Free radical addition in conjugated dienes MCQs With Answer

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Free radical addition in conjugated dienes MCQs With Answer offers B. Pharm students a focused review of radical mechanisms, regioselectivity, and reaction control in 1,3-dienes. This introduction covers key concepts such as initiation, propagation and termination steps, peroxide effect, allylic radical stabilization, and the kinetic (1,2) vs thermodynamic (1,4) product distribution. Understanding these principles helps predict product ratios, reaction conditions, and common inhibitors or initiators used in radical chemistry. Targeted practice with multiple-choice questions reinforces mechanism-based reasoning essential for medicinal chemistry and organic synthesis topics in the B.Pharm curriculum. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. In the free radical addition of HBr to a conjugated diene, which intermediate is primarily responsible for the formation of both 1,2- and 1,4-products?

  • Carbocation intermediate
  • Allylic radical intermediate
  • Diradical intermediate
  • Carbanion intermediate

Correct Answer: Allylic radical intermediate

Q2. Which step in a typical radical chain reaction is mainly responsible for product formation and radical propagation?

  • Initiation
  • Propagation
  • Termination
  • Isomerization

Correct Answer: Propagation

Q3. In peroxide-initiated addition of HBr to 1,3-butadiene at low temperature, which product predominates?

  • 1,2-addition product (kinetic)
  • 1,4-addition product (thermodynamic)
  • Only polymerization occurs
  • No reaction occurs

Correct Answer: 1,2-addition product (kinetic)

Q4. Which reagent is commonly used to generate radicals in free radical addition reactions?

  • Peroxides (ROOR)
  • Strong acids (H2SO4)
  • Nucleophilic catalysts (NaOH)
  • Hydrogen gas (H2) without catalyst

Correct Answer: Peroxides (ROOR)

Q5. What effect does increasing temperature generally have on product distribution for addition to conjugated dienes?

  • Favors kinetic product exclusively
  • Favors thermodynamic product (1,4)
  • Prevents any addition
  • Only increases the rate without changing selectivity

Correct Answer: Favors thermodynamic product (1,4)

Q6. Which radical species initiates chain formation by abstracting hydrogen from HBr in peroxide-initiated reactions?

  • Bromine radical (Br•)
  • Alkoxy radical (RO•)
  • Hydroxyl radical (HO•)
  • Alkyl radical (R•)

Correct Answer: Alkoxy radical (RO•)

Q7. Which of the following best explains why allylic radicals lead to both 1,2 and 1,4 addition?

  • Allylic radicals are chiral
  • Allylic radicals are resonance-stabilized with two equivalent resonance forms
  • Allylic radicals are highly unstable and fragment
  • Allylic radicals cannot abstract hydrogen

Correct Answer: Allylic radicals are resonance-stabilized with two equivalent resonance forms

Q8. Which condition would most strongly inhibit a free radical addition to a conjugated diene?

  • Addition of a radical initiator
  • Presence of oxygen or radical scavengers like TEMPO
  • Lowering temperature slightly
  • Using a polar solvent

Correct Answer: Presence of oxygen or radical scavengers like TEMPO

Q9. In the radical chain mechanism of HBr addition, what is the role of Br• after it is generated?

  • It abstracts hydrogen from HBr to form H•
  • It adds to the alkene to form a carbon-centered radical
  • It acts as a Lewis acid to activate HBr
  • It terminates the chain by dimerization only

Correct Answer: It adds to the alkene to form a carbon-centered radical

Q10. Which product is labeled “thermodynamic” in conjugated diene addition reactions?

  • Product formed fastest at low temperature
  • Product with lower overall free energy (more stable)
  • Product formed only in the absence of radical initiators
  • Product that immediately decomposes

Correct Answer: Product with lower overall free energy (more stable)

Q11. Which experimental observation indicates kinetic control in a diene addition?

  • Product ratio changes with temperature to favor more stable product at high temp
  • Same product ratio regardless of temperature
  • Major product forms rapidly at low temperature
  • Reaction stops when initiator is removed

Correct Answer: Major product forms rapidly at low temperature

Q12. Which of the following best describes the termination step in radical chain reactions?

  • Formation of a radical species from initiator decomposition
  • Chain propagation by transfer of an atom
  • Combination or disproportionation of two radicals to give stable molecules
  • Resonance stabilization of radical intermediates

Correct Answer: Combination or disproportionation of two radicals to give stable molecules

Q13. In free radical addition to conjugated dienes, why does 1,4-addition often increase at equilibrium?

  • Because of irreversible polymerization
  • Because 1,4-product is usually more stable due to internal substitution or conjugation
  • Because radicals cannot form at higher temperature
  • Because 1,2-product immediately undergoes rearrangement to 1,4

Correct Answer: Because 1,4-product is usually more stable due to internal substitution or conjugation

Q14. Which experimental reagent would you add to favor anti‑Markovnikov addition in simple alkenes via a radical pathway?

  • A strong acid like H2SO4
  • Peroxides (ROOR)
  • Palladium catalyst
  • Lewis base such as pyridine

Correct Answer: Peroxides (ROOR)

Q15. For resonance-stabilized allylic radicals, which carbon positions can bond formation occur at to give different constitutional isomers?

  • Only the terminal carbon
  • Only the central carbon
  • Either terminal or internal carbon due to delocalization
  • Only via hydrogen abstraction, not addition

Correct Answer: Either terminal or internal carbon due to delocalization

Q16. Which factor does NOT significantly influence the 1,2 vs 1,4 product ratio in conjugated diene radical addition?

  • Temperature
  • Nature of radical initiator
  • Solvent polarity in most radical cases
  • Resonance stabilization of intermediate radical

Correct Answer: Solvent polarity in most radical cases

Q17. Which of the following statements about allylic radical stabilization is correct?

  • Allylic radicals are destabilized compared to primary radicals
  • Allylic radicals are stabilized by resonance delocalization of the unpaired electron
  • Allylic radicals cannot form in conjugated systems
  • Allylic radicals always lead to epoxide formation

Correct Answer: Allylic radicals are stabilized by resonance delocalization of the unpaired electron

Q18. Which termination pathway would produce a single non-radical molecule by radical combination?

  • Disproportionation
  • Reversible addition
  • Combination of two radicals
  • Hydrogen abstraction from solvent only

Correct Answer: Combination of two radicals

Q19. Which radical initiator is commonly used in laboratory radical reactions alongside peroxides?

  • AIBN (azobisisobutyronitrile)
  • Lewis acid BF3
  • Sodium metal
  • Hydrochloric acid

Correct Answer: AIBN (azobisisobutyronitrile)

Q20. In the radical chain mechanism for HBr addition, which propagation step regenerates Br•?

  • RO• + HBr → ROH + Br•
  • Alkyl radical + HBr → alkyl-H + Br•
  • Br• + alkene → bromoalkyl radical
  • Two bromoalkyl radicals combine

Correct Answer: Alkyl radical + HBr → alkyl-H + Br•

Q21. Which statement is true about stereochemistry when radicals add to alkenes?

  • Radical addition is always stereospecific giving only syn products
  • Radical intermediates are planar, often leading to racemic or non-stereospecific outcomes
  • Radical addition always gives enantiomerically pure products
  • Radical addition cannot produce chiral centers

Correct Answer: Radical intermediates are planar, often leading to racemic or non-stereospecific outcomes

Q22. Which of the following best describes the energy profile for formation of a resonance-stabilized allylic radical?

  • Very high activation energy, endergonic formation
  • Lower activation energy than for non-stabilized radicals, making formation easier
  • No activation energy required
  • Formation requires rearrangement by ionic intermediates only

Correct Answer: Lower activation energy than for non-stabilized radicals, making formation easier

Q23. How does the presence of electron-withdrawing substituents on a diene influence radical addition?

  • They always speed up radical formation
  • They stabilize adjacent radical centers less, altering regioselectivity
  • They convert radical mechanism to ionic mechanism
  • They have no effect on radical stability or regioselectivity

Correct Answer: They stabilize adjacent radical centers less, altering regioselectivity

Q24. In mechanistic terms, why is peroxide required to observe anti-Markovnikov HBr addition to simple alkenes?

  • Peroxide acts as a Lewis acid to flip regiochemistry
  • Peroxide generates radicals (RO•) that start a radical chain leading to anti-Markovnikov pathway
  • Peroxide protonates the double bond
  • Peroxide deactivates HBr making it nucleophilic

Correct Answer: Peroxide generates radicals (RO•) that start a radical chain leading to anti-Markovnikov pathway

Q25. Which observation supports a radical rather than ionic mechanism in diene addition?

  • Reaction rate increases with added nucleophile
  • Reaction is inhibited by oxygen or radical traps
  • Strong acids accelerate the reaction dramatically
  • Only one regioisomer is observed regardless of conditions

Correct Answer: Reaction is inhibited by oxygen or radical traps

Q26. Which of these is a likely fate of a carbon-centered radical in a propagation step?

  • Loss of electrons to form cation instantly
  • Abstraction of hydrogen from HBr to form product and regenerate Br•
  • Immediately rearranges to an alkene without reaction
  • Becomes permanently stabilized and stops chain

Correct Answer: Abstraction of hydrogen from HBr to form product and regenerate Br•

Q27. Why are benzyl radicals usually more stabilized than simple alkyl radicals?

  • Because they are sp-hybridized
  • Because of resonance delocalization into the aromatic ring
  • Because they cannot undergo termination
  • Because they are always protonated

Correct Answer: Because of resonance delocalization into the aromatic ring

Q28. In free radical addition to conjugated dienes, which product distribution suggests thermodynamic control after equilibration?

  • Higher amount of less substituted, less stable product
  • Dominance of more substituted, more stable product (often 1,4)
  • Equal amounts of all possible products
  • Only polymeric material is present

Correct Answer: Dominance of more substituted, more stable product (often 1,4)

Q29. Which solvent property is least critical when selecting a medium for radical addition reactions?

  • Ability to dissolve initiator
  • Polarity that stabilizes ionic intermediates
  • Low oxygen solubility or degassing options
  • Inertness toward radicals (no H-atom donation)

Correct Answer: Polarity that stabilizes ionic intermediates

Q30. Which reagent would you use as a radical inhibitor in an experimental setup?

  • Peroxide
  • Oxygen or TEMPO
  • Lewis acid
  • Excess alkene

Correct Answer: Oxygen or TEMPO

Q31. During peroxide initiation, which bond in ROOR breaks to form radicals?

  • O–H bond
  • O–O bond
  • R–O bond
  • R–R bond

Correct Answer: O–O bond

Q32. Which experimental change would most likely increase the proportion of 1,4-product in HBr addition to 1,3-butadiene?

  • Lowering the temperature to -78 °C
  • Heating the reaction mixture to higher temperature
  • Removing peroxide initiator
  • Adding radical inhibitors

Correct Answer: Heating the reaction mixture to higher temperature

Q33. Which is a characteristic feature of allylic radicals compared to simple alkyl radicals?

  • They cannot participate in chain propagation
  • They have delocalized electron density over multiple carbons
  • They are always less stable than methyl radicals
  • They are not formed in conjugated systems

Correct Answer: They have delocalized electron density over multiple carbons

Q34. What is the usual order of steps in a radical chain mechanism?

  • Propagation → Termination → Initiation
  • Initiation → Propagation → Termination
  • Termination → Initiation → Propagation
  • Single concerted step only

Correct Answer: Initiation → Propagation → Termination

Q35. Which experimental evidence would indicate that product distribution is under thermodynamic control?

  • Different product mixture at different reaction times without temperature change
  • Product ratio shifts toward the more stable isomer upon heating or longer reaction time
  • Product ratio is fixed immediately and never changes
  • No products are formed

Correct Answer: Product ratio shifts toward the more stable isomer upon heating or longer reaction time

Q36. In radical chemistry, what is the effect of a bulky substituent adjacent to a radical center?

  • It always destabilizes the radical by steric hindrance
  • It can slow reaction rates by steric hindrance and influence regioselectivity
  • It converts radical into an ionic species
  • It has no effect on radical reactions

Correct Answer: It can slow reaction rates by steric hindrance and influence regioselectivity

Q37. Which statement about the peroxide effect (Kharasch effect) is correct in the context of HBr addition?

  • Peroxide prevents radical pathways and enforces Markovnikov addition
  • Peroxide promotes radical chain leading to anti-Markovnikov addition for simple alkenes
  • Peroxide is only relevant for polymerization, not addition
  • Peroxide converts HBr into a base

Correct Answer: Peroxide promotes radical chain leading to anti-Markovnikov addition for simple alkenes

Q38. Why might a researcher degas a solvent before performing a radical addition to a conjugated diene?

  • To increase solvent polarity
  • To remove oxygen which can quench radicals and inhibit chain reactions
  • To add more initiator
  • To shift the equilibrium toward ionic mechanisms

Correct Answer: To remove oxygen which can quench radicals and inhibit chain reactions

Q39. For 1,3-butadiene, which resonance form of the allylic radical places the unpaired electron on the terminal carbon?

  • A form with the radical at C2 only
  • A terminal resonance form delocalizing the radical between C1 and C3
  • No resonance forms exist for allylic radicals
  • A form where radical is on an sp-hybridized carbon only

Correct Answer: A terminal resonance form delocalizing the radical between C1 and C3

Q40. Which factor most directly stabilizes an allylic radical?

  • Hyperconjugation only
  • Resonance delocalization across the conjugated system
  • Hydrogen bonding
  • Coordination with metal catalysts exclusively

Correct Answer: Resonance delocalization across the conjugated system

Q41. When a bromine radical adds to the more substituted end of a conjugated diene, what is the immediate type of intermediate formed?

  • Carbocation intermediate
  • Bromo-substituted allylic radical
  • Carbanion intermediate
  • Epoxide intermediate

Correct Answer: Bromo-substituted allylic radical

Q42. How does chain transfer affect radical polymerization compared to simple radical addition?

  • Chain transfer increases molecular weight drastically
  • Chain transfer can terminate one chain and start another, reducing molecular weight and altering product distribution
  • Chain transfer is identical to initiation
  • Chain transfer only occurs in ionic reactions

Correct Answer: Chain transfer can terminate one chain and start another, reducing molecular weight and altering product distribution

Q43. Which measurement would help determine whether 1,2 or 1,4 isomer is formed predominantly in a reaction mixture?

  • NMR spectroscopy to analyze regiochemistry
  • Measuring pH of the solution
  • Measuring only mass of product without structure data
  • Observing color change only

Correct Answer: NMR spectroscopy to analyze regiochemistry

Q44. In the radical addition mechanism, which species is regenerated to sustain the chain after product formation?

  • Initiator molecule unchanged
  • Radical species such as Br• or R•
  • Only stable ions are regenerated
  • Peroxide remains intact and unchanged

Correct Answer: Radical species such as Br• or R•

Q45. Which of these is a likely reason why HCl does not show a peroxide effect like HBr for simple alkenes?

  • HCl forms stronger radicals than HBr
  • H–Cl bond is stronger and less easily abstracted by radicals, so chain cannot propagate effectively
  • Chlorine radicals are more selective
  • HCl is insoluble in organic solvents

Correct Answer: H–Cl bond is stronger and less easily abstracted by radicals, so chain cannot propagate effectively

Q46. Which type of spectroscopy can directly show the presence of radical intermediates in some cases?

  • IR spectroscopy
  • EPR (electron paramagnetic resonance) spectroscopy
  • UV-Vis only
  • Polarimetry

Correct Answer: EPR (electron paramagnetic resonance) spectroscopy

Q47. In an experiment, adding excess HBr to a radical addition of a diene leads to which expected effect?

  • Reduced rate due to product inhibition
  • Increase in propagation efficiency since HBr is the hydrogen donor regenerating Br•
  • Complete suppression of radical formation
  • Immediate polymerization without product formation

Correct Answer: Increase in propagation efficiency since HBr is the hydrogen donor regenerating Br•

Q48. Which outcome is expected if a radical reaction is run under high concentrations of radical inhibitor?

  • Enhanced radical chain length
  • Suppressed reaction or much lower yields of addition products
  • Shift to ionic pathways producing exactly the same products
  • Faster formation of thermodynamic product only

Correct Answer: Suppressed reaction or much lower yields of addition products

Q49. When analyzing mechanism, which computational property helps rationalize the stability of allylic vs simple radicals?

  • Calculated bond angles only
  • Spin density distribution and resonance stabilization energies
  • Dielectric constant of the solvent
  • Optical rotation values

Correct Answer: Spin density distribution and resonance stabilization energies

Q50. For medicinal chemists in B.Pharm, why is understanding free radical addition to conjugated dienes important?

  • It has no relevance to drug synthesis or degradation
  • It helps predict site-selective functionalization, stability of intermediates, and potential degradation pathways of conjugated systems in drug molecules
  • It only matters for polymer science and not pharmaceuticals
  • Because radicals always produce toxic impurities

Correct Answer: It helps predict site-selective functionalization, stability of intermediates, and potential degradation pathways of conjugated systems in drug molecules

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