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A Facile Solvent-Free Cannizzaro Reaction. an Instructional Model for Introductory Organic Chemistry Laboratory Supplemental Materials A Facile Solvent-free Cannizzaro Reaction : An Instructional Model for Introductory Organic Chemistry Laboratory. Sonthi Phonchaiya1 and Bhinyo Panijpan2 Institute for Innovation and Development of Learning Process, Mahidol University, Rama 6 Rd., Bangkok, 10400, Thailand Tel. +662-2015104-8, Fax +662-3547345 E-mail: 1. [email protected] 2. [email protected] Shuleewan Rajviroongit Department of Chemistry, Mahidol University, Rama6 Rd., Bangkok 10400 Thailand Tel. + 662-2015134 Fax + 662-3547151 E-mail: [email protected] Tony Wright The School of Education, The University of Queensland, St Lucia, QLD 4072, Australia Tel. +617-33656465 (St Lucia), +617-33811507 (Ipswich) Email: [email protected] Joanne T. Blanchfield The School of Molecular and Microbial Science, The University of Queensland, QLD 4072, Australia Tel. +617-33653622 Fax +617-33654273 E-mail: [email protected] A Facile Solvent-free Cannizzaro Reaction : An Instructional Model for Introductory Organic Chemistry Laboratory Table of Contents Supplements for Students 3 - 21 • Lab Instruction 3 • Pre – Lab Questions 15 • Results sheet 17 • Post - Lab exercise 21 Answer Key 22 – 28 • Pre – Lab Questions 22 • Results sheet 24 • Post - Lab exercise 28 Supplements for instructors 29 - 33 • Safety Information 29 • Lab Preparation 30 • Experimental Tips and Comments 31 • Additional Identification Activities 32 • Thermodynamics activity comments 33 CAS Registry Numbers 34 Product Information 34 IR spectra 35 1H NMR spectra 36 13C NMR spectra 37 2 A Facile Solvent-free Cannizzaro Reaction : An Instructional Model for Introductory Organic Chemistry Laboratory SUPPLEMENTS FOR STUDENTS An Environmentally Friendly Redox Reaction Background Objectives In this experiment you will 1. To perform a solvent-free self-redox investigate a Cannizzaro reaction which is a reaction of 2-chlorobenzaldehyde. self-redox reaction or a disproportionation 2. To monitor the reaction progress by reaction. thin layer chromatography (TLC). 3. To design experimental procedures The disproportionation reaction is for separation and identification of fascinating because a single reactant gives products. rise to two products. Strong bases are used to 4. To learn green chemistry concepts. catalyse the disproportionation reactions of some aldehydes to their corresponding alcohols and carboxylic acids. This experiment provides you with an opportunity to design experimental procedures and characterize the oxidation and reduction products. After performing the experiment you will be introduced to green chemistry concepts and use them to evaluate the environmental impact of the reaction. This experiment consists of four parts; 1. A solvent-free reaction 2. Separation and Identification of products 3. Thermodynamics 4. Green chemistry SAFETY Always wear goggles, gloves and apron when conducting this experiment 3 A Facile Solvent-free Cannizzaro Reaction : An Instructional Model for Introductory Organic Chemistry Laboratory PROCEDURE PART 1: The Solvent-Free Reaction 1. In a fume hood, 2-chlorobenzaldehyde (17.8 mmol, 2 mL) and potassium hydroxide (26.7 mmol, 1.5 g) are added to the mortar. 2. The mixture is ground with a pestle. If solid materials begin to coat the mortar or the pestle, dislodge them with a spatula and put back into the mixture. 3. About 30-40 minutes of grinding are required for completion of the reaction at which point the gummy mixture turns into a paste. starting material + KOH grinding products Figure 1 Performing the reaction in a mortar 4. The reaction is monitored for completion using thin layer chromatography. Thin Layer Chromatography (TLC) TLC is a primary tool for rapid qualitative analysis. It is convenient for monitoring the progress of the reaction. Preparing a TLC chamber o Place a piece of filter paper (size 4.5 x 9 cm) inside a 50-mL beaker. o Pour 5 mL of 30% ethyl acetate – hexane (eluent) into the beaker and cover it with a Petri dish or watch glass. o The chamber is again covered for equilibration of the system, it becomes saturated with solvent vapour in a few minutes and is ready for use. Petri dish or watch glass beaker filter paper Figure 2 The TLC chamber 4 A Facile Solvent-free Cannizzaro Reaction : An Instructional Model for Introductory Organic Chemistry Laboratory Marking the TLC plate o A TLC plate (2.5 x 5 cm) will be provided for each of you in the laboratory. o Mark 3 well-separated points (A, B and C) on the TLC plate with a pencil about 1 cm from the bottom. Figure 3 The TLC with 3 spotting points Monitoring the reaction progress Preparing a sample for monitoring o A very tiny amount of the reaction mixture is transferred to a small vial by spatula. o Dissolve that mixture with a small amount of dichloromethane. (It may not all dissolve, one product is insoluble.) a tiny bit of products Figure 4 Preparing a sample for TLC 5 A Facile Solvent-free Cannizzaro Reaction : An Instructional Model for Introductory Organic Chemistry Laboratory Spotting the TLC o Dip a capillary tube into the reaction mixture. o At position A and B, gently place the tip of the capillary tube onto the surface of the TLC plate and immediately withdraw it. A small amount of mixture in the capillary tube will be adsorbed on the TLC plate (repeat this step if too little of the sample is adsorbed). o Dip a new capillary tube into a sample of 2-chlorobenzaldehyde (provided by the instructor or teaching assistants) and then spot onto the TLC plate at position B and C. o Leave the TLC plate to dry for a few seconds. reaction mixture reaction mixture + 2-chlorobenzaldehyde 2-chlorobenzaldehyde Figure 5 Spotting the TLC plate Developing the TLC plate o Carefully place the TLC plate in the TLC chamber, the level of the eluent in the chamber should be lower than the spots on TLC. o Close the TLC chamber and leave the eluent to progress to the top of the TLC plate. Figure 6 Developing the TLC plate 6 A Facile Solvent-free Cannizzaro Reaction : An Instructional Model for Introductory Organic Chemistry Laboratory Visualizing the substance spots on the TLC plate o Remove the TLC plate from the chamber and leave it to dry for one minute. o Place the TLC plate under the UV lamp (wavelength 366 nm). o The appearance of new spots and the disappearance of 2-chlorobenzaldehyde at position A, compared to position C, indicates the completion of the reaction (Figure 7a). If the reaction is not complete, additional grinding is required. The position A will show two spots corresponding to 2-chlorobenzaldehyde and one of the products (Figure 7b). The other product fails to show on the TLC due to its insolubility in the dichloromethane solvent. The spot for the reactant is “absent”. b) 2-chlorobenzaldehyde a) Reduction product A B C A B C a) the reaction is complete b) the reaction is not complete Figure 7 The developed TLC plate under a UV lamp 7 A Facile Solvent-free Cannizzaro Reaction : An Instructional Model for Introductory Organic Chemistry Laboratory PART 2 Separation and Identification of Products You will design your own procedure to separate, isolate and identify your products. Discuss your experimental plan with your instructor or teaching assistants before starting. Separation 1. Vacuum filtration (Appendix A) and acidification (Appendix B) steps are recommended. 2. Solubility properties of three substances are given in Table 1. 3. Outline your procedure for separation and isolation of products and explain how it may work. Table 1 The solubility of potassium 2-chlorobenzoate, 2-chlorobenzoic acid and 2-chlorobenzyl alcohol Substances Solubility (in water) potassium 2-chlorobenzoate soluble 2-chlorobenzoic acid partially soluble 2-chlorobenzyl alcohol partially soluble Identification 1. Authentic samples of 2-chlorobenzoic acid, 2-chlorobenzyl alcohol and their melting points are provided. 2. You are required to consider physical properties of the substances such as adsorption to the TLC matrix (Rf value), IR, NMR spectra and melting points. 3. Write your method for identification of products and explain how it works. PART 3: Thermodynamics: Energy Involved in the Reactions 1. Use the molecular modelling software (Spartan 06 programme) to provide the internal energy (heat of formation) and entropy change for each substance in the reaction (T = 298.15 K). Calculate the energy of the reaction and explain why the reaction should occur. - O O H(l) OH (s) OH (s) 2 + H2O (l) + Cl Cl Cl 8 A Facile Solvent-free Cannizzaro Reaction : An Instructional Model for Introductory Organic Chemistry Laboratory 2. Search thermodynamic data from NIST webBook (http://webbook.nist.gov/chemistry) for the given reaction. - O O H (l) OH (s) OH (l) 2 + H2O (l) + Then answer questions: 2.1 Is the reaction exothermic or endothermic? 2.2 Does the reaction proceed spontaneously? 2.3 Write the enthalpy diagram (the Hess’s diagram*) of the reactions using the given equations. 14 C(s) + 7 H2(g) + (3/2) O2(g) C7H8O2(l) + C7H6O2(s) 14 C(s) + 7 H2(g) + (3/2) O2(g) 2 C7H6O(l) + H2O(l) 14 C(s) + 7 H2(g) + (3/2) O2(g) C7H8O(l) + C7H6O(l) + (1/2) O2(g) 14 C(s) + 7 H2(g) + (3/2) O2(g) C7H6O2(s) + C7H6O(l) + H2(g) 2 C7H6O(l) + H2O(l) C7H8O2(l) + C7H6O2(s) C7H8O(l) + C7H6O(l) + (1/2) O2(g) C7H8O2(l) + C7H6O2(s) C7H6O2(s) + C7H6O(l) + H2(g) C7H8O2(l) + C7H6O2(s) * Hess’s law is implied in the 1st law of thermodynamics. For a review of thermodynamics, please read 1. Atkins, P.; Paula, J. Physical Chemistry for the Life Sciences, W.H. Freeman and Company: 2006. 2. Chang, R. Chemistry, 9 ed.; Mc Graw Hill: 2007. 9 A Facile Solvent-free Cannizzaro Reaction : An Instructional Model for Introductory Organic Chemistry Laboratory PART 4: Green Chemistry Reactions with a green chemistry consideration commonly emphasize the following criteria: • Minimizing waste • Limiting the use of toxic compounds • Working safely • Minimizing cost By analysis of the chemicals and processes involved in this laboratory, you should be able to evaluate its “greenness” compared to those in the conventional method.
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