Leaving Certificate Chemistry Organic Chemistry The Esters Leaving Certificate Chemistry

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

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

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

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.

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

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

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.

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

Mandatory Experiment Preparation of Soap

Esterification reaction Hydrolysis reaction + H20 + Base + acid

Hydrolysis reaction used to make soap C15H31COO  CH2 C15H31COOK CH2OH  +  C15H31COO  CH + 3KOH  C15H31COOK + CHOH  +  potassium palmitate glycerol Soap is made by boiling animal or vegetable fats with potassium hydroxide.

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

Reflux the mixture for 15 – 30 minutes

What’s left in the reflux mixture? Ester from OIL Potassium hydroxide C15H31COO  CH2 C15H31COOK CH2OH  +  C15H31COO  CH + 3KOH  C15H31COOK + CHOH  +  C15H31COO  CH2 C15H31COOK CH2OH potassium palmitate glycerol SOAP SALT Dissolved in ethanol and water

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. 351-352 °C palmitic acid Gycerol – 290 degreees too high

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)

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.

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.

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.

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.