Photochemical Reactions: photo-oxidation, photo-addition, photo-fragmentation MCQs With Answer
This quiz collection focuses on photochemical reactions — photo-oxidation, photo-addition and photo-fragmentation — tailored for M.Pharm students studying MPC 202T Advanced Organic Chemistry II. Questions emphasize mechanistic aspects (singlet and triplet states, energy and electron transfer), common named photoreactions (Paternò–Büchi, Norrish, Wolff, Barton), roles of photosensitizers and quenchers, and practical detection or outcome of photoprocesses. These MCQs are designed to deepen conceptual understanding required for research and pharmaceutical applications, including oxidative degradation, photostability, and photochemical synthesis, while testing the ability to apply mechanistic reasoning to predict products and intermediates.
Q1. Which reactive oxygen species is most commonly responsible for photo-oxidation of organic substrates under photosensitized conditions?
- Superoxide radical anion (O2•–)
- Hydroxyl radical (•OH)
- Singlet oxygen (1O2)
- Ozone (O3)
Correct Answer: Singlet oxygen (1O2)
Q2. Which compound is a classical photosensitizer for generation of singlet oxygen in solution?
- Benzophenone
- Rose Bengal
- Tetramethylsilane
- AIBN (azobisisobutyronitrile)
Correct Answer: Rose Bengal
Q3. Photochemical [2+2] cycloaddition between two alkenes generally proceeds via which mechanistic pathway?
- Concerted pericyclic ground-state pathway
- Stepwise diradical intermediate
- Polar ionic intermediate
- Carbene insertion into double bond
Correct Answer: Stepwise diradical intermediate
Q4. The Paternò–Büchi reaction (photoaddition of a carbonyl to an alkene) yields which type of heterocyclic product?
- Dioxetane
- Oxetane
- Cyclobutane
Correct Answer: Oxetane
Q5. Norrish Type II photoreaction is characterized by which initial step?
- α-C–C bond homolysis adjacent to carbonyl
- γ-hydrogen abstraction to give a 1,4-biradical
- Concerted [2+2] cycloaddition
- Loss of molecular nitrogen (N2)
Correct Answer: γ-hydrogen abstraction to give a 1,4-biradical
Q6. In a photoinduced electron transfer (PET) where a substrate acts as the electron donor, what species is formed from that substrate?
- Radical anion
- Carbocation
- Radical cation
- Carbene
Correct Answer: Radical cation
Q7. Photochemical α-cleavage of a carbonyl compound (producing an acyl and an alkyl radical) is known as which reaction?
- Norrish Type I
- Norrish Type II
- Wolff rearrangement
- Barton decarboxylation
Correct Answer: Norrish Type I
Q8. Which of the following cycloadditions is symmetry-forbidden thermally but becomes allowed under photochemical excitation?
- Diels–Alder [4+2] cycloaddition
- [2+2] cycloaddition
- SN2 substitution
- Electrophilic aromatic substitution
Correct Answer: [2+2] cycloaddition
Q9. Photooxygenation of an allylic substrate by singlet oxygen typically gives which primary product?
- Allylic hydroperoxide
- Allylic alcohol
- 1,2-diol
- Epoxide
Correct Answer: Allylic hydroperoxide
Q10. Which compound is widely used as a triplet sensitizer to induce hydrogen abstraction from substrates under UV irradiation?
- Rose Bengal
- Benzophenone
- Chlorobenzene
- Pyridine
Correct Answer: Benzophenone
Q11. Photolysis of aryl diazonium salts commonly leads to formation of an aryl radical by extrusion of which small molecule?
- Nitrogen monoxide (NO)
- Carbon monoxide (CO)
- Nitrogen (N2)
- Hydrogen gas (H2)
Correct Answer: Nitrogen (N2)
Q12. In the Paternò–Büchi reaction the initially excited chromophore is typically which functional group?
- Alkene π-bond
- Carbonyl group
- Alcohol hydroxyl
- Nitrile group
Correct Answer: Carbonyl group
Q13. Singlet oxygen can be directly detected by its characteristic phosphorescence at approximately what wavelength?
- 420 nm
- 680 nm
- 1270 nm
- 254 nm
Correct Answer: 1270 nm
Q14. Which of the following most commonly decreases the quantum yield of a photochemical reaction involving a triplet excited state?
- Presence of a good triplet quencher such as molecular oxygen
- Use of a photosensitizer with long triplet lifetime
- Low substrate concentration
- Use of monochromatic light matching absorption maximum
Correct Answer: Presence of a good triplet quencher such as molecular oxygen
Q15. The Barton decarboxylation is a photochemical method to generate radicals from carboxylic acids via which type of intermediate?
- Thiohydroxamic ester (Barton ester)
- Diazo compound
- Nitrate ester
- Acyl chloride
Correct Answer: Thiohydroxamic ester (Barton ester)
Q16. Regioselectivity in the Paternò–Büchi reaction is largely governed by which interaction?
- Steric hindrance only
- Frontier orbital interaction between the excited carbonyl π* orbital and the alkene HOMO
- Solvent polarity exclusively
- Acid–base catalysis at the carbonyl oxygen
Correct Answer: Frontier orbital interaction between the excited carbonyl π* orbital and the alkene HOMO
Q17. Photooxidation of a thioether (sulfide) by singlet oxygen typically yields which product as the primary oxidation state?
- Sulfone
- Sulfoxide
- Sulfonic acid
- Disulfide
Correct Answer: Sulfoxide
Q18. Photochemical [2+2] cycloaddition between two simple alkenes generally shows what stereochemical behaviour with respect to alkene geometry?
- Completely racemized product
- Stereospecific: cis-alkene and trans-alkene give different stereochemical cyclobutanes
- Always gives the same stereochemistry regardless of alkene geometry
- Only gives achiral products
Correct Answer: Stereospecific: cis-alkene and trans-alkene give different stereochemical cyclobutanes
Q19. A good photosensitizer for energy transfer reactions should possess which key property?
- Very short excited-state lifetime
- Long-lived excited triplet state capable of energy transfer
- Strong basicity in the ground state
- High fluorescence quantum yield but no intersystem crossing
Correct Answer: Long-lived excited triplet state capable of energy transfer
Q20. The Wolff rearrangement, often induced photochemically from α-diazoketones, yields which reactive intermediate?
- Carbene that inserts to give esters
- Ketenes
- Epoxides
- Aryl cations
Correct Answer: Ketenes
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