Synthetic evidences for structure of benzene MCQs With Answer

Synthetic evidences for structure of benzene MCQs With Answer offers B. Pharm students a focused review of the experimental and synthetic data that established benzene’s unique aromatic structure. This introduction connects important keywords—benzene structure, Kekulé model, resonance stabilization, electrophilic aromatic substitution, hydrogenation enthalpy, and isomer counts—with practical synthetic observations and reaction patterns. It emphasizes how substitution reactions, hydrogenation energetics, lack of typical alkene addition, and spectroscopic data provide converging evidence for aromaticity. Designed for pharmacy undergraduates, these concise MCQs reinforce reaction mechanisms, orientation effects, and laboratory synthesis relevant to drug chemistry and pharmaceutical analysis. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which synthetic observation most strongly challenged Kekulé’s alternating single–double bond model for benzene?

• Pattern of substitution giving only three isomers for disubstituted benzenes
• Existence of benzene as a liquid at room temperature
• Benzene’s ability to undergo electrophilic substitution
• Formation of benzene from acetylene trimerization

Correct Answer: Pattern of substitution giving only three isomers for disubstituted benzenes

Q2. Which reaction demonstrates benzene’s reluctance to undergo addition compared to alkenes?

• Hydrogenation under mild conditions without catalyst
• Bromination without catalyst at room temperature
• Hydrogenation requiring catalyst and heat
• Hydration to form alcohols

Correct Answer: Hydrogenation requiring catalyst and heat

Q3. The extra stabilization energy of benzene compared to a hypothetical cyclohexatriene is called:

• Ring strain energy
• Aromatic stabilization energy (resonance energy)
• Conjugation penalty
• Hyperconjugation energy

Correct Answer: Aromatic stabilization energy (resonance energy)

Q4. Which synthetic route historically prepared benzene by trimerization of acetylene?

• Kolbe synthesis
• Bouveault–Blanc reduction
• Berthelot synthesis
• Wurtz coupling

Correct Answer: Berthelot synthesis

Q5. Which piece of experimental evidence supports equal C–C bond lengths in benzene?

• Different melting points of benzene derivatives
• X-ray crystallography and electron diffraction data
• Observation of three isomers for disubstitution
• Rate of nitration at different positions

Correct Answer: X-ray crystallography and electron diffraction data

Q6. Why does benzene predominantly undergo electrophilic aromatic substitution (EAS) instead of addition?

• EAS allows retention of aromatic stabilization
• Addition reactions are always forbidden by quantum rules
• Benzene cannot form sigma complexes
• EAS produces saturated products

Correct Answer: EAS allows retention of aromatic stabilization

Q7. Which reagent combination is commonly used for nitration of benzene?

• HNO3 and H2 with Pt
• HNO3 and H2SO4
• Br2 and FeBr3
• HCl and Zn

Correct Answer: HNO3 and H2SO4

Q8. Friedel–Crafts alkylation of benzene requires which catalytic species?

• Lewis base like pyridine
• Lewis acid like AlCl3
• Radical initiator like peroxides
• Strong base like NaOH

Correct Answer: Lewis acid like AlCl3

Q9. Which experimental result supports resonance delocalization in benzene?

• Different chemical shifts for all six protons in 1H NMR
• Equal proton chemical shift for all aromatic hydrogens in 1H NMR
• Formation of multiple structural isomers upon monobromination
• Rapid hydrogenation at room temperature

Correct Answer: Equal proton chemical shift for all aromatic hydrogens in 1H NMR

Q10. Heat of hydrogenation measurements of benzene compared to three isolated double bonds indicate:

• Benzene is less stable than three isolated double bonds
• Benzene has extra stabilization (resonance energy)
• Heat of hydrogenation is identical in both cases
• Benzene gives polymeric hydrogenation products

Correct Answer: Benzene has extra stabilization (resonance energy)

Q11. Which observation about disubstituted benzene is a key synthetic evidence for its symmetric ring?

• There are six possible disubstituted isomers
• Only three distinct disubstituted isomers exist: ortho, meta, para
• Disubstituted benzenes cannot be isolated
• Disubstituted benzenes react only by addition

Correct Answer: Only three distinct disubstituted isomers exist: ortho, meta, para

Q12. Catalytic bromination of benzene requires which condition?

• No catalyst, proceeds rapidly at room temperature
• Light irradiation only
• Lewis acid catalyst such as FeBr3
• Strong base catalyst like NaOH

Correct Answer: Lewis acid catalyst such as FeBr3

Q13. Which mechanism intermediate is formed during electrophilic aromatic substitution?

• Carbanion intermediate
• Arenium (σ-complex) carbocation intermediate
• Radical cation exclusively
• Pericyclic transition state without intermediates

Correct Answer: Arenium (σ-complex) carbocation intermediate

Q14. Which synthetic transformation converts benzene to chlorobenzene efficiently?

• Radical chlorination with Cl2 under UV
• Electrophilic aromatic substitution with Cl2 and FeCl3
• Nucleophilic aromatic substitution with Cl-
• Hydrohalogenation of benzene

Correct Answer: Electrophilic aromatic substitution with Cl2 and FeCl3

Q15. The lack of addition products when benzene reacts with bromine in the dark indicates:

• Benzene is highly reactive toward radicals
• Benzene preserves aromaticity and resists addition
• Benzene forms bromonium ions readily
• Benzene is saturated

Correct Answer: Benzene preserves aromaticity and resists addition

Q16. Which experimental synthesis shows benzene can be prepared by dehydrogenation of cyclohexane?

• Catalytic oxidative dehydrogenation at high temperature
• Hydrogenation over Pd/C
• Acid-catalyzed hydration
• Grignard synthesis

Correct Answer: Catalytic oxidative dehydrogenation at high temperature

Q17. Which is a synthetic method to introduce a nitro group selectively on benzene?

• Nitration using HNO3 alone at low temperature
• Nitration using HNO3 and H2SO4 to generate NO2+ electrophile
• Direct radical nitration with NO gas
• Base-catalyzed substitution with NO2-

Correct Answer: Nitration using HNO3 and H2SO4 to generate NO2+ electrophile

Q18. Which observation from hydrogenation experiments quantifies benzene’s resonance energy?

• Comparing benzene hydrogenation enthalpy with cyclohexane
• Comparing benzene hydrogenation with three isolated alkenes gives a deficit
• Measuring hydrogen uptake only
• Observing color change during hydrogenation

Correct Answer: Comparing benzene hydrogenation with three isolated alkenes gives a deficit

Q19. Which is true about benzene’s 1H NMR spectrum as synthetic evidence?

• Shows six signals for six protons
• Shows a single sharp signal for all aromatic protons
• Shows no resonance because benzene is NMR silent
• Shows only broad unresolved peaks

Correct Answer: Shows a single sharp signal for all aromatic protons

Q20. Which named reaction forms benzene derivatives via electrophilic aromatic substitution?

• Wurtz reaction
• Friedel–Crafts acylation
• Buchner ring expansion
• Diels–Alder addition to benzene

Correct Answer: Friedel–Crafts acylation

Q21. Which result from synthetic studies supports the delocalized pi system of benzene?

• Alternate single and double bonds observed in NMR
• Equal reactivity at all six positions under identical substituent effects
• Identical C–C bond lengths and uniform reactivity pattern
• Ability to form stable addition products easily

Correct Answer: Identical C–C bond lengths and uniform reactivity pattern

Q22. Which catalyst is commonly used for catalytic hydrogenation of benzene to cyclohexane?

• Pd/C or Pt catalysts under high pressure
• NaBH4 in methanol
• AlCl3 in dry ether
• KMnO4 in aqueous medium

Correct Answer: Pd/C or Pt catalysts under high pressure

Q23. How does the number of substitution isomers for tri-substituted benzenes support benzene’s symmetry?

• It shows there are always six distinct isomers
• Symmetry reduces possible isomers compared to an open chain
• It proves benzene has alternating localized double bonds
• It indicates benzene is non-aromatic

Correct Answer: Symmetry reduces possible isomers compared to an open chain

Q24. Which experimental fact about benzene derivatives indicates resonance stabilization?

• Rapid polymerization in presence of acids
• Relative stabilization of para products and identical spectra for symmetry-related protons
• Complete lack of substitution reactions
• High tendency for radical halogenation at benzylic positions exclusively

Correct Answer: Relative stabilization of para products and identical spectra for symmetry-related protons

Q25. Which oxidizing agent can oxidize alkyl side chains on aromatic rings to carboxylic acids, supporting benzene’s ring stability?

• KMnO4 under heating
• LiAlH4 in ether
• NaBH4 in methanol
• H2 and Pd/C

Correct Answer: KMnO4 under heating

Q26. Which observation from substitution patterns indicates benzene’s conjugated cyclic structure?

• Substitution yields only meta products regardless of substituent
• Directing effects of substituents (ortho/para or meta) reflect electron distribution
• Substituents always direct to benzylic position
• Substitution leads to ring opening always

Correct Answer: Directing effects of substituents (ortho/para or meta) reflect electron distribution

Q27. Which technique provides bond length measurements that support benzene’s equalized bonds?

• Thin-layer chromatography (TLC)
• X-ray diffraction and electron diffraction
• Melting point determination
• Infrared spectroscopy alone

Correct Answer: X-ray diffraction and electron diffraction

Q28. The inability of benzene to undergo addition with cold KMnO4 indicates:

• Benzene is a strong reducing agent
• Benzene’s pi electrons are stabilized by aromaticity and resist oxidation by mild reagents
• Benzene is easily oxidized to aliphatic products
• Benzene lacks pi electrons

Correct Answer: Benzene’s pi electrons are stabilized by aromaticity and resist oxidation by mild reagents

Q29. Which phenomenon explains why benzene has lower reactivity in addition than typical alkenes?

• Higher steric hindrance only
• Aromatic stabilization energy makes loss of aromaticity unfavorable
• Presence of heteroatoms in the ring
• Very high ring strain that prevents additions

Correct Answer: Aromatic stabilization energy makes loss of aromaticity unfavorable

Q30. What product results from exhaustive catalytic hydrogenation of benzene?

• Methane
• Cyclohexane
• Hexane
• Phenol

Correct Answer: Cyclohexane

Q31. Which spectroscopic evidence supports delocalized pi-electrons in benzene?

• IR stretching frequencies typical of isolated C=C only
• UV-Vis absorption showing conjugation and specific π→π* transitions
• No peaks in UV-Vis for benzene
• Only aliphatic proton signals in 1H NMR

Correct Answer: UV-Vis absorption showing conjugation and specific π→π* transitions

Q32. Which reagent pair converts benzene to benzene diazonium salts directly?

• NaNO2 and HCl on aniline derivatives (via aromatic amine)
• Direct reaction of benzene with NaNO2 and HCl
• Treatment with NaN3
• Reaction with NBS under light

Correct Answer: NaNO2 and HCl on aniline derivatives (via aromatic amine)

Q33. Which reaction is inhibited on benzene but proceeds on alkenes, showing aromatic stability?

• Electrophilic aromatic substitution
• Radical substitution at benzylic hydrogen
• Bromine addition across a double bond without catalyst
• Friedel–Crafts acylation

Correct Answer: Bromine addition across a double bond without catalyst

Q34. In the context of benzene structure, what does “aromaticity” principally refer to?

• Saturated nature of the ring
• Planar, cyclic, conjugated system with (4n+2) π electrons and extra stability
• Only presence of alternating single and double bonds
• Ability to undergo radical polymerization easily

Correct Answer: Planar, cyclic, conjugated system with (4n+2) π electrons and extra stability

Q35. Which experimental synthesis indicates benzene’s resistance to addition by halogens without a catalyst?

• Bromination of cyclohexene in dark
• Bromination of benzene in dark without FeBr3 shows no reaction
• Hydrogenation of benzene with H2 at room temperature
• Oxidation with H2O2

Correct Answer: Bromination of benzene in dark without FeBr3 shows no reaction

Q36. Which of these supports the delocalized bonding model over alternating Kekulé structures?

• Odd–even alternation of bond lengths around the ring
• All C–C bonds having intermediate and equal length between single and double bonds
• Different chemical reactivity for each carbon atom
• Formation of stable cyclohexatriene in isolation

Correct Answer: All C–C bonds having intermediate and equal length between single and double bonds

Q37. Which reagent would oxidize a benzylic methyl group to benzoic acid, illustrating stability of the aromatic ring?

• LiAlH4
• KMnO4 (hot, acidic or basic conditions)
• H2 and Pd/C
• PBr3

Correct Answer: KMnO4 (hot, acidic or basic conditions)

Q38. Which property of benzene observed in synthesis supports planarity of the ring?

• Chirality of unsubstituted benzene
• Equal chemical environment for all protons seen in NMR implying a symmetric planar structure
• Inability to form sigma complexes
• Preference for sp3 hybridization at carbon

Correct Answer: Equal chemical environment for all protons seen in NMR implying a symmetric planar structure

Q39. Which mechanism step involves re-aromatization in electrophilic aromatic substitution?

• Formation of radical intermediate
• Deprotonation of the σ-complex to restore aromaticity
• Nucleophilic attack on the ring carbon
• Concerted pericyclic ring closure

Correct Answer: Deprotonation of the σ-complex to restore aromaticity

Q40. Which observation from synthetic chemistry helped establish benzene’s cyclic conjugation rather than acyclic polyene?

• Easy addition reactions akin to alkenes
• Lower than expected reactivity and unique substitution patterns compared to polyenes
• Formation of multiple radical polymers
• Complete absence of π electrons

Correct Answer: Lower than expected reactivity and unique substitution patterns compared to polyenes

Q41. Which reagent pair is used in Friedel–Crafts acylation to introduce an acyl group into benzene?

• R–COCl and AlCl3
• R–OH and H2SO4
• R–Br and NaOH
• R–COOH and H2O

Correct Answer: R–COCl and AlCl3

Q42. Which experimental fact demonstrates benzene’s exceptional chemical stability relevant to pharmaceuticals?

• Benzene readily forms alcohols by addition
• Benzene requires harsh conditions or catalysts for many transformations, preserving aromatic core
• Benzene cannot be functionalized at all
• Benzene decomposes at room temperature

Correct Answer: Benzene requires harsh conditions or catalysts for many transformations, preserving aromatic core

Q43. Which reaction is commonly used to introduce an aldehyde or ketone via formylation of benzene derivatives?

• Gattermann–Koch or Vilsmeier–Haack formylation
• Wurtz coupling
• Sandmeyer reaction directly on benzene
• Ozonolysis of benzene

Correct Answer: Gattermann–Koch or Vilsmeier–Haack formylation

Q44. How does benzylic oxidation selectivity support aromatic ring resilience?

• Benzylic positions are inert and never oxidized
• Side chains oxidize to acids while aromatic core remains intact under many conditions
• Oxidation always destroys the aromatic ring first
• Oxidation yields only ring-opening products

Correct Answer: Side chains oxidize to acids while aromatic core remains intact under many conditions

Q45. Which synthetic observation demonstrates that substitution on benzene is positionally selective based on directing effects?

• All substituents give only para products
• Electron-donating groups direct ortho/para, electron-withdrawing groups direct meta
• Directing effects are random and unpredictable
• Positions are determined solely by steric hindrance

Correct Answer: Electron-donating groups direct ortho/para, electron-withdrawing groups direct meta

Q46. Which evidence from chlorination of benzene indicates the need for a Lewis acid?

• Chlorination proceeds rapidly without catalysts
• Chlorination of benzene is slow or non-existent without FeCl3
• Chlorination produces aliphatic chlorides only
• Chlorination cleaves the ring immediately

Correct Answer: Chlorination of benzene is slow or non-existent without FeCl3

Q47. Which analytical method shows multiple identical carbon environments for benzene, supporting symmetry?

• Gas chromatography only
• 13C NMR showing a single carbon resonance for all six carbons in benzene
• Mass spectrometry showing no molecular ion
• IR showing distinct C–C single and double bond stretches

Correct Answer: 13C NMR showing a single carbon resonance for all six carbons in benzene

Q48. Which pericyclic process is unfavorable for benzene because it would destroy aromaticity?

• Electrophilic aromatic substitution
• Concerted [4+2] cycloaddition (Diels–Alder) with benzene as diene under mild conditions
• Benzylic radical halogenation
• Friedel–Crafts alkylation

Correct Answer: Concerted [4+2] cycloaddition (Diels–Alder) with benzene as diene under mild conditions

Q49. Which historical synthetic observation supported the concept of aromaticity before modern spectroscopy?

• Equal boiling points for all aromatic compounds
• Resistance to addition and characteristic substitution patterns for benzene derivatives
• Immediate combustion of benzene in air
• High solubility of benzene in water

Correct Answer: Resistance to addition and characteristic substitution patterns for benzene derivatives

Q50. For pharmaceutical synthesis, why is understanding benzene’s synthetic evidences important?

• It helps design reactions that preserve or functionalize the aromatic core predictably
• It proves aromatic rings are always biologically inactive
• It indicates benzene cannot be part of drug molecules
• It shows aromatic rings are always unstable under physiological conditions

Correct Answer: It helps design reactions that preserve or functionalize the aromatic core predictably

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