An ester is an organic compound where the hydrogen in the compound's carboxyl group is replaced with a hydrocarbon group. Esters are derived from carboxylic acids and (usually) alcohol. While carboxylic acid has the -COOH group, the hydrogen is replaced by a hydrocarbon in an ester. The chemical formula of an ester takes the form RCO2R', where R is the hydrocarbon parts of the carboxylic acid, and R' is the alcohol.
The term "ester" was coined by the German chemist Leopold Gmelin in 1848. It is likely the term was a contraction of the German word essigäther, which means "acetic ether."
Examples of Esters
Ethyl acetate (ethyl ethanoate) is an ester. The hydrogen on the carboxyl group of acetic acid is replaced with an ethyl group.
Other examples of esters include ethyl propanoate, propyl methanoate, propyl ethanoate, and methyl butanoate. Glycerides are fatty acid esters of glycerol.
Fats Versus Oils
Fats and oils are examples of esters. The difference between them is the melting point of their esters. If the melting point is below room temperature, the ester is considered to be an oil (e.g., vegetable oil). On the other hand, if the ester is a solid at room temperature, it is considered to be a fat (e.g., butter or lard).
The naming of esters can be confusing new to organic chemistry students because the name is opposite of the order in which the formula is written. In the case of ethyl ethanoate, for example, the ethyl group is listed before the name. "Ethanoate" comes from ethanoic acid.
While the IUPAC names of esters come from the parent alcohol and acid, many common esters are called by their trivial names. For example, ethanoate is commonly called acetate, methanoate is formate, propanoate is called propionate, and butanoate is called butyrate.
Esters are somewhat soluble in water because they can act as hydrogen-bond acceptors to form hydrogen bonds. However, they cannot act as hydrogen-bond donors, so they don't self-associate. Esters are more volatile than comparable-sized carboxylic acids, more polar than ethers and less polar than alcohols. Esters tend to have a fruity fragrance. They may be distinguished from each other using gas chromatography because of their volatility.
Polyesters are an important class of plastics, consisting of monomers linked by esters. Low molecular weight esters act as fragrance molecules and pheromones. Glycerides are lipids that are found in vegetable oil and animal fat. Phosphoesters form the DNA backbone. Nitrate esters are commonly used as explosives.
Esterification and Transesterification
Esterification is the name given to any chemical reaction that forms an ester as a product. Sometimes the reaction may be recognized by the fruity or floral fragrance released by the reaction. An example of an ester synthesis reaction is Fischer esterification, in which a carboxylic acid is treated with alcohol in the presence of a dehydrating substance. The general form of the reaction is:
RCO2H + R'OH ⇌ RCO2R' + H2O
The reaction is slow without catalysis. The yield may be improved by adding an excess of alcohol, using a drying agent (e.g., sulfuric acid), or removing water.
Transesterification is a chemical reaction that changes one ester into another. Acids and bases catalyze the reaction. The general equation for the reaction is:
RCO2R' + CH3OH → RCO2CH3 + R'OH