Synthesis of polynuclear hydrocarbons MCQs With Answer

Synthesis of polynuclear hydrocarbons MCQs With Answer is a concise, student-focused overview for B.Pharm learners exploring the preparation, mechanisms, and applications of polynuclear hydrocarbons (PNHs) and polycyclic aromatic hydrocarbons (PAHs). This introduction highlights synthetic strategies — Diels–Alder, Scholl, Mallory, oxidative coupling, cross-coupling, photocyclization — along with regioselectivity, aromatic stabilization, and challenges in functionalization and purification. Emphasis is on reagents, catalysts, reaction conditions, stereochemistry, and spectral characterization (NMR, UV, MS) relevant to pharmaceutical frameworks. Clear MCQs reinforce concepts required for drug design and organic synthesis labs. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the primary driving force for aromatization during the synthesis of polynuclear hydrocarbons?

• Formation of sigma bonds
• Increase in molecular weight
• Stabilization by conjugated pi-electron delocalization
• Hydrogen bonding interactions

Correct Answer: Stabilization by conjugated pi-electron delocalization

Q2. Which reaction is commonly used to construct fused six-membered rings en route to polynuclear hydrocarbons?

• Buchwald–Hartwig amination
• Diels–Alder cycloaddition
• Aldol condensation
• Wittig reaction

Correct Answer: Diels–Alder cycloaddition

Q3. The Scholl reaction is typically used for which transformation in PAH synthesis?

• Oxidative aromatic C–C bond formation to fuse rings
• Reduction of nitro groups to amines
• Formation of carbonyl groups from alcohols
• Conversion of alkenes to epoxides

Correct Answer: Oxidative aromatic C–C bond formation to fuse rings

Q4. Which oxidant is frequently used in Scholl-type oxidative cyclizations?

• Pd/C with hydrogen
• Ferric chloride (FeCl3)
• Sodium borohydride (NaBH4)
• LiAlH4

Correct Answer: Ferric chloride (FeCl3)

Q5. In Mallory photocyclization, what is the common prefunctionalized substrate?

• Trans-stilbene derivatives
• Alkyl halides
• Carboxylic acids
• Enamines

Correct Answer: Trans-stilbene derivatives

Q6. Which catalyst is central to Heck coupling used to construct ring precursors for polynuclear hydrocarbons?

• Palladium(0) complexes
• Iron(III) chloride
• Aluminium chloride
• Chromium(VI) oxide

Correct Answer: Palladium(0) complexes

Q7. Which property of PAHs is most directly assessed by UV-Vis spectroscopy?

• Mass-to-charge ratio
• Conjugation and electronic transitions
• Proton exchange rates
• Melting point

Correct Answer: Conjugation and electronic transitions

Q8. In retrosynthetic planning for PNHs, what is a common disconnection strategy?

• Disconnecting carbonyl groups
• Retrosynthetic ring-opening of fused rings to simpler aromatic building blocks
• Removing protecting groups first
• Cleaving ester linkages

Correct Answer: Retrosynthetic ring-opening of fused rings to simpler aromatic building blocks

Q9. Which named reaction directly forms polyaromatic frameworks via oxidative cyclodehydrogenation?

• Friedel–Crafts acylation
• Scholl reaction
• Grignard formation
• Claisen rearrangement

Correct Answer: Scholl reaction

Q10. What challenge is most associated with regioselectivity in polynuclear hydrocarbon synthesis?

• Choosing the right solvent for distillation
• Controlling which aromatic positions undergo C–C bond formation
• Measuring the pH of reaction mixture
• Separation of enantiomers

Correct Answer: Controlling which aromatic positions undergo C–C bond formation

Q11. Which technique is most useful for distinguishing different aromatic proton environments in a newly synthesized PAH?

• IR spectroscopy
• 1H NMR spectroscopy
• TLC with UV light only
• Rotary evaporation

Correct Answer: 1H NMR spectroscopy

Q12. What is the role of a directing group in regioselective electrophilic aromatic substitution during ring annulation?

• It converts aromatic rings to aliphatics
• It guides electrophile approach to specific positions on the ring
• It increases the molecular weight
• It initiates radical polymerization

Correct Answer: It guides electrophile approach to specific positions on the ring

Q13. Which coupling reaction is commonly used to join two aromatic fragments in PAH synthesis with boronic acids?

• Stille coupling
• Suzuki–Miyaura coupling
• Ullmann coupling
• Esterification

Correct Answer: Suzuki–Miyaura coupling

Q14. What is a common problem when performing oxidative cyclizations to build polynuclear frameworks?

• Excessive hydration of product
• Over-oxidation and decomposition of the substrate
• Formation of stable carbanions
• Inability to dissolve starting materials

Correct Answer: Over-oxidation and decomposition of the substrate

Q15. Which solvent property is most important for high-temperature cyclodehydrogenation reactions?

• High polarity to stabilize ionic intermediates
• Appropriate boiling point and thermal stability
• Ability to act as a strong oxidant
• Low viscosity for chromatography

Correct Answer: Appropriate boiling point and thermal stability

Q16. What does the term “peri-annulation” refer to in polynuclear hydrocarbon synthesis?

• Annulation at non-adjacent positions creating bridgehead bonds
• Introduction of peroxo groups
• Formation of saturated side chains
• Cleavage of fused rings

Correct Answer: Annulation at non-adjacent positions creating bridgehead bonds

Q17. Why is microwave irradiation sometimes used in PAH synthesis?

• To selectively form only carbonyl compounds
• To accelerate reactions and improve yields by rapid heating
• To separate isomers during chromatography
• To measure melting points accurately

Correct Answer: To accelerate reactions and improve yields by rapid heating

Q18. In mass spectrometry of PAHs, which fragmentations are commonly observed?

• Loss of small neutral fragments like H and C2H2 from aromatic systems
• Formation of hydrated adducts only
• Base peak always corresponds to intact molecular ion for large PAHs
• No fragmentation due to extreme stability

Correct Answer: Loss of small neutral fragments like H and C2H2 from aromatic systems

Q19. What characteristic of polynuclear hydrocarbons increases with extended conjugation?

• Polarity and water solubility
• Red-shift in UV-Vis absorption (bathochromic shift)
• Basicity of ring protons
• Boiling point decreases sharply

Correct Answer: Red-shift in UV-Vis absorption (bathochromic shift)

Q20. Which safety concern is particularly relevant when handling many PAHs in the lab?

• Flammability only
• Potential carcinogenicity and long-term toxicity
• Explosive decomposition at room temperature
• They are all inert and non-toxic

Correct Answer: Potential carcinogenicity and long-term toxicity

Q21. In building angular fused PAHs, which cyclization approach is commonly applied?

• Intramolecular [4+2] cycloaddition or electrophilic cyclization of alkynyl arenes
• Simple ester hydrolysis
• Nucleophilic substitution at saturated carbons
• Pericyclic [2+2] thermal cycloaddition of alkanes

Correct Answer: Intramolecular [4+2] cycloaddition or electrophilic cyclization of alkynyl arenes

Q22. Which factor primarily influences the regioselectivity of intramolecular Friedel–Crafts cyclizations?

• Nature of the Lewis acid catalyst and substituent effects on the aromatic ring
• Molecular weight of reagent only
• Presence of tertiary amines exclusively
• Amount of light exposure

Correct Answer: Nature of the Lewis acid catalyst and substituent effects on the aromatic ring

Q23. Which reagent is commonly used for dehydrogenation to convert saturated frameworks into aromatic rings?

• Pd/C under hydrogen atmosphere
• 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)
• Sodium cyanoborohydride
• Hydrazine hydrate

Correct Answer: 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)

Q24. What is the primary use of directing groups like methoxy or alkyl in PAH precursors?

• To deactivate rings completely
• To guide electrophiles and stabilize intermediates during annulation
• To increase volatility for GC analysis
• To promote radical polymerization

Correct Answer: To guide electrophiles and stabilize intermediates during annulation

Q25. Which spectroscopic technique best distinguishes between different fused-ring topologies (linear vs angular)?

• Infrared spectroscopy (IR)
• 1D and 2D NMR combined with UV-Vis and computational chemical shift analysis
• Refractive index measurement
• Elemental analysis alone

Correct Answer: 1D and 2D NMR combined with UV-Vis and computational chemical shift analysis

Q26. What role do transition-metal catalysts play in modern PAH syntheses?

• They always oxidize substrates to CO2
• They facilitate C–C bond formation and coupling reactions with controlled selectivity
• They serve only as solvents
• They prevent formation of aromatic rings

Correct Answer: They facilitate C–C bond formation and coupling reactions with controlled selectivity

Q27. Which structural feature contributes most to the planarization of polynuclear hydrocarbons?

• Presence of many sp3 carbons
• Extended sp2 conjugated networks and fused aromatic rings
• Large aliphatic side chains
• High content of heteroatoms

Correct Answer: Extended sp2 conjugated networks and fused aromatic rings

Q28. Which method is preferred to introduce substituents to tune solubility of PAHs for pharmaceutical applications?

• Late-stage functionalization via cross-coupling or electrophilic substitution
• Complete hydrogenation of the aromatic core
• Free radical polymerization of PAHs
• Thermal cracking

Correct Answer: Late-stage functionalization via cross-coupling or electrophilic substitution

Q29. Which named reaction couples aryl halides with organotin reagents and can be used in PAH assembly?

• Stille coupling
• Suzuki coupling
• Heck reaction
• Grignard formation

Correct Answer: Stille coupling

Q30. What is the main advantage of building PAHs via modular cross-coupling of smaller aryl units?

• It avoids the need for any catalysts
• It allows diversity, regiocontrol, and late-stage modification
• It always yields only a single product without side products
• It converts PAHs into peptides directly

Correct Answer: It allows diversity, regiocontrol, and late-stage modification

Q31. In oxidative photocyclization methods, what is the typical function of oxygen or an acceptor?

• To act as a base
• To reoxidize an intermediate and restore aromaticity
• To reduce the substrate
• To protect the product from light

Correct Answer: To reoxidize an intermediate and restore aromaticity

Q32. When designing a synthetic route to a large PAH for drug discovery, which consideration is least important?

• Scalability and reproducibility
• Cost and availability of building blocks
• Regioselectivity and functional group compatibility
• Color of the final product

Correct Answer: Color of the final product

Q33. Which analytical technique helps estimate the degree of aromatic conjugation by measuring HOMO–LUMO gap indirectly?

• Gas chromatography
• UV-Visible spectroscopy
• Titration with base
• Optical rotation

Correct Answer: UV-Visible spectroscopy

Q34. Why are protecting groups sometimes necessary when synthesizing polynuclear hydrocarbons?

• To permanently block reactivity so no reactions proceed
• To mask reactive functional groups and allow selective transformations on other parts of the molecule
• To increase volatility for distillation
• To convert aromatics into aliphatic chains

Correct Answer: To mask reactive functional groups and allow selective transformations on other parts of the molecule

Q35. Which intermediate is commonly involved in electrophilic cyclodehydrogenation reactions?

• Carbocationic sigma-complex on the aromatic ring
• Stable free carbanions only
• Metal hydride complexes exclusively
• Peroxide radicals that form esters

Correct Answer: Carbocationic sigma-complex on the aromatic ring

Q36. What is the significance of Clar’s aromatic sextet theory in PAH chemistry?

• It predicts solubility in water
• It helps rationalize stability, reactivity, and localization of aromatic sextets in PAHs
• It is used to determine melting points
• It describes hydrogen-bonding networks

Correct Answer: It helps rationalize stability, reactivity, and localization of aromatic sextets in PAHs

Q37. Which reaction would you choose to convert a 1,2-diarylalkene into a fused aromatic ring system photochemically?

• Mallory photocyclization
• Wolff rearrangement
• Baeyer–Villiger oxidation
• Hydroboration–oxidation

Correct Answer: Mallory photocyclization

Q38. Which solvent is commonly avoided in Friedel–Crafts-type annulations due to coordination with Lewis acids?

• Dry dichloromethane
• Ether solvents like diethyl ether
• Nitromethane
• Carbon tetrachloride

Correct Answer: Ether solvents like diethyl ether

Q39. Which structural motif is often targeted in pharmaceutical discovery within polynuclear hydrocarbons?

• Highly water-soluble ionic salts only
• Planar aromatic cores that enable pi–pi stacking for target binding
• Long flexible alkyl chains exclusively
• Polymers with high molecular weight

Correct Answer: Planar aromatic cores that enable pi–pi stacking for target binding

Q40. When purifying PAHs, which method is frequently used because of strong pi–pi interactions and low polarity?

• Normal phase silica gel chromatography with nonpolar eluents and careful gradient
• Simple aqueous extraction
• Ion-exchange chromatography
• Dialysis

Correct Answer: Normal phase silica gel chromatography with nonpolar eluents and careful gradient

Q41. Which approach helps prevent polymerization or overreaction during oxidative cyclizations?

• Using excess strong oxidant at high temperature
• Careful control of stoichiometry, temperature, and addition rate of oxidant
• Adding large amounts of water to reaction mixture
• Running the reaction under strong base

Correct Answer: Careful control of stoichiometry, temperature, and addition rate of oxidant

Q42. Which computational method is most useful for predicting aromatic stabilization and reactive sites in PAHs?

• Simple molecular mechanics without electronic terms
• Density functional theory (DFT) calculations
• Classical reservoir simulation
• Empirical pKa tables only

Correct Answer: Density functional theory (DFT) calculations

Q43. What is the typical consequence of introducing heteroatoms into polynuclear hydrocarbon frameworks?

• Complete loss of aromaticity always
• Modulation of electronic properties, reactivity, and possible improved solubility
• Instant polymerization
• Conversion into inorganic salts

Correct Answer: Modulation of electronic properties, reactivity, and possible improved solubility

Q44. Which step is essential to confirm the successful formation of a new fused ring in PAH synthesis?

• Measuring refractive index only
• Collecting combined spectroscopic evidence: NMR, HRMS, and UV-Vis and, if possible, X-ray crystallography
• Weighing the reaction mixture directly
• Checking for smell change

Correct Answer: Collecting combined spectroscopic evidence: NMR, HRMS, and UV-Vis and, if possible, X-ray crystallography

Q45. Which reagent is commonly used to introduce alkynyl groups that later undergo cyclization to form PAHs?

• Terminal alkynes via Sonogashira coupling with aryl halides
• Diazonium salts alone
• Grignard reagents with carbon dioxide
• Sodium borohydride reductions

Correct Answer: Terminal alkynes via Sonogashira coupling with aryl halides

Q46. In the context of medicinal chemistry, why might one prefer smaller fused PAHs over very large extended PAHs?

• Smaller PAHs typically have better drug-like properties including solubility and metabolic profiles
• Large PAHs are always cheaper
• Small PAHs cannot interact with biological targets
• Large PAHs never fluoresce

Correct Answer: Smaller PAHs typically have better drug-like properties including solubility and metabolic profiles

Q47. What synthetic tactic can reduce unwanted regioisomers during ring formation?

• Performing reactions at very high dilution without considering kinetics
• Using steric blocking groups or temporary directing groups to steer reactivity
• Ignoring reagent stoichiometry
• Heating to extremes to force product formation

Correct Answer: Using steric blocking groups or temporary directing groups to steer reactivity

Q48. Which experimental evidence would most directly indicate planarization after cyclodehydrogenation?

• Loss of all UV-Vis absorption
• Downfield shifts of aromatic protons in 1H NMR and increased conjugation in UV-Vis
• Complete disappearance in NMR
• Large increase in molecular weight measured by balance

Correct Answer: Downfield shifts of aromatic protons in 1H NMR and increased conjugation in UV-Vis

Q49. What is a common environmental concern when synthesizing and disposing of PAHs?

• They readily biodegrade in wastewater
• Their persistence and potential toxicity leading to soil and water contamination
• They neutralize all acids
• They are completely safe and unregulated

Correct Answer: Their persistence and potential toxicity leading to soil and water contamination

Q50. Which strategy can be used to increase the yield of a key annulation step in a multistep PAH synthesis?

• Optimize catalyst, solvent, temperature, and additive profile through systematic screening
• Double the substrate concentration without changing other parameters
• Exclude purification steps to save time
• Use only water as the solvent for all steps

Correct Answer: Optimize catalyst, solvent, temperature, and additive profile through systematic screening

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