ESTERIFICATIN: PREPARATIN F BENZYL ACETATE MICRSCALE EXPERIMENT IV 39 R C R' The ester group is an important functional group that can be synthesized in a number of different ways. The low molecular-weight esters have very pleasant odours and indeed are major components of the flavour and odour aspects of a number of fruits. Although the natural flavour may contain nearly a hundred different compounds, single esters approximate the natural odours and are often used in the food industry for artificial flavours and fragrances (see Table on p. 40). Esters can be prepared by the reaction of a carboxylic acid with an alcohol in the presence of a catalyst such as concentrated sulfuric acid, hydrogen chloride, p-toluenesulfonic acid, or by using the acid form of an ion-exchange resin, for example: HCl CH 3 C H CH 3 H CH 3 C CH 3 acetic acid methanol methyl acetate H 2 This Fischer esterification reaction reaches equilibrium after a few hours of refluxing. The position of the equilibrium can be shifted by adding more of the acid or of the alcohol depending on cost or availability. The mechanism of the reaction involves initial protonation of the carboxyl group, attack by the nucleophilic hydroxyl group of the alcohol, a proton transfer, and loss of water, followed by deprotonation to give the ester. Because each of these steps is completely reversible, this process is also, in reverse, the mechanism for the hydrolysis of an ester.
40 TABLE. Fragrances and Boiling Points of Esters Ester Formula Boiling Point ( C) Fragrance CH 3 Isobutyl formate HCCH 2 CHCH 3 98 Raspberry n-propyl acetate CH 3 CCH 2 CH 2 CH 3 102 Pear Methyl butanoate CH 3 CH 2 CH 2 CCH 3 102 Apple Ethyl butanoate CH 3 CH 2 CH 2 CCH 2 CH 3 121 Pineapple Isobutyl propanoate CH 3 CH 3 CH 2 CCH 2 CHCH 3 137 Rum CH 3 Isoamyl acetate CH 3 CCH 2 CH 2 CHCH 3 142 Banana Benzyl acetate CH 3 CCH 2 206 Peach ctyl acetate CH 3 CCH 2 (CH 2 ) 6 CH 3 210 range Methyl salicylate CCH 3 222 Wintergreen H
ther methods are available for the synthesis of esters, most of them more expensive but 41 readily carried out on a small scale. For example, alcohols react with anhydrides and with acid chlorides: CH 3 CH 2 H + CH 3 C CCH 3 Ethanol Acetic anhydride CH 3 CCH 2 CH 3 + CH 3 CH Ethyl acetate Acetic acid CH 3 CH 2 CH 2 H + CH 3 CCl 1-Propanol Acetyl chloride CH 3 CCH 2 CH 2 CH 3 + HCl n-propyl acetate Pyridine is usually added to react with the hydrogen chloride. A number of other methods can be used to synthesize the ester group. Among these are the addition of 2-methylpropene to an acid to form t-butyl esters and the reaction of a silver salt with an alkyl halide: CH 2 CH 3 CCH 3 + CH 3 CH 2 CH H + CH 3 CH 3 CH 2 CCCH 3 2-Methylpropene (isobutylene) Propanoic acid (propionic acid) CH 3 t-butyl propanoate CH 3 CH 3 C - Ag + + BrCh 2 CH 2 CHCH 3 Silver acetate 1-Bromo-3-methylbutane CH 3 CH 3 CCH 2 CH 2 CHCH 3 Isoamyl acetate As noted above, Fischer esterification is an equilibrium process. Consider the reaction of acetic acid with 1-butanol to give n-butyl acetate: CH 3 CH + HCH 2 CH 2 CH 2 CH 3 H + CH 3 CCH 2 CH 2 CH 2 CH 3 + H 2
42 The equilibrium constant is given by K eq = [ n BuAc][ H 2] [ n BuH][ H Ac] 2 For primary alcohols reacting with unhindered carboxylic acids, Keq. 4. If equal quantities of 1-butanol and acetic acid are allowed to react, at equilibrium the theoretical yield of ester is only 67%. To upset the equilibrium we can, by Le Châtelier s principle, increase the concentration of either the alcohol or acid, as noted above. If either one is doubled, the theoretical yield increases to 85%. When one is tripled, it goes to 90%. But note that in the example cited, the boiling point of the relatively nonpolar ester is only about 8 C higher than the boiling points of the polar acetic acid and 1-butanol, so a difficult separation problem exists if either starting material is increased in concentration and the product is isolated by distillation. Another way to upset the equilibrium is to remove water. This can be done by adding to the reaction mixture molecular sieves, an artificial zeolite, which preferentially adsorb water. Most other drying agents, such as anhydrous sodium sulfate or calcium chloride, will not remove water at the temperatures used to make esters. Reference: rganic Chemistry (5 th edn., 2000) by J. McMurry; Sections 21.3, 21.6.
Experimental Procedure Preparation of Benzyl Acetate from Acetic Anhydride and Benzyl 43 Alcohol CH 2 H 2 + CH 3 C CCH 3 CH 2 CCH 3 + CH 3 CH To a round bottomed flask add 108 mg of benzyl alcohol and 102 mg of acetic anhydride and a boiling chip. Reflux the mixture for at least 1 hour, cool the mixture to room temperature, and chromatograph the liquid as follows: Assemble the column as shown in the Figure, being sure it is clamped (with one clamp) in a vertical position. Close the valve and fill ¼ of the column with dichloromethane DICHLRMETHANE IS VERY VLATILE to the bottom of the funnel. Prepare a slurry of 10 ml silica gel in 4 ml of dichloromethane in a small beaker. Stir the slurry gently to get rid of air bubbles and gently swirl, pour and scrape the slurry into the funnel. After some of the silica gel has been added to the column, allow the solvent to drain slowly into an Erlenmeyer flask. Use this dichloromethane to rinse the beaker containing the silica gel. As the silica gel is being added, VERY GENTLY tap the column with a pencil so the adsorbent will pack tightly into the column. Continue to tap the column while cycling the dichloromethane through the column once more and then add a sample of anhydrous potassium carbonate to the top of the silica gel. The potassium carbonate will remove water from the esterification mixture as well as react with any carboxylic acid present. Run the solvent down to the surface of the potassium carbonate. Using a Pasteur pipet, add the sample to the column and let it run into the adsorbent, stopping when the solution reaches the top of the potassium carbonate. The flask is rinsed twice
44 with 0.5 ml portions of dichloromethane that are run into the column, with the eluent being collected in a tared sample bottle. The elution is completed with 4 ml more dichloromethane. Evaporate the dichloromethane on the steam bath in the hood. Since the dichloromethane boils at 40 C and the product at 206 C, separation of the two is easily accomplished. Determine the weight of the product and calculate the yield. The ester should be a perfectly clear, homogeneous liquid. An IR spectrum and a 1 H NMR spectrum of benzyl acetate are supplied. Analyze it for the presence of unreacted alcohol and acetic anhydride. (These spectra can be found on page 45-47.)
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