4.5: Esters

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Learning Objectives

Identify the general structure for an ester.
Use common names to name esters.
Name esters according to the IUPAC system.

Esters are carboxylic acid derivatives formed by replacing the H in the hydroxyl group with a carbon group. Esters have the general formula RCOOR′, where R may be a hydrogen atom, an alkyl group, or an aryl group, and R′ may be an alkyl group or an aryl group but not a hydrogen atom (if it were a hydrogen atom, the compound would be a carboxylic acid). Figure \(\PageIndex{1}\) shows the general structure for an ester, with the functional group shown in red.

general structure of an ester — Figure \(\PageIndex{1}\): The structure of an ester. Esters feature a carbon-oxygen double bond that is also singly bonded to a second oxygen atom, which is then joined to an alkyl or an aryl group.

Esters occur widely in nature. Unlike carboxylic acids, esters generally have pleasant odors and are often responsible for the characteristic fragrances of fruits and flowers. Once a flower or fruit has been chemically analyzed, flavor chemists can attempt to duplicate the natural odor or taste. Both natural and synthetic esters are used in perfumes and as flavoring agents.

Fats and vegetable oils are esters of long-chain fatty acids and glycerol. Esters of phosphoric acid are of the utmost importance to life.

Naming Esters

Although esters are covalent compounds and salts are ionic, esters are named in a manner similar to that used for naming salts.

naming esters

Figure \(\PageIndex{2}\): Guide for naming esters.

The common and International Union of Pure and Applied Chemistry (IUPAC) rules for naming esters are similar. In both systems, the identity of the R' carbon group (that replaced the hydrogen) is given and is followed by the name of the acid portion. Another similarity among the systems is that the -ic acid ending of the parent acid is replaced by the suffix -ate (Table \(\PageIndex{1}\)).

If the ester molecule has substituents, as with carboxylic acids, the carbonyl carbon atom is always assigned to C1. It is unnecessary to designate this group by number.

Table \(\PageIndex{1}\): Nomenclature of Esters
Condensed Structural Formula	Common Name	IUPAC Name
HCOOCH₃	methyl formate	methyl methanoate
CH₃COOCH₃	methyl acetate	methyl ethanoate
CH₃COOCH₂CH₃	ethyl acetate	ethyl ethanoate
CH₃CH₂COOCH₂CH₃	ethyl propionate	ethyl propanoate
CH₃CH₂CH₂COOCH(CH₃)₂	isopropyl butyrate	isopropyl butanoate
	ethyl benzoate	ethyl benzoate

Example \(\PageIndex{1}\)

Give the common and IUPAC names for each compound.

Solution

The alkyl group attached directly to the oxygen atom is a butyl group (in green).

The part of the molecule derived from the carboxylic acid (in red) has three carbon atoms. It is called propionate (common) or propanoate (IUPAC). The ester is therefore butyl propionate or butyl propanoate.
An alkyl group (in green) is attached directly to the oxygen atom by its middle carbon atom; it is an isopropyl group. The part derived from the acid (that is, the benzene ring and the carbonyl group, in red) is benzoate. The ester is therefore isopropyl benzoate (both the common name and the IUPAC name).

Exercise \(\PageIndex{1}\)

Give the common and IUPAC names for each compound.

Example \(\PageIndex{2}\)

Draw the structure for ethyl pentanoate.

Solution

Start with the portion from the acid. Draw the pentanoate (five carbon atoms) group first; keeping in mind that the last carbon atom is a part of the carboxyl group.

Then attach the ethyl group to the bond that ordinarily holds the hydrogen atom in the carboxyl group.

Exercise \(\PageIndex{2}\)

Draw the structure for phenyl pentanoate.

Key Takeaway

An ester has an OR' group attached to the carbon atom of a carbonyl group.
The R' cannot be H, because this will represent a carboxylic acid.
The common and IUPAC names for esters identify the R' group that replaced the H followed by the name of the acid portion of the molecule.