Free radical addition reactions of alkenes MCQs With Answer

Free radical addition reactions of alkenes MCQs With Answer provide B. Pharm students a focused review of radical chemistry relevant to pharmaceutical synthesis and drug stability. This introduction explains key concepts: radical initiation, propagation and termination steps, common initiators (peroxides, AIBN), peroxide-induced anti-Markovnikov HBr addition, radical stability, selectivity (bromine vs chlorine), inhibitors (O2, TEMPO), and relevance to polymerization and allylic functionalization. Understanding mechanisms, regioselectivity and practical factors (solvent, temperature, radical traps) helps predict reaction outcomes in medicinal chemistry and formulation. Clear practice through targeted multiple-choice questions strengthens problem-solving and exam readiness. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the first step in a free radical addition reaction?

• Propagation
• Termination
• Initiation
• Rearrangement

Correct Answer: Initiation

Q2. Which initiator is commonly used to generate radicals thermally in laboratory reactions?

• Benzoyl peroxide
• Sodium borohydride
• Hydrochloric acid
• Pyridinium chlorochromate

Correct Answer: Benzoyl peroxide

Q3. In radical chain reactions, which step typically involves radical species adding to the alkene?

• Initiation
• Propagation
• Termination
• Isomerization

Correct Answer: Propagation

Q4. Which statement best describes the peroxide effect (Kharasch effect) in HBr addition to alkenes?

• Peroxides promote Markovnikov addition of HBr
• Peroxides cause anti-Markovnikov addition of HBr
• Peroxides inhibit HBr addition entirely
• Peroxides convert HBr into Br2

Correct Answer: Peroxides cause anti-Markovnikov addition of HBr

Q5. Which radical is most stable?

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

Correct Answer: Tertiary alkyl radical

Q6. Which of the following acts as a radical inhibitor in many reactions?

• Hydrogen peroxide
• Oxygen (O2)
• Sodium metal
• Aluminum chloride

Correct Answer: Oxygen (O2)

Q7. What type of termination yields a single product from coupling of two radicals?

• Disproportionation
• Combination (coupling)
• Chain transfer
• Initiation

Correct Answer: Combination (coupling)

Q8. In radical halogenation, which halogen is most reactive but least selective?

• Bromine
• Chlorine
• Iodine
• Fluorine

Correct Answer: Fluorine

Q9. Why is bromine often used in radical additions for selective substitution?

• Because bromine radicals are extremely reactive and nonselective
• Because bromine radicals are less reactive but more selective for stable radicals
• Because bromine forms ionic intermediates only
• Because bromine cannot form radicals

Correct Answer: Because bromine radicals are less reactive but more selective for stable radicals

Q10. Which intermediate is formed when a radical adds to an alkene?

• Carbocation
• Carbanion
• Carbon-centered radical
• Carbene

Correct Answer: Carbon-centered radical

Q11. During propagation, after the alkyl radical forms by addition to the alkene, what is the next typical step in hydrohalogenation?

• Reaction with solvent to form carbocation
• Abstraction of a hydrogen atom from HBr
• Direct elimination to regenerate alkene
• Spontaneous decomposition to smaller fragments

Correct Answer: Abstraction of a hydrogen atom from HBr

Q12. Which reagent is often used for allylic bromination via a radical mechanism?

• Br2 in dark
• N-Bromosuccinimide (NBS) with peroxide or light
• HBr with peroxide only
• Sodium bromide in water

Correct Answer: N-Bromosuccinimide (NBS) with peroxide or light

Q13. What is the usual stereochemical outcome when radicals add to monosubstituted alkenes in absence of stereocontrol?

• Enantioselective retention
• Formation of a single stereoisomer
• Racemic or a mixture of stereoisomers
• Only trans product forms

Correct Answer: Racemic or a mixture of stereoisomers

Q14. Which mechanism explains why tertiary radicals are formed preferentially in hydrogen abstraction?

• Polar effects only
• Hyperconjugation and greater radical stabilization
• Solvent stabilization of primary radicals
• Steric hindrance prevents tertiary radical formation

Correct Answer: Hyperconjugation and greater radical stabilization

Q15. In radical polymerization of styrene, what initiates the chain growth?

• Carbocation formation
• Radical generation from an initiator like AIBN
• Nucleophilic attack by base
• Condensation reaction

Correct Answer: Radical generation from an initiator like AIBN

Q16. Which of the following best describes chain transfer in radical polymerization?

• Termination by combination
• Transfer of radical activity to another molecule, limiting chain length
• Initiation of radical by light
• Conversion of radical to carbocation

Correct Answer: Transfer of radical activity to another molecule, limiting chain length

Q17. Which reagent is commonly used to trap and detect radical intermediates experimentally?

• TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)
• Sodium hydroxide
• Pyridine
• Sodium sulfate

Correct Answer: TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)

Q18. What is the role of light in free radical addition reactions?

• To quench radicals
• To generate radicals by homolytic cleavage of initiators
• To protonate alkenes
• To form carbocations

Correct Answer: To generate radicals by homolytic cleavage of initiators

Q19. Which process is NOT a typical termination pathway in radical chain reactions?

• Combination of two radicals
• Disproportionation between two radicals
• Chain transfer to solvent
• Oxidative addition to metal that regenerates radicals

Correct Answer: Oxidative addition to metal that regenerates radicals

Q20. In anti-Markovnikov addition of HBr (peroxide present), which species adds first to the double bond?

• Proton (H+)
• Bromide anion (Br-)
• Bromine radical (Br•)
• Hydride (H-)

Correct Answer: Bromine radical (Br•)

Q21. Which factor increases the rate of radical addition to alkenes?

• Electron-poor alkene and nucleophilic radical
• Electron-rich alkene and electrophilic radical
• No radical present
• Low temperature only

Correct Answer: Electron-rich alkene and electrophilic radical

Q22. Which radical species is resonance-stabilized?

• Primary alkyl radical without conjugation
• Allylic radical
• Ethyl radical
• Methyl radical

Correct Answer: Allylic radical

Q23. What is the approximate order of radical stability?

• Methyl > primary > secondary > tertiary
• Primary > methyl > tertiary > secondary
• Tertiary > secondary > primary > methyl
• Secondary > tertiary > primary > methyl

Correct Answer: Tertiary > secondary > primary > methyl

Q24. Which reagent combination would favor formation of a radical at an allylic position?

• NBS and radical initiator
• HCl and zinc
• PCC oxidation
• NaBH4 reduction

Correct Answer: NBS and radical initiator

Q25. Atom transfer radical addition (ATRA) involves which transition?

• Polar ionic addition only
• Reversible transfer of a halogen atom between radical and substrate
• Hydride transfer between two radicals
• Formation of carbocations exclusively

Correct Answer: Reversible transfer of a halogen atom between radical and substrate

Q26. In radical bromination of alkanes, which hydrogen is preferentially abstracted?

• Least substituted hydrogen
• Most substituted hydrogen (leading to most stable radical)
• Only benzylic hydrogens
• Hydrogens bonded to heteroatoms

Correct Answer: Most substituted hydrogen (leading to most stable radical)

Q27. What does a high chain length (number of propagation cycles per initiation) indicate in radical polymerization?

• Many terminations occur
• Efficient propagation with long polymer chains
• Poor initiation efficiency
• Rapid decomposition of monomer

Correct Answer: Efficient propagation with long polymer chains

Q28. Radical addition to alkenes often proceeds with which regioselectivity principle when radicals are stabilized?

• Formation of the least stable radical intermediate
• Formation of the more stable radical intermediate
• Always Markovnikov product
• Regioselectivity is random

Correct Answer: Formation of the more stable radical intermediate

Q29. Which experimental condition would suppress radical reactions?

• Adding radical initiator
• Excluding oxygen and performing at high temperature
• Adding radical scavenger like TEMPO
• Irradiation with UV light

Correct Answer: Adding radical scavenger like TEMPO

Q30. What is the effect of polar substituents on radical addition rates according to the polar effect?

• Electron-withdrawing substituents always increase rate
• Electron-donating substituents accelerate addition of electrophilic radicals
• Substituents have no effect
• Only steric effects matter

Correct Answer: Electron-donating substituents accelerate addition of electrophilic radicals

Q31. Which compound is a common thermal radical initiator that decomposes to give nitrogen gas?

• Hydrogen peroxide
• AIBN (azobisisobutyronitrile)
• Benzene
• Sodium azide

Correct Answer: AIBN (azobisisobutyronitrile)

Q32. How does solvent polarity affect radical addition to alkenes?

• Polar solvents always stop radical reactions
• Solvent polarity can influence rates via stabilization of polarizable transition states
• Only temperature matters, not solvent
• Nonpolar solvents convert radicals to ions

Correct Answer: Solvent polarity can influence rates via stabilization of polarizable transition states

Q33. Which technique can be used to detect radical intermediates directly in a reaction?

• NMR only
• Electron paramagnetic resonance (EPR) spectroscopy
• IR spectroscopy only
• Mass spectrometry without trapping

Correct Answer: Electron paramagnetic resonance (EPR) spectroscopy

Q34. What is disproportionation as a radical termination pathway?

• Two radicals combine to give one product
• One radical abstracts a hydrogen from another radical, yielding an alkene and a saturated product
• Radical reacts with solvent to form polymer
• Radical undergoes homolytic cleavage

Correct Answer: One radical abstracts a hydrogen from another radical, yielding an alkene and a saturated product

Q35. In radical hydrohalogenation with HBr under peroxide, which carbon receives hydrogen?

• The more substituted carbon
• The less substituted carbon
• Hydrogen attaches randomly
• Only terminal carbons receive hydrogen

Correct Answer: The more substituted carbon

Q36. Why is HCl not typically subject to anti-Markovnikov addition via peroxides like HBr is?

• Because Cl• is not a radical
• Because hydrogen abstraction by Cl• is too unfavorable and chlorine radical reactivity differs
• Because HCl forms stable carbocations only
• Because peroxides cannot decompose in presence of HCl

Correct Answer: Because hydrogen abstraction by Cl• is too unfavorable and chlorine radical reactivity differs

Q37. What role do antioxidants play in pharmaceutical formulations regarding radical reactions?

• They promote radical polymerization
• They scavenge radicals to prevent oxidative degradation
• They act as radical initiators
• They replace active pharmaceutical ingredients

Correct Answer: They scavenge radicals to prevent oxidative degradation

Q38. Which of the following best describes a radical clock experiment?

• Measuring ionization potential of radicals
• Using a substrate that undergoes a known intramolecular radical rearrangement to estimate radical lifetimes
• Timing how fast a radical reacts with oxygen
• Heating radicals to measure decomposition time

Correct Answer: Using a substrate that undergoes a known intramolecular radical rearrangement to estimate radical lifetimes

Q39. Which outcome indicates radical chain reaction propagation is efficient?

• High initiation rate but no product
• Long chain length and high product yield per initiator molecule
• Immediate termination after initiation
• Complete inhibition by trace oxygen only

Correct Answer: Long chain length and high product yield per initiator molecule

Q40. Which species commonly abstracts a hydrogen atom during propagation in radical hydrohalogenation?

• Alkene
• Halogen radical (e.g., Br•)
• Hydride ion
• Carbocation

Correct Answer: Halogen radical (e.g., Br•)

Q41. In radical additions, what influences regioselectivity most strongly?

• Radical stability of the intermediate and polar effects
• Only temperature
• Presence of acid catalysts
• Absolute absence of substituents

Correct Answer: Radical stability of the intermediate and polar effects

Q42. Why are iodination reactions seldom used for radical addition to alkenes?

• Iodine radicals are too reactive and unselective
• Iodine forms stable radicals that rarely propagate addition efficiently
• Iodine cannot form radicals under any conditions
• Iodine gives only Markovnikov products

Correct Answer: Iodine forms stable radicals that rarely propagate addition efficiently

Q43. Which of the following best describes polar effects in radical reactions?

• Radicals are unaffected by electronic character of substrates
• Electrophilic radicals add faster to electron-rich alkenes, and nucleophilic radicals add faster to electron-poor alkenes
• Polar effects only apply to ionic reactions
• Polar effects reverse radical stability order

Correct Answer: Electrophilic radicals add faster to electron-rich alkenes, and nucleophilic radicals add faster to electron-poor alkenes

Q44. What is the main difference between ionic and radical addition of HBr to alkenes?

• Ionic addition always gives anti-Markovnikov product
• Radical addition (with peroxides) gives anti-Markovnikov while ionic addition gives Markovnikov
• Both give identical products under all conditions
• Ionic addition requires radical initiators

Correct Answer: Radical addition (with peroxides) gives anti-Markovnikov while ionic addition gives Markovnikov

Q45. Which experimental change would most likely increase selectivity in a radical bromination?

• Using higher temperatures and excess light
• Switching from bromine to chlorine
• Using dilute bromine and lower temperature
• Adding strong acid

Correct Answer: Using dilute bromine and lower temperature

Q46. In pharmaceutical synthesis, why is control of radical reactions important?

• Because radicals always give a single predictable product
• To prevent undesired degradation and to achieve selective functionalization
• Radical control is not relevant to pharmaceuticals
• Radicals only affect polymer excipients, not drugs

Correct Answer: To prevent undesired degradation and to achieve selective functionalization

Q47. Which step is generally rate-determining in radical chain reactions?

• Propagation is always rate-determining
• Initiation or the slowest propagation step can be rate-determining
• Termination is always rate-determining
• There is no rate-determining step in chain reactions

Correct Answer: Initiation or the slowest propagation step can be rate-determining

Q48. What happens when oxygen is present during a radical addition reaction?

• Oxygen acts as a radical scavenger, often inhibiting the reaction
• Oxygen promotes chain propagation always
• Oxygen converts radicals into carbocations
• Oxygen has no effect on radicals

Correct Answer: Oxygen acts as a radical scavenger, often inhibiting the reaction

Q49. Which structural feature in an alkene would favor addition of electrophilic radicals?

• Electron-withdrawing substituents on the double bond
• Electron-donating substituents on the double bond
• Substitution with halogens only
• Only terminal alkynes favor electrophilic radicals

Correct Answer: Electron-donating substituents on the double bond

Q50. For a B. Pharm student, why is understanding free radical addition to alkenes clinically relevant?

• It has no clinical relevance
• It helps predict drug metabolism, degradation, and targeted synthesis of pharmacophores
• It only matters for inorganic chemistry
• Because radicals always sterilize formulations

Correct Answer: It helps predict drug metabolism, degradation, and targeted synthesis of pharmacophores

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com