Organic Intermediates: Free radicals formation, stability and applications MCQs With Answer

Introduction:
Organic Intermediates: Free radicals formation, stability and applications MCQs With Answer is designed to help M.Pharm students master radical chemistry concepts relevant to advanced organic processes and pharmaceutical applications. This set focuses on mechanisms of radical generation, factors governing radical stability (hyperconjugation, resonance, captodative effects, inductive influences), kinetics (rate constants, radical clocks), detection (ESR/EPR), inhibition and trapping, and practical uses in polymerization, drug synthesis and radiolysis. The MCQs include reasoning-based choices and concise answers to reinforce learning, prepare for exams in MPC 102T Advanced Organic Chemistry I, and build problem-solving skills required in drug development and analytical interpretation of radical-mediated transformations.

Q1. Which factor most increases the thermodynamic stability of an alkyl radical?

• Inductive withdrawal by electronegative substituents
• Hyperconjugation
• Steric hindrance
• Solvent polarity alone

Correct Answer: Hyperconjugation

Q2. Among the following C–H bonds, which typically has the lowest homolytic bond dissociation energy (BDE)?

• Primary C–H bond
• Secondary C–H bond
• Tertiary C–H bond
• Methyl C–H bond

Correct Answer: Tertiary C–H bond

Q3. Which condition most directly promotes homolytic bond cleavage to form radicals?

• Presence of a radical initiator (peroxides, heat or light)
• High concentrations of nucleophiles
• Strong Bronsted acidity
• Polar protic solvent without energy input

Correct Answer: Presence of a radical initiator (peroxides, heat or light)

Q4. The captodative effect describes radical stabilization by:

• An electronegative substituent alone
• A donating substituent alone
• Simultaneous presence of both electron-donating and electron-withdrawing groups
• Steric protection only

Correct Answer: Simultaneous presence of both electron-donating and electron-withdrawing groups

Q5. Which of the following radicals is generally the most stabilized by resonance?

• Allyl radical (CH2=CH–CH2•)
• Benzyl radical (PhCH2•)
• tert-Butyl radical ((CH3)3C•)
• Phenyl radical (Ph•)

Correct Answer: Benzyl radical (PhCH2•)

Q6. Regarding radical halogenation of alkanes, which statement is correct about chlorination versus bromination?

• Chlorination is more selective than bromination
• Bromination is more selective and favors formation of the most stable radical
• Both give identical regioselectivity
• Neither proceeds via a radical chain mechanism

Correct Answer: Bromination is more selective and favors formation of the most stable radical

Q7. The persistent radical effect (PRE) in radical chemistry is best described as:

• The rapid dimerization of both reactive and persistent radicals equally
• Accumulation of a long-lived (persistent) radical that selectively reacts with transient radicals, controlling product distribution
• A phenomenon where radicals are immediately quenched by oxygen
• A method to generate radicals photochemically only

Correct Answer: Accumulation of a long-lived (persistent) radical that selectively reacts with transient radicals, controlling product distribution

Q8. Which experimental technique is most commonly used to directly detect and characterize organic free radicals?

• Infrared (IR) spectroscopy
• Nuclear magnetic resonance (NMR) spectroscopy
• Electron spin resonance (ESR/EPR) spectroscopy
• Ultraviolet-visible (UV-Vis) spectroscopy

Correct Answer: Electron spin resonance (ESR/EPR) spectroscopy

Q9. A common thermal radical initiator used in free radical polymerizations is:

• Benzoyl chloride
• Azobisisobutyronitrile (AIBN)
• Sodium azide
• Hydrochloric acid

Correct Answer: Azobisisobutyronitrile (AIBN)

Q10. The “radical clock” method is primarily used to determine which of the following in radical chemistry?

• Absolute reaction enthalpies
• Rate constants or lifetimes of radical intermediates
• The product stereochemistry only
• Bond dissociation energies directly

Correct Answer: Rate constants or lifetimes of radical intermediates

Q11. Which statement about captodative-stabilized radicals is correct?

• They are destabilized relative to unsubstituted radicals because of steric strain
• The combination of donor and acceptor substituents lowers the SOMO energy and increases stabilization
• Only the donor substituent contributes significantly to stabilization
• They are only relevant in ionic reaction mechanisms

Correct Answer: The combination of donor and acceptor substituents lowers the SOMO energy and increases stabilization

Q12. Which of the following is not classified as a termination step in radical polymerization?

• Combination of two radical chain ends
• Disproportionation between two radical chain ends
• Chain transfer to monomer
• Reaction of two radicals to form a single dead molecule

Correct Answer: Chain transfer to monomer

Q13. Primary “chain-breaking” antioxidants like hindered phenols stabilize radical chains mainly by:

• Chelating metal ions
• Donating a hydrogen atom to a radical (H-atom donation)
• Increasing solvent polarity
• Forming stable carbocations

Correct Answer: Donating a hydrogen atom to a radical (H-atom donation)

Q14. Photochemical Norrish Type I reactions in carbonyl compounds produce radicals by which event?

• Alpha cleavage of the carbonyl (homolytic C–C bond cleavage adjacent to C=O)
• Gamma-hydrogen abstraction only
• Formation of carbocations via heterolysis
• Electrocyclic ring closure

Correct Answer: Alpha cleavage of the carbonyl (homolytic C–C bond cleavage adjacent to C=O)

Q15. Thermal decomposition of AIBN as a radical initiator produces which primary species?

• Nitrogen gas and two cyanoisopropyl radicals
• Oxygen and benzoyl radicals
• Hydrogen gas and methyl radicals
• Chlorine gas and alkyl radicals

Correct Answer: Nitrogen gas and two cyanoisopropyl radicals

Q16. Which C–H bond is least susceptible to hydrogen abstraction under typical radical chain conditions?

• Allylic C–H bond
• Benzylic C–H bond
• Aromatic (aryl) C–H bond
• Tertiary alkyl C–H bond

Correct Answer: Aromatic (aryl) C–H bond

Q17. The peroxide (Kharasch) effect in HBr addition to alkenes gives which regiochemistry and why?

• Markovnikov addition due to ionic mechanism
• Anti-Markovnikov addition because radical addition leads to the most stable radical intermediate
• Syn addition only due to concerted pathway
• No addition occurs in presence of peroxides

Correct Answer: Anti-Markovnikov addition because radical addition leads to the most stable radical intermediate

Q18. Which substituent trend generally decreases the stability of a carbon-centered radical?

• Conjugation with an adjacent pi-system
• Electron-donating alkyl groups (hyperconjugation)
• Strong electron-withdrawing substituents by induction (e.g., –NO2)
• Resonance donation into the radical center

Correct Answer: Strong electron-withdrawing substituents by induction (e.g., –NO2)

Q19. Which of the following is widely used as a radical-trapping agent (spin trap) in mechanistic studies?

• TEMPO (2,2,6,6-Tetramethylpiperidin-1-oxyl)
• Sodium chloride
• Hydrochloric acid
• Triphenylphosphine

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

Q20. In the radiolysis of water, which primary radical species is the strongest oxidizing agent?

• Hydrated electron (e–(aq))
• Hydrogen atom (H•)
• Hydroxyl radical (•OH)
• Perhydroxyl radical (•OOH)

Correct Answer: Hydroxyl radical (•OH)

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