Reactions of polynuclear hydrocarbons MCQs With Answer

Reactions of polynuclear hydrocarbons MCQs With Answer

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Reactions of polynuclear hydrocarbons MCQs With Answer are essential for B. Pharm students studying the chemistry and biological impact of polycyclic aromatic compounds. This introduction covers core concepts such as electrophilic substitution, oxidation, reduction, peri and benzylic reactivity, regioselectivity in naphthalene, anthracene and phenanthrene, and functionalization methods used in drug impurity and metabolism studies. Understanding mechanisms, common reagents (KMnO4, HNO3, H2/Pd, FeBr3) and outcomes (quinones, sulfonic acids, epoxides) helps predict transformations and toxicology. These SEO-focused notes include relevant keywords to boost revision efficiency for exams and practicals. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which position in naphthalene is termed the “alpha” position and is more reactive toward electrophilic substitution?

  • Carbon-1 (adjacent to the fusion)
  • Carbon-2 (adjacent but not at fusion)
  • Carbon-3 (remote from fusion)
  • Carbon-4 (remote ring)

Correct Answer: Carbon-1 (adjacent to the fusion)

Q2. In electrophilic nitration of naphthalene under mild conditions, the major product is nitration at which position?

  • Alpha position (1-nitronaphthalene)
  • Beta position (2-nitronaphthalene)
  • 3-nitronaphthalene
  • 4-nitronaphthalene

Correct Answer: Alpha position (1-nitronaphthalene)

Q3. Anthracene undergoes Diels–Alder type addition predominantly at which positions?

  • 9,10-positions forming 9,10-adducts
  • 1,2-positions forming 1,2-adducts
  • 2,3-positions forming 2,3-adducts
  • 3,4-positions forming 3,4-adducts

Correct Answer: 9,10-positions forming 9,10-adducts

Q4. Phenanthrene is more reactive than anthracene in electrophilic substitution at which ring?

  • Central ring (ring B)
  • Terminal ring (ring A)
  • Terminal ring (ring C)
  • All rings equally reactive

Correct Answer: Central ring (ring B)

Q5. Which reagent is commonly used to oxidize benzylic side chains of polynuclear hydrocarbons to carboxylic acids?

  • Potassium permanganate (KMnO4)
  • Lindlar’s catalyst
  • Sodium borohydride (NaBH4)
  • Hydrogen iodide (HI)

Correct Answer: Potassium permanganate (KMnO4)

Q6. Birch reduction of polycyclic aromatics typically yields which type of product?

  • Partially reduced dihydro or tetrahydro aromatic systems
  • Fully hydrogenated cycloalkanes
  • Carboxylic acids
  • Alcohols at ring carbons

Correct Answer: Partially reduced dihydro or tetrahydro aromatic systems

Q7. Which fused polycyclic hydrocarbon is most likely to form a stable quinone upon oxidation?

  • Anthracene
  • Naphthalene
  • Pyrene
  • Coronene

Correct Answer: Anthracene

Q8. In Friedel–Crafts acylation of naphthalene under kinetic conditions, acylation occurs mainly at which carbon?

  • Alpha carbon (1-position)
  • Beta carbon (2-position)
  • 3-position
  • 4-position

Correct Answer: Alpha carbon (1-position)

Q9. Which rule helps predict the most aromatic resonance form in polycyclic aromatic hydrocarbons?

  • Clar’s sextet rule
  • Markovnikov’s rule
  • Hückel’s rule only
  • Fajans’ rule

Correct Answer: Clar’s sextet rule

Q10. Which reagent combination is typically used for nitration of polynuclear aromatics?

  • Nitric acid and sulfuric acid (HNO3/H2SO4)
  • Nitrosyl chloride and AlCl3
  • Sodium nitrite and acetic acid
  • Hydrogen peroxide and acetic acid

Correct Answer: Nitric acid and sulfuric acid (HNO3/H2SO4)

Q11. Which position in anthracene is least aromatic and therefore most reactive toward addition reactions?

  • 9,10-positions
  • 1,2-positions
  • 2,3-positions
  • 1,4-positions

Correct Answer: 9,10-positions

Q12. Sulfonation of naphthalene with fuming sulfuric acid primarily gives which product under equilibrium favoring high temperature?

  • 2-naphthalenesulfonic acid (beta)
  • 1-naphthalenesulfonic acid (alpha)
  • 3-naphthalenesulfonic acid
  • 4-naphthalenesulfonic acid

Correct Answer: 2-naphthalenesulfonic acid (beta)

Q13. Which polycyclic aromatic hydrocarbon is planar and highly symmetric, often used as a model for extended conjugation?

  • Coronene
  • Anthracene
  • Naphthalene
  • Fluorene

Correct Answer: Coronene

Q14. Halogenation of polynuclear aromatics generally requires a Lewis acid catalyst; which catalyst is common for bromination?

  • FeBr3
  • Pd/C
  • CuBr
  • NiCl2

Correct Answer: FeBr3

Q15. Benzylic oxidation of alkyl-substituted polycyclic aromatics proceeds readily when benzylic hydrogen atoms are present; which reagent is most specific for converting benzylic methyl to carboxyl?

  • KMnO4 (hot, aqueous)
  • H2, Pd/C
  • NBS (N-bromosuccinimide)
  • LiAlH4

Correct Answer: KMnO4 (hot, aqueous)

Q16. The carcinogenicity of many PAHs is linked to metabolic activation to which reactive intermediate?

  • Epoxides (aryl epoxides)
  • Alcohols
  • Carboxylic acids
  • Alkanes

Correct Answer: Epoxides (aryl epoxides)

Q17. Pyrene oxidation often yields which type of functionalized product used in studies of toxicity?

  • Quinones
  • Alkyl halides
  • Imides
  • Thioethers

Correct Answer: Quinones

Q18. For regioselectivity in electrophilic substitution, resonance stabilization of the intermediate carbocation is most important. Which carbocation is more stabilized in naphthalene substitution at C1?

  • Carbocation stabilized by more resonance forms
  • Carbocation with fewer resonance forms
  • Carbocation stabilized by hyperconjugation only
  • Carbocation stabilized by inductive withdrawal

Correct Answer: Carbocation stabilized by more resonance forms

Q19. In aromatic stabilization energy, which is generally true comparing benzene, naphthalene and anthracene?

  • Per ring aromatic stabilization decreases as rings are fused
  • Per ring stabilization increases with ring fusion
  • All have equal per ring stabilization
  • Naphthalene is more stable per ring than benzene

Correct Answer: Per ring aromatic stabilization decreases as rings are fused

Q20. Friedel–Crafts alkylation of polynuclear aromatics is often problematic due to what major side reaction?

  • Polyalkylation and carbocation rearrangement
  • Oxidative cleavage
  • Radical polymerization
  • Formation of epoxides

Correct Answer: Polyalkylation and carbocation rearrangement

Q21. Which polynuclear hydrocarbon readily undergoes singlet oxygen addition at the 9,10-position to form endoperoxides?

  • Anthracene
  • Naphthalene
  • Phenanthrene
  • Pyrene

Correct Answer: Anthracene

Q22. Electrophilic aromatic substitution in phenanthrene gives the most resonance-stabilized sigma complex when substitution occurs at which position?

  • 9-position
  • 1-position
  • 2-position
  • 3-position

Correct Answer: 9-position

Q23. The oxidation of anthracene with dichromate yields which type of product at the 9,10-positions?

  • Anthraquinone (9,10-anthraquinone)
  • 9,10-diol
  • 9,10-dichloroanthracene
  • Carboxylic acid derivatives

Correct Answer: Anthraquinone (9,10-anthraquinone)

Q24. Which experimental method is commonly used to determine regiochemistry of substitution in polynuclear aromatic products?

  • NMR spectroscopy (1H and 13C)
  • TLC only
  • Refractive index measurement
  • Melting point alone

Correct Answer: NMR spectroscopy (1H and 13C)

Q25. Which factor reduces reactivity of a ring in a fused polycyclic aromatic system toward electrophilic substitution?

  • Loss of aromatic sextet in that ring’s resonance forms
  • Presence of electron-donating substituents
  • High electron density due to resonance
  • Steric accessibility

Correct Answer: Loss of aromatic sextet in that ring’s resonance forms

Q26. N-oxidation is not a reaction of hydrocarbons; which metabolic transformation is most relevant for PAHs in drug metabolism?

  • Cytochrome P450-mediated epoxidation
  • Direct glucuronidation of hydrocarbons
  • Phosphorylation
  • Sulfation of parent hydrocarbon

Correct Answer: Cytochrome P450-mediated epoxidation

Q27. Which product results from ozonolysis of an exposed double bond formed after partial hydrogenation of an aromatic ring in a PAH?

  • Carbonyl compounds (aldehydes/ketones)
  • Alcohols
  • Amides
  • Nitriles

Correct Answer: Carbonyl compounds (aldehydes/ketones)

Q28. Which descriptor best explains why 1-naphthol directs electrophiles to the 2-position?

  • Activation by resonance from the hydroxyl group favors ortho/para
  • Inductive electron withdrawal by OH
  • Steric blocking at 1-position
  • OH deactivates the ring entirely

Correct Answer: Activation by resonance from the hydroxyl group favors ortho/para

Q29. In lab synthesis, selective monobromination of naphthalene at C2 often requires which approach?

  • Controlled temperature and Lewis acid to favor beta substitution
  • Excess bromine at high temperature
  • Strong base and bromine radical conditions
  • Photochemical conditions only

Correct Answer: Controlled temperature and Lewis acid to favor beta substitution

Q30. Which statement about resonance energy distribution in polycyclic aromatics is correct?

  • Some rings in a fused system can be more aromatic than others
  • All rings always share energy equally
  • Only terminal rings are aromatic
  • Fused rings eliminate aromaticity entirely

Correct Answer: Some rings in a fused system can be more aromatic than others

Q31. Which reagent is preferred for hydrogenation of aromatic rings under mild conditions to avoid over-reduction of sensitive functionalities?

  • H2 with Pd/C under controlled pressure
  • LiAlH4 in ether
  • KMnO4 (hot)
  • HNO3/H2SO4

Correct Answer: H2 with Pd/C under controlled pressure

Q32. Which mechanism describes electrophilic aromatic substitution in polycyclic systems?

  • Formation of sigma complex (Wheland intermediate) followed by deprotonation
  • Radical addition followed by elimination
  • Nucleophilic aromatic substitution
  • Pericyclic concerted substitution

Correct Answer: Formation of sigma complex (Wheland intermediate) followed by deprotonation

Q33. Sulfonation of anthracene tends to occur preferentially at which positions under standard conditions?

  • 9-position forming 9-anthracenesulfonic derivatives
  • 1-position only
  • 2-position only
  • Any position equally

Correct Answer: 9-position forming 9-anthracenesulfonic derivatives

Q34. Which polycyclic aromatic hydrocarbon is known to exhibit “peri” hydrogen interactions affecting reactivity?

  • Phenanthrene (peri positions 1 and 8)
  • Naphthalene
  • Anthracene
  • Pyrene

Correct Answer: Phenanthrene (peri positions 1 and 8)

Q35. In the context of industrial synthesis, why is regioselective functionalization of PAHs important in pharmaceutical impurity profiling?

  • Different regioisomers can have distinct toxicity and metabolic profiles
  • All regioisomers are pharmacologically identical
  • Regioselectivity is irrelevant for impurities
  • Only physical properties change, not biological activity

Correct Answer: Different regioisomers can have distinct toxicity and metabolic profiles

Q36. Which analytical technique is most useful for identifying oxidation products like quinones from PAHs?

  • Mass spectrometry coupled with chromatography (LC-MS)
  • Simple colorimetry
  • Beilstein test only
  • Distillation

Correct Answer: Mass spectrometry coupled with chromatography (LC-MS)

Q37. Which covalent metabolite formation pathway increases DNA adduct formation risk for PAHs?

  • Formation of arene oxides (epoxides) that react with nucleophiles
  • Simple glucuronidation and excretion
  • Sulfation leading to inactive metabolites
  • Oxidation to stable carboxylic acids

Correct Answer: Formation of arene oxides (epoxides) that react with nucleophiles

Q38. Which reaction is characteristic of phenanthrene under oxidation to give phenanthrenequinone derivatives?

  • Oxidation at 9,10 positions to give phenanthrene-9,10-quinone
  • Hydrogenation at 1,2 positions only
  • Halogenation at the central ring exclusively
  • Nitration at position 3 only

Correct Answer: Oxidation at 9,10 positions to give phenanthrene-9,10-quinone

Q39. Which structural feature makes pyrene highly photoreactive compared to naphthalene?

  • Extended conjugation and higher excited-state lifetimes
  • Lack of conjugation
  • Presence of heteroatoms
  • Non-planarity

Correct Answer: Extended conjugation and higher excited-state lifetimes

Q40. Halogenation at benzylic positions of PAHs is often achieved using which reagent for radical bromination?

  • NBS (N-bromosuccinimide) with light or radical initiator
  • FeCl3
  • HNO3/H2SO4
  • KMnO4

Correct Answer: NBS (N-bromosuccinimide) with light or radical initiator

Q41. Which statement about electrophilic substitution reactivity order is correct for naphthalene vs benzene?

  • Naphthalene is more reactive than benzene at the alpha position
  • Benzene is always more reactive than naphthalene
  • Naphthalene is unreactive compared to benzene
  • Both have identical reactivity

Correct Answer: Naphthalene is more reactive than benzene at the alpha position

Q42. Nitration of anthracene under vigorous conditions often leads to decomposition because:

  • The aromatic system is disrupted and unstable sigma complexes form
  • Nitration is always harmless
  • Anthracene forms stable nitro derivatives easily
  • Nitration only occurs at benzylic positions

Correct Answer: The aromatic system is disrupted and unstable sigma complexes form

Q43. Which reagent set is suitable for converting a polycyclic aromatic ketone to the corresponding alcohol (reduction)?

  • NaBH4 in methanol
  • KMnO4 (hot)
  • HNO3/H2SO4
  • Br2 and FeBr3

Correct Answer: NaBH4 in methanol

Q44. What determines whether a fused ring behaves as an aromatic sextet according to Clar’s rule?

  • Distribution of isolated benzene-like sextets giving maximum number of sextets
  • Only the number of rings irrespective of electron distribution
  • Presence of substituents only
  • Molten state of the compound

Correct Answer: Distribution of isolated benzene-like sextets giving maximum number of sextets

Q45. Which oxidation method selectively converts 1- or 2-alkylnaphthalenes to naphthalene carboxylic acids?

  • Oxidation with hot KMnO4
  • Hydrogenation with H2/Pd
  • Photochemical ring opening
  • Alkylation with Friedel–Crafts

Correct Answer: Oxidation with hot KMnO4

Q46. In PAH chemistry, what is a common consequence of peri-substitution in phenanthrene?

  • Steric strain and altered reactivity at peri positions
  • Complete loss of aromaticity
  • Enhanced solubility in water
  • Spontaneous polymerization

Correct Answer: Steric strain and altered reactivity at peri positions

Q47. Which reagent can be used to convert a nitro-substituted PAH to an amine under laboratory conditions?

  • Sn/HCl or catalytic hydrogenation (H2/Pd)
  • KMnO4 oxidation
  • NBS bromination
  • Conc. HNO3 only

Correct Answer: Sn/HCl or catalytic hydrogenation (H2/Pd)

Q48. Which factor most influences site-selectivity in electrophilic substitution of polycyclic aromatics?

  • Relative resonance stabilization of the intermediate cation
  • Only steric hindrance
  • Only reagent concentration
  • Phase of moon

Correct Answer: Relative resonance stabilization of the intermediate cation

Q49. Metabolic detoxification of PAHs often involves phase II conjugation; which conjugation increases solubility for renal excretion?

  • Glucuronidation
  • Hydrogenation
  • Oxidative ring fusion
  • Dehydrogenation

Correct Answer: Glucuronidation

Q50. Which laboratory safety concern is especially important when handling polynuclear aromatic compounds during reaction and purification?

  • Many PAHs are toxic and potentially carcinogenic; avoid inhalation and skin contact
  • They are completely non-toxic and require no precautions
  • Only flammability is a concern, not toxicity
  • They readily dissolve in water so aqueous disposal is safe

Correct Answer: Many PAHs are toxic and potentially carcinogenic; avoid inhalation and skin contact

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