Carboxylic acids and esters: TCE Chemistry (Tasmania)
“Describe the acidity of carboxylic acids and the esterification and hydrolysis of esters.”
Carboxylic acids are weak acids that react with bases, carbonates and reactive metals. Heated with an alcohol and a concentrated sulfuric acid catalyst, they undergo a condensation called esterification to form an ester plus water; this is reversible. Esters can be hydrolysed back to the acid and alcohol, either by acid (reversible) or by base (saponification, which goes to completion).
What this dot point is asking
TASC expects you to describe the acidity and reactions of carboxylic acids, and the formation and hydrolysis of esters, with correct equations.
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 are weak because the equilibrium lies well to the left, although the carboxylate ion is stabilised by delocalisation of its negative charge over both oxygen atoms, which makes carboxylic acids more acidic than alcohols. They show the usual acid reactions:
- with a base: salt and water, for example ;
- with a carbonate or hydrogencarbonate: salt, water and carbon dioxide (effervescence);
- with a reactive metal: salt and hydrogen gas.
The effervescence test with sodium carbonate distinguishes a carboxylic acid (reacts) from a phenol or alcohol (no reaction).
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.
For example, ethanoic acid plus ethanol gives ethyl ethanoate plus water. The sulfuric acid is a catalyst and also helps remove water, shifting the equilibrium towards the ester.
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, fragrances and solvents.
The name has two parts: the alkyl group from the alcohol comes first, then the carboxylate from the acid (ending in ). Ethanol plus ethanoic acid gives ethyl ethanoate. The acid always provides the second word.
Hydrolysis of esters
Esters can be split back into their components by hydrolysis:
- Acid hydrolysis uses water with an acid catalyst and simply reverses esterification, giving the carboxylic acid and the alcohol. Because it is the reverse of an equilibrium reaction, it does not go to completion.
- Base hydrolysis (saponification) uses sodium hydroxide and gives the carboxylate salt and the alcohol. It goes to completion because the carboxylate salt cannot react back to the ester. Saponification of fats and oils produces soap.
Identify the alcohol part
Name the ester formed from propan-1-ol and ethanoic acid. The alcohol is propan-1-ol, giving the alkyl group propyl, which comes first.
Identify the acid part
The acid is ethanoic acid, giving the carboxylate ethanoate, which comes second.
Combine and state the by-product
The ester is propyl ethanoate, , and the by-product is water.
Structure and properties
A carboxylic acid molecule can hydrogen bond strongly (it has both and ), so carboxylic acids have high boiling points and the smaller ones are very soluble in water. Esters cannot donate a hydrogen bond (they have no ), so for similar size they boil lower than the parent acid and are less soluble in water, which is why esters separate as an oily layer.
A frequent error is to name the ester with the acid part first. The alkyl group from the alcohol always comes first, then the carboxylate from the acid ending in . Methanol plus ethanoic acid gives methyl ethanoate, not ethyl methanoate. A second slip is forgetting that base hydrolysis gives the carboxylate salt, not the free acid.
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 questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Practice3 marksEthyl ethanoate is prepared by heating ethanol and ethanoic acid with a concentrated sulfuric acid catalyst. Afterwards, unreacted ethanoic acid is removed with sodium carbonate solution. (a) Write a balanced equation for the reaction of ethanoic acid with sodium carbonate. (b) State what is observed and why this confirms the acid has reacted.Show worked answer →
(a) Carboxylic acids react with carbonates to give a salt, water and carbon dioxide: . (2 marks)
(b) Effervescence (bubbles of ) is observed. This shows the carboxylic acid has reacted with the carbonate, leaving the neutral ester behind in the organic layer. (1 mark)
Practice3 marksUsing structural formulae for all organic compounds, write an equation for the preparation of the ester propyl methanoate, and name the by-product.Show worked answer →
An ester forms by condensation (esterification) of an alcohol with a carboxylic acid, losing water, with a concentrated catalyst. Propyl methanoate is the ester of methanoic acid () with propan-1-ol (). (1 mark)
. (1 mark)
The acid contributes the part and the alcohol the propyl group; the from the acid and the from the alcohol's hydroxyl combine to form water as the by-product. (1 mark)
Practice2 marksButanoic acid reacts with sodium metal. Write a balanced equation and identify the products.Show worked answer →
The reactive site is the acidic of the carboxyl group. Sodium displaces this hydrogen, giving an ionic sodium carboxylate and hydrogen gas. (1 mark)
. The products are sodium butanoate and hydrogen gas. (1 mark)