Nitration: reagents, kinetics, equipment and mixed-acid processes MCQs With Answer

Nitration: reagents, kinetics, equipment and mixed-acid processes MCQs With Answer

This set of multiple-choice questions is designed for M.Pharm students studying MPC 204T Pharmaceutical Process Chemistry. The quiz focuses on nitration — an essential electrophilic aromatic substitution — covering mixed-acid reagents, nitronium ion formation, reaction kinetics and mechanisms, practical equipment and process design, safety and environmental controls. Questions probe deeper into rate laws, rate-determining steps, effects of acid composition and temperature on selectivity, calorimetry, quench and effluent handling, and modern continuous nitration approaches. Answers are provided to help students evaluate understanding of both theoretical principles and industrial practice in nitration processes.

Q1. What is the primary role of sulfuric acid in a mixed-acid nitration system?

• Oxidizing agent that converts aromatic ring to epoxide
• Dehydrating agent and proton donor that generates the nitronium ion
• Phase-transfer catalyst to solubilize organics in water
• Reducing agent that scavenges NOx

Correct Answer: Dehydrating agent and proton donor that generates the nitronium ion

Q2. Which balanced equation best represents nitronium ion generation in a typical mixed-acid nitration?

• HNO3 + H2O → NO2 + 2 H2O
• HNO3 + 2 H2SO4 → NO2+ + H3O+ + 2 HSO4-
• 2 HNO3 → N2O5 + H2O
• HNO2 + H2SO4 → NO+ + SO4 2-

Correct Answer: HNO3 + 2 H2SO4 → NO2+ + H3O+ + 2 HSO4-

Q3. The overall molecularity for the electrophilic nitration of benzene (formation of sigma complex) is best described as:

• Unimolecular (first-order)
• Bimolecular (second-order)
• Termolecular (third-order)
• Catalytic chain reaction (autocatalytic)

Correct Answer: Bimolecular (second-order)

Q4. For aromatic nitration such as benzene → nitrobenzene, which step is generally considered the rate-determining step?

• Deprotonation of the sigma complex
• Formation of the nitronium ion from nitric and sulfuric acids
• Nucleophilic attack of benzene on NO2+ to form the sigma (arenium) complex
• Rearrangement of the sigma complex to give nitro product

Correct Answer: Nucleophilic attack of benzene on NO2+ to form the sigma (arenium) complex

Q5. How does increasing sulfuric acid concentration (keeping nitric acid constant) usually affect the rate of aromatic nitration?

• Decreases the rate by diluting nitronium ion
• Has no effect on the rate
• Increases the rate by increasing nitronium ion concentration
• Converts the mechanism to radical nitration

Correct Answer: Increases the rate by increasing nitronium ion concentration

Q6. To maximize mononitration selectivity and minimize dinitration for an activated aromatic substrate, the preferred process conditions are:

• High temperature and excess nitric acid
• High temperature and dilute sulfuric acid
• Low temperature and excess sulfuric acid
• Use of strong oxidizer and high pressure

Correct Answer: Low temperature and excess sulfuric acid

Q7. Which reactor material is commonly recommended for large-scale mixed-acid nitration to resist strong acids and ensure longevity?

• Ordinary carbon steel without lining
• Glass-lined steel or PTFE-lined stainless steel
• Aluminum alloy reactors
• Pure copper vessels

Correct Answer: Glass-lined steel or PTFE-lined stainless steel

Q8. What is the primary purpose of reaction calorimetry in process development of nitration reactions?

• To determine catalyst lifetime
• To measure heat release and adiabatic temperature rise for safe scale-up
• To analyze product boiling point
• To quantify nitronium concentration directly

Correct Answer: To measure heat release and adiabatic temperature rise for safe scale-up

Q9. Which gaseous emission is characteristically produced during mixed-acid nitration and requires control/abatement?

• Sulfur hexafluoride (SF6)
• Chlorine gas (Cl2)
• Nitrogen dioxide (NO2)
• Methane (CH4)

Correct Answer: Nitrogen dioxide (NO2)

Q10. Compared to batch nitration, a continuous-flow microreactor approach primarily offers:

• Lower capital cost and higher manual handling
• Poorer temperature control but higher selectivity
• Improved heat and mass transfer, enhancing safety and selectivity
• Requirement for larger volumes of mixed acid

Correct Answer: Improved heat and mass transfer, enhancing safety and selectivity

Q11. What is the chemical formula of the active electrophile in classical mixed-acid aromatic nitration?

• NO3-
• NO2 (neutral radical)
• NO2+
• NH3+

Correct Answer: NO2+

Q12. Mixed-acid nitration is primarily employed for which class of substrates in pharmaceutical process chemistry?

• Aromatic compounds (electrophilic aromatic substitution)
• Simple alkanes under radical conditions
• Primary alcohol oxidation to aldehydes
• Peptide coupling reactions

Correct Answer: Aromatic compounds (electrophilic aromatic substitution)

Q13. The recommended immediate quench procedure for a typical nitration reaction involves:

• Adding concentrated nitric acid to the hot reaction mixture
• Slowly pouring the reaction mixture into ice-cold water and neutralizing
• Distilling off sulfuric acid at reflux
• Heating to decompose residual nitronium ions

Correct Answer: Slowly pouring the reaction mixture into ice-cold water and neutralizing

Q14. A significantly negative activation entropy (ΔS‡) observed for a nitration reaction suggests:

• The transition state is more disordered than reactants
• The reaction proceeds by a radical chain mechanism
• The transition state is more ordered, consistent with an associative electrophilic attack
• There is no temperature dependence for the rate

Correct Answer: The transition state is more ordered, consistent with an associative electrophilic attack

Q15. The methyl substituent on toluene directs nitration predominantly to which positions on the aromatic ring?

• Meta only (deactivating)
• Ortho and para (activating, electron-donating)
• Para only due to steric hindrance
• It prevents nitration entirely

Correct Answer: Ortho and para (activating, electron-donating)

Q16. Mechanistically, how does sulfuric acid facilitate formation of the nitronium ion from nitric acid?

• By acting as an oxidant to remove an electron from HNO3
• By protonating HNO3 and promoting loss of water to give NO2+
• By forming a covalent adduct that directly nitrates the aromatic ring
• By acting purely as a solvent with no chemical role

Correct Answer: By protonating HNO3 and promoting loss of water to give NO2+

Q17. In mixed-acid nitration, which species is most commonly considered the direct nitrating agent that attacks the aromatic substrate?

• Nitric acid molecule (HNO3)
• Nitronium ion (NO2+)
• Nitrite ion (NO2-)
• Nitrogen monoxide (NO)

Correct Answer: Nitronium ion (NO2+)

Q18. From a process control perspective, the most direct measure of nitrating power of a mixed acid is:

• Measured viscosity of the mixed acid
• Concentration of nitronium ion (NO2+) or its practical proxies
• Color of the acid mixture
• Electrical conductivity of the mixture

Correct Answer: Concentration of nitronium ion (NO2+) or its practical proxies

Q19. What is a common industrial method for abating NOx emissions from nitration plants?

• Direct release to the atmosphere at night
• Wet alkaline scrubbing to remove NOx as nitrates/nitrites
• Flaring without scrubbing
• Neutralization with concentrated hydrochloric acid

Correct Answer: Wet alkaline scrubbing to remove NOx as nitrates/nitrites

Q20. Why is semi-batch addition of nitric acid to an aromatic substrate frequently used in industrial nitration?

• It increases the concentration of nitronium ion abruptly to speed reaction
• It allows better heat control and reduces local excess of nitrating agent, improving safety and selectivity
• It prevents formation of any nitro product at all
• It eliminates the need for cooling systems

Correct Answer: It allows better heat control and reduces local excess of nitrating agent, improving safety and selectivity

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