Esters
7 marks · HardA student mixes methanol and propanoic acid in the presence of a few drops of concentrated sulfuric acid and heats the mixture.
Describe the type of reaction taking place and explain what is meant by 'esterification'.
Draw the displayed formula of the ester formed in this reaction.
State one common use for this ester.
Write the balanced chemical equation (using molecular formulae, not names) for the formation of the ester in part (b).
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) — · must show Esterification is a reaction between an alcohol and a carboxylic acid to form an ester and water., It is a condensation reaction since water is lost.
(b) — · must show Displayed structure of methyl propanoate: CH3CH2COOCH3, all bonds shown.
(c) used as a flavouring · must show Accept food flavouring, perfume, or fragrance
(d) CH3CH2COOH + CH3OH → CH3CH2COOCH3 + H2O · must show Balanced formula equation for methyl propanoate formation
Also accept: Food flavouring; Perfume
Common mistakes: Incorrectly joining the molecules; Incorrect balanced equation; Swapping the reactants for the ester linkage
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States that esterification reacts an alcohol and carboxylic acid to give ester and water.
A1 (a) · 1 mark · Correctly explains that water is lost so it is a condensation reaction.
D1 (b) · 1 mark · Correct structure of methyl propanoate with all carbon, hydrogen, oxygen atoms and bonds.
D2 (b) · 1 mark · Ester linkage (COO) shown correctly.
D3 (b) · 1 mark · No atoms or bonds missing; hydrogen positions all correct.
B1 (c) · 1 mark · Suitable use given, e.g. food flavouring or perfume.
A2 (d) · 1 mark · Balanced formula equation for methyl propanoate formation.
Worked solution
a. This is an esterification reaction, in which an alcohol and a carboxylic acid react to form an ester and water. It is a condensation reaction because water is eliminated as the molecules join together.
b. The displayed formula for methyl propanoate is:
All atoms and bonds shown. The COO group is the ester linkage.
c. Use: Methyl propanoate can be used as a flavouring (or perfume/fragrance).
d. The balanced chemical equation is: