Overview

Esters

The structure of an ester is a carbonyl with an alkyl or aryl group (R) on one side, and an oxygen bound to another alkyl or aryl group (R’) on the other side, represented by the general formula: RCOOR’. Esters can be commonly derived by an esterification reaction between a carboxylic acid and an alcohol. The hydroxyl group of the carboxylic acid is replaced by an alkyl or aryl group. Simple esters, which have low molecular weight and small R and R’ functional groups, have smells and flavors of fruits and flowers.

The simplest carboxylic acid is formic acid (HCOOH), with the simplest alcohol being methanol. Thus, the simplest ester formed from the esterification of these two molecules is methyl formate (HCOOCH₃). Methyl formate is considered a derivative of formic acid because the OH group is replaced by the OCH₃ group. The structural differences between methyl formate and formic acid dictate the difference in their properties. This is because the properties of the molecule depend heavily on the R and R’ groups.

Esters that are found in fruits and flowers are known as simple esters due to the low complexity of their structure in comparison to other esters. The alkyl and aryl groups found on these esters greatly influences the fragrance they emit; thus, the addition of a carbon or a different functional group can impact the scent produced by the ester. For example, propyl acetate (C₅H₁₀O₂) has the scent of pears. However, butyl acetate (C₆H₁₂O₂), which has only one more carbon in its chain, smells like apples.

Long-chain esters, meaning those with long alkyl chains, are called fatty acids. Fatty acids are important biological compounds that are essential for certain biological processes. Fatty acids are also a biological form of energy storage. When three fatty acids react with glycerol, they form a triglyceride — the most common form of fatty acids. Triglycerides have a very high molecular weight and can be highly branched. Their function and properties are dependent on the length and types of alkyl chains incorporated into their structure. One of the most common industrial uses for triglycerides is in the production of soaps and detergents.

Esterification Reaction

Esters are formed from an esterification reaction, with simple esters being formed through Fisher esterification. This reaction converts a carboxylic acid and alcohol into an ester with water as a by-product. Fisher esterification is a reversible reaction that proceeds very slowly. An acid catalyst, typically in the form of sulfuric acid, is added to increase the rate of the reaction while also acting as a dehydrating agent.

The mechanism of action for a Fisher esterification begins by the carbonyl oxygen attacking the sulfuric acid and deprotonating its OH group. This leads to a positively charged carbonyl oxygen. To better stabilize the positive charge, one electron pair of the carbonyl double bond is pushed to the carbonyl oxygen while simultaneously creating an electron-poor carbon center. Next, the hydroxy group of the alcohol acts as a nucleophile and attacks the electron-poor carbon — the electrophile — forming an intermediate. Then, the positively-charged hydroxyl group from the alcohol is deprotonated by the hydroxyl group of the carboxylic acid, stabilizing the oxygen atom of the alcohol. In the last step, the carbonyl oxygen is deprotonated by the conjugated base, and the free electron pair is moved towards the carbonyl center while the protonated carboxyl group leaves as a water molecule.

Since the esterification reaction is reversible, a 1:1 mixture of the carboxylic acid and the alcohol will reach equilibrium with about 70% yield of the ester. However, as with any equilibrium process, the reaction can be driven in one direction by changing the concentration, pressure, temperature, or volume of the reagents. This change causes the reaction to adjust to a new equilibrium to counteract the change, which is the basis of Le Chatelier’s principle.

When the concentration of one of the reactants is increased, the equilibrium shifts in the direction that will decrease its concentration. Thus, by increasing the concentration of one reactant, the equilibrium produces more of the product, thereby resulting in a higher yield of the ester product.

One common method utilized in organic chemistry labs is to provide an excess of one of the reagents, typically the alcohol. As mentioned earlier, Fisher esterification can utilize sulfuric acid as a catalyst. However, one of the other functions of sulfuric acid is to act as a dehydrating agent, sequestering water molecules away from the reaction. Applying Le Chatelier’s principle demonstrates that as one of the products is removed — in this case, water — the reaction is pushed to making more product. This increases the yield of the ester formed.

Reference

1. Streitwieser, A., Heathcock, C.H., Kosower, E.M. (1998). Introduction to Organic Chemistry. Upper Saddle River, NJ: Prentice Hall.

Procedure

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.