Organic Chemistry - Part X

Esters
We have seen how different functional groups attached to parent hydrocarbon, change the properties of the hydrocarbon. When the functional group is –COOR, where R is an alkyl group, the resultant compound is called an ester. While learning about the reactions of alcohols with carboxylic or organic acids, we have seen how an ester is formed. Ester and water are the reaction products of this reaction. Such a reaction is called esterification. Esterification is done in the presence of concentrated sulphuric acid, which is a catalyst for the reaction. The ester tests are performed to identify if an organic mixture has alcohol or an acid.

Esters have a very sweet fruity smell. Naturally occurring esters are found in fruits. The structure of a typical ester namely ethyl ethanoate is shown below.  

Naming of esters
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 IPUAC 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.

Examples below will clarify the way the IUPAC nomenclature is used.

Example 1: Let us see how methyl methanoate HCOOCH₃ is named by IUPAC method.  

The functional group –COOR, R here is a methyl group CH₃. This is 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.

Example 2: Let us see how ethyl methanoate HCOOC₂H₅ is named by IUPAC method.  

The functional group –COOR, R here is a 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 HCOO C₂H₅ is named ethyl methanoate by the rules laid down for  IUPAC nomenclature for esters. The common name for this ester is ethyl formate.

Example 3: Let us see how methyl ethanoate CH₃COOCH₃ is named by IUPAC method. 

The functional group –COOR, R here is 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 CH₃COOCH₃ is named methyl ethanoate by the rules laid down for  IUPAC nomenclature for esters. The common name of this ester is methyl acetate.

Example 4: Let us see how ethyl ethanoate CH₃COOC₂H₅ is named by IUPAC method.

The functional group –COOR, R here is a 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 CH₃COOC₂H₅ is named ethyl ethanoate by the rules laid down for  IUPAC nomenclature for esters. The common name of this ester is ethyl acetate.

Properties of esters

Physical Properties
1. Physical State : Lower molecular weighted esters are colourless volatile liquids. Higher esters are colourelss waxy solids.
2. Odour : All esters have a strong fruity smell.
3. Solubility : Lower members of esters are soluble in water. The solubility decreases with increase in molecular weight of the esters. Esters are soluble in organic solvents. Esters themselves are good organic solvents.
4. Acidic nature : Esters are neutral to litmus tests.

Chemical Properties
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 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 colouration. Heat the solution. When the ester has reacted completely the pink colour will disappear.  

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