Esterification

Esters are a class of organic molecules that can have a fruity or flowery aroma. The structure of an ester is a carbonyl with an alkyl or aryl group on one side and an oxygen bound to another alkyl or aryl group on the other side.

Depending on the alkyl or aryl groups, the ester can take on many different characteristics. For example, the reaction of glycerol and three fatty acids, which are carboxylic acids with long alkyl chains, results in a triglyceride, which has three ester groups. The long alkyl chains from the fatty acids give triglycerides a very high molecular weight. In contrast, simple esters have a low molecular weight and small functional groups.

The smells and flavors of fruits and flowers are attributed to simple esters. Even small changes to the structure of a simple ester greatly affect its fragrance. For example, changing a hydroxyl group to an amine changes the scent of this ester from wintergreen to grape. Similarly, propyl acetate smells like pears, while butyl acetate, which has only one more carbon in its chain, smells like apples.

One common way to make an ester is Fischer esterification, where a carboxylic acid and an alcohol react in the presence of an acid catalyst to form the ester and water. The R group and the carbonyl come from the carboxylic acid, and the alkoxy or aryloxy group with the R' comes from the alcohol. This esterification reaction is reversible. With a 1 to 1 mixture of the carboxylic acid and the alcohol, it tends to reach equilibrium with about a 70% yield of the ester at best.

However, Le Chatelier's principle allows us to increase the yield of the ester beyond that. Le Chatelier's principle states that any system at chemical equilibrium that is subjected to a change in concentration, pressure, temperature, or volume will adjust to a new equilibrium that counteracts the change.

So, if we increase the concentration of one of the reactants in a reversible reaction, the equilibrium will shift in the direction that decreases its concentration. This results in a higher yield of the ester product at equilibrium. Thus, we can improve the yield of esterification by using a 3:1 or 1:3 molar ratio of carboxylic acid to alcohol.

In this experiment, a Fischer esterification will be performed using alcohol in excess and sulfuric acid as the catalyst. In this reaction, the carboxylic acid reactivity is enhanced by sulfuric acid, which protonates the oxygen of the carbonyl. The alcohol is a nucleophile that attacks the carbon of the carbonyl to form an intermediate. Next, the hydrogen of the alcohol is transferred to a nearby hydroxyl. The carbonyl then reforms, eliminating a water molecule. Finally, deprotonation results in a neutral ester.

In this lab, you'll perform a Fischer esterification reaction with an unknown alcohol and carboxylic acid in a molar ratio of 3:1. You'll then identify the ester using its odor, determine the yield, and identify the two unknown reagents based on the structure of the ester.