Electrophilic+Aromatic+Substitution+(EAS)+Reactions

Electrophilic Aromatic Iodination

Intro: This week’s lab was to conduct an electrophilic substitution reaction. The aromatic compound used in this lab was 4-hydroxy-3-methoxybenzaldehyde (vanillin). Extract from the vanilla bean. The greener approach for this lab was in the use of iodine in place of Cl2 or Br2 as the substitutant. Bromine as a liquid and chlorine as a gas can cause serious and possibly fatal burns upon contact with the human integument. The procedure is conducted by use of powerful oxidizing agent (NaOCl) to facilitate the reaction since iodine alone is unreactive. The electrophilic substitution reaction is stereo selective. The selectivity is aided by stabilization from the benzene ring resonance structures. Procedure: I. An ice bath is prepared and set upon a stir plate. A ring stand is placed behind the plate and one ring and one clamp are attached. Clamp a 100ml round bottom flask in place and then insert the separatory funnel into the ring. II. 1 gram of vanillin is added to the round bottom flask. Add 20ml of ethanol and then the stir bar. Begin stirring and lower flask into the ice bath to begin cooling to 0°C. Then pour in grams of sodium iodide. The color will be light yellowish at this point. III. Measure out 11ml of aqueous sodium hypochlorite (NaOCl) and add it to the separatory funnel making sure that the stop cock is closed. Then cap the funnel. IV. After mixture in the round bottom flask is sufficiently cooled, begin to add NaOCl to the mixture at a rate of one drop approximately every three seconds for ten minutes. The color of the solution will change at this point to a reddish brown color. V. Remove the round bottom flask from the ice bath and place it into a bowl of lukewarm water and continue to stir slowly warming to room temperature. Remove from water bath. Add 10 ml sodium thiosulfate to the solution. The solution will become slightly brown/orange similar to a carrot and pea baby food jar. VI. Pour in approximately 3ml of HCl to make the solution acidic again. Add small amounts of HCl to the solution testing frequently until it is acidic. The color will change to a lighter version of milky carrot/pea baby food. VII. Take round bottom flask to the Roto-Vap and the instructor will use the vacuum device to remove the ethanol from the flask. The flask minus the ethanol is now cooled in an ice bath and then the solution is poured into the vacuum filtration device. Wash the flask out with a small amount of cold water to remove any leftover product and empty into the filtration device. Then wash the product with 1 ml of cold ethanol. IX. Empty the solution again into a vacuum filtration device and allow drying. The crystals are very small and slightly yellow/tan in color. There are brown specks mixed throughout the sample. When the product is thoroughly dry collect the crystals and weigh the sample. Data: Product weight. First drying. 1.464g Second drying. 1.033g Percent yield. Vanillin 152.15g/mol 4-hydroxy-3-iodo-5-methoxybenzaldehyde 278.04g/mol 0.006572 mol vanillin X 278.04 g/mol C8H7IO3 = 1.827g theoretical yield 1.033g/1.827g =56.5% yield calculation is correct, although your sig figs are off a bit. You could have reported one more digit in your % yield.
 * 1) Empty the crystals into a clean 125ml Erlenmeyer flask and add 2-propanol first to dissolve the product and then to obtain a solution that is slightly cloudy and yellowish in color. Place the solution into an ice bath and allow to recrystalize.

Melting point data. Melting point range of product 181.9°C -183.0°C Literature accepted value for product 183°C -185°C =Analysis/Discussion:= The percentage yield 56.5% reflects there was room for error in the procedure resulting in loss of products. The melting point range 181.9°C -183.0°C show relative success at the idonation of vanillin. =Conclusion:= The overall purpose of this lab was to conduct an electrophilic substitution reaction, by iodination of vanillin showing students a more safer greener approach. According to our data the percentage yield of 56.5% this could be answered by loss of product during filtration and transferring. The melting point range181.9°C -183.0°C concludes the product recovered it proportionetly close to the finish products literature melting point. ...which indicates it is quite pure. The main point is the formation of aromatic compound went into a electrophilic -elimination mechanism, the pi bond of the aromatic ring attacked the electrophilic reagent, forming a cationic intermediate, which then eliminated the leaving group to form the substituted product of idonation of vanillin.The reagents and solvents used in this reaction are much greener approach and minimizes the formations of byproducts, which then eliminates byproducts. Saves a lot of material and solvents in processing the pure substance.

=Error:= The rate at which the bleach was added was either too fast or too slow within the ten minute time lapse for adding the bleach, which could have slowed the process of recrystallization. Or the temperature of the mixture in 100ml round-bottom flask was not keep at 0.0°C during procedure.During the filtration there was possible loss of products and when removing the product from the filter paper, small pieces of paper were lifted along with the product causing impurities and excess material. When removing the product with aspiration, there was a large amount of product remaining in the collection flask resulting in loss of products.

=Post-lab Questions:= Describe physical properties color and state product appeared to me a milky color and semi solid form the mass recovered 1.033g and yield recovery was 56.5%. Lower yield due to loss of product during filtration, and transferring product. The melting point of recovered product appeared to be relatively close to literature value. Collected product melting point range 181.9°C -183.0°C, and the literature 183°C -185°C.

=Reference:= Gilbertson, R.; Parent, K. E.; McKenzie, L. C.; Hutchison, J. E. //Electrophilic Aromatic Iodination of 4'-Hydroxyacetophenone// Greener Approaches to Undergraduate Chemistry Experiments, Print Kirchhoff, M., Ryan, M., Eds.; American Chemical Society: Washington D.C., 2002; pp 1-3. [|http://spot.pcc.edu/~gbackes/ORGANIC/CH%20243/Labs243/Aromatic%20Iodination%20vanillin.htm] °C