CIE A Level Chemistry Carboxylic Acids and Derivatives Exam Questions

About CIE A Level Chemistry Carboxylic Acids and Derivatives

Carboxylic acids and derivatives in Cambridge International 9701 runs across AS and A Level. At AS you learn how carboxylic acids are made, by oxidising primary alcohols or aldehydes and hydrolysing nitriles, and their reactions with metals, bases and carbonates, reduction with LiAlH4 and esterification. Esters are made and hydrolysed using acid or alkali. At A Level you explain relative acidities of chlorine-substituted acids, the oxidation of methanoic and ethanedioic acids, and the preparation and reactions of acyl chlorides with water, alcohols, phenols, ammonia and amines. The topic links to alcohols, nitrogen compounds and polymers, and examiners focus on reagents, products and explaining acid strength.

What you need to know

• Making carboxylic acids Oxidise primary alcohols or aldehydes with acidified K2Cr2O7 under reflux, hydrolyse nitriles by heating with dilute acid, or hydrolyse esters.
• Acid reactions Carboxylic acids react with reactive metals to give hydrogen, with alkalis to give salts and water, and with carbonates to give CO2, which is a test for the COOH group.
• Esters Formed by heating a carboxylic acid with an alcohol and concentrated H2SO4 catalyst. Acid hydrolysis is reversible; hydrolysis with NaOH goes to completion, giving the carboxylate salt and alcohol.
• Reduction LiAlH4 in dry ether reduces carboxylic acids to primary alcohols.
• Relative acidity (A Level) Chlorine atoms withdraw electrons, delocalising and stabilising the carboxylate ion, so acid strength increases from ethanoic to chloroethanoic to trichloroethanoic acid. Closer and more Cl atoms have a bigger effect.
• Acyl chlorides (A Level) Made from carboxylic acids with PCl5, PCl3 with heat, or SOCl2. They react vigorously with water (carboxylic acid), alcohols and phenols (esters), ammonia (amides) and amines (N-substituted amides), releasing HCl.
• Addition-elimination The nucleophile attacks the delta positive carbonyl carbon, the C=O reforms and Cl- is eliminated. Acyl chlorides hydrolyse much more easily than chloroalkanes because the carbon is attached to both O and Cl.

Common mistakes in Carboxylic Acids and Derivatives

• Saying ester hydrolysis with NaOH gives a carboxylic acid; it gives the carboxylate salt.
• Explaining acid strength only by saying chlorine is electronegative; say it withdraws electrons and stabilises the anion by spreading the negative charge.
• Forgetting HCl as a product of acyl chloride reactions, or writing them with water as a product.
• Esterifying phenol with a carboxylic acid; phenol esters are made using acyl chlorides.
• Naming esters with the alcohol part second; the alkyl group from the alcohol comes first, as in ethyl ethanoate.

Try three Carboxylic Acids and Derivatives exam-style questions