Orbital picture of benzene MCQs With Answer

Orbital picture of benzene MCQs With Answer provides B.Pharm students a focused exploration of benzene’s molecular orbital framework, aromatic stabilization, and how delocalized 2p electrons determine reactivity and spectroscopy. This introduction covers key concepts such as sp2 hybridization, the six 2pz-derived π molecular orbitals, HOMO–LUMO features, Hückel’s 4n+2 rule, resonance energy, and implications for NMR and UV–Vis behavior. Clear, exam-relevant MCQs reinforce understanding of orbital symmetry, nodal patterns, and reaction preferences like electrophilic aromatic substitution. The set is tailored to bridge theory and pharmaceutical applications, improving problem-solving for medicinal chemistry and drug-design contexts. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which atomic orbitals overlap to form the π system in benzene?

• Carbon 2s orbitals
• Carbon 2pz orbitals
• Hydrogen 1s orbitals
• Carbon sp2 hybrid orbitals

Correct Answer: Carbon 2pz orbitals

Q2. What is the hybridization of each carbon atom in benzene?

• sp
• sp2
• sp3
• sp3d

Correct Answer: sp2

Q3. How many π electrons are present in benzene’s delocalized system?

• 4 π electrons
• 5 π electrons
• 6 π electrons
• 8 π electrons

Correct Answer: 6 π electrons

Q4. Which rule explains benzene’s aromatic stability quantitatively?

• Baird’s rule
• Aromatic sextet rule
• Hückel’s rule (4n+2)
• Nernst rule

Correct Answer: Hückel’s rule (4n+2)

Q5. How many molecular orbitals result from combining six 2pz atomic orbitals in benzene?

• 3 molecular orbitals
• 4 molecular orbitals
• 6 molecular orbitals
• 12 molecular orbitals

Correct Answer: 6 molecular orbitals

Q6. How many doubly degenerate sets of π molecular orbitals does benzene have?

• One doubly degenerate set
• Two doubly degenerate sets
• Three doubly degenerate sets
• No degenerate sets

Correct Answer: Two doubly degenerate sets

Q7. How many electrons occupy the HOMO level(s) of benzene?

• 2 electrons
• 4 electrons
• 6 electrons
• 0 electrons

Correct Answer: 4 electrons

Q8. What is the degeneracy of benzene’s LUMO?

• Non-degenerate
• Doubly degenerate
• Triply degenerate
• Quadruply degenerate

Correct Answer: Doubly degenerate

Q9. How many nodal planes (around the ring) are present in benzene’s highest-energy π molecular orbital?

• Zero nodal planes
• One nodal plane
• Two nodal planes
• Three nodal planes

Correct Answer: Three nodal planes

Q10. Which concept best accounts for benzene’s extra stability compared with localized cyclohexatriene?

• Inductive effect
• Hyperconjugation
• Delocalization of π electrons (aromatic stabilization)
• Steric hindrance

Correct Answer: Delocalization of π electrons (aromatic stabilization)

Q11. Which resonance contributors are principal in describing benzene’s π electronic structure?

• A single Kekulé structure
• Two equivalent Kekulé structures
• Three localized double-bond structures
• Only ionic structures

Correct Answer: Two equivalent Kekulé structures

Q12. How does the diamagnetic ring current in benzene affect proton NMR chemical shifts of ring protons?

• It causes significant shielding (upfield shifts)
• It causes deshielding (downfield shifts)
• No effect on chemical shift
• It splits the signals into many peaks

Correct Answer: It causes deshielding (downfield shifts)

Q13. What is the approximate C–C bond length in benzene due to delocalization?

• 1.20 Å
• 1.34 Å
• 1.39 Å
• 1.54 Å

Correct Answer: 1.39 Å

Q14. According to Clar’s sextet theory, how many π sextets does benzene contain?

• Zero sextets
• One sextet
• Two sextets
• Three sextets

Correct Answer: One sextet

Q15. Compared to Kekulé structures, what does the molecular orbital picture emphasize?

• Localized double bonds only
• Delocalized π orbitals spanning the whole ring
• Only σ-bonding interactions
• Ionic character of benzene

Correct Answer: Delocalized π orbitals spanning the whole ring

Q16. The dominant UV–Vis transition in benzene associated with π electrons is classified as:

• n→σ* transition
• σ→σ* transition
• π→π* transition
• n→π* transition

Correct Answer: π→π* transition

Q17. Ionization of benzene (removal of an electron) primarily removes an electron from which orbital?

• The lowest π orbital
• The σ C–H orbital
• The HOMO (highest occupied π orbital)
• The LUMO

Correct Answer: The HOMO (highest occupied π orbital)

Q18. When benzene is reduced by addition of an electron, which orbital is the incoming electron placed into?

• The lowest σ orbital
• The HOMO
• The LUMO (lowest unoccupied π orbital)
• A nonbonding orbital

Correct Answer: The LUMO (lowest unoccupied π orbital)

Q19. Which set of conditions is required by Hückel’s rule for aromaticity?

• Cyclic, nonplanar, conjugated, 4n electrons
• Cyclic, planar, conjugated, 4n+2 π electrons
• Acyclic, planar, conjugated, 4n+2 π electrons
• Cyclic, planar, saturated, 4n+2 electrons

Correct Answer: Cyclic, planar, conjugated, 4n+2 π electrons

Q20. Which substituent increases electron density on the benzene ring through resonance (+R effect)?

• Nitro (−NO2)
• Fluoro (−F)
• Hydroxyl (−OH)
• Trifluoromethyl (−CF3)

Correct Answer: Hydroxyl (−OH)

Q21. The lowest-energy π molecular orbital in benzene has how many nodal planes around the ring?

• Zero nodal planes
• One nodal plane
• Two nodal planes
• Three nodal planes

Correct Answer: Zero nodal planes

Q22. Which simple theoretical method is widely used to illustrate benzene π molecular orbitals qualitatively?

• Hartree–Fock only
• Hückel molecular orbital (HMO) method
• Möller–Plesset perturbation theory
• Coupled cluster theory

Correct Answer: Hückel molecular orbital (HMO) method

Q23. In the Hückel approximation, what does the parameter β represent?

• The Coulomb integral
• The resonance (interaction) integral between adjacent p orbitals
• The kinetic energy of σ electrons
• The energy of hydrogen atoms

Correct Answer: The resonance (interaction) integral between adjacent p orbitals

Q24. Why does benzene prefer electrophilic aromatic substitution (EAS) over addition reactions?

• Addition increases conjugation and stability
• Addition would disrupt aromatic stabilization
• EAS breaks aromaticity temporarily and is irreversible
• Substitution involves σ-bond cleavage exclusively

Correct Answer: Addition would disrupt aromatic stabilization

Q25. Which species is aromatic and isoelectronic in π count with benzene (6 π electrons)?

• Cyclobutadiene
• Cyclopentadienyl anion
• Butadiene
• Cyclohexadienyl cation with 5 π electrons

Correct Answer: Cyclopentadienyl anion

Q26. Which cyclic conjugated system is classically antiaromatic due to having 4 π electrons?

• Cyclopropenyl cation
• Cyclobutadiene
• Benzene
• Cyclopentadienyl anion

Correct Answer: Cyclobutadiene

Q27. In a π molecular orbital diagram, what does a node between adjacent p orbitals indicate?

• In-phase overlap
• Out-of-phase overlap (phase change)
• σ-bonding interaction
• Hydrogen bonding

Correct Answer: Out-of-phase overlap (phase change)

Q28. Where is π electron density primarily located in benzene?

• In the plane of the ring on carbon atoms
• Above and below the ring plane
• Only at the C–C bond centers in the plane
• On the hydrogen atoms

Correct Answer: Above and below the ring plane

Q29. Are benzene π molecular orbitals localized on individual C=C bonds or delocalized over the whole ring?

• Localized on alternating C=C bonds only
• Delocalized over all six carbon atoms
• Localized on hydrogen atoms
• Localized between nonadjacent carbons exclusively

Correct Answer: Delocalized over all six carbon atoms

Q30. Photoelectron spectroscopy (PES) of benzene typically shows how many major ionization peaks for π electrons corresponding to occupied π levels?

• One peak
• Two peaks
• Three peaks
• Six peaks

Correct Answer: Three peaks

Q31. A relatively large HOMO–LUMO energy gap in benzene contributes to which property?

• High chemical reactivity toward addition
• Low chemical stability
• Thermodynamic stability and low tendency for π→π* excitations at long wavelengths
• Strong radical character

Correct Answer: Thermodynamic stability and low tendency for π→π* excitations at long wavelengths

Q32. Which reaction type is most commonly observed for benzene under normal conditions?

• Radical addition
• Electrophilic aromatic substitution (EAS)
• Nucleophilic addition to the ring π system
• Pericyclic cycloaddition on the ring

Correct Answer: Electrophilic aromatic substitution (EAS)

Q33. Aromatic stabilization arises primarily from which orbital interaction?

• Overlap of sp3 orbitals around the ring
• Continuous overlap of adjacent 2pz orbitals forming a cyclic π system
• Hydrogen bonding between ring hydrogens
• Dipole–dipole interactions only

Correct Answer: Continuous overlap of adjacent 2pz orbitals forming a cyclic π system

Q34. How many σ bonds are present in benzene (counting C–C and C–H σ bonds)?

• 6 σ bonds
• 9 σ bonds
• 12 σ bonds
• 18 σ bonds

Correct Answer: 12 σ bonds

Q35. In the benzene radical cation, how is the unpaired electron best described?

• Localized on a single carbon atom
• Delocalized over the π system
• Located on a hydrogen atom
• Confined to the σ framework

Correct Answer: Delocalized over the π system

Q36. Are the π electrons in benzene localized in alternating double bonds in the ground state?

• Yes, they are fully localized
• No, they are delocalized across the ring
• They are localized on hydrogens
• They are only present in σ orbitals

Correct Answer: No, they are delocalized across the ring

Q37. Which spectroscopic technique directly probes molecular orbital ionization energies experimentally?

• Infrared (IR) spectroscopy
• Ultraviolet–visible (UV–Vis) spectroscopy
• Photoelectron spectroscopy (PES)
• Nuclear magnetic resonance (NMR)

Correct Answer: Photoelectron spectroscopy (PES)

Q38. The resonance (stabilization) energy of benzene relative to hypothetical cyclohexatriene is approximately:

• 5 kcal/mol
• 20 kcal/mol
• 36 kcal/mol
• 100 kcal/mol

Correct Answer: 36 kcal/mol

Q39. Bonding between adjacent p orbitals in benzene requires which phase relationship?

• Out-of-phase overlap only
• In-phase overlap (same sign lobes overlapping)
• No overlap at all
• Overlap through the hydrogen atoms

Correct Answer: In-phase overlap (same sign lobes overlapping)

Q40. How many formal π bonds are counted in benzene when using simple Lewis structures?

• 1 π bond
• 2 π bonds
• 3 π bonds
• 6 π bonds

Correct Answer: 3 π bonds

Q41. Are the σ-framework and π-system in benzene orthogonal to each other?

• No, they are colinear
• Yes, σ bonds lie in the ring plane while π orbitals are perpendicular
• They are both perpendicular to the ring plane
• They overlap strongly causing σ–π mixing

Correct Answer: Yes, σ bonds lie in the ring plane while π orbitals are perpendicular

Q42. Electron-donating substituents typically direct electrophilic aromatic substitution to which positions?

• Meta only
• Ortho and para positions
• Only para position
• Anti positions

Correct Answer: Ortho and para positions

Q43. According to Frontier Molecular Orbital (FMO) theory, which orbitals interact principally during electrophilic aromatic substitution?

• Nucleophile LUMO with electrophile HOMO
• Aromatic HOMO with electrophile LUMO
• Both HOMOs only
• σ orbitals of benzene only

Correct Answer: Aromatic HOMO with electrophile LUMO

Q44. Benzene’s response to an external magnetic field reveals a ring current; this behavior indicates what magnetic property?

• Paramagnetism
• Diamagnetism associated with aromatic ring currents
• Ferromagnetism
• Antiferromagnetism

Correct Answer: Diamagnetism associated with aromatic ring currents

Q45. How many occupied π molecular orbitals are present in ground-state benzene?

• One occupied π orbital
• Two occupied π orbitals
• Three occupied π orbitals
• Six occupied π orbitals

Correct Answer: Three occupied π orbitals

Q46. Which of the following best describes benzene’s highest occupied π MOs in terms of degeneracy?

• They are non-degenerate
• They form a doubly degenerate pair
• They form a triply degenerate set
• They are all singly degenerate with different energies

Correct Answer: They form a doubly degenerate pair

Q47. The highest-energy π molecular orbital of benzene displays what pattern of phases around the ring?

• All lobes in-phase
• Alternating in-phase and out-of-phase creating three sign changes
• No sign changes around the ring
• Random phase distribution with no nodes

Correct Answer: Alternating in-phase and out-of-phase creating three sign changes

Q48. Which computational approach is commonly used for accurate visualization and quantitative energies of benzene molecular orbitals?

• Classical mechanics
• Density functional theory (DFT)
• Empirical boiling point estimation
• Simple ball-and-stick modeling

Correct Answer: Density functional theory (DFT)

Q49. The HOMO of benzene is best described as which of the following in terms of degeneracy?

• Non-degenerate and singly occupied
• Doubly degenerate and fully occupied
• Triply degenerate and unoccupied
• Non-degenerate and unoccupied

Correct Answer: Doubly degenerate and fully occupied

Q50. How many equivalent Kekulé resonance structures are required to represent benzene’s delocalization in simple valence-bond terms?

• One
• Two
• Three
• Six

Correct Answer: Two

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