Markownikoff’s orientation, often called Markovnikov’s rule, is fundamental for B.Pharm students studying regioselective electrophilic additions to alkenes. The rule predicts which carbon receives the electrophile based on carbocation stability, guiding hydrohalogenation, acid-catalyzed hydration, and alkyne additions. Understanding mechanism, carbocation rearrangements, and exceptions—such as anti-Markovnikov outcomes from peroxides or hydroboration–oxidation—is critical for predicting drug-intermediate formation and avoiding synthetic pitfalls. This concise, mechanism-focused overview emphasizes practical examples, stereochemical consequences, and reagent-specific behavior relevant to medicinal chemistry and pharmaceutical synthesis. Practice with mechanism-based MCQs improves exam performance and synthetic reasoning. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What does Markownikoff’s orientation (Markovnikov’s rule) predict in electrophilic addition to alkenes?
- The electrophile adds to the less substituted carbon
- The electrophile adds to the more substituted carbon
- The nucleophile always adds first
- It predicts stereochemistry only
Correct Answer: The electrophile adds to the more substituted carbon
Q2. Which intermediate is central to classical Markovnikov addition mechanisms?
- Bromonium ion
- Carbocation
- Radical
- Nucleophile
Correct Answer: Carbocation
Q3. Order the following carbocations by stability (most to least): tertiary, secondary, primary, methyl.
- Primary > Secondary > Tertiary > Methyl
- Tertiary > Secondary > Primary > Methyl
- Methyl > Primary > Secondary > Tertiary
- Secondary > Tertiary > Primary > Methyl
Correct Answer: Tertiary > Secondary > Primary > Methyl
Q4. In hydrohalogenation of propene with HBr (no peroxides), the major product is:
- 1-bromopropane (anti-Markovnikov)
- 2-bromopropane (Markovnikov)
- Propan-1-ol
- Propyl bromide mixture equally
Correct Answer: 2-bromopropane (Markovnikov)
Q5. Which reagent combination gives anti-Markovnikov alcohol from alkene?
- H2O / H2SO4 (acid-catalyzed hydration)
- BH3·THF followed by H2O2/NaOH
- Hg(OAc)2 / NaBH4
- Br2 / H2O
Correct Answer: BH3·THF followed by H2O2/NaOH
Q6. Which addition proceeds with syn stereochemistry and anti-Markovnikov regiochemistry?
- Hydroboration–oxidation of alkenes
- Halogenation with Br2
- Acid-catalyzed hydration
- Oxymercuration-demercuration
Correct Answer: Hydroboration–oxidation of alkenes
Q7. Oxymercuration–demercuration of an alkene yields which regioselectivity and which important advantage?
- Anti-Markovnikov product with rearrangement
- Markovnikov product with no carbocation rearrangement
- Markovnikov product with radical intermediate
- Equal mixture of regioisomers
Correct Answer: Markovnikov product with no carbocation rearrangement
Q8. Which statement about the peroxide (Kharasch) effect is true?
- It causes anti-Markovnikov addition for HBr via radical chain
- It causes Markovnikov addition for HBr via carbocation
- It has the same effect for HCl and HI
- It only affects oxymercuration reactions
Correct Answer: It causes anti-Markovnikov addition for HBr via radical chain
Q9. Which reagent set hydrates an alkyne to give a ketone following Markovnikov addition?
- BH3, then H2O2/NaOH
- H2O / H2SO4 with HgSO4
- H2 / Pd-C
- O3 / (CH3)2S
Correct Answer: H2O / H2SO4 with HgSO4
Q10. Which reaction typically avoids carbocation rearrangement when forming Markovnikov alcohols?
- Acid-catalyzed hydration
- Oxymercuration-demercuration
- Hydrohalogenation
- Free-radical hydration
Correct Answer: Oxymercuration-demercuration
Q11. In hydrohalogenation in presence of peroxides, which step governs regiochemistry?
- Carbocation formation stability
- Radical addition to alkene forming more stable radical
- Nucleophilic attack by halide
- Electrophile first forming a π-complex
Correct Answer: Radical addition to alkene forming more stable radical
Q12. Which of these halogen additions proceeds via a bridged halonium ion?
- Addition of HBr to alkenes
- Addition of Br2 to alkenes
- Hydroboration of alkenes
- Oxymercuration
Correct Answer: Addition of Br2 to alkenes
Q13. Which reagent pair will give Markovnikov alcohol without rearrangement from an alkene?
- HBr with peroxides
- BH3/THF then H2O2/NaOH
- Hg(OAc)2 then NaBH4
- H2O/H2SO4
Correct Answer: Hg(OAc)2 then NaBH4
Q14. Which statement about hydroboration–oxidation is correct?
- It proceeds via carbocation intermediate
- It gives anti-Markovnikov alcohol by syn addition
- It gives Markovnikov product by anti addition
- It requires strong acid to proceed
Correct Answer: It gives anti-Markovnikov alcohol by syn addition
Q15. Why do tertiary carbocations form preferentially in electrophilic additions?
- Tertiary carbocations are less stabilized by hyperconjugation
- Tertiary carbocations are stabilized by hyperconjugation and inductive effects
- Because tertiary carbons are less accessible sterically
- Tertiary carbocations are resonance-stabilized only
Correct Answer: Tertiary carbocations are stabilized by hyperconjugation and inductive effects
Q16. Which reaction gives an alcohol with anti-Markovnikov regiochemistry but via a different mechanism than hydroboration?
- HBr with peroxides
- Radical hydration (e.g., MHAT processes)
- Acid-catalyzed hydration
- Oxymercuration-demercuration
Correct Answer: Radical hydration (e.g., MHAT processes)
Q17. In HBr addition with peroxides, which species initiates the radical chain?
- Carbocation
- Bromine radical
- Hydride ion
- Mercurinium ion
Correct Answer: Bromine radical
Q18. Which addition to alkenes typically gives anti stereochemistry between substituents?
- Hydroboration–oxidation
- Syn hydrogenation with H2/Pd
- Halogenation with Br2
- Oxymercuration followed by NaBH4
Correct Answer: Halogenation with Br2
Q19. Which of the following will undergo hydride shift during acid-catalyzed hydration?
- 2-methylpropene to form tertiary carbocation
- Ethene only
- Alkenes that form only primary carbocations
- Tertiary alkenes that already form tertiary carbocations
Correct Answer: 2-methylpropene to form tertiary carbocation
Q20. For addition to a conjugated diene, which products reflect kinetic vs thermodynamic control?
- 1,2-addition is thermodynamic; 1,4-addition is kinetic
- 1,2-addition is kinetic; 1,4-addition is thermodynamic
- Both are only kinetic
- Both are only thermodynamic
Correct Answer: 1,2-addition is kinetic; 1,4-addition is thermodynamic
Q21. Which reagent will hydrate an alkene to give a Markovnikov alcohol without acidic rearrangement?
- H2O/H2SO4
- BH3, then H2O2/NaOH
- Hg(OAc)2 followed by NaBH4
- Br2/H2O
Correct Answer: Hg(OAc)2 followed by NaBH4
Q22. In addition to HBr, which hydrogen halide shows anti-Markovnikov behavior in radical conditions?
- HCl in presence of peroxides
- HI in presence of peroxides
- Only HBr shows significant peroxide-induced anti-Markovnikov addition
- All HX behave the same under peroxides
Correct Answer: Only HBr shows significant peroxide-induced anti-Markovnikov addition
Q23. Which feature of a substrate increases likelihood of rearrangement during electrophilic addition?
- Formation of a more stable carbocation via shift
- Alkene being terminal only
- Presence of peroxide
- Syn addition preference
Correct Answer: Formation of a more stable carbocation via shift
Q24. Which product results from hydroboration–oxidation of 1-butene?
- 2-butanol (Markovnikov)
- 1-butanol (anti-Markovnikov)
- Butanone
- 1,2-dibromobutane
Correct Answer: 1-butanol (anti-Markovnikov)
Q25. Why does oxymercuration avoid rearrangement compared to acid hydration?
- Because it proceeds via a bridged mercurinium ion rather than a free carbocation
- Because it uses radicals instead of carbocations
- Because it forms a free hydride first
- Because it requires peroxides
Correct Answer: Because it proceeds via a bridged mercurinium ion rather than a free carbocation
Q26. Which of the following best describes Markovnikov addition to an unsymmetrical alkyne?
- Addition places hydrogen on more substituted carbon
- Addition places hydrogen on less substituted carbon
- Addition gives a terminal alkene only
- Addition always yields anti-Markovnikov products
Correct Answer: Addition places hydrogen on more substituted carbon
Q27. In hydrohalogenation, which factor most influences regioselectivity?
- Solvent polarity exclusively
- Stability of the developing carbocation or radical
- Color of reagent
- Presence of base only
Correct Answer: Stability of the developing carbocation or radical
Q28. Adding HBr to 2-methylpropene in presence of peroxides yields primarily:
- 2-bromo-2-methylpropane (tertiary bromide)
- 1-bromo-2-methylpropane (primary bromide)
- Alcohols instead of bromides
- No reaction
Correct Answer: 1-bromo-2-methylpropane (primary bromide)
Q29. Which mechanism characterizes addition of Br2 to an alkene in aqueous solution to form a halohydrin?
- Carbocation intermediate with rearrangement
- Bromonium ion followed by nucleophilic attack by water
- Radical chain propagation exclusively
- Concerted syn addition
Correct Answer: Bromonium ion followed by nucleophilic attack by water
Q30. For pharmaceutical synthesis, why is understanding Markovnikov orientation important?
- It predicts only the color of products
- It helps predict regioselectivity, potential rearrangements, and product outcomes critical for active ingredient synthesis
- It is only of historical interest
- It dictates melting points exclusively
Correct Answer: It helps predict regioselectivity, potential rearrangements, and product outcomes critical for active ingredient synthesis
Q31. Which reagent combination converts an alkene into a vicinal dihalide (no Markovnikov concern)?
- HBr alone
- Br2 in inert solvent
- BH3 then H2O2
- H2O/H2SO4
Correct Answer: Br2 in inert solvent
Q32. Which addition to an alkene gives products via a free radical mechanism under normal conditions?
- HBr in presence of peroxides
- Oxymercuration-demercuration
- Acid-catalyzed hydration
- Hydration with HgSO4
Correct Answer: HBr in presence of peroxides
Q33. Which outcome favors formation of Markovnikov product during electrophilic addition?
- Formation of the less stable carbocation
- Formation of the more stable carbocation intermediate
- Strong radical initiators present
- Hydroboration conditions
Correct Answer: Formation of the more stable carbocation intermediate
Q34. In hydration of alkynes under Hg2+ catalysis, the enol tautomerizes to give:
- An aldehyde for internal alkynes
- A ketone for terminal alkynes (Markovnikov)
- An alcohol that remains as enol
- An alkyne oxide
Correct Answer: A ketone for terminal alkynes (Markovnikov)
Q35. Which reagent is used to promote anti-Markovnikov addition of HBr specifically?
- Peroxides like ROOR
- Hg(OAc)2
- Strong acid like H2SO4 only
- H2 with Pd/C
Correct Answer: Peroxides like ROOR
Q36. In electrophilic addition to styrene (vinyl benzene), regioselectivity is influenced by:
- Resonance stabilization of benzylic carbocation favoring Markovnikov addition
- Only steric factors
- Peroxide effect exclusively
- Formation of bromonium ions only
Correct Answer: Resonance stabilization of benzylic carbocation favoring Markovnikov addition
Q37. Which process describes a 1,2-hydride shift?
- Methyl group moves from adjacent carbon to stabilize charge
- Hydride (H-) moves from adjacent carbon to the carbocation center
- Proton transfer to solvent only
- Radical abstraction of hydrogen
Correct Answer: Hydride (H-) moves from adjacent carbon to the carbocation center
Q38. Which addition method is preferred to avoid rearrangements when preparing sensitive pharmaceutical intermediates?
- Acid-catalyzed hydration
- Oxymercuration-demercuration or hydroboration-oxidation depending on regiochemical need
- HBr with peroxides
- High temperature radical conditions
Correct Answer: Oxymercuration-demercuration or hydroboration-oxidation depending on regiochemical need
Q39. Which of the following is NOT an exception to typical Markovnikov addition?
- Hydroboration–oxidation giving anti-Markovnikov alcohols
- HBr addition with peroxides giving anti-Markovnikov product
- Oxymercuration–demercuration giving Markovnikov alcohols without rearrangement
- Acid-catalyzed hydration causing carbocation rearrangement
Correct Answer: Oxymercuration–demercuration giving Markovnikov alcohols without rearrangement
Q40. What is the primary driving factor for anti-Markovnikov regiochemistry in hydroboration?
- Formation of the most stable carbocation
- Concerted addition where boron attaches to less hindered carbon
- Formation of bromonium ion
- Use of strong acids
Correct Answer: Concerted addition where boron attaches to less hindered carbon
Q41. How does resonance stabilization affect regioselectivity in electrophilic addition?
- Resonance always destabilizes carbocations
- Resonance-stabilized carbocations form preferentially, influencing Markovnikov addition
- Resonance only affects stereochemistry, not regiochemistry
- Resonance prevents any addition reaction
Correct Answer: Resonance-stabilized carbocations form preferentially, influencing Markovnikov addition
Q42. Which statement about stereochemical outcome of oxymercuration-demercuration is true?
- It proceeds by syn addition exclusively
- It proceeds with anti addition through mercurinium ion opening, but stereochemistry depends on nucleophile approach
- It yields racemic mixtures only
- It proceeds via free radicals causing random stereochemistry
Correct Answer: It proceeds with anti addition through mercurinium ion opening, but stereochemistry depends on nucleophile approach
Q43. In hydrobromination of an asymmetric alkene via carbocation pathway, the bromide ends up:
- On the carbon bearing more hydrogens
- On the more substituted carbon (Markovnikov)
- Equally distributed always
- Only at the terminal carbon
Correct Answer: On the more substituted carbon (Markovnikov)
Q44. Which of the following predicts product regioselectivity for addition to an alkene that forms an allylic carbocation?
- Allylic carbocations are less stabilized; ignore resonance
- Resonance-stabilized allylic carbocations change regioselectivity toward resonance-stabilized center
- Only steric hindrance matters
- Radical mechanisms always override resonance
Correct Answer: Resonance-stabilized allylic carbocations change regioselectivity toward resonance-stabilized center
Q45. What is the outcome when HBr adds to an alkene in absence vs presence of peroxides?
- Absence: anti-Markovnikov; Presence: Markovnikov
- Absence: Markovnikov via carbocation; Presence: anti-Markovnikov via radical
- Both always give same product
- Both pathways always give bromonium ions
Correct Answer: Absence: Markovnikov via carbocation; Presence: anti-Markovnikov via radical
Q46. Which structural feature reduces the likelihood of rearrangement during addition?
- Substrate that forms a very unstable carbocation
- Using mechanisms that avoid free carbocations (e.g., oxymercuration)
- High acidity and heat
- Presence of radical initiators
Correct Answer: Using mechanisms that avoid free carbocations (e.g., oxymercuration)
Q47. For drug synthesis, which advantage does hydroboration–oxidation offer?
- Gives Markovnikov alcohols with rearrangement
- Provides anti-Markovnikov alcohols with predictable syn stereochemistry and mild conditions
- Always requires strong acids and high temperatures
- Produces vicinal dihalides primarily
Correct Answer: Provides anti-Markovnikov alcohols with predictable syn stereochemistry and mild conditions
Q48. Which of these additions to an alkene is most likely to produce a racemic mixture when forming a new stereocenter in achiral conditions?
- Syn hydrogenation with chiral catalyst
- Acid-catalyzed hydration via planar carbocation intermediate
- Hydroboration with chiral borane
- Oxymercuration–demercuration with chiral ligand
Correct Answer: Acid-catalyzed hydration via planar carbocation intermediate
Q49. Which reagent is suitable to achieve Markovnikov hydration of a terminal alkyne to give a methyl ketone?
- BH3 then H2O2/NaOH
- H2O/H2SO4 with HgSO4
- H2 with Lindlar catalyst
- Br2 in CCl4
Correct Answer: H2O/H2SO4 with HgSO4
Q50. When predicting products in exam questions, which step best ensures correct Markownikoff orientation?
- Assume anti-Markovnikov for all additions
- Identify mechanism (carbocation vs radical vs concerted) and stabilize intermediate to determine regiochemistry
- Always choose the most sterically hindered product
- Ignore reagent identity and predict randomly
Correct Answer: Identify mechanism (carbocation vs radical vs concerted) and stabilize intermediate to determine regiochemistry
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