Preparation of Isopentyl Acetate

INTRODUCTION

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).

EXPERIMENTAL

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