Esters Exam Questions

(a) An ester is an organic compound formed by the reaction between a carboxylic acid and an alcohol [1]

(b) A carboxylic acid and an alcohol [1]

(c) Concentrated sulfuric acid [1]

(d) Water [1]

Examiner note: Esterification is a condensation reaction — a small molecule (water) is eliminated when the two reactant molecules join. Concentrated sulfuric acid acts as both a catalyst (speeds up the reaction) and a dehydrating agent (removes water to push the equilibrium towards products).

(a) Methyl ethanoate [1]

(b) Ethyl methanoate [1]

(c) Propyl ethanoate [1]

(d) Methyl propanoate [1]

Examiner note: The naming rule is systematic: the alcohol provides the alkyl part (first word), and the acid provides the -anoate part (second word). Methanol → methyl, ethanol → ethyl, propanol → propyl. Methanoic acid → methanoate, ethanoic acid → ethanoate, propanoic acid → propanoate. Reversing the order is a common mistake.

(a) CH3COOH + C2H5OH → CH3COOC2H5 + H2O [2] (1 mark for correct reactants and products, 1 mark for correct formulae/balancing)

(b) The ester has a distinctive sweet/fruity smell [1]. Carefully warm the mixture gently, then pour it into cold water in a beaker — the ester forms an oily layer on top of the water because it is immiscible with and less dense than water [1]

Examiner note: Never directly sniff chemicals — waft the vapour towards your nose with your hand. The fruity smell is characteristic of esters. Pouring into cold water allows the ester layer to separate, confirming its formation. Do not heat the mixture too strongly or the volatile ester will evaporate.

Any three from:

• Flavourings in food — esters have fruity/sweet smells and tastes that mimic natural fruit flavours [1]
• Perfumes — esters have pleasant smells and are volatile enough to evaporate from skin [1]
• Solvents (in nail varnish remover, paints, adhesives) — esters dissolve many organic substances and evaporate readily [1]
• Plasticisers — esters can be added to polymers to make them more flexible [1]

Examiner note: Different esters have different smells: ethyl ethanoate has a glue-like smell, propyl ethanoate smells of pears, and octyl ethanoate smells of oranges. Linking the use to a specific property of esters is essential for full marks.

(a) The -COO- group / the C(=O)-O- linkage [1]

(b) Propyl ethanoate [1]

(c) Acid: ethanoic acid (CH3COOH) [1] Alcohol: propan-1-ol / propanol (CH3CH2CH2OH) [1]

Examiner note: To identify the parent acid and alcohol, break the ester at the -COO- linkage. The part bonded to the C=O side (with the extra oxygen) comes from the acid. The part bonded to the single-bond oxygen (the alkyl chain) comes from the alcohol. Add -H to the acid fragment and -OH to the alcohol fragment to recover the original molecules.

• Esterification is reversible because the ester and water can react to re-form the acid and alcohol (hydrolysis) [1]
• The reaction reaches an equilibrium where the rate of the forward reaction equals the rate of the reverse reaction [1]
• The yield is less than 100% because not all the reactants are converted to products / some reactants remain at equilibrium [1]

Examiner note: To increase yield, you can remove the water as it forms (using concentrated sulfuric acid as a dehydrating agent) or use excess of one reactant. The reverse reaction (hydrolysis of an ester) can be catalysed by dilute acid or by heating with an alkali (saponification in the case of fats).