A carboxylic acid reacts with an alcohol, with concentrated sulfuric acid as catalyst, to form an ester and water: CH3COOH(l) + C2H5OH(l) → CH3COOC2H5(l) + H2O(l). The reaction is reversible and produces a sweet, fruity smell.
Esterification is the reaction between a carboxylic acid and an alcohol to make an ester and water. Esters are the sweet-smelling compounds behind the aromas of fruits and flowers and are used in perfumes and food flavourings, so this reaction closes the Form 5 Carbon Compounds chapter neatly. The exam tests the balanced equation, the catalyst and conditions, and the unmistakable smell that signals an ester has formed.
The balanced equation
Taking the common example of ethanoic acid and ethanol, which make the ester ethyl ethanoate:
CH3COOH(l) + C2H5OH(l) ⇌ CH3COOC2H5(l) + H2O(l)
The double arrow shows the reaction is reversible: ester and water can react back to the acid and alcohol. To check the balance, count atoms: ethanoic acid CH3COOH is C2H4O2 and ethanol C2H5OH is C2H6O, giving C4 H10 O3 on the left; the ester CH3COOC2H5 is C4H8O2 and water is H2O, giving C4 H10 O3 on the right. It balances. The naming rule is worth memorising: the ester name takes the “-yl” part from the alcohol and the “-oate” part from the acid, so ethanol + ethanoic acid gives ethyl ethanoate.
Conditions required
- A few drops of concentrated sulfuric acid are added as the catalyst.
- The mixture is warmed gently (heated in a water bath).
Concentrated sulfuric acid does two jobs: it catalyses the reaction and, as a dehydrating agent, it absorbs the water produced, which shifts the reversible reaction towards making more ester.
Observations
- After warming and pouring the mixture into water, a sweet, fruity or pleasant smell is detected, this is the ester.
- A colourless, oily liquid layer of ester often floats on the surface of the water because esters are less dense than water and only slightly soluble.
The sweet smell is the classic evidence, and describing it correctly is usually required for the mark.
Hydrolysis: the reverse reaction
Because esterification is reversible, an ester can be hydrolysed back to its acid and alcohol by heating with water, faster with an acid or alkali catalyst. This link is worth knowing because the saponification of fats, boiling an ester with sodium hydroxide to make soap, is an alkaline hydrolysis of an ester, tying esterification to the Consumer Chemistry chapter.
How esterification fits the organic sequence
Esterification sits at the end of a chain of conversions the exam likes to test as one story: ethene is hydrated to ethanol, ethanol is oxidised to ethanoic acid, and the acid and an alcohol are then esterified to an ester. Being able to state the reagent and condition for each step lets you answer a long structured question that walks through several functional groups in one go.
Common mistakes to avoid
Show the reaction as reversible with the double arrow, and remember water is a product, a frequent error is to leave it out. Name the ester the right way round (alcohol part first, then acid part). Say the catalyst is concentrated sulfuric acid, not dilute, and that the mixture is warmed. Do not confuse the ester’s sweet smell with the sharp vinegar smell of the acid.
How it appears in the SPM exam
In Paper 2 (4541/2) you may write the balanced, reversible equation, name the ester, state the catalyst and condition, and give the smell observation. In Paper 1 (4541/1), objective items test the reactants, the catalyst and the naming rule. In Paper 3 (4541/3), preparing an ester and detecting its smell is a straightforward practical. Keep acid + alcohol → ester + water at the centre, and every esterification question becomes routine.
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