Anti-Markownikoff’s orientation MCQs With Answer

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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

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