Matthew Seidler of 5

Transcription

1 Matthew Seidler of 5 JN ABBTT CLLEGE LABRATRY #8 TE CANNIZZAR REACTIN Instructor: D. Fenwick biectives: To carry out a Cannizzaro reaction using p-chlorobenzaldehyde as the substrate and to identify product collected after the workup. The Cannizzaro reaction is a named organic reaction that is described in most textbooks. Additional information on the reaction can be obtained from the Named rganic Reactions link on our web site. Procedure: In a small vial dissolve 1.07 mmol of p-chlorobenzaldehyde in ml of methanol. In a 5 ml vial equipped with a spinning vane dissolve 4.3 mmol of potassium hydroxide in ml of water. Cool the solution to room temperature and then add dropwise the p- chlorobenzaldehyde solution. Attach a reflux condenser and heat at 55 to 60 C for 1 hour. After cooling, transfer the contents to a centrifuge tube add 2 ml water and extract with methylene chloride (3 x 0.5 ml portions). The methylene chloride layers are to be discarded. The aqueous layer is transferred to a 10 ml beaker and acidified with concentrated hydrochloric acid (added dropwise, approximately 0.5 ml). The precipitate is collected at the pump, rinsed with cold water (3 x 0.5 ml portions) and dried thoroughly with the aid of a heat lamp. Record the mass of the product and measure its melting point. The NMR and IR spectra will be available. (Spectra were not provided as they were not necessary to determine the product) Additional Question: Suppose the methylene chloride layers were combined, rather than discarded, extracted with saturated sodium bicarbonate solution and then dried by passing through a plugged Pasteur pipette containing anhydrous magnesium sulfate. Evaporation of the methylene chloride would leave a solid product with a melting point of C. Name and provide the structure of this other product of the Cannizzaro reaction that we did not have the time to collect during our experiment.

2 Experiment #8 The Cannizzaro Reaction 2 of 5 Matthew Seidler Chemistry 202 DDB 05 EXPERIMENT #8 TE CANNIZZAR REACTIN BJECTIVES/ABUT TE REACTIN The main objective of this experiment is to carry out a Cannizzaro reaction using p- chlorobenzaldehyde and to identify the product that is collected. If an aldehyde with no α- hydrogens reacts with sodium hydroxide, half of the aldehyde gets converted to a carboxylic acid (that gives up a proton), and the other half gets turned into an alcohol. [Bruice, 3/E] In this experiment, p-chlorobenzaldehyde in methanol will react with potassium hydoxide to create p-chlorobenzyl alcohol and p-chlorobenzoic acid: K Cannizzaro Reaction - TE MECANISM Acid Base p-chlorobenzoic acid p-chlorobenzyl alcohol RELEVANT INFRMATIN FR CEMICALS USED 1 : p-chlorobenzaldehyde Density Boiling Point g/mol 1.196/mL 214 o C 1 accessed 04/17/07

3 Experiment #8 The Cannizzaro Reaction 3 of 5 Potassium ydroxide Density p-chlorobenzoic acid Melting Point Comments p-chlorobenzyl alcohol Boiling Point Melting Point /mol g/ml g/mol 243 o C Triclinic crystals /mol 234 o C 71 o C EXPERIMENTAL DATA Mass of p-chlorobenzaldehyde g Volume of Methanol 0.4 ml Mass of K g Volume of Water 0.5 ml Melting Point of the Product 238 o C Mass of Empty Watch Glass g [1] Mass of Watch Glass Product g [2] ANALYSIS F RESULTS Calculating the Number of Grams of p-chlorobenzaldehyde n = 1.07 mmol = mol, and m = nm = ( mol)( g/mol) = g Calculating the Number of Grams of Potassium ydroxide n = 4.3 mmol = mol, and m = nm = ( mol)( g/mol) = g Mass of product formed = [2] - [1] = g

4 Experiment #8 The Cannizzaro Reaction 4 of 5 Theoretical Yield Note that the product we isolated was p-chlorobenzoic acid (see Additional Question for more information). Also note one mole of the aldehyde used yields one mole of product; that is, ½ of a mol of the acid and ½ of a mole of the alcohol. The reaction is 1:1 (i.e. 1:0.5:0.5). K I R ½ ( ) ½ ( ) F (mol) Mass of p-chlorobenzoic acid = ( mol)( g/mol) = g % yield = Percent Yield actual yield g "100% = "100% = 24% theoretical yield g Percent Yield = 24%

5 Experiment #8 The Cannizzaro Reaction 5 of 5 ADDITINAL QUESTIN: Suppose the methylene chloride layers were combined, rather than discarded, extracted with saturated sodium bicarbonate solution and then dried by passing through a plugged Pasteur pipette containing anhydrous magnesium sulfate. Evaporation of the methylene chloride would leave a solid product with a melting point of C. Name and provide the structure of this other product of the Cannizzaro reaction that we did not have the time to collect during our experiment. The product that we were able to collect in the lab was in the form of crystals, which had a melting point of 238 o C. As made clear in the mechanism and in the Theoretical Yield calculations on the preceding page, there are two products of the Cannizzaro reaction: the p- chlorobenzaldehyde, when reacted with base, produces p-chlorobenzoic acid and p-chlorobenzyl alcohol. The literature indicates that the acid product is composed of triclinic crystals that melt at 243 o C. The alcohol, however, melts at 71 o C. Because the difference between the two melting points is very large, we can safely assume that since the melting point of the product we extracted was so close to that of the acid, our product is p-chlorobenzoic acid. Therefore, the other product of the Cannizzaro reaction that we did not have time to collect during our experiment was the p-chlorobenzyl alcohol, whose structure is shown below: p-chlorobenzyl alcohol CNCLUSIN In this experiment, a Cannizzaro reaction was successfully carried out. p-chlorobenzaldehyde was reacted with potassium hydroxide, and after a workup, the product extracted was p- chlorobenzoic acid. The product extracted was found to have a melting point of 238 o C, which is relatively close to the accepted value of 243 o C. It was clear that the extracted product was the acid because the other product of the reaction, which is an alcohol, melts 71 o C, which is significantly different than the melting point of the product obtained. A percent yield of 24% was achieved, which is relatively satisfactory due to one source of error: the mixture was refluxed for an hour, and some of our product burned away (it was stuck to the sides of the vial and we were unable to retrieve it.) We did not have time to collect the other product of this reaction, p- chlorobenzyl alcohol (see ADDITINAL QUESTIN).