Introduction to Esters

Esters have a very sweet fruity smell. Naturally occurring esters are found in fruits. An ester is a product of the reaction of an acid (usually organic) and an alcohol (the hydrogen of the acid R-COOH is replaced by an alkyl group R'). Esters mainly result from the condensation (a reaction that produces water) of a carboxylic acid and an alcohol. The process is called esterification. This reaction can be catalyzed by the presence of H⁺ ions. Sulphuric acid, H₂SO₄, is often used as a catalyst for this reaction. The name ester is derived from the German Essig-Aether, an old name for acetic acid ethyl ester (ethyl acetate). Esters have the general formula R-COOR',

Esters are named in the same manner as salts (although esters and salts have completely different properties): two-word names are used. Note that in the general formula, R-COOR' (the carbon is double-bonded to one oxygen atom and single-bonded to another), the alkyl group (R') is always attached to an oxygen atom. This alkyl group (R') is named as the first word of the two-word name. The second word is derived by adding the ending -oate to the stem of the acid name (-oic in the acid name is replaced by -oate).

A reversible reaction between an alcohol and a carboxylic acid causes loss of water and the formation of an ester:

Esters are named as derivatives of the carboxylic acid from which they are formed. Condensation of ethanoic acid with methanol will produce methyl ethanoate. As stated above the ending of the acid -oic is changed to -oate, much as if the ester were a salt of the acid. The esterification reactions are generally easily reversible by addition of water; the reverse reaction is called the hydrolysis of the ester and proceeds in the presence of aqueous base.

The esterification process will proceed more nearly to completion if a substance which removes water without reacting with the acid or the alcohol is added to the reaction, such as sulfuric acid. For example, the reaction between ethanoic acid and ethanol produces the ester ethyl ethanoate.

The concentrated H₂SO₄ removes water from the products and is a dehydrating agent. Most esters have very pleasant odors (see below). Many flavoring and scenting agents are made from esters. Esters are volatile liquids which are not ionized and they are soluble in organic solvents but not in water.

In the laboratory, methyl methanoate can be produced by the condensation reaction of methanol and methanoic acid, as follows:

Industrial methyl methanoate, however, is usually produced by the combination of methanol and carbon monoxide in the presence of a strong base:

As shown in the diagram above the hydrogen atom on the right can be replaced with a CH₃ group or additional CH₂ units, producing other methyl esters, such as:

For esters derived from the simplest carboxylic acids, the traditional names are recommended by IUPAC, such as, formate, acetate, propionate, butyrate, though out of these only acetate may carry further substituents. For esters from higher acids, the alkane name with an -oate ending is generally preferred, e.g., hexanoate. Common esters of aromatic acids include benzoates such as methyl benzoate, with substitution allowed in the name.

Common names of esters are derived from the organic acid and the alcohol from which they are derived. For example, when acetic acid reacts with ethyl alcohol, the ester formed is called ethyl acetate. The IUPAC name is different. Acetic acid is called ethanoic acid by the IUPAC rules. Thus the ester formed is called ethyl ethanoate. IUPAC names ester from two words: first from the prefix of the alcohol and the second from the name of the acid.

General formula RCOOR'. In the following example R is H- while R' is a methyl group -CH₃. This will produce the simplest of esters. The reaction of methanoic acid HCOOH and methanol CH₃OH can form this ester. From the IUPAC rules, the ester will take its first name from the prefix of the alcohol, in this case methyl, and the second name from the acid, in this case it is methanoate. Thus HCOOCH₃ is named methyl methanoate by the rules laid down for IUPAC nomenclature for esters. The common name of this ester is methyl formate.

General formula RCOOR'. In the following example R is H- while R' is an ethyl group -C₂H₅. The reaction of methanoic acid HCOOH and ethanol C₂H₅OH can form this ester. From the IUPAC rules, the ester will take its first name from the prefix of the alcohol, in this case ethyl, and the second name from the acid, in this case it is methanoate. Thus HCOOC₂H₅ is named ethyl methanoate by the rules laid down for IUPAC nomenclature for esters. The common name for this ester is ethyl formate.

General formula RCOOR'. In the following example R is CH₃- while R' is also a methyl group -CH₃. The reaction of ethanoic acid CH₃COOH and methanol CH₃OH can form this ester. From the IUPAC rules, the ester will take its first name from the prefix of the alcohol, in this case methyl, and the second name from the acid, in this case it is ethanoate. Thus CH₃COOCH₃is named methyl ethanoate by the rules laid down for IUPAC nomenclature for esters. The common name of this ester is methyl acetate.

General formula RCOOR'. In the following example R is CH₃- while R' is an ethyl group -C₂H₅. The reaction of ethanoic acid CH₃COOH and ethanol C₂H₅OH can form this ester. From the IUPAC rules, the ester will take its first name from the prefix of the alcohol, in this case ethyl, and the second name from the acid, in this case it is ethanoate. Thus CH₃COOC₂H₅is named ethyl ethanoate by the rules laid down for IUPAC nomenclature for esters. The common name of this ester is ethyl acetate.

General formula RCOOR'. In the following example R is CH₃-CH₂- while R' is a methyl group -CH₃. The reaction of propanoic acid CH₃CH₂COOH and methanol CH₃OH can form this ester. From the IUPAC rules, the ester will take its first name from the prefix of the alcohol, in this case methyl, and the second name from the acid, in this case it is propanoate. Thus CH₃CH₂COOCH₃ is named methyl propanoate by the rules laid down for IUPAC nomenclature for esters.

In the following example particular exceptions have been made regarding the IUPAC rules. A six membered ring with alternating double and single bonds is 1,3,5-cyclohexatriene. However, this has been accepted simply as benzene. When an alcohol group is attached rather than call it 1,3,5-cyclohexatrienol, it is referred to as 2-hydroxybenzene, or phenol. Further when this molecule becomes a carboxylic acid with the inclusion of the -COOH group it can be referred to as 2-hydroxy-1,3,5-cyclohexatrienoic acid or 2-hydroxybenzenoic acid with the latter generally utilized as its "chemical" name. It also has a "common" of salicylic acid.

When salicylic acid combines with methanol it becomes the ester known as methyl salicylate or oil of wintergreen. Methanol is also known as methyl alcohol and wood alcohol.

General formula RCOOR'. The reaction of salicylic acid C₆H₄(OH)CO₂H and methanol CH₃OH forms this ester. From the IUPAC rules, the ester will take its first name from the prefix of the alcohol, in this case methyl, and the second name from the acid. Thus C₆H₄(HO)CO₂CH₃is named methyl salicylate by the exceptions laid down for IUPAC nomenclature for esters. The common name of this ester is methyl salicylate (oil of wintergreen).

2-hydroxy-1,3,5-cyclohexatrienoic acid + methanol

methyl 2-hydroxy-1,3,5-cyclohexatrienoate + water

CH₃(CH₂)₃COOH + CH₃CH₂CH₂OH

CH₃CH₂CH₂CH₂COOCH₂CH₂CH₃+ H₂O

Condensation of methanoic acid with ethanol will produce ethyl methanoate, an ester having the odor of rum.

Among the most important of the natural esters are fats such as lard, tallow, and butter, and oils such as linseed, cottonseed, and olive. These are made up of esters of such complex acids as palmitic, C₁₅H₃₁COOH, stearic, C₁₇H₃₅COOH, and oleic, C₁₇H₃₃COOH, with the trihydroxyl alcohol, glycerol, C₃H₅(OH)₃.

Soaps are made by boiling natural fats and oils with strong bases such as sodium and potassium hydroxides. When animal fat is treated with sodium hydroxide, glycerol and sodium salts of the fatty acids, palmitic, stearic, and oleic acid, are formed. The reaction in the case of glyceryl stearate (the fat) is given by

(C₁₇H₃₅COO)₃C₃H₅+ 3 NaOH 3 C₁₇H₃₅COONa + C₃H₅(OH)₃

To obtain the soap (the sodium salt of the fatty acid) free from glycerol and water, sodium chloride is added which causes the soap to precipitate, or "salt out". The soap, being lighter, collects as a crust on the top where it is removed, partially dried, and pressed into cakes for use.

Hydrolysis of esters : Esters break down into their respective organic acid and alcohol from which they are formed. This process is called hydrolysis.

When sodium hydroxide is added to an ester, say for example to ethyl ethanoate, a salt sodium ethanoate is formed along with ethyl alcohol. The reaction is shown below.

Hydrolysis of an ester with an alkaline solution like sodium hydroxide is known as saponification (soap making). This reaction is used in the preparation of soaps.

The above reaction is a test for checking if esters are present in any solution. Few drops of indicator phenolphthalein is added to a solution of ester and NaOH. The solution shows pink coloration. Heat the solution. When the ester has reacted completely the pink color will disappear.

Esters participate in hydrogen bonds as hydrogen-bond acceptors, but cannot act as hydrogen-bond donors, unlike their parent alcohols. This ability to participate in hydrogen bonding makes them more water-soluble than their parent hydrocarbons. However, the limitations on their hydrogen bonding also make them more hydrophobic than either their parent alcohols or parent acids. Their lack of hydrogen-bond-donating ability means that ester molecules cannot hydrogen-bond to each other, which makes esters generally more volatile than a carboxylic acid of similar molecular weight. This property makes them very useful in organic analytical chemistry: unknown organic acids with low volatility can often be esterified into a volatile ester, which can then be analysed using gas chromatography, gas liquid chromatography, or mass spectrometry. Many esters have distinctive odors, which has led to their use as artificial flavorings and fragrances. For example:

Ethyl Methanoate	H-CO₂-CH₂-CH₃	Rum
Methyl Phenyl Ethanoate	C₆H₅-CH₂-CO₂-CH₃	Honey
Benzyl Ethanoate	C₆H₅-CH₂-CO₂CH₃	Jasmine
Pentyl Ethanoate	CH₃-CO₂-CH₂-CH₂-CH₂-CH₂-CH₃	Pear
3-Methylbutyl Ethanoate	CH₃-CO₂-CH₂-CH₂-CH(CH₃)-CH₃	Banana
Octyl Ethanoate	CH₃-CO₂-CH₂(CH₂)₆-CH₃	Orange
Methyl Butanoate	CH₃-CH₂-CH₂-CO₂-CH₃	Pineapple
Ethyl Butanoate	CH₃-CH₂-CH₂-CO₂CH₂CH₃	Peach
3-Methylbutyl-3-Methylbutanoate	CH₃CH(CH₃)CH₂CO₂CH₂CH₂CH(CH₃)CH₃	Apple
Pentyl Butanoate	CH₃-CH₂-CH₂CO₂-CH₂-(CH₂)₃-CH₃	Apricot
Methyl Salicylate	C₆H₄(OH)-CO₂-CH₃	Oil of Wintergreen
Methyl Anthranilate	C₆H₄(NH₂)-CO₂-CH₃	Grapes
Ethyl Cinnamate	C₆H₅-CH=CH-CO₂-CH₂-CH₃	Oil of Cinnamon


benzene	2-hydroxybenzene	benzoic acid	2-hydroxybenzenoic acid
	phenol		salicylic acid

	+	CH₃OH		+	H₂O
2-hydroxybenzoic acid		methanol	methyl-2-hydroxybenzoate		water
salicylic acid		methanol	methyl salicylate		water


ethyl butanoate (apple)	octyl ethanoate (orange)