Esters
6 marks · NormalA group of students carried out an experiment to produce an ester from ethanol and propanoic acid. They left the product to stand after completing the practical.
Explain how the students could confirm that an ester had formed, even if the reactants and product are colourless.
Ethyl propanoate is an ester. Give the name of the alcohol used to manufacture this ester.
The mixture was poured into a beaker containing sodium carbonate solution. Suggest a reason for this step.
Explain why small esters are easy to separate from the reaction mixture compared to the carboxylic acid and alcohol used.
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(a) Smell the mixture using the wafting technique; esters have distinct sweet or fruity smells different from alcohols and acids. · must show wafting, distinctive smell
(b) ethanol
(c) To neutralise excess acid and allow the ester layer to separate and float, making it easier to detect. · must show neutralise acid, layer of ester
(d) Small esters are less soluble in water than alcohols and acids, so they form a separate layer and float. Alcohols and acids are highly soluble due to hydrogen bonding; esters lack the -OH and -COOH groups needed to form strong hydrogen bonds. · must show esters less soluble, float on water, explain hydrogen bonding
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Marking points
M1 (a) · 1 mark · Mentions using wafting or similar safe method to detect smell.
A1 (a) · 1 mark · States that esters have a fruity or distinctive smell.
A2 (b) · 1 mark · Correct alcohol: ethanol.
A3 (c) · 1 mark · States that sodium carbonate is added to neutralise acid and separate ester.
A4 (d) · 1 mark · Mentions lower solubility of ester in water/ester forms its own layer.
M2 (d) · 1 mark · Explains difference is due to absence of hydrogen bonding in esters compared to alcohols/acids.
Worked solution
a) The students can gently waft the air above the reaction toward their nose. The ester will have a distinctive fruity or sweet smell, which is evidence of its formation.
b) The alcohol is ethanol.
c) Sodium carbonate neutralises the excess acid and helps the ester layer to separate, making it easier to identify and smell.
d) Small esters do not dissolve well in water because they lack -OH or -COOH groups that allow strong hydrogen bonding. Alcohols and acids are highly soluble in water due to hydrogen bonding but esters form a separate layer and float.