Organic Synthesis and Analysis Practice Questions PDF

Questions: 1. Which of the following techniques is used to separate liquids with different boiling points? * A) Filtration * B) Distillation * C) Recrystallisation * D) Chromatography 2. What is the purpose of re uxing a reaction mixture? * A) To separate a...

Questions: 1. Which of the following techniques is used to separate liquids with different boiling points? * A) Filtration * B) Distillation * C) Recrystallisation * D) Chromatography 2. What is the purpose of re uxing a reaction mixture? * A) To separate a solid from a liquid. * B) To purify a solid. * C) To heat a reaction mixture without losing volatile reactants or products. * D) To measure the melting point of a solid. 3. In simple distillation, where should the thermometer bulb be placed? * A) In the reaction mixture. * B) At the bottom of the condenser. * C) At the junction of the condenser and the still head. * D) In the receiving ask. 4. What is the purpose of the condenser in distillation? * A) To heat the reaction mixture. * B) To cool the vapours back into a liquid. * C) To prevent evaporation. * D) To remove impurities. 5. What is the purpose of anti-bumping granules in distillation? * A) To speed up the reaction. * B) To prevent vigorous, uneven boiling. * C) To increase the yield. * D) To purify the product. 6. Which technique is used to purify an impure solid? * A) Distillation * B) Re ux * C) Recrystallisation * D) Titration fl fl fl  7. What is the rst step in recrystallisation? * A) Cool the solution slowly. * B) Dissolve the solid in a minimum volume of hot solvent. * C) Filter the hot solution. * D) Dry the crystals. 8. Why is a minimum volume of hot solvent used in recrystallisation? * A) To ensure all the solid dissolves. * B) To prevent the formation of crystals. * C) To obtain a saturated solution and maximise yield on cooling. * D) To speed up the process. 9. Why is the hot solution ltered during recrystallisation? * A) To remove insoluble impurities. * B) To remove soluble impurities. * C) To collect the crystals. * D) To cool the solution. 10. Why are the crystals washed with cold solvent after ltration in recrystallisation? * A) To dissolve the crystals. * B) To remove soluble impurities. * C) To dry the crystals. * D) To make the crystals bigger. 11. What is a suitable method to dry the crystals after recrystallisation? * A) Heating in a Bunsen burner ame. * B) Leaving them in a warm oven. * C) Drying in a dessicator or between lter papers * D) Washing with more solvent. 12. What technique can be used to assess the purity of a solid? * A) Re uxing. * B) Melting Point Determination * C) Distillation. * D) Recrystallisation. 13. How does the melting point of an impure substance compare to that of the pure substance? * A) It is higher and over a wider range. fl fi fi fl fi fi * B) It is lower and over a wider range. * C) It is the same. * D) It is higher and sharper. 14. What is the mobile phase in thin-layer chromatography (TLC)? * A) The stationary phase * B) The solvent * C) The sample * D) The TLC plate 15. What is the stationary phase in thin-layer chromatography (TLC)? * A) The solvent * B) The sample * C) A thin layer of silica or alumina on a glass plate * D) The mobile phase 16. What is the Rf value in chromatography? * A) The distance travelled by the solvent. * B) The distance travelled by the spot divided by the distance travelled by the solvent. * C) The distance travelled by the solvent divided by the distance travelled by the spot. * D) The time taken for the spot to move. 17. How can colourless spots be visualised on a TLC plate? * A) Heating the plate. * B) Using a UV lamp or a locating agent. * C) Adding water to the plate. * D) Using a microscope. 18. Which of the following functional groups can be identi ed using 2,4- dinitrophenylhydrazine (2,4-DNPH)? * A) Alcohols * B) Carboxylic acids * C) Aldehydes and ketones * D) Alkenes 19. What is observed when 2,4-DNPH reacts with a carbonyl compound? * A) A silver mirror forms. fi * B) A yellow/orange precipitate forms. * C) The solution turns from orange to green. * D) Effervescence. 20. How can infrared (IR) spectroscopy be used to distinguish between different functional groups? * A) Different functional groups absorb at different wavenumbers. * B) Different functional groups have different colours. * C) Different functional groups have different melting points. * D) Different functional groups have different Rf values. Answers and Explanations: 1. B - Distillation separates liquids based on boiling points. 2. C - Re uxing allows for prolonged heating without loss of volatile substances. 3. C - The thermometer should measure the temperature of the vapour entering the condenser. 4. B 5. B - Anti-bumping granules provide a surface for small bubbles to form, preventing large, disruptive bubbles. 6. C - Recrystallisation puri es solids by exploiting differences in solubility. 7. B 8. C - This ensures that the solution is saturated when hot, so crystals form upon cooling. 9. A - Insoluble impurities are removed by hot ltration. 10. B - Washing with *cold* solvent removes soluble impurities without dissolving the desired product. 11. C 12. B - A pure substance has a sharp and distinct melting point. 13. B - Impurities lower and broaden the melting point range. 14. B - The solvent moves up the TLC plate. 15. C 16. B - Rf = (distance moved by spot) / (distance moved by solvent) 17. B - UV light or locating agents (e.g., iodine) make colourless spots visible. 18. C - 2,4-DNPH reacts with the carbonyl group (C=O) in aldehydes and ketones. 19. B fl fi fi 20. A - Different functional groups absorb speci c IR frequencies (wavenumbers), creating a unique spectrum. fi