How do carboxylic acids behave and how are esters formed?
Describe the acidity of carboxylic acids and the esterification and hydrolysis of esters.
The acidity of carboxylic acids, their reactions with bases and carbonates, esterification with alcohols, and the hydrolysis of esters.
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What this dot point is asking
You must describe the acidity of carboxylic acids and the formation and hydrolysis of esters.
Carboxylic acids as weak acids
The carboxyl group donates a proton to give a carboxylate ion, but only partly, so carboxylic acids are weak acids:
They show the usual acid reactions. With a base they form a salt and water; with a carbonate or hydrogencarbonate they release carbon dioxide; with a reactive metal they release hydrogen. The carbon dioxide test (effervescence with sodium carbonate) helps distinguish a carboxylic acid from a phenol or alcohol.
Esterification
When a carboxylic acid is heated with an alcohol and a concentrated sulfuric acid catalyst, an ester forms and water is eliminated. This is a condensation reaction.
The reaction is reversible and reaches equilibrium, so it does not go to completion. Esters often have sweet, fruity smells and are used as flavourings and solvents.
Hydrolysis of esters
Esters can be split back into their components by hydrolysis. Acid hydrolysis with water and an acid catalyst reverses esterification, giving the carboxylic acid and the alcohol. Base hydrolysis (saponification) with sodium hydroxide gives the carboxylate salt and the alcohol, and goes to completion because the salt does not react back. Saponification of fats and oils produces soap.
In the exam, show esterification as a reversible reaction with water as a product and a sulfuric acid catalyst, name the ester alcohol-part first, and distinguish acid hydrolysis (reversible) from base hydrolysis (complete).
Exam-style practice questions
Practice questions written in the style of TASC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2022 TASC2 marksEthyl ethanoate can be prepared by heating ethanol and ethanoic acid with a catalyst. After the reaction, unreacted ethanoic acid is removed by reacting with sodium carbonate solution. Using molecular formulas, write a balanced chemical equation for the reaction between sodium carbonate and ethanoic acid.Show worked answer →
Carboxylic acids react with carbonates to give a salt, water and carbon dioxide. Ethanoic acid is CH3COOH (molecular formula C2H4O2) and sodium carbonate is Na2CO3.
Balanced equation: 2CH3COOH + Na2CO3 -> 2CH3COONa + H2O + CO2.
(Using strict molecular formulas: 2C2H4O2 + Na2CO3 -> 2C2H3O2Na + H2O + CO2.) The effervescence of CO2 confirms the acid has reacted, leaving the neutral ester behind. (2 marks: correct products and balancing.)
2022 TASC3 marksUsing structural formulas for all organic compounds, write a chemical equation to show the preparation of the ester propyl methanoate.Show worked answer →
An ester is formed by condensation (esterification) of an alcohol with a carboxylic acid, with the loss of water and using a strong acid catalyst (such as concentrated H2SO4).
Propyl methanoate is the ester of methanoic acid (HCOOH) with propan-1-ol (CH3CH2CH2OH).
Equation: HCOOH + CH3CH2CH2OH reversible HCOOCH2CH2CH3 + H2O.
The acid contributes the -C(=O)-O- part and the alcohol contributes the propyl group; the -OH from the acid and the -H from the alcohol's hydroxyl combine to form water. (3 marks: correct reactants, ester structure, and water as the by-product.)
2023 TASC2 marksButanoic acid reacts with sodium. Identify the products of this reaction.Show worked answer →
The reactive site is the acidic O-H of the carboxylic acid group. Sodium is a reactive metal that displaces this hydrogen, producing an ionic sodium carboxylate salt and hydrogen gas.
2CH3CH2CH2COOH + 2Na -> 2CH3CH2CH2COO-Na+ + H2.
The products are sodium butanoate (the sodium salt of butanoic acid) and hydrogen gas. (2 marks: salt and hydrogen gas.)