Continuous Flow Reactors: advantages and synthetic applications MCQs With Answer

Continuous flow reactors have transformed synthetic organic chemistry by offering precise control over reaction parameters, superior heat and mass transfer, and enhanced safety for hazardous processes. This MCQ set is designed for M.Pharm students studying Advanced Organic Chemistry II, focusing on the advantages, reactor types, operational parameters, and real-world synthetic applications of continuous flow technology. Questions explore key concepts such as residence time distribution, microreactors, photochemical and electrochemical flow reactions, scale-up strategies, and continuous downstream processing like crystallization and inline analysis. The quiz balances theory and practice to reinforce understanding and prepare students for applying flow methodologies in pharmaceutical synthesis and process development.

Q1. Which primary advantage of continuous flow reactors directly contributes to improved temperature control for highly exothermic organic reactions?

• Longer residence time
• High surface-area-to-volume ratio
• Lower reagent consumption
• Batch-to-batch variability

Correct Answer: High surface-area-to-volume ratio

Q2. In the context of continuous flow chemistry, what does residence time distribution (RTD) primarily affect?

• Purity of final product by influencing mixing and selectivity
• The wavelength of light used in photochemistry
• The electrode potential in electrochemical cells
• The boiling point of solvents

Correct Answer: Purity of final product by influencing mixing and selectivity

Q3. Which reactor type behaves closest to an ideal plug flow reactor (PFR) commonly used in flow synthesis?

• Well-stirred batch flask
• Long tubular microreactor with laminar flow
• Stirred tank packed with inert beads
• Open-air photoreactor

Correct Answer: Long tubular microreactor with laminar flow

Q4. Which synthetic application is particularly advantaged by continuous flow photochemistry?

• High-temperature thermal rearrangements
• Reactions requiring uniform photon flux and short irradiation times
• Slow heterogeneous catalysis requiring long contact times
• Standard SN1 substitutions at ambient light

Correct Answer: Reactions requiring uniform photon flux and short irradiation times

Q5. Why are continuous flow reactors considered safer than batch reactors for hazardous reagents such as diazomethane or azides?

• They increase the total inventory of hazardous reagent
• They eliminate the need for any solvent
• They minimize on-line concentration and exposure by using small reactor volumes
• They always operate at room temperature

Correct Answer: They minimize on-line concentration and exposure by using small reactor volumes

Q6. Which parameter is critical to control when performing gas–liquid reactions in continuous flow, such as hydrogenation?

• Electrode surface area
• Gas–liquid interfacial area and mass transfer rate
• Optical path length for irradiation
• Crystallization seeding temperature

Correct Answer: Gas–liquid interfacial area and mass transfer rate

Q7. What is a key benefit of telescoped multi-step synthesis in continuous flow without intermediate isolation?

• Increased solvent consumption
• Reduced overall yield due to accumulation of impurities
• Reduced process time and improved overall efficiency
• Necessity for larger batch reactors

Correct Answer: Reduced process time and improved overall efficiency

Q8. In flow chemistry scale-up, which principle is most often used to maintain product selectivity when moving from lab to production?

• Maintaining identical reactor surface area only
• Geometric scaling by multiplying reactor length regardless of flow rates
• Maintaining constant space velocity or residence time and mixing characteristics
• Switching from laminar to turbulent regime without design changes

Correct Answer: Maintaining constant space velocity or residence time and mixing characteristics

Q9. Which of the following is a common design for immobilized catalyst use in continuous flow organic synthesis?

• Homogeneous catalyst dissolved in large batch reactors
• Packed-bed reactor containing catalyst particles
• Static photoreactor with no catalyst
• Open stirred tank with free catalyst crystals

Correct Answer: Packed-bed reactor containing catalyst particles

Q10. How does segmented (slug) flow improve mixing in microreactors?

• By creating alternating liquid and gas segments that generate internal circulation within slugs
• By increasing electrode polarization
• By ensuring complete laminar flow without internal circulation
• By eliminating phase boundaries entirely

Correct Answer: By creating alternating liquid and gas segments that generate internal circulation within slugs

Q11. Which continuous flow approach is best suited for highly exothermic nitration reactions to improve safety and control?

• Large-volume batch reactor with slow cooling
• Microreactor with rapid heat dissipation and short residence time
• Open-air bench stirrer
• Static crystallizer

Correct Answer: Microreactor with rapid heat dissipation and short residence time

Q12. Which inline analytical technique is frequently integrated with continuous flow to enable real-time reaction monitoring?

• Offline thin-layer chromatography only
• Inline IR, NMR, or HPLC for immediate analysis
• End-of-day mass balance weighing
• Manual sampling followed by long delays

Correct Answer: Inline IR, NMR, or HPLC for immediate analysis

Q13. In electrochemical continuous flow cells, what factor most directly influences current density and reaction rate?

• Residence time distribution only
• Distance between electrodes and electrode surface area
• Volume of downstream collection vessel
• Type of solvent crystallization behavior

Correct Answer: Distance between electrodes and electrode surface area

Q14. Which phenomenon must be considered in microreactors due to dominant laminar flow when designing mixing strategies?

• Turbulent eddy diffusion
• Molecular diffusion and chaotic advection techniques
• Natural convection from boiling
• Complete lack of any concentration gradients

Correct Answer: Molecular diffusion and chaotic advection techniques

Q15. Which continuous process is commonly coupled with flow synthesis for the production of well-controlled pharmaceutical solids?

• Batch distillation
• Continuous crystallization for narrow particle size distribution
• Open-air evaporation on trays
• Large-scale manual filtration

Correct Answer: Continuous crystallization for narrow particle size distribution

Q16. Photoredox catalysis in flow benefits from which of the following compared to batch photochemistry?

• Uneven light exposure throughout a thick batch reactor
• More efficient photon utilization and controlled irradiation time
• Requirement for significantly higher photocatalyst loadings
• Slower kinetics due to reduced surface area

Correct Answer: More efficient photon utilization and controlled irradiation time

Q17. What is a major challenge when running heterogeneous suspensions in microreactors and how is it addressed?

• Excessive vapor pressure; addressed by removing solvent
• Particle clogging; addressed by using larger channels, segmented flow, or packed-bed designs
• Inability to control temperature; addressed by adding more solvent
• Insufficient mixing; addressed by operating at extremely low flow rates only

Correct Answer: Particle clogging; addressed by using larger channels, segmented flow, or packed-bed designs

Q18. Which metric is most useful for comparing heat transfer performance between batch and continuous reactors?

• Residence time distribution only
• Heat transfer coefficient (U) and surface-area-to-volume ratio
• Optical transparency of reactor walls
• Total mass of reactor vessel

Correct Answer: Heat transfer coefficient (U) and surface-area-to-volume ratio

Q19. Which type of reaction has been successfully translated to continuous flow for safer on-demand production in pharmaceutical contexts?

• Large-scale uncontrolled polymerizations with runaway risk
• Hazardous gas-generating processes such as diazotization and subsequent Sandmeyer reactions
• Simple room-temperature SN2 substitutions with benign reagents only
• Open-flame thermal decompositions

Correct Answer: Hazardous gas-generating processes such as diazotization and subsequent Sandmeyer reactions

Q20. When designing a continuous flow route for an organometallic cross-coupling reaction, which factor is crucial to maintain reagent integrity?

• Ensuring exposure to air and moisture at all times
• Oxygen- and moisture-free conditions, often achieved by inline degassing and inert solvent handling
• Using the largest possible reactor volume to dilute reagents
• Frequent manual addition of reagents mid-run

Correct Answer: Oxygen- and moisture-free conditions, often achieved by inline degassing and inert solvent handling

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