug synthesis and metabolism. This concise introduction explains core concepts—allylic cations, resonance stabilization, SN1′ and SN2′ pathways, and sigmatropic shifts like Cope and Claisen—while emphasizing stereochemical outcomes and reagent effects. Understanding allylic rearrangements helps predict regioselectivity, reaction intermediates, and product distributions in organic transformations relevant to medicinal chemistry and formulation sciences. These MCQs focus on mechanism details, experimental evidence, and practical examples to build strong problem-solving skills. Keywords: Allylic rearrangement MCQs With Answer, allylic cation, SN1′, SN2′, Cope, Claisen, pi-allyl complex. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the defining feature of an allylic carbocation?
- A positive charge located on a carbon directly bonded to a carbon–carbon double bond
- A positive charge exclusively on a vinylic carbon within the double bond
- A negative charge delocalized over an aromatic ring
- A radical centered on an sp3 carbon adjacent to a triple bond
Correct Answer: A positive charge located on a carbon directly bonded to a carbon–carbon double bond
Q2. Which statement best explains why allylic cations are stabilized?
- Hyperconjugation with adjacent sp3 carbons only
- Resonance delocalization of the positive charge across the pi system
- Inductive withdrawal by neighboring halogens
- Steric hindrance preventing nucleophile approach
Correct Answer: Resonance delocalization of the positive charge across the pi system
Q3. In an SN1′ reaction at an allylic center, what is the key intermediate or species involved?
- Direct displacement at the vinylic carbon without intermediates
- A bridged or delocalized allylic carbocation
- A free radical formed by homolysis only
- A cyclic bromonium ion
Correct Answer: A bridged or delocalized allylic carbocation
Q4. SN2′ reactions at allylic systems typically lead to nucleophilic attack at which position?
- The same carbon as the leaving group (direct substitution)
- The terminal carbon of the allylic system, leading to transposition
- The carbonyl carbon in conjugated systems
- The aromatic ring substituent
Correct Answer: The terminal carbon of the allylic system, leading to transposition
Q5. Which reagent is commonly used to promote a palladium-catalyzed allylic substitution via a pi-allyl complex?
- LiAlH4 without catalyst
- Pd(PPh3)4 or Pd(0) complexes
- H2 with PtO2 only
- Br2 in acetic acid
Correct Answer: Pd(PPh3)4 or Pd(0) complexes
Q6. The Cope rearrangement is classified as which type of reaction?
- An ionic SN2′ process
- A [3,3]-sigmatropic pericyclic rearrangement
- A nucleophilic acyl substitution
- A radical chain polymerization
Correct Answer: A [3,3]-sigmatropic pericyclic rearrangement
Q7. Which of the following is true about the Claisen rearrangement?
- It converts allylic alcohols directly to alkanes
- It is a [3,3]-sigmatropic rearrangement of allyl vinyl ethers to carbonyl-containing products
- It proceeds through a discrete allylic carbocation intermediate
- It is catalyzed only by strong acids
Correct Answer: It is a [3,3]-sigmatropic rearrangement of allyl vinyl ethers to carbonyl-containing products
Q8. In a pi-allyl palladium intermediate, where is the electrophilic center located?
- Exclusively on the palladium atom
- Delocalized over the three carbon atoms of the allyl fragment and interacting with palladium
- Only at the terminal double bond far from metal
- On an adjacent oxygen atom rather than the carbon chain
Correct Answer: Delocalized over the three carbon atoms of the allyl fragment and interacting with palladium
Q9. Which factor most favors formation of the thermodynamic product in allylic rearrangements?
- Very low temperature and fast quench
- High temperature and longer reaction time allowing equilibration
- Strong nucleophile under kinetic control only
- Use of radical initiators exclusively
Correct Answer: High temperature and longer reaction time allowing equilibration
Q10. Which substituent on an allylic system increases stabilization of an allylic cation most effectively?
- An electron-donating group (e.g., -OMe) conjugated with the double bond
- A strong electron-withdrawing group (e.g., -NO2) adjacent to the cation
- A bulky tert-butyl group remote from the double bond
- A saturated alkyl chain with no resonance participation
Correct Answer: An electron-donating group (e.g., -OMe) conjugated with the double bond
Q11. Which experimental technique provides direct evidence for resonance-stabilized allylic cations?
- UV–visible spectroscopy only
- NMR spectroscopy showing equivalent environments for allylic carbons
- Mass spectrometry alone without fragmentation pattern
- Thin-layer chromatography Rf values
Correct Answer: NMR spectroscopy showing equivalent environments for allylic carbons
Q12. In the context of allylic rearrangements, what does SN1′ denote?
- An SN1 reaction feeding into an aromatic substitution
- An unimolecular ionization giving a delocalized allylic cation followed by nucleophilic attack at a different carbon
- A radical substitution pathway involving hydrogen abstraction
- An electrophilic aromatic substitution variant
Correct Answer: An unimolecular ionization giving a delocalized allylic cation followed by nucleophilic attack at a different carbon
Q13. Which outcome indicates an SN2′ pathway rather than SN2 in allylic substitution?
- Nucleophile replaces leaving group at the same carbon with inversion
- Nucleophile attacks the adjacent carbon of the double bond resulting in allylic transposition
- Formation of a radical polymer instead of substitution
- No reaction due to steric hindrance
Correct Answer: Nucleophile attacks the adjacent carbon of the double bond resulting in allylic transposition
Q14. Which mechanistic feature differentiates vinylic and allylic carbocations?
- Vinylic carbocations are resonance-stabilized like allylic ones
- Allylic carbocations are resonance-stabilized; vinylic carbocations are much less stable due to positive charge on sp2 carbon of double bond
- Vinylic carbocations are always more stable due to hyperconjugation
- There is no difference in stability
Correct Answer: Allylic carbocations are resonance-stabilized; vinylic carbocations are much less stable due to positive charge on sp2 carbon of double bond
Q15. Which product is expected from the acid-catalyzed rearrangement of an allylic alcohol (tertiary allylic alcohol) under SN1-type conditions?
- Alkane via hydrogenation only
- Allylicly rearranged carbocation-derived substitution or elimination products
- Unchanged alcohol due to acid inhibition
- Internal alkyne formation exclusively
Correct Answer: Allylicly rearranged carbocation-derived substitution or elimination products
Q16. The oxy-Cope rearrangement requires what additional condition compared with the neutral Cope?
- Addition of a radical initiator
- Deprotonation (base) to generate an alkoxide, which accelerates the [3,3]-sigmatropic shift
- Presence of strong Lewis acid only
- UV irradiation at 300 nm exclusively
Correct Answer: Deprotonation (base) to generate an alkoxide, which accelerates the [3,3]-sigmatropic shift
Q17. Which stereochemical outcome is typical for a concerted [3,3]-sigmatropic rearrangement like the Claisen?
- Random racemization always occurs
- Suprafacial migration with predictable stereochemical relationships when concerted
- Complete inversion at every stereocenter
- No stereochemical predictability due to radical intermediates
Correct Answer: Suprafacial migration with predictable stereochemical relationships when concerted
Q18. In an allylic bromide subjected to nucleophilic attack, which factor favors SN2 over SN2′?
- A very soft nucleophile and polarizable leaving group
- A highly basic, small nucleophile attacking rapidly at the carbon bearing the leaving group
- Very high temperature to promote equilibration
- Presence of Pd(0) catalyst to form pi-allyl complex
Correct Answer: A highly basic, small nucleophile attacking rapidly at the carbon bearing the leaving group
Q19. Which observation supports a stepwise mechanism through an allylic cation vs. a concerted SN2′?
- Complete inversion of configuration at reacting center only
- Formation of racemic or partially racemic product from chiral allylic substrate indicating planar carbocation intermediate
- No change in regioisomer distribution under different nucleophiles
- Strict retention of stereochemistry in all conditions
Correct Answer: Formation of racemic or partially racemic product from chiral allylic substrate indicating planar carbocation intermediate
Q20. Which is a common laboratory test for identifying allylic rearrangement products?
- Comparison of melting points only
- NMR analysis showing shifted chemical shifts and coupling patterns consistent with allylic transposition
- Simple visual inspection of color change only
- Gas evolution measurement without spectroscopic data
Correct Answer: NMR analysis showing shifted chemical shifts and coupling patterns consistent with allylic transposition
Q21. Which of these rearrangements involves a concerted pericyclic transition state with cyclic electron flow?
- SN1′ via discrete carbocation
- Cope and Claisen rearrangements ([3,3]-sigmatropic processes)
- Radical chain autoxidation only
- Nucleophilic acyl substitution
Correct Answer: Cope and Claisen rearrangements ([3,3]-sigmatropic processes)
Q22. In an allylic substitution catalyzed by palladium, what determines whether attack occurs at the more substituted or less substituted terminus?
- Only the solvent polarity
- Electronic and steric factors of ligands and nucleophile, and ligand-controlled regioselectivity
- Molecular weight of the substrate only
- Temperature exclusively without ligand effects
Correct Answer: Electronic and steric factors of ligands and nucleophile, and ligand-controlled regioselectivity
Q23. Which statement about the Cope rearrangement equilibrium is correct?
- Cope rearrangement equilibria are always completely to the right (product-favored)
- Equilibrium position depends on substitution pattern and thermodynamic stability of isomers
- Equilibrium is irrelevant because Cope never reaches it
- Cope rearrangement only proceeds under photochemical conditions
Correct Answer: Equilibrium position depends on substitution pattern and thermodynamic stability of isomers
Q24. Which reagent combination promotes allylic transposition by forming a pi-allyl complex in situ?
- NaBH4 in methanol only
- Pd(0) catalyst with appropriate ligand and weak nucleophile
- H2O2 alone without metal
- Strong base without metal catalysts
Correct Answer: Pd(0) catalyst with appropriate ligand and weak nucleophile
Q25. How does conjugation affect the position of an allylic rearrangement product?
- Products never favor conjugation
- Conjugated products are often thermodynamically favored due to increased resonance stabilization
- Conjugation always prevents rearrangement
- Conjugation only affects solubility, not stability
Correct Answer: Conjugated products are often thermodynamically favored due to increased resonance stabilization
Q26. Which kinetic feature favors a concerted sigmatropic rearrangement over stepwise ionic pathways?
- High ionic strength in polar protic solvents
- Pericyclic selection rules permitting allowed thermal pathways and lack of strong ionic stabilizing conditions
- Strong acid promoting carbocation formation
- High concentrations of nucleophiles that trap intermediates immediately
Correct Answer: Pericyclic selection rules permitting allowed thermal pathways and lack of strong ionic stabilizing conditions
Q27. What is the role of ligands like PPh3 in Pd-catalyzed allylic substitution?
- They act as oxidants to generate radicals
- They modulate electron density at Pd, affecting formation and reactivity of pi-allyl complexes and regio-/stereoselectivity
- They hydrolyze the substrate before reaction
- They irreversibly bind the substrate preventing turnover
Correct Answer: They modulate electron density at Pd, affecting formation and reactivity of pi-allyl complexes and regio-/stereoselectivity
Q28. Which type of allylic rearrangement is most relevant to synthesizing substituted carbonyl compounds from allyl vinyl ethers?
- SN2′ nucleophilic substitution
- Claisen rearrangement
- Free radical allylic oxidation only
- Hydroboration–oxidation exclusively
Correct Answer: Claisen rearrangement
Q29. When an allylic halide reacts via an SNi mechanism, what is characteristic of the stereochemical outcome?
- Complete racemization due to planar cation
- Retention of configuration due to internal nucleophilic participation creating a cyclic intermediate
- Exclusive inversion as in SN2
- No reaction ever occurs via SNi
Correct Answer: Retention of configuration due to internal nucleophilic participation creating a cyclic intermediate
Q30. Which solvent type generally stabilizes carbocation intermediates and may favor SN1′ pathways?
- Nonpolar solvents like hexane
- Polar protic solvents like water or alcohols
- Dry inert gases only
- Solids like silica gel without solvent
Correct Answer: Polar protic solvents like water or alcohols
Q31. Which rearrangement is involved when an allylic alcohol is converted to a carbonyl compound under acidic conditions with migration?
- Wagner–Meerwein–type allylic rearrangement leading to stabilized carbocation and subsequent loss to give carbonyl-containing products
- Direct photochemical dimerization only
- Simple SN2 substitution at the alcohol
- Hydrogenation to saturated alcohol
Correct Answer: Wagner–Meerwein–type allylic rearrangement leading to stabilized carbocation and subsequent loss to give carbonyl-containing products
Q32. In enzymatic allylic rearrangements (biological systems), which factor commonly directs regioselectivity?
- Random diffusion without active site control
- Active site architecture and specific binding interactions directing substrate orientation
- Temperature alone controls which bond shifts
- Only metal cofactors with no protein influence
Correct Answer: Active site architecture and specific binding interactions directing substrate orientation
Q33. Which observation would indicate formation of a pi-allyl intermediate in a Pd-catalyzed reaction?
- Immediate precipitation of palladium metal
- Isolation or spectroscopic detection (e.g., NMR) of a stabilized allyl–Pd complex or change in regioselectivity consistent with allyl delocalization
- Complete lack of reactivity under all conditions
- Formation of a peroxides exclusively
Correct Answer: Isolation or spectroscopic detection (e.g., NMR) of a stabilized allyl–Pd complex or change in regioselectivity consistent with allyl delocalization
Q34. Which product is expected from a thermal Claisen rearrangement of allyl phenyl ether?
- Alkyl halide by SN2
- Ortho-allylphenol (allyl group migrates to aromatic ortho position)
- Oxidized quinone only
- Polymerized ether chains
Correct Answer: Ortho-allylphenol (allyl group migrates to aromatic ortho position)
Q35. Which property of a nucleophile favors attack at the less substituted terminus of a pi-allyl complex?
- High steric bulk which forces attack at the more exposed position
- Small size and high nucleophilicity favoring attack at the less hindered terminus
- Complete lack of nucleophilicity
- Exclusive oxidation potential without nucleophilic character
Correct Answer: Small size and high nucleophilicity favoring attack at the less hindered terminus
Q36. Which mechanistic pathway is associated with rearrangements that preserve stereochemistry through cyclic intermediates?
- Open planar carbocation formation leading to racemization
- Neighboring group participation (anchimeric assistance) forming cyclic intermediates that lead to net retention
- Free radical chain processes yielding random stereochemistry
- Radical polymerization that erases stereocenters
Correct Answer: Neighboring group participation (anchimeric assistance) forming cyclic intermediates that lead to net retention
Q37. In allylic oxidation (formation of allylic alcohols or carbonyls), which reagent is commonly used selectively?
- PCC acting on allylic positions exclusively
- SeO2 which selectively oxidizes allylic positions under many conditions
- NaBH4 as an oxidant
- HCl alone as oxidant
Correct Answer: SeO2 which selectively oxidizes allylic positions under many conditions
Q38. What determines whether an allylic rearrangement follows kinetic or thermodynamic control?
- Only the solvent color
- Reaction temperature, time, and reversibility; low temperature favors kinetic products, high temperature favors thermodynamic ones
- The brand of glassware used
- Whether the substrate is crystalline
Correct Answer: Reaction temperature, time, and reversibility; low temperature favors kinetic products, high temperature favors thermodynamic ones
Q39. Which is true about allylic halide reactivity trends?
- Allylic halides are generally less reactive than primary alkyl halides in substitution
- Allylic halides are often more reactive toward nucleophiles than saturated alkyl halides due to resonance-stabilized transition states
- Allylic halides never undergo substitution and are inert
- Only radical conditions permit substitution of allylic halides
Correct Answer: Allylic halides are often more reactive toward nucleophiles than saturated alkyl halides due to resonance-stabilized transition states
Q40. In the context of drug synthesis, why are allylic rearrangements important to B. Pharm students?
- They are irrelevant to medicinal chemistry
- They influence regio- and stereochemistry of synthetic routes, affecting biological activity and metabolite formation
- They only affect color of pharmaceutical formulations
- They exclusively produce toxic byproducts and are avoided
Correct Answer: They influence regio- and stereochemistry of synthetic routes, affecting biological activity and metabolite formation
Q41. Which of the following best describes the energy profile of a concerted pericyclic allylic rearrangement?
- Multiple high-energy ionic intermediates separated by deep wells
- A single transition state connecting reactant and product without intermediate minima
- An endless series of radicals forming polymer
- No activation energy required
Correct Answer: A single transition state connecting reactant and product without intermediate minima
Q42. Which effect does an allylic oxygen substituent (e.g., OR) typically have on the allylic system?
- It withdraws electron density strongly and destabilizes allylic cations
- It donates electron density by resonance and can stabilize adjacent allylic cations
- It prevents any rearrangement from occurring
- It transforms the system into an aromatic ring
Correct Answer: It donates electron density by resonance and can stabilize adjacent allylic cations
Q43. Which analytical sign indicates that an allylic rearrangement produced a conjugated diene?
- New IR absorption around 1700 cm−1 only
- UV–visible absorption increase and characteristic NMR coupling patterns for conjugated dienes
- No spectroscopic changes at all
- Disappearance of all hydrogen signals in NMR
Correct Answer: UV–visible absorption increase and characteristic NMR coupling patterns for conjugated dienes
Q44. How does temperature influence concerted vs. stepwise allylic rearrangements?
- Lower temperatures universally favor stepwise ionic mechanisms
- Higher temperatures can promote pericyclic, concerted rearrangements if thermally allowed; also can shift equilibria to thermodynamic products
- Temperature has no effect on mechanism
- Only photochemical energy determines concerted pathways
Correct Answer: Higher temperatures can promote pericyclic, concerted rearrangements if thermally allowed; also can shift equilibria to thermodynamic products
Q45. Which leaving group improves the likelihood of SN1′ allylic substitution?
- Poor leaving groups like -NH2 without activation
- Good leaving groups such as tosylate or bromide that allow ionization to form an allylic cation
- Non-leaving hydrogen atoms only
- Inert groups like methyl that cannot leave
Correct Answer: Good leaving groups such as tosylate or bromide that allow ionization to form an allylic cation
Q46. Which product results from the rearrangement of 1,5-hexadiene via Cope rearrangement under thermal conditions?
- Polymeric material only
- Isomeric 1,5-hexadiene isomers or substituted hexadienes depending on substitution pattern (reversible isomerization)
- Complete oxidation to hexanone automatically
- Conversion to an aromatic ring
Correct Answer: Isomeric 1,5-hexadiene isomers or substituted hexadienes depending on substitution pattern (reversible isomerization)
Q47. Which descriptor best applies to the transition state of a pericyclic allylic rearrangement obeying Woodward–Hoffmann rules?
- Forbidden and always high-energy
- Symmetry-allowed for thermal suprafacial [3,3]-sigmatropic shifts and therefore lower in energy when allowed
- Independent of orbital symmetry considerations
- Always involves ionic fragmentation
Correct Answer: Symmetry-allowed for thermal suprafacial [3,3]-sigmatropic shifts and therefore lower in energy when allowed
Q48. In allylic substitution, what outcome results from using a soft nucleophile with a soft metal catalyst (e.g., Pd) vs. a hard nucleophile?
- Soft nucleophiles often promote formation of pi-allyl complexes and lead to controlled regio- and stereoselectivity, whereas hard nucleophiles favor direct displacement
- Soft nucleophiles never react with allylic substrates
- Hard nucleophiles form pi-allyl complexes exclusively
- Both give identical product distributions regardless of conditions
Correct Answer: Soft nucleophiles often promote formation of pi-allyl complexes and lead to controlled regio- and stereoselectivity, whereas hard nucleophiles favor direct displacement
Q49. Which mechanistic probe can distinguish between SN2′ and SN1′ pathways experimentally?
- Measuring only product color change
- Using stereochemically defined, enantiomerically enriched substrates to see inversion vs racemization patterns and kinetics under varying nucleophile strength and solvent
- Only checking melting point of products
- Assessing pH without structural analysis
Correct Answer: Using stereochemically defined, enantiomerically enriched substrates to see inversion vs racemization patterns and kinetics under varying nucleophile strength and solvent
Q50. Which precaution is important when applying allylic rearrangement knowledge to drug metabolite prediction?
- Assume all rearrangements will be instantaneous in vivo
- Consider enzyme selectivity, possible allylic oxidation or rearrangement pathways, and stereochemical consequences for metabolites and activity
- Ignore stereochemistry because metabolites are always achiral
- Only consider hydrolysis; rearrangements are irrelevant biologically
Correct Answer: Consider enzyme selectivity, possible allylic oxidation or rearrangement pathways, and stereochemical consequences for metabolites and activity
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