Organic Chemistry The Esters Leaving Certificate Chemistry An

Organic Chemistry The Esters Leaving Certificate Chemistry

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

Esterification H H O C C O H Ethanoic Acid CH 3 COOH H H Ethyl Group Ethyl Ethanoate CH 3 COOC 2 H 5 H CH 3 COOH H CH 3 COOC 2 H 5 O C C 2 H 5 OH H C H H H 2 SO 4 Ethanol O CH H 2 2 5 OH

Esterification H H H O C C C H Propanoic Acid C 2 H 5 COOH H H O Methyl Propanoate H C 2 H 5 COOCH 3 H O C H H H 2 O Methyl Group Methanol CH 3 OH

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 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 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. . . 1. 2. 3. 4. How is an ester made? How are esters named? Uses of esters? Physical properties of the esters?

Esterification reaction Hydrolysis reaction + H 20 + acid + Base

Hydrolysis reaction used to make soap C 15 H 31 COO CH 2 C 15 H 31 COO CH + 3 KOH C 15 H 31 COO CH 2 C 15 H 31 COOK + + C 15 H 31 COOK potassium palmitate CH 2 OH 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 C 15 H 31 COO CH 2 C 15 H 31 COO CH + 3 KOH C 15 H 31 COO CH 2 Dissolved in ethanol and water C 15 H 31 COOK + + C 15 H 31 COOK potassium palmitate SOAP SALT CH 2 OH glycerol

• Remove the ethanol by distillation at 78 OC 2 • Glycerol does not distil over because its boiling point is very 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 AND VAN DER WAALS)

Extracting the soap from the reaction mixture 1. Dissolve the residue left after distillation in a minimum of hot water 2. Add this solution to the brine (saturated salt and water solution) 3. The soap should precipitate out of solution. Glycerol stays in the water. 4. Filter the soap. 5. 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 do. How you explain cleansing action of doesthe soap work? soap? • The polar of the soap molecule is watersoluble 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.