2.7 Ester

General formula of ester
■ Ester

	►	Ester can be represented by the general formula: ○ R – COO-R' (R and R' are alkyl groups such as methyl, ethyl, propyl and butyl) ○ CnH2n+1COOCmH2m+1 with n = 0,1,2,3, .... and m = 1, 2, 3, .......
	►	The functional group of esters is -COO-. Structure of this function is as follows:

■ Several members in the ester homologous series

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Ester Molecular formula Structural formula Methyl methanoate HCOOCH3 Methyl ethanoate CH3COOH3 Ethyl propanoate C2H5COOC2H5 Propyl butanoate C3H7COOC3H7

■ Nomenclature of the ester

	►	Esters are compounds derived from alcohol and carboxylic group. Therefore the name of ester will have part of alcohol and carboxylic group name.
	►	The prefix of the ester is an alkyl group name found in alcohol.
	►	The second part of the ester name is derived from carboxylic acids, replacing the ending “-oic” with “-oate”. ○ Alkyl group(Originate from alcohol) + Second part(Originate from carboxylic acid)

■ This video contains information on the feneral formula of ester.

✍ Worked-example 2.7(a) Name the structural formulae of the following esters. Solution: Alkyl group originate from alcohol: propyl Part originate from carboxylic acid: ethanoate Name of ester: Propyl ethanoate Solution: Alkyl group originate from alcohol: methyl Part originate from carboxylic acid: propanoate Name of ester: methyl propanoate

✍ Worked-example 2.7(b) Based on the molecular formula of the given esters, draw their structural formulae. Butyl propanoate Ethyl butanoate Solution: (a) Butyl propanoate $\begin{array}{c}\text{butyl}\\ \downarrow \\ C_4{H}_9\end{array}+\begin{array}{c}\text{propanoate}\\ \downarrow \\ C_2{H}_5{\mathit{COO}}^{\text{-}}\end{array}$ Structural formula : C2H5COOC4H9 (b) Ethyl butanoate $\begin{array}{c}\text{ethyl}\\ \downarrow \\ C_2{H}_5\end{array}+\begin{array}{c}\text{butanoate}\\ \downarrow \\ C_3{H}_7{\mathit{COO}}^{\text{-}}\end{array}$ Structural formula : C3H7COOC2H5

Preparation of ester in laboratory
■ Preparation of ester in laboratory

	►	Ester can be prepared through the reaction of carboxylic acid and alcohol with concentrated sulphuric acid as a catalyst. ○ $\text{Alcohol}+\text{carboxylic acid}\begin{array}{c}\mathrm{\rightleftharpoons }\\ \text{concentrated sulphuric acid}\end{array}\text{Ester}+\text{water}$
	►	Function of sulphuric acid: ○ to absorb the water that is produced during reaction. ○ act as a catalyst to increase the rate of reaction.
	►	Heating under reflux is used to avoid loss of volatile substances.
	►	All members of the ester homologous series are prepared using the same method (Esterification reaction).

Laboratory Activity 2.7.1 : Preparation of Ester in Laboratory

Preparation of esters
■ Properties of esters

	►	Colourless liquids with pleasant, fruity aromas
	►	Volatile substance with the a boiling point
	►	Less dense than water
	►	Insoluble in water but soluble in organic solvents

Source of ester
■ Natural esters

	►	Aromas from fruits like apples, pears, and strawberries are esters.
	►	Fats and oils: an ester of fatty acid and glycerol.
	►	Pheromones (animal sex hormone) are natural esters.
	►	Essential oil like lavender are esters.

Uses of ester
■ Uses of esters

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As food additive To make perfumes and cosmetics As fragrances To make medicines To make clothes, tyres and plastic bottles

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