9701 · 18.2
Esters flashcards
Revision flashcards for Cambridge 9701 Esters (syllabus 18.2). Flip, recall, then mark a real past-paper question.
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What is the general structure of an ester functional group?
The ester functional group is –COO–. The general formula is RCOOR', where R and R' are alkyl or aryl groups. The R group comes from the carboxylic acid and the R' group comes from the alcohol.
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What are the reactants for forming an ester?
A carboxylic acid and an alcohol.
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What are the essential conditions for esterification?
Heat (often under reflux) and a strong acid catalyst, typically concentrated sulfuric acid ($H_2SO_4$).
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What are the two roles of concentrated sulfuric acid in esterification?
1. It acts as a catalyst to speed up the reaction. 2. It acts as a dehydrating agent, removing the water produced to shift the equilibrium to the right, favouring the ester product.
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How do you name an ester, e.g., the one from ethanol and propanoic acid?
Using the 'alkyl alkanoate' format. The first part comes from the alcohol (ethanol → ethyl) and the second from the carboxylic acid (propanoic acid → propanoate). So, it is ethyl propanoate.
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What is hydrolysis?
Hydrolysis is the breakdown of a compound by reaction with water. For esters, it splits the ester link.
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What are the products of the acid hydrolysis of ethyl ethanoate?
Ethanoic acid and ethanol. The reaction is reversible, reaching an equilibrium.
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What are the products of the alkaline hydrolysis of ethyl ethanoate?
Sodium ethanoate (a carboxylate salt) and ethanol. The reaction is irreversible.
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What is the common name for alkaline hydrolysis of an ester?
Saponification, as it is the process used to make soap from fats and oils (which are large esters).
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Why is alkaline hydrolysis of an ester irreversible?
The carboxylic acid produced reacts with the alkali (e.g., NaOH) in an acid-base neutralisation reaction to form a stable carboxylate salt. This salt does not react with the alcohol, preventing the reverse reaction.
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Why are esters used as solvents for non-polar compounds?
Although they have a polar C=O bond, the two non-polar alkyl chains (R and R') make the overall molecule largely non-polar, allowing it to dissolve other non-polar substances.
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Why are many esters suitable for use in perfumes?
They are volatile (evaporate easily to be smelled) and often have pleasant, fruity or sweet smells.