CCEA A Level Chemistry 4.9 Derivatives of Carboxylic Acids Exam Questions

About CCEA A Level Chemistry 4.9 Derivatives of Carboxylic Acids

Section 4.9 of Unit A2 1 covers esters and acyl chlorides. You need to name and draw esters and acyl chlorides, explain the boiling points and solubility of monoesters, and form esters from alcohols with either carboxylic acids or acyl chlorides. CCEA requires the laboratory preparation of a liquid ester, the description of fats and oils as triesters of propane-1,2,3-triol (glycerol) and fatty acids, transesterification to make biodiesel and margarines, and hardening of oils by catalytic hydrogenation with nickel. You also need the reactions of acyl chlorides with water and alcohols. This content links to carboxylic acids (4.8), amides (5.8) and polymers (5.10).

What you need to know

• Naming esters Esters are named from the alcohol part first, then the acid part as -oate, for example ethyl ethanoate CH3COOCH2CH3 and methyl propanoate CH3CH2COOCH3.
• Properties Esters cannot hydrogen bond with each other, so they have lower boiling points than carboxylic acids of similar Mr. Small esters are slightly soluble in water and have fruity smells.
• Preparing an ester Heat the acid and alcohol with concentrated sulfuric acid under reflux, distil, wash with sodium carbonate solution in a separating funnel to remove acid, dry with anhydrous calcium chloride or sodium sulfate, then redistil, collecting at the boiling point.
• Acyl chlorides Acyl chlorides react vigorously and irreversibly: with water to give a carboxylic acid and HCl (steamy fumes), and with alcohols to give an ester and HCl. This gives a better yield than using the acid.
• Fats and oils Triesters of glycerol and long chain fatty acids. Oils contain more unsaturated fatty acids. Hardening by hydrogen with a finely divided nickel catalyst raises the melting point for margarines.
• Transesterification Reacting a vegetable oil with methanol, using a catalyst such as KOH, swaps the glycerol for methyl groups, producing methyl esters of fatty acids (biodiesel) and glycerol.

Common mistakes in 4.9 Derivatives of Carboxylic Acids

• Naming esters with the acid part first. Ethyl ethanoate comes from ethanol and ethanoic acid.
• Saying esters form hydrogen bonds between molecules. They have no O-H group, so only weaker intermolecular forces act.
• Washing an ester with sodium hydroxide. NaOH would hydrolyse the ester; use sodium carbonate solution.
• Saying acyl chloride reactions are reversible. They go to completion, which is an advantage over using carboxylic acids.
• Saying hardening removes all double bonds in margarine. Partial hydrogenation is often used to get the desired texture.

Try three 4.9 Derivatives of Carboxylic Acids exam-style questions