Exp t 83 Synthesis of Benzyl Acetate from Acetic Anhydride from K L Williamson Macroscale and Microscale Organic Experiments 2nd Ed 1994 Houghton Mifflin Boston p385 revised 10 14 00 Prelab Exercise Give the detailed mechanism for the acid catalyzed hydrolysis of benzyl acetate Introduction 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 odors and indeed are the major components of the flavor and odor aspects of a number of fruits Although the natural flavor may contain nearly a hundred different compounds single esters approximate the natural odors and are often used in the food industry for artificial flavors and fragrances 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 the acid form of an ion exchange resin O C H3C CH OH 3 OH O H H3C O CH3 H O 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 a proton transfer and loss of water followed by loss of the catalyzing proton 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 Other methods are available for the synthesis of esters most of them more expensive but readily carried out on a small scale For example alcohols react with acid anhydrides to form esters and this is the method used in this experiment O CH3CH2OH Ethanol O O C C H3C CH3 O Acetic anhydride H3C O CH2CH3 CH3COOH Ethyl acetate Acetic acid Acid chlorides form esters by reaction with alcohols O CH3CH2CH2OH C H3C Cl Acetyl chloride 1 Propanol O C H3C O CH2CH2CH3 HCl n Propyl acetate In the latter reaction an organic base such as 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 the addition of ketene to make CH3 O CH3CH2C OH Propionic Acid C CH2 CH3 O H CH3CH2C O C CH3 CH3 CH3 t Butyl propionate 2 Methylpropene isobutylene O CH2 C O Ketene Silver acetate CH3 C OCH2 Benzyl Acetate Benzyl alcohol O CH3C O Ag CH3 HOCH2 BrCH2CH2CH CH3 O CH3 CH3C O CH2CH2CH CH3 1 Bromo 3 methylbutane Isoamyl acetate acetates and the reaction of a silver salt with an alkyl halide As noted above Fischer esterification is an equilibrium process Consider the reaction of acetic acid with 1 butanol to give n butyl acetate O H3C C OH Acetic acid HOCH2CH2CH2CH3 n Butanol H The equilibrium expression for this reaction is shown below O C CH2CH2CH2CH3 O H3C n Butylacetate H2O O H3C C Keq O CH2 CH2CH2 CH3 O H3C C OH H2O HOCH2CH2CH2CH3 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 Chatelier 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 A third way to upset the equilibrium is to preferentially remove the water as an azeotrope The information in the table below can be found in any chemistry handbook table of ternary three component azeotropes The Ternary Azeotrope of Boiling Point 90 7 C Percentage Composition of Azeotrope Compound l Butanol n Butyl acetate Water Boiling Point of Pure Compound C 117 7 126 7 100 0 Vapor Phase Upper Layer Lower Layer 8 0 63 0 29 0 11 0 86 0 3 0 2 0 1 0 97 0 These data tell us that the vapor that distills from a mixture of 1 butanol n butyl acetate and water will boil at 90 7 C and the vapor contains 8 alcohol 63 ester and 29 water The vapor is homogeneous but when it condenses it separates into two layers The upper layer is composed of 11 alcohol 86 ester and 3 water but the lower layer consists of 97 water with only traces of alcohol and ester If some ingenious way to remove the lower layer from the condensate and still return the upper layer to the reaction mixture can be devised then the equilibrium can be upset and nearly 100 of the ester can be produced in the reaction flask The Dean Stark Apparatus shown below is one such solution Dean Stark trap for removing water through azeotropic distillation The apparatus shown modeled after that of Dean and Stark achieves the desired separation of the two layers The mixture of equimolar quantities of 1 butanol and acetic acid is placed in the flask along with an acid catalyst Stirring reduces bumping The vapor the temperature of which is 90 7 C condenses and runs down to the sidearm which is closed with a cork The layers separate with the denser water layer remaining in the sidearm while the lighter ester plus alcohol layer runs down into the reaction flask As soon as the theoretical quantity of water has collected the reaction is over and the product in the flask should be ester of high purity 3 way connector Esterfication using a carboxylic acid and an alcohol requires an acid catalyst Often the acid form of an ionexchange resin is used This resin in the form of small beads is a cross linked polystyrene that bears sulfonic acid groups on some of the phenyl groups Essentially it is an immobilized form of p toluenesulfonic acid an organicsubstituted sulfuric acid This catalyst has the distinct advantage that at the end of the reaction it can be removed simply by filtration Immobilized catalysts of this type are