Preparation of Esters
Esters are the product of a condensation reaction between alchohols and carboxylic acids, meaning that when they react together they form a molecule of water.
The way they can be prepared can be shown by mixing two such substances : Methanol and Ethanoic Acid.
Test Tubes, Test Tube Rack
1 Large Beaker
2 Small Beakers
A Bunsen Burner
Concentrated Sulphuric Acid
1mol/ l Sodium hydrogencarbonate
*Methanol is toxic! Ethanoic Acid and Sulphuric acid are corrosive!*
What is Actually Going On
1. Put some water into the large beaker and sit it on the tripod. Heat the water until it boils then turn off the bunsen. (This is important! you must not leave the bunsen along due to the flammability of the chemicals)
2. Add about 1cm of the Methanol to the test tube and then add the same volume of ethanoic acid.
3. Add about 5 drops of concentrated sulphuric acid to the mixture.
4. Soak the paper towel in cold water, fold it and secure it around the neck of the test tube with the rubber band. the purpose of this is to act as a condenser.
5. Place a loose plug of cotton wool in the mouth of the test tube to stop any chemicals coming out whilst heating is occuring.
6. Place the test tube into the beaker of hot water. Leave for ten minuets.
7. Wake the small beaker and add 20cm2 of sodium hydrogencarbonate to it.
8. Once heated, take the test tube from the hot water and remove the cotton wool. Pour the contents of the test tube into the small beaker. The sodium hydrogencarbonate neutralises any remaining sulphuric/ carboxylic acid. (This means the smell of the carboxylic acid is also removed.)
9. The beaker should be gently swirled, the ester forming on the surface of the solution. Methyl Ethanoate.
10. Take a breath of fresh, uncontaminated air. Hold the beaker a little away and waft the scent towards your nose, sniffing only lightly. You should smell a fruity flavour.
When the molecule
s are placed together they react (With the help of the sulphuric acid and the heating) by losing a hydrogen from the alchohol and a Hydrogen
and Oxygen atom
from the carboxylic acid
Ethanoic Acid + Methanol -> Methyl Ethanoate
COOH + CH3
OH -> CH3
H O H H O
| // | | //
H-C-C + H-O-C-H ---> H-C-C H + H-O
| \ | | \ | |
H O-H H H O-C-H H
The H is lost from the carbonyl (COOH) group from the alchohol and the OH is the hydroxyl group from the acid, shown below.
H O | | H
| // | | |
H-C-C | H- | O-C-H
| | \ + | |
H | O-H | H
A new bond is formed by the rest of the molecule
H O | | H H |O | H
| // | | | | ||| | |
H-C-C | + | O-C-H ---> H-C- |C-O| -C-H
| | | | | |_ _| |
H | | H H H
This is the new Ester Group :
C = Carbon
H = Hydrogen
O = Oxygen
Some Common Esters and their Scents :
Ethyl Butanoate Pineapple
Propyl Ethanoate Pears
2-Methylpropyl Ethanoate Raspberries
Experimental Data Sheets.