25.5 Esters – Structure, Properties and Naming

Esters contain a carbonyl group with a second oxygen atom bonded to the carbon atom in the carbonyl group by a single bond. In an ester, the second oxygen atom bonds to another carbon atom (Figure 25.5a.). The names for esters include prefixes that denote the lengths of the carbon chains in the molecules and are derived following nomenclature rules similar to those for inorganic acids and salts. The functional groups for an ester are shown in red below.

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 hydrogen atom, the compound would be a carboxylic acid.) Figure 25.5b. shows models for two common esters.

Esters are produced by the reaction of acids with alcohols. For example, the ester ethyl acetate, CH₃CO₂CH₂CH₃, is formed when acetic acid reacts with ethanol (Figure 25.5c.).

Properties of Esters

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

Ester molecules are polar but have no hydrogen atom attached directly to an oxygen atom. They are therefore incapable of engaging in intermolecular hydrogen bonding with one another and thus have considerably lower boiling points than their isomeric carboxylic acids counterparts. Because ester molecules can engage in hydrogen bonding with water molecules, however, esters of low molar mass are somewhat soluble in water. Borderline solubility occurs in those molecules that have three to five carbon atoms. Table 25.5a. lists the physical properties of some common esters.

Source: “15.6: Physical Properties of Esters” In Basics of GOB Chemistry (Ball et al.), CC BY-NC-SA 4.0.

Names of Esters

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

Here are some basic rules for naming esters from the International Union of Pure and Applied Chemistry (IUPAC):

1. Write the name for the carbon chain from the alcohol as an alkyl group. For example if the alcohol is ethanol, the first word in ester naming will be ethyl.
2. The second word for naming an ester involves naming the acid but substituting the -oic ending for -oate. For example if the acid is ethanoic acid, the second word in the name becomes ethanoate.

The group name of the alkyl or aryl portion is given first and is followed by the name of the acid portion. In both common and International Union of Pure and Applied Chemistry (IUPAC) nomenclature, the –ic ending of the parent acid is replaced by the suffix –ate (Table 25.5b.). 

Table 25.5b. 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

Table and image credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0.

Example 25.5a

Give the common and IUPAC names for each compound.

Solution:

a. 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.

b. 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 & Image credits: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0

Exercise 25.5a

Give the common and IUPAC names for each compound.

Check Your Answers:^[1]

Exercise & Image credits: Introduction to Chemistry: GOB., CC BY-NC-SA 3.0.

Exercise 25.5b

Draw the structure for ethyl pentanoate.

Check Your Answer^[2]

Exercise & solution image credits: Introduction to Chemistry: GOB., CC BY-NC-SA 3.0.

Indigenous Perspectives: The Strawberry

The strawberry or ken’niiohontésha in Mohawk language, is a symbol of importance in woman’s medicine and for naming babies in the longhouse. The strawberry is one of several festivals in Haudenosaunee’s cycle of ceremonies to give thanks to the natural world.

For more details on the importance of the strawberry read: CBC News – Strawberry harvest has cultural and ceremonial significance for Kahnawake community [New tab].  The interview below with Elder Duke Redbird also looks at the importance of the strawberry “heart berry”: City News – The Indigenous story of the Strawberry Moon [New tab].

Spotlight on Everyday Chemistry: Julian Silverman’s Research on Oil Spill Cleanup

Attribution & References

Except where otherwise noted, this page is adapted by Caryn Fahey from:

• “18.3 Aldehydes, Ketones, Carboxylic Acids, and Esters” In General Chemistry 1 & 2 by Rice University, a derivative of Chemistry (Open Stax) by Paul Flowers, Klaus Theopold, Richard Langley & William R. Robinson and is licensed under CC BY 4.0. ​Access for free at Chemistry (OpenStax)
• “15.5: Esters – Structures and Names” & “15.6: Physical Properties of Esters” In Basics of General, Organic, and Biological Chemistry (Ball et al.)by David W. Ball, John W. Hill, and Rhonda J. Scott via LibreTexts, CC BY-NC-SA 4.0./ A LibreTexts version of Introduction to Chemistry: GOB (v. 1.0), CC BY-NC 3.0.