ESTERIFICATION AND HYDROLYSIS


Experiment #1: Methyl Benzoate by Fisher Esterification


Post – Lab


Organic Chemistry II Lab – Section 7515


February 25, 2004


INTRODUCTION


The Fischer esterification reaction reaches equilibrium after a few hours. This equilibrium can be shifted by adding more of the acid or of the alcohol. The mechanism of the reaction involves initial protonation of the carboxyl group, followed by attack by nucleophilic hydroxyl, a proton transfer, and finally loss of water followed by loss of the catalyzing proton to give ester. All these steps are also reversable.


The purpose of this experiment is to obtain methyl benzoate through fisher esterification by using benzoic acid, methanol, and concentrated sulfuric acid.


EXPERIMENTAL PROCEDURE


10g of pre-weighed benzoic acid and 25ml of methanol were placed into the 125ml round bottomed flask. 3ml of concentrated sulfuric acid was added to the flask very carefully after the mixture cooled off (in the ice bath) to approximately room temperature and the whole mixture was then swirled. After the reflux condenser was attached, the boiling chip was added, the mixture was refluxed for approximately 1 hour. The solution was then cooled and was poured into a separatory funnel that contained 50ml of water. The flask was rinsed with 35ml of ethyl ether, and this was then added to the separatory funnel. The separatory funnel was shaken well and the water layer (containing sulfuric acid and methanol) was drained off. The ether in separatory funnel was washed with 25ml of water and then with 25ml of 0.5M sodium bicarbonate (to remove unreacted benzoic acid). The funnel was shaken well again with frequent release of pressure (inverting the separatory funnel and opening the stopcock) until no reaction was visible and the bicarbonate layer was drained off into a beaker. The ether layer in separatory funnel was washed with saturated sodium chloride solution and dried with anhydrous calcium chloride in Erlenmeyer flask. After about 10 minutes, the dry ether solution was poured into a flask, while the drying agent was washed with 5ml of ether, and poured out again. The ether was removed by evaporation. When evaporation was over, methyl benzoate was poured into 50ml round bottomed flask for the simple distillation process. The boiling point of ester is very high, around 199 degrees Celsius. This high temperature may cause the condenser to crack. The material was collected into three pre-weighed bottles (the first two bottles had around 2-3ml, while the third had the rest of the distilled solution) when there was boiling above 190 degrees Celsius.


IR analysis was done on the three collected samples.


DATA











MW(g/mol)


BP(degrees C)


MP(degrees C)


Density(g/cm^3)


Benzoic acid


122.12


122


Methanol


32.04


64.6


0.791


Methyl benzoate


136.15


198-199


1.094


Sample Bottle #1


Sample


Bottle #2


Sample


Bottle #3


Volume (ml)


2.5


2.5


14


weight of the


empty bottle


without cap (g)


13.150


13.117


13.191


weight of the


bottle without


cap with the


collected sample (g)


14.003


14.017


18.033


CALCULATION









































































































































DISCUSSION


OBSERVATION AND EXPLANATION





When the IR analysis was done, the following information was obtained:


Sample #1


- the peaks at 3073.91 cm-1, 2947.83cm-1, and 1717.39cm-1.


Sample #2


- the peaks at 3065.22cm-1, 2952.17cm-1, and 1730.43cm-1.


Sample #3


- the peaks at 3069.57cm-1, 2947.83cm-1, and 1721.74cm-1.


The frequency at which esters appear on IR analysis is 1730-1750 cm-1, aromatic C-H is just to the left of 3000cm-1 and sp3 C-H is 2850-2950cm-1.


The results obtained are in very close range with the given primary standard of methyl benzoate obtained from the instructor.


The actual yield obtained for the methyl benzoate is 6.593g. The theoretical yield is 11.15g. The percent yield for methyl benzoate is 59%.


Every experiment conducted has certain percentage of error. In this case, that can be caused by many different variables: some of the product may be poured out when the solutions are poured out of the separatory funnel, some of the benzoic acid might have stayed in there as well as water, not all of the ether was evaporated on the steam bath, etc.


Problem 1: In the preparation of methyl benzoate what is the purpose of:


a) washing the organic layer with sodium bicarbonate solution?


b) washing the organic layer with saturated sodium chloride solution?


c) treating the organic layer with anhydrous calcium chloride pellets?


a) the reason the organic layer is washed with sodium bicarbonate solution is to remove the benzoic acid that did not react


b) washing the organic layer with sodium chloride solution is used to extract dissolved water in the organic solution. When it is washed, the water molecules that are dissolved in the organic solvent move into aqueous solution to