Anti-Markownikoff’s orientation MCQs With Answer is a focused guide for B.Pharm students covering anti-Markovnikov regiochemistry in alkene additions. This introduction explains key concepts like the peroxide (Kharasch) effect, hydroboration–oxidation (BH3/9-BBN → H2O2/NaOH), radical HBr addition, and syn vs. anti addition mechanisms. Learn how reagents, radical initiation, transition states, and stereochemistry determine product distribution and why HBr (not HCl/HI) shows peroxide-driven anti-Markovnikov behavior. The set targets problem-solving, mechanism insight, and practical reagents to help you master regioselectivity and predict products confidently. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What does “anti-Markownikoff’s orientation” mean in alkene addition reactions?
- The substituent adds to the more substituted carbon of the double bond
- The substituent adds to the less substituted carbon of the double bond
- The addition proceeds via a carbocation intermediate
- The product is always racemic
Correct Answer: The substituent adds to the less substituted carbon of the double bond
Q2. Which reagent sequence reliably gives anti-Markovnikov alcohols from terminal alkenes?
- HBr in presence of peroxide
- Hg(OAc)2, H2O followed by NaBH4
- BH3·THF followed by H2O2/NaOH
- H2SO4, H2O
Correct Answer: BH3·THF followed by H2O2/NaOH
Q3. Which name is commonly used for peroxide-initiated anti-Markovnikov addition of HBr?
- Kharasch reaction
- Wagner–Meerwein rearrangement
- Oxymercuration
- Friedel–Crafts alkylation
Correct Answer: Kharasch reaction
Q4. What is the stereochemical outcome of hydroboration-oxidation on an alkene?
- Anti addition
- Syn addition
- Retention at one center and inversion at the other
- Racemization
Correct Answer: Syn addition
Q5. Which hydrogen halide displays the peroxide (anti-Markovnikov) effect most commonly?
- HCl
- HF
- HBr
- HI
Correct Answer: HBr
Q6. In the peroxide-initiated addition of HBr to alkenes, what species adds first to the double bond?
- H• radical
- Br• radical
- HBr molecule
- BH3
Correct Answer: Br• radical
Q7. What is the main reason BH3 adds to the less substituted carbon of an alkene in hydroboration?
- Because boron is highly nucleophilic and attacks the more substituted carbon
- Because boron prefers the less hindered carbon due to steric and transition-state factors
- Because hydroboration proceeds via a free carbocation
- Because BH3 is a bulky reagent that avoids terminal positions
Correct Answer: Because boron prefers the less hindered carbon due to steric and transition-state factors
Q8. Which oxidant is used in the oxidation step after hydroboration to obtain the alcohol?
- KMnO4, heat
- H2O2 in NaOH (hydrogen peroxide in base)
- O3 followed by Zn
- Chromic acid (H2CrO4)
Correct Answer: H2O2 in NaOH (hydrogen peroxide in base)
Q9. Which experimental condition promotes the peroxide effect in HBr addition?
- Strict absence of oxygen and light
- Addition of strong acid catalyst
- Presence of organic peroxides, heat or light
- Use of polar protic solvents only
Correct Answer: Presence of organic peroxides, heat or light
Q10. Which of the following is true about hydroboration-oxidation compared to acid-catalyzed hydration?
- Hydroboration-oxidation proceeds via a carbocation intermediate
- Hydroboration-oxidation gives Markovnikov alcohols
- Hydroboration-oxidation gives anti-Markovnikov alcohols with syn stereochemistry
- Hydroboration-oxidation causes frequent rearrangements
Correct Answer: Hydroboration-oxidation gives anti-Markovnikov alcohols with syn stereochemistry
Q11. Which bulky borane reagent is often used to improve regioselectivity and reduce side reactions in hydroboration?
- LiAlH4
- 9-BBN (9-borabicyclo[3.3.1]nonane)
- BCl3
- NaBH4
Correct Answer: 9-BBN (9-borabicyclo[3.3.1]nonane)
Q12. In the radical chain mechanism of HBr addition under peroxides, what regenerates the bromine radical?
- Addition of Br• to H2O2
- Abstraction of hydrogen from HBr by the carbon radical
- Oxidation by molecular oxygen
- Reaction with BH3
Correct Answer: Abstraction of hydrogen from HBr by the carbon radical
Q13. Which statement explains why HCl does not show a peroxide-induced anti-Markovnikov addition like HBr?
- Chlorine radicals are too selective and always give anti-Markovnikov products
- The H–Cl bond is too weak to donate hydrogen to carbon radicals efficiently in propagation
- The propagation step (R• + HCl → RH + Cl•) is energetically unfavorable compared to termination
- Chlorine cannot form radicals under any conditions
Correct Answer: The propagation step (R• + HCl → RH + Cl•) is energetically unfavorable compared to termination
Q14. Hydroboration is best described mechanistically as:
- A stepwise radical process
- An ionic, carbocation-mediated pathway
- A concerted syn addition via a four-membered transition state
- A two-step nucleophilic substitution
Correct Answer: A concerted syn addition via a four-membered transition state
Q15. For propene, addition of HBr in presence of peroxides yields which major product?
- 2-bromopropane (Markovnikov product)
- 1-bromopropane (anti-Markovnikov product)
- Isopropyl alcohol
- Propyl peroxide
Correct Answer: 1-bromopropane (anti-Markovnikov product)
Q16. Which of the following is NOT a characteristic of hydroboration-oxidation?
- No carbocation rearrangements
- Anti-Markovnikov regiochemistry
- Formation of organoborane intermediates
- Requirement of strong acid to protonate alkene first
Correct Answer: Requirement of strong acid to protonate alkene first
Q17. What is the immediate organic intermediate formed after hydroboration before oxidation?
- Alkyl bromide
- Alkylborane (organoborane)
- Carbocation
- Alcohol
Correct Answer: Alkylborane (organoborane)
Q18. In radical HBr addition to an unsymmetrical alkene, the regiochemistry is determined mainly by:
- Carbocation stability
- Thermodynamics of final product only
- Stability of the radical intermediate formed during propagation
- Solvent polarity exclusively
Correct Answer: Stability of the radical intermediate formed during propagation
Q19. Which of the following is a common radical initiator used to start peroxide-type anti-Markovnikov reactions?
- Benzoyl peroxide
- Sulfuric acid
- Pd/C and H2
- LiAlH4
Correct Answer: Benzoyl peroxide
Q20. Which reaction most reliably avoids rearrangement of carbon skeleton during hydration of alkenes?
- Acid-catalyzed hydration (H2SO4)
- Oxymercuration-demercuration
- SN1 substitution
- Free radical polymerization
Correct Answer: Oxymercuration-demercuration
Q21. Hydroboration of a cis-alkene followed by oxidation gives which relative stereochemistry at the new stereocenters?
- Anti stereochemistry
- Syn stereochemistry
- Random stereochemistry
- Inverted configuration at both centers
Correct Answer: Syn stereochemistry
Q22. Which product results from hydroboration-oxidation of 1-hexene?
- 2-hexanol (Markovnikov)
- 1-hexanol (anti-Markovnikov)
- Hexane
- Hexanone
Correct Answer: 1-hexanol (anti-Markovnikov)
Q23. During the peroxide-initiated addition of HBr, which step is considered the propagation step that forms the organic product?
- Initiator decomposition to radicals
- R• + HBr → RH + Br•
- Termination by radical recombination
- Oxidation of organoborane
Correct Answer: R• + HBr → RH + Br•
Q24. Which reaction gives a carbonyl compound from a terminal alkyne via anti-Markovnikov addition?
- Acid-catalyzed hydration (H2SO4/HgSO4)
- Hydroboration–oxidation (R2BH then H2O2/NaOH) to give an aldehyde
- Ozonolysis
- Radical HBr addition
Correct Answer: Hydroboration–oxidation (R2BH then H2O2/NaOH) to give an aldehyde
Q25. Predict the major product of isobutylene (2-methylpropene) with HBr in presence of peroxides.
- 2-bromo-2-methylpropane (tert-bromide)
- 1-bromo-2-methylpropane (primary bromide, anti-Markovnikov)
- 2-methylpropan-1-ol
- No reaction
Correct Answer: 1-bromo-2-methylpropane (primary bromide, anti-Markovnikov)
Q26. Which reagent combination would NOT produce an anti-Markovnikov alcohol from a terminal alkene?
- BH3·THF followed by H2O2/NaOH
- HBr in presence of peroxides
- Hydroboration using 9-BBN then oxidation
- Oxymercuration-demercuration (Hg(OAc)2, H2O then NaBH4)
Correct Answer: Oxymercuration-demercuration (Hg(OAc)2, H2O then NaBH4)
Q27. Why are rearrangements common in Markovnikov (acid-catalyzed) hydration but not in hydroboration?
- Because hydroboration proceeds via a radical intermediate that rearranges less
- Because acid-catalyzed hydration forms carbocations which can rearrange, while hydroboration is concerted
- Because hydroboration uses high temperatures that prevent rearrangement
- Because boron forces rearrangement into less stable products
Correct Answer: Because acid-catalyzed hydration forms carbocations which can rearrange, while hydroboration is concerted
Q28. Which experimental observation supports a radical mechanism for peroxide-induced HBr addition?
- Product distribution identical to acid-catalyzed hydration
- Reaction inhibited by oxygen or radical scavengers
- Formation of carbocation rearrangement products
- Requirement of strong acid catalysts
Correct Answer: Reaction inhibited by oxygen or radical scavengers
Q29. Which of the following best describes the selectivity of Br• addition to an alkene in peroxide-initiated HBr addition?
- Br• always adds to the more substituted carbon
- Br• tends to add to the less substituted carbon to form the more stabilized radical
- Br• adds randomly with no preference
- Br• does not add to alkenes
Correct Answer: Br• tends to add to the less substituted carbon to form the more stabilized radical
Q30. Which of the following is the correct sequence for hydroboration-oxidation?
- BH3·THF → H2O2/NaOH
- HBr/ROOR → H2O
- Hg(OAc)2 → NaBH4
- KMnO4 → H3O+
Correct Answer: BH3·THF → H2O2/NaOH
Q31. What product results when 1-hexene undergoes peroxide-initiated addition of HBr?
- 2-bromohexane (Markovnikov)
- 1-bromohexane (anti-Markovnikov)
- 1-hexanol
- Hexane
Correct Answer: 1-bromohexane (anti-Markovnikov)
Q32. Which of the following will most effectively inhibit a peroxide-initiated anti-Markovnikov HBr addition?
- Adding excess HBr
- Removing oxygen from the reaction
- Adding a radical scavenger such as TEMPO
- Increasing temperature
Correct Answer: Adding a radical scavenger such as TEMPO
Q33. Which factor primarily determines regioselectivity in hydroboration?
- Stability of a carbocation intermediate
- Steric and electronic preference of boron to add to the less hindered carbon
- Solvent polarity
- Presence of acid catalyst
Correct Answer: Steric and electronic preference of boron to add to the less hindered carbon
Q34. In a radical anti-Markovnikov addition, which radical stability order influences product formation?
- Primary > Secondary > Tertiary
- Tertiary > Secondary > Primary
- Secondary > Primary > Tertiary
- All radicals have equal stability
Correct Answer: Tertiary > Secondary > Primary
Q35. What is a major safety concern when performing peroxide-initiated anti-Markovnikov reactions in the lab?
- Peroxides are inert and pose no risk
- Organic peroxides can be shock-sensitive and explosive if mishandled
- They form only harmless by-products
- They always require cryogenic conditions
Correct Answer: Organic peroxides can be shock-sensitive and explosive if mishandled
Q36. Which statement correctly contrasts oxymercuration-demercuration with hydroboration-oxidation?
- Both give anti-Markovnikov alcohols
- Oxymercuration gives Markovnikov alcohols without rearrangement; hydroboration gives anti-Markovnikov alcohols
- Hydroboration requires mercuric acetate
- Oxymercuration always causes rearrangements
Correct Answer: Oxymercuration gives Markovnikov alcohols without rearrangement; hydroboration gives anti-Markovnikov alcohols
Q37. During hydroboration-oxidation, the OH group ends up on which carbon relative to the original double bond?
- The more substituted carbon (Markovnikov)
- The less substituted carbon (anti-Markovnikov)
- Always on a terminal carbon regardless of starting alkene
- Randomly distributed
Correct Answer: The less substituted carbon (anti-Markovnikov)
Q38. Which of the following reactions is most likely to produce a mixture of stereoisomers due to non-concerted steps?
- Hydroboration-oxidation of a simple alkene
- Acid-catalyzed hydration via carbocation
- Syn-addition hydrogenation over Pd/C
- Oxidation of an organoborane with H2O2/NaOH
Correct Answer: Acid-catalyzed hydration via carbocation
Q39. Which reagent combination yields an aldehyde from a terminal alkyne by anti-Markovnikov hydration?
- HgSO4/H2SO4 (acid-catalyzed)
- R2BH (e.g., disiamylborane) then H2O2/NaOH
- O3 then H2O
- HBr/ROOR
Correct Answer: R2BH (e.g., disiamylborane) then H2O2/NaOH
Q40. Which observation would indicate that hydroboration proceeded by a concerted mechanism rather than stepwise?
- Formation of carbocation rearrangement products
- Retention of relative stereochemistry leading to syn addition
- Radical trapping changes product distribution
- Requirement of strong acids
Correct Answer: Retention of relative stereochemistry leading to syn addition
Q41. In peroxide-initiated HBr addition, what is the role of heat or light?
- To stabilize carbocations
- To decompose peroxide into radicals (initiation)
- To convert Br– to Br2
- To protonate the alkene
Correct Answer: To decompose peroxide into radicals (initiation)
Q42. Which type of alkene will most strongly favor anti-Markovnikov addition under radical HBr conditions?
- Alkene where radical formed is resonance-stabilized (e.g., styrene)
- Alkene that gives only primary radicals
- Tetrasubstituted alkene that cannot form radicals
- Alkene in very polar protic solvents only
Correct Answer: Alkene where radical formed is resonance-stabilized (e.g., styrene)
Q43. Which of the following is a correct propagation sequence in peroxide-initiated HBr addition?
- R• + O2 → ROO• ; ROO• + HBr → no reaction
- Br• + alkene → R• ; R• + HBr → RH + Br•
- BH3 + alkene → carbocation ; carbocation + H2O → alcohol
- Carbocation formation followed by hydride shift then capture by Br–
Correct Answer: Br• + alkene → R• ; R• + HBr → RH + Br•
Q44. What is the effect of using bulky borane reagents (like 9-BBN) in hydroboration?
- Decrease regioselectivity and increase rearrangements
- Increase selectivity for terminal (less substituted) carbon and reduce side reactions
- Convert reaction into ionic hydration
- Eliminate the need for oxidation
Correct Answer: Increase selectivity for terminal (less substituted) carbon and reduce side reactions
Q45. Which of the following best describes the difference between Markovnikov and anti-Markovnikov additions?
- Markovnikov places substituent at less substituted carbon; anti-Markovnikov at more substituted
- Markovnikov and anti-Markovnikov are the same
- Markovnikov places substituent at more substituted carbon; anti-Markovnikov places it at less substituted carbon
- Both always lead to racemic mixtures
Correct Answer: Markovnikov places substituent at more substituted carbon; anti-Markovnikov places it at less substituted carbon
Q46. Why does peroxide-promoted HBr addition not favor HCl or HI analogs?
- Because Cl• and I• radicals are too reactive to form selectively stabilized organic radicals in propagation
- Because propagation with HCl is less favorable energetically and HI leads to different side reactions
- Because Cl and I cannot form radicals
- Because HCl and HI are solids
Correct Answer: Because propagation with HCl is less favorable energetically and HI leads to different side reactions
Q47. Which experimental technique can help determine if hydroboration-oxidation occurred with syn addition?
- NMR analysis of stereochemical relationships (e.g., coupling constants and NOE)
- Measuring boiling point only
- Adding base to the product
- Performing IR to detect B–H stretching exclusively
Correct Answer: NMR analysis of stereochemical relationships (e.g., coupling constants and NOE)
Q48. Which product results from addition of HBr (no peroxides) to propene?
- 1-bromopropane (anti-Markovnikov)
- 2-bromopropane (Markovnikov)
- 1-propanol
- Propane
Correct Answer: 2-bromopropane (Markovnikov)
Q49. Which of the following is TRUE about organoborane intermediates?
- They are ionic carbocations
- They readily rearrange via hydride shifts
- They react with hydrogen peroxide under basic conditions to give alcohols
- They are completely inert toward oxidation
Correct Answer: They react with hydrogen peroxide under basic conditions to give alcohols
Q50. For safe lab practice, which precaution is most important when using organic peroxides for anti-Markovnikov reactions?
- Store peroxides at high temperature to keep them active
- Dilute peroxides in flammable solvents without controls
- Handle peroxides with minimal mechanical shock and follow storage temperature limits
- Always add peroxides to strong reducing agents first
Correct Answer: Handle peroxides with minimal mechanical shock and follow storage temperature limits
