25.13: Esters

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Perfumes contain ingredients from a number of sources. Musk is obtained from animals, but the vast majority of perfume components are obtained from plants. Approximately 2,000 plant species have been used as sources for perfume materials. The needed chemicals are extracted using solvent extraction or distillation. The oils are diluted with ethanol to varying degrees, depending on the price of the finished product—the less ethanol present (meaning there is a higher percentage of the active scent ingredients), the more expensive the perfume.

Esters

An ester is an organic compound that is a derivative of a carboxylic acid, in which the hydrogen atom of the hydroxyl group has been replaced with an alkyl group. The structure is the product of a carboxylic acid (the \(\ce{R}\)-portion) and an alcohol (the \(\ce{R'}\)-portion). The general formula for an ester is shown below.

Structure of an ester

The \(\ce{R}\) group can either be a hydrogen or a carbon chain. The \(\ce{R'}\) group must be a carbon chain since a hydrogen atom would make the molecule a carboxylic acid. The steps for naming esters, along with two examples, are shown below.

Identify and name the alkyl group \(\left( \ce{R'} \right)\) that has replaced the hydrogen of the hydroxyl group. This is the first part of the ester name.
Name the carboxylic acid portion of the molecule \(\left( \ce{R-COO} \right)\), but change the ending of the name from -oic acid to -oate. This is the second part of the ester name.
Leave a space between the alkyl group name and the name of the carboxylic acid derivative.

Nomenclature of esters

Esters have lower boiling points than the carboxylic acids from which they were derived because they cannot form hydrogen bonds with each other. This is because there are no hydrogen atoms bonded to oxygen atoms, as in the acid. Esters with shorter carbon chains are soluble in water, while those with longer chains are less soluble.

Esters are very commonly found in plants, and are responsible for many distinct odors and flavors. For example, methyl salicylate has the odor and flavor of oil of wintergreen, while propyl ethanoate has that of a pear.

Esters

Summary