Halogenation of alkanes MCQs With Answer

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Halogenation of alkanes MCQs With Answer

Halogenation of alkanes is a fundamental topic in organic chemistry and essential for B. Pharm students preparing for exams. This introduction covers free radical halogenation, chlorination and bromination mechanisms, regioselectivity, reaction kinetics, bond dissociation energies, and practical reagents like Cl2/hν, Br2/hν and N-bromosuccinimide (NBS). Understanding selectivity (primary, secondary, tertiary), factors affecting product distribution, and experimental considerations helps in drug synthesis and impurity control. These MCQs focus on mechanism depth, problem-solving, and real lab implications to strengthen your conceptual and applied knowledge. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the first step in the free radical halogenation of an alkane using Cl2 and hv?

  • Propagation: hydrogen abstraction from alkane by Cl·
  • Termination: combination of two radicals
  • Initiation: homolytic cleavage of Cl2 to 2 Cl·
  • Recombination of alkyl radicals

Correct Answer: Initiation: homolytic cleavage of Cl2 to 2 Cl·

Q2. In radical halogenation, which step typically determines regioselectivity for bromination?

  • Termination step
  • Initiation step
  • Hydrogen abstraction (propagation) step
  • Recombination with halogen radical

Correct Answer: Hydrogen abstraction (propagation) step

Q3. Which hydrogen is most reactive toward bromination in 2-methylbutane?

  • Primary hydrogen at terminal methyl
  • Secondary hydrogen on CH2 adjacent to methyl
  • Tertiary hydrogen
  • Secondary hydrogen at CH adjacent to methyl group

Correct Answer: Tertiary hydrogen

Q4. Which reagent is commonly used for selective allylic bromination of alkenes?

  • N-bromosuccinimide (NBS) in CCl4 with hv
  • Br2 in water
  • HBr with peroxide
  • N-chlorosuccinimide (NCS)

Correct Answer: N-bromosuccinimide (NBS) in CCl4 with hv

Q5. Why is bromination more selective than chlorination?

  • Bromine radicals are smaller than chlorine radicals
  • Bromination is more exothermic so kinetically controlled
  • H-abstraction by Br· is endothermic and transition state resembles the radical
  • Chlorination cannot occur via radicals

Correct Answer: H-abstraction by Br· is endothermic and transition state resembles the radical

Q6. Which relative reactivity ratio (approx.) best describes bromination for tertiary:secondary:primary H?

  • 5 : 3.8 : 1
  • 1600 : 82 : 1
  • 1 : 1 : 1
  • 10 : 2 : 1

Correct Answer: 1600 : 82 : 1

Q7. What is the typical bond dissociation energy (BDE) trend for C–H bonds?

  • Primary > secondary > tertiary
  • Tertiary > secondary > primary
  • Primary = secondary = tertiary
  • Secondary > primary > tertiary

Correct Answer: Primary > secondary > tertiary

Q8. Which statement best describes the termination step in radical halogenation?

  • Formation of radicals from light
  • Abstraction of hydrogen by halogen radical
  • Combination of two radicals to form stable molecules
  • Isomerization of alkyl radical

Correct Answer: Combination of two radicals to form stable molecules

Q9. In chlorination of propane, which product predominates?

  • 1-chloropropane (primary substituted)
  • 2-chloropropane (secondary substituted)
  • 3-chloropropane
  • No reaction occurs

Correct Answer: 2-chloropropane (secondary substituted)

Q10. What experimental condition promotes radical halogenation?

  • Strong base and low temperature
  • UV light or thermal initiation
  • Acid catalysis
  • Hydrogenation catalyst

Correct Answer: UV light or thermal initiation

Q11. Which halogenation is least favorable under radical conditions?

  • Fluorination due to uncontrolled reactivity
  • Chlorination because it never occurs
  • Bromination because it’s too exothermic
  • Iodination because radical iodine reactions are fast

Correct Answer: Fluorination due to uncontrolled reactivity

Q12. Why is iodination of alkanes not commonly used?

  • Iodine radicals are too reactive and over-halogenate
  • Iodine radical formation is highly endothermic and unfavourable
  • Iodination yields are always quantitative
  • Iodine is too expensive

Correct Answer: Iodine radical formation is highly endothermic and unfavourable

Q13. Which factor does NOT significantly affect radical halogenation selectivity?

  • Stability of the intermediate alkyl radical
  • Bond dissociation energy (C–H)
  • Solvent polarity for gas phase radical halogenation
  • Temperature and light intensity

Correct Answer: Solvent polarity for gas phase radical halogenation

Q14. According to Hammond’s postulate, which type of H-abstraction has a transition state resembling the radical product?

  • Highly exothermic H-abstraction
  • Endothermic H-abstraction
  • Isoenergetic H-abstraction
  • Termolecular H-abstraction

Correct Answer: Endothermic H-abstraction

Q15. Which reagent pair is commonly used for photochemical chlorination of alkanes in the lab?

  • HCl and peroxide
  • Cl2 and hv
  • NaCl and light
  • NBS and hv

Correct Answer: Cl2 and hv

Q16. What is the major product of bromination of isobutane?

  • 1-bromobutane
  • 2-bromobutane
  • tert-butyl bromide
  • 1,2-dibromobutane

Correct Answer: tert-butyl bromide

Q17. Which radical is more stabilized?

  • Primary alkyl radical
  • Secondary alkyl radical
  • Tertiary alkyl radical
  • Methyl radical

Correct Answer: Tertiary alkyl radical

Q18. In radical chlorination of methane, what is the product?

  • Chloroform
  • Methyl chloride
  • Carbon tetrachloride
  • Methanol

Correct Answer: Methyl chloride

Q19. Which condition helps increase bromination selectivity over chlorination when both halogens are present?

  • High temperature
  • Excess chlorine
  • Use of Br2 with hv and controlled conditions
  • Adding a strong acid

Correct Answer: Use of Br2 with hv and controlled conditions

Q20. Which step in propagation forms the alkyl radical?

  • Combination of two halogen atoms
  • Hydrogen abstraction by halogen radical from alkane
  • Recombination of alkyl radicals
  • Initiation of halogen molecule

Correct Answer: Hydrogen abstraction by halogen radical from alkane

Q21. Which of the following is a common radical initiator besides hv?

  • AIBN (azobisisobutyronitrile)
  • NaBH4
  • LiAlH4
  • H2SO4

Correct Answer: AIBN (azobisisobutyronitrile)

Q22. Which phenomenon explains the preference for tertiary hydrogen abstraction in radical halogenation?

  • Hyperconjugation and inductive stabilization of the radical
  • Greater steric hindrance at tertiary center
  • Higher C–H BDE at tertiary carbon
  • Lower activation energy for primary hydrogen

Correct Answer: Hyperconjugation and inductive stabilization of the radical

Q23. How does temperature affect the selectivity of chlorination?

  • Higher temperature increases selectivity dramatically
  • Higher temperature decreases selectivity (more random substitution)
  • Temperature has no effect
  • Lower temperature always stops the reaction

Correct Answer: Higher temperature decreases selectivity (more random substitution)

Q24. What is the expected major product when cyclohexane is chlorinated under hv?

  • Chloroform
  • Chlorocyclohexane
  • Bicyclic chlorinated product
  • No reaction

Correct Answer: Chlorocyclohexane

Q25. Which reagent is used for radical allylic chlorination selectively?

  • N-chlorosuccinimide (NCS)
  • HCl gas
  • SOCl2
  • NaCl in water

Correct Answer: N-chlorosuccinimide (NCS)

Q26. What product distribution would you expect from chlorination of propane?

  • Only 1-chloropropane
  • Only 2-chloropropane
  • Mixture with more 2-chloropropane than 1-chloropropane
  • Equal amounts of 1- and 2-chloropropane

Correct Answer: Mixture with more 2-chloropropane than 1-chloropropane

Q27. Which is true about the rate-determining step in bromination?

  • Initiation is rate-determining
  • Termnation is rate-determining
  • Hydrogen abstraction (propagation) is rate-determining
  • Combination of radicals is rate-determining

Correct Answer: Hydrogen abstraction (propagation) is rate-determining

Q28. Radical inhibitors such as oxygen affect halogenation by:

  • Enhancing radical formation
  • Quenching radicals and slowing reaction
  • Converting radicals to ionic species
  • Acting as chain carriers

Correct Answer: Quenching radicals and slowing reaction

Q29. In the presence of peroxides, addition of HBr to alkenes follows which rule?

  • Markovnikov addition
  • Anti-Markovnikov addition due to radical mechanism
  • Stereospecific syn addition
  • No addition occurs

Correct Answer: Anti-Markovnikov addition due to radical mechanism

Q30. Which product forms from radical bromination of toluene at benzylic position using NBS?

  • Bromination at aromatic ring only
  • Benzylic bromide (benzyl bromide)
  • No reaction under radical conditions
  • Oxidation to benzaldehyde

Correct Answer: Benzylic bromide (benzyl bromide)

Q31. What is the effect of a strong electron-withdrawing group near a C–H bond on radical halogenation at that site?

  • Increases radical stability and reactivity
  • Decreases radical stability and reactivity at that site
  • No effect
  • Causes immediate elimination

Correct Answer: Decreases radical stability and reactivity at that site

Q32. In which case is chlorination preferred over bromination in synthesis?

  • When high regioselectivity is required
  • When a fast, inexpensive reagent is needed despite lower selectivity
  • For exclusive tertiary substitution only
  • When allylic selectivity is essential

Correct Answer: When a fast, inexpensive reagent is needed despite lower selectivity

Q33. Which of the following best describes the energy profile of bromination H-abstraction?

  • Highly exothermic with early transition state
  • Endothermic with late transition state
  • Barrierless
  • Impossible under normal conditions

Correct Answer: Endothermic with late transition state

Q34. Which laboratory safety concern is most important during radical halogenation?

  • Handling flammable solvents and toxic halogen gases
  • Avoiding oxidizing agents entirely
  • Keeping reactions below 0°C
  • Using only aqueous media

Correct Answer: Handling flammable solvents and toxic halogen gases

Q35. Which reagent selectively provides chlorination at allylic sites under radical conditions?

  • Cl2 in water
  • NCS with radical initiator
  • SOCl2
  • Conc. HCl

Correct Answer: NCS with radical initiator

Q36. What role does solvent play in radical halogenation of alkanes?

  • Often inert; can influence radical lifetime and cage effects
  • Always nucleophilic and participates in mechanism
  • Determines product via acid catalysis
  • No solvent is ever used

Correct Answer: Often inert; can influence radical lifetime and cage effects

Q37. Which term describes a sequence of radical steps that regenerates a radical species?

  • Termination
  • Initiation
  • Propagation
  • Isomerization

Correct Answer: Propagation

Q38. What is the expected product when methane is exposed to excess Cl2 and hv?

  • Only chloromethane forms
  • Mixture of chloromethane, dichloromethane, chloroform and carbon tetrachloride
  • Methanol forms
  • No chlorination occurs

Correct Answer: Mixture of chloromethane, dichloromethane, chloroform and carbon tetrachloride

Q39. Which observation supports a radical mechanism for halogenation?

  • Reaction is inhibited by radical scavengers like O2 or TEMPO
  • Reaction rate is independent of light
  • Products are ionic salts
  • Only electrophilic substitution products observed

Correct Answer: Reaction is inhibited by radical scavengers like O2 or TEMPO

Q40. During propagation, which reaction regenerates the halogen radical?

  • Alkyl radical abstracts a hydrogen atom
  • Alkyl radical reacts with halogen molecule to give alkyl halide and halogen radical
  • Two radicals combine to give a stable molecule
  • Halogen radical recombines with another halogen radical

Correct Answer: Alkyl radical reacts with halogen molecule to give alkyl halide and halogen radical

Q41. Which statement about selectivity in chlorination is true?

  • Chlorination shows extremely high selectivity similar to bromination
  • Chlorination is less selective and more statistically controlled than bromination
  • Chlorination only gives tertiary products
  • Chlorination does not depend on radical stability

Correct Answer: Chlorination is less selective and more statistically controlled than bromination

Q42. For a substrate with equivalent hydrogens, what determines product distribution in radical halogenation?

  • Electronic differences only
  • Statistical number of equivalent hydrogens and radical stabilities
  • Only solvent polarity
  • Only the initiator used

Correct Answer: Statistical number of equivalent hydrogens and radical stabilities

Q43. Which radical species is responsible for hydrogen abstraction in halogenation?

  • Alkyl radical
  • Halogen atom (Cl· or Br·)
  • Proton (H+)
  • Hydride ion (H−)

Correct Answer: Halogen atom (Cl· or Br·)

Q44. What is the effect of branching on C–H bond strength?

  • Branching increases C–H BDE making abstraction harder
  • Branching decreases C–H BDE at tertiary centers making abstraction easier
  • Branching has no influence on BDE
  • Branching always prevents halogenation

Correct Answer: Branching decreases C–H BDE at tertiary centers making abstraction easier

Q45. Which technique can be used to detect radical intermediates experimentally?

  • NMR of stable products only
  • Electron paramagnetic resonance (EPR) spectroscopy
  • Melting point determination
  • IR of solvents only

Correct Answer: Electron paramagnetic resonance (EPR) spectroscopy

Q46. In pharmaceutical synthesis, why must halogenation of alkanes be controlled?

  • To avoid formation of toxic halogenated impurities and over-halogenation
  • Because halogenation never affects biological activity
  • There is no regulatory concern
  • Halogenation is always 100% selective

Correct Answer: To avoid formation of toxic halogenated impurities and over-halogenation

Q47. Which process competes with hydrogen abstraction in radical reactions and can reduce yield?

  • Radical recombination (termination)
  • Only ionic reactions
  • Direct substitution without radicals
  • Hydrogenation by H2 gas

Correct Answer: Radical recombination (termination)

Q48. Which reagent is used for benzylic bromination under radical conditions with high selectivity?

  • Br2 with hv only
  • N-bromosuccinimide (NBS) in presence of radical initiator
  • HBr in aqueous solution
  • SOBr2

Correct Answer: N-bromosuccinimide (NBS) in presence of radical initiator

Q49. Which statement best describes cage effect in radical halogenation?

  • Radicals are instantly trapped by solvent cage leading to recombination before escaping
  • It refers to ionic pairing in solution
  • It enhances product distribution evenly
  • Irrelevant to radical reactions

Correct Answer: Radicals are instantly trapped by solvent cage leading to recombination before escaping

Q50. For selective monohalogenation of a complex alkane, which strategy is most useful?

  • Use excess halogen and long irradiation
  • Employ radical initiator without control
  • Choose a selective reagent (e.g., NBS for benzylic/allylic) and optimize stoichiometry and light
  • Always use fluorine for selectivity

Correct Answer: Choose a selective reagent (e.g., NBS for benzylic/allylic) and optimize stoichiometry and light

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