Thursday 26 February 2015

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

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.

  1. Butyl propanoate
  2. Ethyl butanoate
Solution:
(a) Butyl propanoate
butyl C4H9+ propanoateC2H5COO-
Structural formula : C2H5COOC4H9
(b) Ethyl butanoate
ethyl C2H5+ butanoateC3H7COO-
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.
Alcohol + carboxylic acid concentrated sulphuric acid Ester + 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

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



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