Chemistry 234 Organic Chemistry Laboratory

back to Table of Contents

Experiment 6: Preparation of methyl benzoate
(40 points)

I. Summary

In this experiment you will prepare methyl benzoate by reacting benzoic acid with methanol using sulfuric acid as a catalyst. Since this is a reversible reaction, it will reach an equilibrium that is described by the equilibrium constant, K_eq .

In this experiment, you will isolate the product, methyl benzoate and any unreacted benzoic acid. Using this data, you will calculate the equilibrium constant for the reaction as described in part III.

II. Procedure

First Period

Obtain a 10g (0.082 mol) sample of benzoic acid from the storeroom (weigh the benzoic acid to obtain an exact weight before you begin). Add to it 0.62 mol of methanol in a 100 mL round-bottomed flask. Cautiously, add 3 mL concentrated H2S04 down the side of the flask. After gently swirling the contents of the flask, attach a reflux condenser and reflux the mixture for about 60 min. Sulfuric acid causes acid burns on the skin. If you get some on your skin be sure to wash with large amounts of water. Contact your TA!

Allow the solution to cool. Add 50 mL of water to a separatory funnel, then add the contents of the flask. The flask should then be washed with 40 mL of dichloromethane (CH₂Cl₂) and the washings transferred to the separatory funnel. Use the separatory funnel to extract the ester into the CH₂Cl₂ (organic) layer. Wash sequentially with 25 mL water and 25 mL 0.6M sodium bicarbonate* (Caution, CO₂ will be given off with effervescence).

After shaking and venting the funnel, remove the sodium bicarbonate layer and test to see if it is basic. Then wash the organic layer with NaCl (salt) solution, separate the organic layer and dry with anhydrous magnesium sulfate. Remove the MgS0₄ by gravity filtration, and concentrate the solution using the same apparatus used to remove CH₂Cl₂ from anthracene in Experiment 4.

*The aqueous layer from the sodium bicarbonate wash should be acidified with concentrated HCl The unreacted benzoic acid should precipitate. Remove the solvent by vacuum filtration. Collect and dry the solid benzoic acid and let it air-dry in your drawer until the next lab period.

Second Period

Distill the crude methyl benzoate (b.p. 199°), collecting anything that boils between 170°C and 200°C. Weigh the sample of methyl benzoate and determine the yield. Weigh the sample of recovered benzoic acid. Place both samples in labeled vials, and give them to your TA.

Put aluminum foil around your flask for insulation because of the high boiling point of methyl benzoate.

Caution:

1. Methanol is flammable and toxic

2. Dichloromethane is toxic

3. Handle concentrated H₂SO₄ and HCl with great care

4. Vent the separatory funnel frequently after swirling and subsequent shaking. CO₂, gas is evolved and will build up pressure in the funnel.

III. Calculation of the Equilibrium Constant, K_eq.

Calculate a value for the equilibrium constant K_eq based upon your weight of recovered benzoic acid. In addition, calculate the percent yield of your synthesis based upon the amount of starting benzoic acid used, and another percent yield based upon the amount of benzoic acid that actually reacted (as determined by the weight of recovered benzoic acid).

Calculation of the Equilibrium Constant K_eq:

Since the volume of the reaction mixture is the same for each component, we can use moles instead of molarities as the units in our K_eq expression. Let B_i = initial moles of benzoic acid, let M_i = initial moles of methanol, and let X = final moles of methyl benzoate produced at equilibrium.

Based upon the stoichiometry of equation 1, we can conclude three things about X, namely that X = final moles of water produced at equilibrium, X = moles of benzoic acid consumed when equilibrium is established, and X = moles of methanol consumed when equilibrium is established. Using our definitions, equation 2 becomes:

We know B_i and M_i. All we need to do before we can calculate K_eq is to Put X in terms of something known, like the amount of benzoic acid recovered. Let B_f = final moles of benzoic acid present at equilibrium. Thus B_f = B_i - X, or X = B_i - B_f. Substituting(B_i - B_f) for X in equation 3 gives:

We can now calculate K_eq by entering the appropriate values of the moles of benzoic acid used (B_i), the moles of methanol used (M_i), and the moles of benzoic acid recovered (B_f) into equation 5.

Calculation of the % Yield based upon initial amount of benzoic acid used:

Let E = moles of methyl benzoate ester obtained. Note that this calculation is valid only if benzoic acid is the limiting reagent.

Calculation of the % Yield based upon amount of benzoic acid that is consumed:

IV. Report Requirements

Your lab report should contain the following information:

back to Table of Contents