Organic Chemistry Microscale Projection Experiments - Chemistry en miniature - Peter Keusch, University of Regensburg

German version Using Leitz-Prado-Universal (Photo 1) and Fantex Multi-Purpose Projector (Photo 2), provided by Leybold Didactic, chemical effects can be visualized in a convincing and efficient manner in a classroom or lecture hall. Photo 1 Photo 2 The objective system is adjustable arranged on two guide rods. A rectangular quartz glass cuvette (80 mm × 60 mm × 25 mm) is positioned in the optical path between the light source and the lens system. Perforated covers placed on the cuvette permit the accurate and upright placement of the test tubes and U-tubes inside the cuvette. The cuvette (partially filled with water) ensures a uniform medium in the beam path. Thus, aberrations (distortions, shadow effects) at the curved surfaces of the test tubes are avoided. Additional instruments (electrodes and switch boxes) allow the presentation of electrochemical experiments. Instruments for the measurement of voltage and current (suitable for projection) are shown with the aid of a further projector. Reversal prisms allow the inversion of the projected image. Thus, the experimental arrangement appears on the projection screen in original position. Using the projectors provided by Leybold Didactic, it is also possible to project horizontally arranged vessels. The vertical projection is particularly suitable for the demonstration of the migration of ions in an electrical field and for the visualization of crystallization processes. Photo 3: Accessories for the projection - pipettes (1), cuvette (2), U-tubes (3), perforated covers (4), mounting plate (5) for the electrodes (6) Micro-scaled chemistry experiments shown with a projector require a modified experimental procedure. A successful projecting of chemical effects is guaranteed by paying attention to specific experimental instructions: ·   Location of the projector: The distance between projector and screen should be 2.50 m. Using an objective with a focal length of 100 mm a projected image results, whose diameter is 0.9 m. ·   Sharpness of projected image: Halogen bulbs (24 V, 150 W) illuminate the projection screen so brightly that daylight projection is permitted. ·   Location of the projected image: The projected image can be lifted with a prism. When projecting at extreme upward angles (deeply placed projector, raised projected image), an inclined projection screen allows the distortion free projection. ·   Cuvette: In order to prevent air bubbles, some drops of a detergent are added to the water in the cuvette. Sometimes it is favourable to fill the cuvette with water freshly heated ('water bath'). In the case of exothermic reactions or reactions running at low temperature, ice should be added to the water in the cuvette. ·   Experimenting, handling reagents: The viewers must be allowed to follow all the activities while the experiment is being carried out. Already the positioning of the test tubes in the optical path should be projected onto the screen. The chemicals are available in 100 mL wide-neck bottles. The filling of test tubes with chemicals takes place with bulbed Pasteur pipettes, which should be available in sufficient number. The fluid level in the test tubes or U-tubes should always be visible in the projected image. Basically only very dilute solutions should be used, since intensive colors or an excessively violent gassing can mask details or the really important effect. If reactants are added in dissolved form, it is advisable to add the solutions dropwise using a Pasteur pipette. If a solution should be layered with another liquid it is convenient to pour the liquid slowly down the inner side of the test tube with the pipette so that the two liquids do not mix. Also solutions can be mixed using a pipette. All operations with the pipette are carried out by gentle squeezing of the pipette bulb. Photo 4: Debromination of meso- (1) and of DL-2,3-Dibromosuccinic acid (2) with NaJ, starch-iodine test Advantages of screen projected experiments: ·   Only small quantities of chemicals are needed. The experimental equipment is simple. ·   The volume of waste is reduced. The disposal problems are virtually eliminated ('green chemistry'). ·   Due to the simple experimental set-up the experimental procedure is problem-free. ·   The chemical effect can be presented in a short time. ·   The observer is not confused by a variety of glasswares and instruments. ·   Light-induced reactions can be shown. ·   The oversized projected image allows to show the substantial of the reaction happening and the outstanding sharpness of image permits an impressive visualization of details e.g. change in color, formation of 'schlieren' (Photo 4), interface between immiscible liquids, formation of bubbles and precipitates on the surface of electrodes, growing of transparent crystals in a saturated solution that is allowed to cool down. Demonstration of a microscale projection experiment - Electrolysis of Viologendichloride Videoclip (Download RealPlayer .rm-Datei) Disadvantages of screen projected experiments: ·   The visualization of precipitation reactions via projecting is not always favourable because solids appear black or brownish in the projected image. ·   Reactions proceeding at high temperature cannot be projected with the instruments provided by Leybold Didactic. ·   The light sensivity of chemicals restricts the projecting of experiments. ·   The direct and motivating contact of the viewers with glass wares and chemicals is missing. ·   The 'microscale projection experiment' ignores the concrete operations of the experimenting chemist. ·   Due to the simple experiment set-up and due to the fast procedure, the experiment attains an abstraction step that can affect negatively the learning process. Note: A general conversion of conventional demonstration experiments into experiments shown with a projector can gratify neither the experimenter nor the viewer. Rather, it seems to be advisable to prefer the 'projection experiment' in regard to the visualization of specific chemical effects. In this sense, the projecting of chemical effects is a methodical enrichment. Reference: Peter Keusch: Medien. In: Konkrete Fachdidaktik Chemie, R. Oldenbourg Verlag, München 1995. Experiments Addition Reactions: Hydroxylation of Propene Objective: Test for the C=C Double Bond Experiment Description (4+2)-Cycloadditions (Diels-Alder) Objective: Diene and Dienophile Components in Diels-Alder Products Experiment Description Elimination Reactions: Dehalogenation of 1,2-Dibromopropane Objective: Elimination Experiment Description Debromination of 2,3-Dibromosuccinic Acid Objective: Elimination Experiment Description Substitution Reactions: Hydrolysis of Butyl Bromide Isomers Objective: Nucleophilic Substitution - SN1 and SN2 Experiment Description Hydrolysis of tertiary Butyl Halides Objective: Nucleophilic Substitution - SN1, Effect of the Leaving Group on Rate Experiment Description Nitration of Aromatics Objective: Electrophilic Aromatic Substitution Experiment Description Reactivity of Oxalic acid diethylester and Ethyl acetate Objectives: Ester Hydrolysis, Nucleophilic Acyl Substitution Experiment Description Reactivity of Ethyl benzoate, Ethyl 4-nitrobenzoate and Methyl 4-methoxybenzoate Objective: Ester Hydrolyis, Nucleophilic Acyl Substitution, Substituent Effect Experiment Description Keto-Enol Tautomerism: Keto-Enol Tautomerism of Ethyl acetoacetate Objective: Test for Enol Form, Fe(III) Complex Experiment Description Redox Reactions: Oxidation of Butanol Isomers Experiment Description Reduction of p-Benzoquinone by Iodide Experiment Description Oxidation of Hydroquinone by Ag Ions Experiment Description Formation of Quinhydrone by Reaction of Hydroquinone with Benzoquinone Experiment Description Oxidation of Hydroquinone by Bromate Experiment Description Reduction of p-Benzoquinone by Disulfite Experiment Description Radicals und Radical-Ions: Goldschmidt-Radical Experiment Description Reduction of Viologen Dications zum Radikalkation Experiment Description Wurster's Blue - Oxidation of N,N,N',N'-Tetramethyl-p-phenylendiamine using Bromine Objective: Comproportionation Experiment Description Oxidation of 1,3,5-Triphenylverdazyl to Verdazylium Ion Experiment Description Dyes: Basic and acid Azo Dyes Objective: Azo Coupling Experiment Description Azo Textile Dyes Experiment Description Effect of Auxochromes on the Light Absorbance of Triphenylmethylium Salts Experiment Description Light Absorbance of Triphenylmethylium Salts Experiment Description Crystal violet - a pH Indicator Objective: Protonation of Crystal Violet Experiment Description Anthocyanins as pH Indicators and Complexing Agents Experiment Description Positive Solvatochromism Objectives: Shift of the Spectrum, Solvent Polarity Experiment Description Negative Solvatochromism Objectives: Shift of the Spectrum, Solvent Polarity Experiment Description Tests for different organic compounds: Test for Aldehydes - Schiff's Reagent Objective: Schiffs Reaction Experiment Description Test for Starch in Foods Objective: Starch-Iodine Complex Experiment Description Test for Sugars with Molisch Test Experiment Description Test for Deoxyribose in DNA Experiment Description Test for Ribose in RNA Experiment Description Test for Vitamin A in Cod-liver-oil Experiment Description Test for Vitamin C in Lemonade Experiment Description