1. Leaving Certificate Chemistry
Organic Chemistry
The Esters
Leaving Certificate Chemistry

2. Acid Alcohol Ester Water
An ester is formed when an acid and an alcohol react together in a substitution (condensation) reaction
Acid
Alcohol
Ester
Water

3. Esterification Ethyl Ethanoate CH3COOC2H5 CH3COOH CH3COOC2H5 H2O
Ethanoic Acid
CH3COOH H C C O H H
Ethyl Ethanoate
CH3COOC2H5
Ethyl Group H H H
CH3COOH
CH3COOC2H5 O C C H
Ethanol
C2H5OH
H2SO4
H2O
C2H5OH H H

4. Esterification Methyl Propanoate C2H5COOCH3 H2O Propanoic Acid
C2H5COOH H H
Methyl Group
Methyl Propanoate
C2H5COOCH3 H H O C H
Methanol
CH3OH
H2O H

5. Physical properties of the esters
Boiling points
Similar to those of aldehydes and ketones with the same number of carbon atoms.( they are all polar molecules and so have dipole-dipole interactions as well as van der Waals dispersion forces.)
However, they don't form hydrogen bonds, and so their boiling points are not as high as an acid with the same number of carbon atoms.

6. Physical properties of the esters
Solubility : water
The small esters are fairly soluble in water - they can form Van Der Waals, Dipole Dipole bonds with water molecules.
But solubility decreases as chain length increases
As chain lengths increase, the hydrocarbon parts of the ester molecules ( non polar) start to get in the way.
By forcing themselves between water molecules, they break the relatively strong hydrogen bonds between water molecules

7. Common esters and their uses
The sweet odour associated with everyday fruits are due to the presence of esters

8. Fats are natural esters
Animal and vegetable fats and oils are just big complicated esters.
The difference between a fat (like butter) and an oil (like sunflower oil) is simply in the melting points of the mixture of esters they contain.

9. Check your learning... How is an ester made? How are esters named?
Uses of esters?
Physical properties of the esters?

10. Mandatory Experiment
Preparation of Soap

11. Esterification reaction
Hydrolysis reaction
+ H20
+ Base
+ acid

12. Hydrolysis reaction used to make soap
C15H31COO  CH C15H31COOK CH2OH
 
C15H31COO  CH KOH  C15H31COOK CHOH
 
potassium palmitate glycerol
Soap is made by boiling animal or vegetable fats with potassium hydroxide.

13. Why use ethanol and water as solvents for the reaction
Ethanol is used as it allows the OIL and KOH to mix
A small amount of water speeds up dissolving

14. Reflux the mixture for 15 – 30 minutes

15. What’s left in the reflux mixture?
Ester from OIL
Potassium hydroxide
C15H31COO  CH C15H31COOK CH2OH
 
C15H31COO  CH KOH  C15H31COOK CHOH
 
C15H31COO  CH C15H31COOK CH2OH
potassium palmitate glycerol
SOAP SALT
Dissolved in ethanol and water

16. Remove the ethanol by distillation at 78OC 2
Glycerol does not distil over because its boiling point is very high
872 = boiling point of potassium palmitate
Boiling point °C palmitic acid
Gycerol – 290 degreees too high

17. Why Glycerol has a high boiling point
Glycerol has 3 hydroxyl groups per molecule so there is lots of hydrogen bonding between its molecules. ( AS WELL AS DIPOLE DIPOLE AND VAN DER WAALS)

18. Extracting the soap from the reaction mixture
Dissolve the residue left after distillation in a minimum of hot water
Add this solution to the brine (saturated salt and water solution)
The soap should precipitate out of solution. Glycerol stays in the water.
Filter the soap.
Test the soap for its lathering qualities by shaking a small sample of it with water.

19. Why the soap precipitates out of solution when brine is added..
Potassium palmitate is soluble in water
Common salt contains some impurities of Magnesium and Calcium chlorides.
A reaction occurs - Magnesium or calcium palmitate salts are formed which are insoluble in water so ppts are observed.

20. Glycerol stays dissolved in water
Glycerol has 3 hydroxyl groups per molecule,
There is also considerable hydrogen bonding between glycerol molecules and water molecules so glycerol will stay dissolved in water, even when brine is added.

21. How does soap work? How do you explain the cleansing action of soap?
The polar of the soap molecule is water-soluble while the non-polar hydrocarbon part is soluble in oil and grease.
Soap therefore allows otherwise insoluble substances such as oil or grease to become soluble in water.