Carboxylic Acids& Esters

Question 1

What are carboxylic acids?

Answer 1

They are weak acids - they partially dissociate in water to form: carboxylate ions and H+ions.
Eg: CH3COOH –> CH3COO- + H+.

Question 2

What are the reactions of carboxylic acids?

Answer 2

1) Metals = salt + hydrogen.
2) Bases = salt + water.
3) Carbonates = salt + CO2 + water.

*The effervescence caused by the production of CO2 can be used as a functional group test for carboxylic acids.

Question 3

Describe the esterification of carboxylic acids with alcohols.

Answer 3

• Presence of concentrated sulphuric acid.
• Forms ester + water.
• Reaction is reversible.
• Heat under reflux(for larger esters) and use fractional distillation to separate the ester from the other compounds.

Question 4

Describe the esterification of acid anhydrides with alcohol.

Answer 4

• Acid anhydride + alcohol –> ester + c.acid.

* Separate products by fractional distillation.

Question 5

What are the uses of esters?

Answer 5

• Esters have a sweet smell, so they can be used in perfumes.
• Used in the food industry for flavour.
• Nail varnish remover; board pens use ethyl ethanoate.

Question 6

How can esters be hydrolysed?

Answer 6

1) Acid hydrolysis : c.acid + alcohol.
• Reversible reaction.
• Dilute acid: HCl or H2SO4.

2) Base hydrolysis: carboxylate salt+ alcohol.
• Non reversible
• Hot aqueous alkali.

*Properties of carboxylates:
•Ionic
•High melting and boiling point
•Dissolve in water

Question 7

What properties must esters have to be used in perfumes?

Answer 7

• Non toxic
• Volatile (turns to gas easily)
• Soluble in solvent, eg: solvent
• Don’t react with water.

Question 8

What are esters?

Answer 8

• Esters are polar molecules - but they don’t form hydrogen bonds as there is no hydrogen bonded to a electronegative atom.
• Lower bpt than their derivative c.acid.
• Almost insoluble in water.

Question 9

What is a triglyceride?

Answer 9

• A triester of glycerol (propane-1,2,3-triol) and fatty acids
• Example of a condensation reaction.

Question 10

How are trans-fatty acids linked to heart disease?

Answer 10

Trans-fatty acids leads to an increase in LDL ‘bad’ cholesterol, which builds up in arteries causing CHD an strokes.

Question 11

How can fats be used to make biodiesel?

Answer 11

• Biodiesel is a renewable fuel made from vegetable oil or animal fats.
• Biodiesel is a mixture of methyl and ethyl esters of fatty acids.
• Biodiesel can be made by reacting triglycerides with ethanol or methanol.
• Made in the presence of a strong alkali catalyst.

Question 12

Explain the water solubility of carboxylic acids.

Answer 12

Smaller carboxylic acids are soluble in water as they can form hydrogen bonds with water.
Larger carboxylic acids the solubility in water rapidly decreases.

*Carboxylic acid bonds: hydrogen and permanent dipole.