Specific acid-base catalysis MCQs With Answer

Specific acid-base catalysis MCQs with Answer are designed for B.Pharm students to master proton-transfer mechanisms and kinetic principles that govern pharmaceutical reactions. This set focuses on specific acid catalysis, specific base catalysis, rate laws, pH-rate profiles, solvent and isotope effects, Bronsted relationships, and buffer-independent proton transfers relevant to drug stability and metabolism. Emphasis is on reaction mechanisms, transition states, catalytic coefficients, experimental determination of catalytic pathway, and implications for enzyme and pharmaceutical chemistry. Clear conceptual understanding of how proton concentration (H+ or OH−) directly affects reaction rates will aid in formulation and drug design. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What defines specific acid catalysis?

• The rate depends only on the concentration of the proton donor H+
• The rate depends on the concentration of all acids present including buffers
• The rate is independent of proton concentration
• The rate depends only on the base concentration OH−

Correct Answer: The rate depends only on the concentration of the proton donor H+

Q2. Which experimental observation indicates specific base catalysis?

• Rate is proportional to OH− concentration and independent of buffer concentration
• Rate increases with general acid concentration
• Rate is unaffected by pH changes
• Rate depends on ionic strength but not on OH−

Correct Answer: Rate is proportional to OH− concentration and independent of buffer concentration

Q3. In a pH-rate profile showing a slope of +1 at high pH, this suggests:

• Specific base catalysis where rate ∝ [OH−]
• Specific acid catalysis where rate ∝ [H+]
• Buffer catalysis dominating the reaction
• No catalytic role of protons or hydroxide

Correct Answer: Specific base catalysis where rate ∝ [OH−]

Q4. Which statement best distinguishes specific from general (buffer) catalysis?

• Specific catalysis depends only on H+ or OH−; general catalysis depends on other proton donors/acceptors such as buffer components
• Specific catalysis involves enzymes; general catalysis does not
• Specific catalysis always gives faster rates than general catalysis
• Specific catalysis requires organic solvents while general catalysis occurs in water

Correct Answer: Specific catalysis depends only on H+ or OH−; general catalysis depends on other proton donors/acceptors such as buffer components

Q5. Which kinetic experiment is most useful to distinguish specific acid catalysis from general acid catalysis?

• Vary buffer concentration at constant pH and observe rate changes
• Measure UV absorbance at a single pH
• Perform reaction in absence of solvent
• Vary ionic strength only

Correct Answer: Vary buffer concentration at constant pH and observe rate changes

Q6. In a Bronsted plot (log k vs pKa of acid catalyst), a slope near zero indicates:

• No significant proton transfer in the rate-determining step
• Strong dependence of rate on acid strength
• Complete proton transfer in the transition state
• Reaction is diffusion-controlled

Correct Answer: No significant proton transfer in the rate-determining step

Q7. The presence of a large solvent kinetic isotope effect (kH2O/kD2O > 2) typically indicates:

• Proton transfer is involved in the rate-determining step
• No proton transfer in the mechanism
• Reaction is controlled purely by diffusion
• Only ionic strength effects are important

Correct Answer: Proton transfer is involved in the rate-determining step

Q8. For ester hydrolysis in acid, specific acid catalysis mechanism involves:

• Protonation of the carbonyl oxygen followed by nucleophilic attack by water
• Direct attack by OH− on the carbonyl carbon
• Concerted removal of proton and leaving group without protonation
• Radical chain mechanism initiated by H+

Correct Answer: Protonation of the carbonyl oxygen followed by nucleophilic attack by water

Q9. Which parameter quantifies the intrinsic ability of H+ to catalyze a reaction (specific acid catalysis)?

• Rate constant proportional to [H+]
• Buffer catalytic coefficient
• Activation entropy only
• Equilibrium constant of substrate

Correct Answer: Rate constant proportional to [H+]

Q10. In buffer catalysis (general acid catalysis), increasing total buffer concentration at constant pH typically:

• Increases the reaction rate
• Decreases the reaction rate
• Has no effect on the rate
• Causes precipitation of substrate

Correct Answer: Increases the reaction rate

Q11. Which observation supports a mechanism with rate-determining proton transfer to solvent rather than to buffer?

• Rate varies with H+ concentration but not with buffer identity or concentration
• Rate changes with buffer identity but not with pH
• Rate is independent of pH and buffer
• Rate is only temperature dependent

Correct Answer: Rate varies with H+ concentration but not with buffer identity or concentration

Q12. When analyzing pH-rate profiles, leveling effect refers to:

• The maximum effective acidity achievable in a solvent due to solvent protonation limitations
• Flattening of rate versus pH due to enzyme saturation
• Buffer capacity limit at high concentrations
• Ionic strength effects masking pH dependence

Correct Answer: The maximum effective acidity achievable in a solvent due to solvent protonation limitations

Q13. A negative slope in a log k vs pH plot indicates:

• Specific acid catalysis where rate decreases as pH increases
• Specific base catalysis where rate increases as pH increases
• No acid-base catalysis involved
• Buffer catalysis dominating the reaction

Correct Answer: Specific acid catalysis where rate decreases as pH increases

Q14. Which example best illustrates specific base catalysis in drug metabolism?

• Deprotonation of a phenolic OH by OH− that accelerates nucleophilic attack
• Protonation of an amide carbonyl by H+
• Enzymatic proton shuttling by histidine without dependence on bulk OH−
• Oxidation by cytochrome P450 unrelated to OH−

Correct Answer: Deprotonation of a phenolic OH by OH− that accelerates nucleophilic attack

Q15. Which experimental control helps rule out general acid catalysis when specific acid catalysis is suspected?

• Keep pH constant while varying buffer concentration
• Change ionic strength while keeping pH and buffer constant
• Vary temperature while keeping pH constant
• Use a nonpolar solvent

Correct Answer: Keep pH constant while varying buffer concentration

Q16. In a reaction showing both specific and general acid catalysis, the observed rate law often is:

• kobs = kH[H+] + kbuffer[Buffer-H]
• kobs = k0 only
• kobs = kOH[OH−] only
• kobs = kT[Temperature]

Correct Answer: kobs = kH[H+] + kbuffer[Buffer-H]

Q17. The primary salt effect often alters observed rates by changing:

• Activity coefficients of charged reactants
• Intrinsic acidity of H+
• Covalent bond strengths
• Solvent viscosity only

Correct Answer: Activity coefficients of charged reactants

Q18. Proton inventory studies (varying H2O/D2O ratios) help determine:

• How many protons are involved in the transition state
• The exact pKa of the substrate
• The molecular weight of intermediates
• The diffusion coefficient of protons

Correct Answer: How many protons are involved in the transition state

Q19. In specific acid catalysis, the transition state is typically stabilized by:

• Protonation of an electronegative atom increasing electrophilicity
• Deprotonation of the leaving group by buffer
• Radical formation
• Coordination to a metal center unrelated to H+

Correct Answer: Protonation of an electronegative atom increasing electrophilicity

Q20. For an SN1-like solvolysis that shows specific base catalysis at high pH, the role of OH− is to:

• Increase nucleophilicity of solvent or substrate via deprotonation
• Protonate the leaving group
• Act as a spectator ion only
• Stabilize the carbocation by direct bonding

Correct Answer: Increase nucleophilicity of solvent or substrate via deprotonation

Q21. Which measurement helps separate contributions of specific acid catalysis from ionic strength effects?

• Vary ionic strength with inert salts while keeping [H+] constant
• Measure rates only at extreme pH values
• Use nonaqueous solvents exclusively
• Perform reactions at a single temperature

Correct Answer: Vary ionic strength with inert salts while keeping [H+] constant

Q22. In enzymatic catalysis, specific acid-base catalysis by bulk H+ or OH− is usually:

• Less important than specific proton transfers mediated by active-site residues
• The dominant catalytic pathway
• Irrelevant to all enzymes
• Always faster than residue-mediated proton transfer

Correct Answer: Less important than specific proton transfers mediated by active-site residues

Q23. A reaction displays first-order dependence on [H+] and zero-order on buffer concentration. This is characteristic of:

• Specific acid catalysis
• General acid catalysis
• Specific base catalysis
• Uncatalyzed reaction

Correct Answer: Specific acid catalysis

Q24. In designing a formulation, why is understanding specific acid-base catalysis important?

• It predicts how pH changes will directly alter degradation rates of drugs
• It tells whether a drug will crystallize
• It identifies flavor of the drug
• It determines the solubility in organic solvents only

Correct Answer: It predicts how pH changes will directly alter degradation rates of drugs

Q25. A Bronsted coefficient (β) near 1 for general base catalysis implies:

• Extensive proton transfer in the transition state
• No proton transfer in the transition state
• Reaction is independent of base strength
• Rate is controlled by diffusion

Correct Answer: Extensive proton transfer in the transition state

Q26. For nucleophilic substitution where OH− acts as specific base, increasing pH will:

• Typically increase the reaction rate
• Always decrease the reaction rate
• Have no effect on rate
• Cause the mechanism to switch to radical pathway

Correct Answer: Typically increase the reaction rate

Q27. Which diagnostic indicates mixed specific and general base catalysis?

• Rate depends on both [OH−] and buffer concentration
• Rate depends only on temperature
• Rate decreases with increasing buffer
• Rate is independent of pH

Correct Answer: Rate depends on both [OH−] and buffer concentration

Q28. How does solvent polarity affect specific acid-base catalysis in aqueous systems?

• Solvent polarity influences stabilization of charged transition states and therefore rate
• Solvent polarity has no role in aqueous reactions
• Only viscosity matters in aqueous catalysis
• Polarity only affects radical reactions

Correct Answer: Solvent polarity influences stabilization of charged transition states and therefore rate

Q29. What does a curved (nonlinear) pH-rate profile generally suggest?

• Multiple mechanistic regimes or involvement of different catalytic species
• Experimental error only
• Purely specific catalysis across entire pH range
• No catalytic involvement of H+ or OH−

Correct Answer: Multiple mechanistic regimes or involvement of different catalytic species

Q30. When determining whether an acid catalysis is specific, which analytical approach is most conclusive?

• Demonstrating rate ∝ [H+] while showing independence from buffer species and concentration
• Measuring only the activation energy
• Observing product distribution at one pH
• Measuring absorbance without kinetic analysis

Correct Answer: Demonstrating rate ∝ [H+] while showing independence from buffer species and concentration