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Preparation Of Isopentyl Acetate

Isopentyl acetate, “banana oil”, is a naturally occurring compound that possesses a distinctive odor. It is found in bananas, as well as many other organisms. This experiment attempts to produce isopentyl acetate by heating under reflux, which involves heating the mixture in a flask with a condenser placed vertically in the neck since any escaping vapours condense and run back into the flask, by combining isopentyl alcohol with acetic acid and an acid catalyst.

The product was isolated using a combination of techniques — acid-base extraction, drying, and distillation — and was characterized by its boiling temperature and its refractive index. Esterification is a condensation reaction where two molecules are joined together to form a larger molecule with the simultaneous loss of water. This ester in this experiment is isopentyl acetate formed from acetic acid and isopentyl alcohol. The reaction is catalyzed by hydrochloric acid, a Fisher esterification process, (McMurry, p780-781) but the catalyst affects only the rate of reaction, and not the extent of reaction.

The desired product accumulates only if the equilibrium constant is favorable. As it happens, the equilibrium constant for this reaction is rather small (4) (comparing bond energies in the reactants and products will tip you off as to why the equilibrium constant is so small). Therefore, simply mixing equal amounts of the starting materials will convert only about 67% of the starting material into product. To drive the equilibrium forward Le Chatelier’s principle is used, in this case there are two ways to adjust reagent concentrations to force isopentyl alcohol to become isopentyl acetate. One way is to remove product as it forms.

The other way is to use a large excess of acetic acid. This experiment is based on the latter approach, but it raises two issues. We can use excess acetic acid only if acetic acid is cheap, and if unreacted acetic acid can be removed easily from the product mixture (Organic chemistry lab. Manual, p32). In this lab had to use acid- base extraction process. Since isopentyl acetate is soluble in diethyl ether, but acetic acid is soluble in both solvents. Therefore, a simple extraction procedure would remove only some of the acetic acid from isopentyl acetate, but it would not completely separate the two compounds.

An acid-base extraction improves on the simple two-solvent extraction scheme by using acid-base reactions to change acetic acid into another compound with different solubility behavior. Hence, we convert acetic acid into, sodium acetate, and obtain a compound that is soluble in water, but not in diethyl ether. Acid-base extraction procedures are practically identical to solvent extractions. The only difference is that, instead of using water and diethyl ether, we use saturated aqueous sodium bicarbonate (NaHCO3) and diethyl ether.

Sodium bicarbonate reacts with acetic acid to make sodium acetate and carbonic acid, H2CO3, but it does not react with isopentyl acetate. Therefore, if we use the proper amount of sodium bicarbonate solution, we can convert all of the acetic acid into water-soluble sodium acetate and no acetic acid will remain in the diethyl ether solution (Organic chemistry lab. Manual, p32-33). Anhydrous MgSO4 can “dry” an ester solution by trapping small numbers of water molecules inside its crystal lattice. When MgSO4 adsorbs water, it changes from a fine powder into larger clumps.

The final product is then obtained by distilling the impure product. Distillation is the process of heating a liquid until it boils, capturing and cooling the resultant hot vapors, and collecting the condensed vapors. Distillation is used to purify a compound by separating it from a non-volatile or less-volatile material. When different compounds in a mixture have different boiling points, they get separated into individual components when the mixture is carefully distilled (Organic chemistry lab. Manual, p40). A reflux operation is first performed in this experiment and the set up is as per the diagram below.

A slow stream of water is passed through the condenser, while heating the flask with a ceramic heater mounted on top of a stirring motor and lab jack. 15 mL of isopentyl alcohol and 20 mL of glacial acetic acid were added to the flask. Then 4 mL of HCl was added slowly to the mixture, the flask was swirled around boiling chips dropped in and the mixture was allowed to reflux for an hour. Once this was done the reaction flask was allowed to cool in an ice bath and was then transferred to a 250 mL separatory funnel. The reaction flask was then rinsed and the rinses were transferred to the separatory funnel.

Then another 55 mL of cold water was added to the funnel. The mixture was then mixed and the phases were allowed to delayer before the aqueous layer was drained off and discarded. Then acid-base extraction was performed, where 25 mL of sodium bicarbonate was added to the separatory funnel. The funnel was gently swirled until no more carbon dioxide gas evolved. The funnel was stoppered and inverted and then allowed to delayer, with the stopper removed. When the two phases separated, the lower layer was drawn off and discarded.

This part was done 3 times, each time with 25 mL of sodium bicarbonate, until the litmus paper turned blue, showing that the solution was basic. After the acetic acid was removed, 25 mL of distilled water was added plus 5 mL of saturated sodium chloride. The funnel was gently swirled around so as to avoid an emulsion forming. Then it was allowed to delayer and the aqueous solution was drained off and discarded. Then the organic layer was poured out from the top of the separatory funnel into a 50 mL Erlenmeyer flask and dried with 5g of MgSO4. This was then swirled around and then allowed to stand.

Finally the ester was decanted, so that the drying agent was excluded and the product was distilled, the distillation set up was done as per the diagram shown, the ester was collected at the boiling fraction of 142 oC in a dry pre-weighed 50mL Erlenmeyer flask. The product is then weighed and the refractive-index calculated using the refractometer. (Organic Chemistry Lab. Manual, p 32-33) Isopentyl acetate was prepared by refluxing isopentyl alcohol with excess acetic acid and hydrochloric acid. The acetate was obtained by distillation giving, 4. 61g a 25. 3% yield.

The boiling point range where isopentyl acetate was collected was at 142oC (lit. 142 oC [CRC, pC-81]). The corrected refractive index was found to be 1. 4044 at 20 0C (lit. 1. 4003 at 20 0C, [CRC, pC-81]). Sources of error during this experiment that may have decreased the yield were: Some of the product could have been lost during the reflux process if the heat was too high, since the heat was continuously being adjusted during the process due to the mantles being old, hence the vapors would go past the condenser part and product would be lost decreasing the yield.

To decrease this sort of error the heat should be kept at a low level. Another source of error that may have decreased the yield was when transferring the solution from separatory funnel to the Erlenmeyer flask or at another part of the procedure during the decanting process where the product may have been left in the Erlenmeyer flask when transferring it to distillation flask. To diminish the error for loss of product during the transference from separatory funnel to Erlenmeyer flask would be wash out the separatory funnel with diethyl ether, this would get all the product left in the funnel.

To decrease the error for the decanting process it may have been better to use a filter paper and separate it out. Yet another source of error that caused the percentage yield to be so low was that the product was collected only at 142 0C. The yield would have been much greater if it were collected at a range of 135 0C – 142 0C. The experimental refractive index as well was not close to the literature refractive index because it is a measure of how pure the product is.

Hence there must be some impurities like alcohol, whose boiling point is lower than isopentyl acetate. This in turn makes the product denser, because the boiling point is lower, and the refractive index is inversely proportional to temperature. Therefore the experimental refractive index is found to be higher than the literature refractive index of pure isopentyl acetate. To decrease this error would need to get a pure product, for this to happen would have to reduce the amount of alcohol in the product so the distillation range would have to be smaller.

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