EXPT #7 ESTERIFICATION REACTIONS OF VANILLIN
Important reading (Zubrick, Chapter 35, NMR)
Vanillin, 4-hydroxy-3-methoxybenzaldehyde, is a multifunctional compound that forms different products (ester #1 and ester #2) when reacted with acetic anhydride under acidic or basic conditions.
One product is a monoester, formed by reaction at the phenol OH group in vanillin. The other is a triester, with one ester forming at the phenol OH group and two esters forming at the aldehyde group. One of your assignments for this experiment is to determine which product is ester #1 and which is ester #2. The structures of the products are shown below.
Preparation of ester #1
Dissolve 304 mg (2.0 mmol) of vanillin in 5 mL of 10% NaOH solution in a 50-mL Erlenmeyer flask. Add 6 g of crushed ice and 0.866 g (0.8 mL; large excess) of acetic anhydride. Cork the flask and shake the reaction mixture several times over a period of 15 minutes. The vanillyl acetate separates as white crystals. Filter the product using a Hirsch funnel, and wash it with three 1 mL portions of cold water. Determine the weight and melting point of the crude product. Recrystallize the crude product from ethanol:water (1:4) and determine the weight and melting point (lit value= 77-79oC). Measure the IR spectrum (DR) of the pure product.
Preparation of ester #2
Dissolve 152 mg (1.0 mmol) of vanillin in 1 mL of acetic anhydride (large excess) in a 13 x 100 mm test tube. Add one microdrop (really small drop!) of concentrated sulfuric acid to the solution of vanillin in acetic anhydride. (If you use too much, the solution will turn a very dark red. Formation of a slight pink or red color is not a problem). Use the squirting technique to thoroughly mix the reaction mixture (suck the reaction mixture in and out of a pipette a few times). Place the test tube in a water bath at 65-70oC and maintain at this temperature for 30 minutes. Cool the reaction mixture in ice and dilute with 3-4 mL of cold water. Cork the test tube and vigorously shake the reaction mixture. Scratch the wall of the test tube to induce crystallization. Stir the mixture to break up any large particles of product. Filter the product using a Hirsch funnel, and wash the flask and filter cake with three 1-mL portions of cold water. Transfer the crude product to a watch glass to air-dry. Determine the weight and melting point of the crude product. Recrystallize the crude product from ethanol:water (1:1) and determine the weight and melting point (lit value= 90-91oC). Measure the IR spectrum (DR) of the pure product.
Get together in groups of four and select a product ester #1 and a product ester #2 for 1H nmr analysis. The selected samples should be placed in the large beaker (in the balance room) labeled "for nmr" . Samples should be submitted in a clearly labeled screw-top vial, with ester number (#1 or #2) and the initials of the person who prepared the sample. An nmr sign up sheet will also be available - each group member should be present when the spectra are acquired.
All other products should be placed in the large beaker (also in the balance room) labeled "vanillin products". Samples should be submitted in a clearly labeled screw-top vial, with ester number (#1 or #2) and the initials of the person who prepared the sample.