Reaction Mechanisms: Types and methods to determine mechanisms MCQs With Answer

Introduction: This quiz set on Reaction Mechanisms: Types and Methods to Determine Mechanisms is tailored for M.Pharm students studying MPC 102T Advanced Organic Chemistry I. It focuses on core mechanistic concepts—ionic, radical, concerted and pericyclic pathways—and practical approaches used to identify them, such as kinetics, isotope effects, stereochemical probes, trapping/crossover experiments, Hammett plots, Eyring/Arrhenius analysis, spectroscopic detection, and computational methods. The questions are designed to test both theoretical understanding and the application of experimental tools that reveal stepwise details of reaction pathways. Use these MCQs to reinforce interpretation of mechanistic evidence commonly encountered in pharmaceutical research and advanced organic synthesis.

Q1. Which mechanism is most consistent with a bimolecular second-order rate law for a nucleophilic substitution?

• SN1 (unimolecular nucleophilic substitution)
• SN2 (bimolecular nucleophilic substitution)
• E1 (unimolecular elimination)
• Radical chain substitution

Correct Answer: SN2 (bimolecular nucleophilic substitution)

Q2. A large primary kinetic isotope effect (kH/kD ≈ 6–8) observed when hydrogen is replaced by deuterium at a reacting center most likely indicates what?

• The C–H bond is not involved in the rate-determining step
• The C–H bond cleavage occurs in the rate-determining step
• A secondary isotope effect due to hybridization change
• An inverse isotope effect due to tunneling

Correct Answer: The C–H bond cleavage occurs in the rate-determining step

Q3. Observation of complete inversion of configuration at a stereogenic carbon in a substitution product is most diagnostic of which pathway?

• SN1 with racemization
• SN2 with backside attack
• Radical recombination
• Concerted pericyclic rearrangement

Correct Answer: SN2 with backside attack

Q4. Formation of a racemic product from an optically active substrate in a substitution reaction is strong evidence for which intermediate?

• Carbocation intermediate (planar)
• Concerted single-step substitution
• Diradical pair tightly bound

Correct Answer: Carbocation intermediate (planar)

Q5. In a Hammett study a positive ρ (rho) value indicates which type of charge development in the transition state?

• Buildup of positive charge at the reaction center
• No significant charge development
• Buildup of negative charge at the reaction center
• Only steric effects dominate, not electronic

Correct Answer: Buildup of negative charge at the reaction center

Q6. A crossover experiment yields mixed (crossover) products when two different labeled substrates are reacted together. What mechanistic conclusion is most reasonable?

• The mechanism is intramolecular and concerted
• The mechanism proceeds via free intermolecular intermediates (e.g., ions or radicals)
• The reaction is purely stereospecific with no intermediates
• The reaction follows a single-electron transfer with no scrambling

Correct Answer: The mechanism proceeds via free intermolecular intermediates (e.g., ions or radicals)

Q7. The rate law experimentally determined for an elimination giving a second-order dependence on substrate and base concentrations is most consistent with which mechanism?

• E1 (unimolecular elimination)
• E2 (bimolecular elimination)
• E1cb (conjugate base mechanism)
• Radical chain elimination

Correct Answer: E2 (bimolecular elimination)

Q8. Which spectroscopic technique directly detects unpaired electron-containing intermediates and is therefore often used to prove radical mechanisms?

• Infrared (IR) spectroscopy
• Electron paramagnetic resonance (EPR) spectroscopy
• Ultraviolet-visible (UV-Vis) spectroscopy
• 13C NMR spectroscopy

Correct Answer: Electron paramagnetic resonance (EPR) spectroscopy

Q9. Which plot is used to extract the activation energy (Ea) of a reaction from the temperature dependence of the rate constant?

• ln(k) vs ln([substrate]) plot
• ln(k) vs 1/T (Arrhenius plot)
• ln(k/T) vs 1/T (Eyring plot)
• Rate vs [substrate] plot

Correct Answer: ln(k) vs 1/T (Arrhenius plot)

Q10. The steady-state approximation assumes which relationship for a short-lived intermediate I in a mechanism?

• The concentration of I rapidly reaches equilibrium with reactants
• The rate of formation of I equals the rate of its consumption
• The intermediate I accumulates to large concentration during reaction
• I has a half-life much longer than the overall reaction

Correct Answer: The rate of formation of I equals the rate of its consumption

Q11. Which mechanistic assumption is appropriate when an initial fast equilibrium is followed by a slower, rate-determining step?

• Steady-state approximation
• Pre-equilibrium approximation
• Transition-state theory cannot be applied
• Radical clock analysis

Correct Answer: Pre-equilibrium approximation

Q12. To determine whether oxygen in a product originates from solvent water or from an oxidant, which experimental approach is most decisive?

• Infrared spectroscopy of the crude mixture
• 18O isotopic labeling combined with mass spectrometry
• Measurement of reaction enthalpy
• Ultraviolet-visible absorption of the product

Correct Answer: 18O isotopic labeling combined with mass spectrometry

Q13. A small normal secondary kinetic isotope effect (kH/kD ≈ 1.1) at a carbon atom undergoing reaction commonly indicates what change?

• Cleavage of the C–H bond in the rate-determining step
• Change in hybridization at carbon (e.g., sp3 → sp2)
• Involvement of radical recombination
• That isotopic substitution has no mechanistic implication

Correct Answer: Change in hybridization at carbon (e.g., sp3 → sp2)

Q14. Which analysis yields both the enthalpy and entropy of activation (ΔH‡ and ΔS‡) for a reaction?

• Arrhenius plot of ln(k) vs 1/T
• Eyring analysis (plot of ln(k/T) vs 1/T)
• Hammett correlation
• Isotope exchange experiments

Correct Answer: Eyring analysis (plot of ln(k/T) vs 1/T)

Q15. Trapping experiments that convert a transient carbocation into a stable adduct support which mechanistic feature?

• That the reaction proceeds by a concerted pericyclic pathway
• Existence of a discrete carbocation intermediate
• Evidence for a direct hydrogen abstraction by radicals
• That the substrate undergoes only surface-catalyzed transformations

Correct Answer: Existence of a discrete carbocation intermediate

Q16. Which observation is most characteristic of a concerted pericyclic reaction (e.g., a Diels–Alder cycloaddition)?

• Formation of long-lived ionic intermediates detected by NMR
• Stereospecific product formation with no interchangeable intermediates
• A rate law that depends on solvent acidity
• Requirement for radical initiators to proceed

Correct Answer: Stereospecific product formation with no interchangeable intermediates

Q17. Which mechanistic probe uses a substrate that undergoes a very rapid unimolecular rearrangement upon formation of a radical to estimate radical lifetimes or capture rates?

• Crossover experiment
• Hammett analysis
• Radical clock experiment
• Isotopic labeling with 13C

Correct Answer: Radical clock experiment

Q18. Which solvent type typically favors an SN1 mechanism by stabilizing a carbocation intermediate?

• Nonpolar aprotic solvents (e.g., hexane)
• Polar protic solvents (e.g., water, alcohols)
• Nonpolar protic solvents
• Supercritical CO2

Correct Answer: Polar protic solvents (e.g., water, alcohols)

Q19. In a linear free energy relationship, a large magnitude of the reaction constant (|ρ|) indicates what about the transition state?

• It is insensitive to electronic substituent effects
• Significant charge development or strong sensitivity to substituent electronics
• That steric effects dominate and electronic effects are negligible
• Reaction is governed entirely by solvent polarity rather than substituents

Correct Answer: Significant charge development or strong sensitivity to substituent electronics

Q20. Which modern method is commonly used to locate transition states, estimate activation barriers, and complement experimental mechanistic studies?

• Classical molecular mechanics with no electronic structure
• Density functional theory (DFT) and quantum chemical calculations
• Thin-layer chromatography monitoring only
• Simple empirical Hammett predictions without computation

Correct Answer: Density functional theory (DFT) and quantum chemical calculations

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