Carboxylic acids, esters, fats and oils
Carboxylic acids are produced by oxidation of alcohols.There are many naturally occuring organic acids - the ones with more carbon atoms often have unpleasant smells, including body odour.
Some of the difficulties that students have with organic chemistry are due to the unfamilar names, structures and confusion over IUPAC and common nomenclature. Often the organic compounds are referred to by their common name rather than their systematic name. It should be pointed out to students that it is conventional to write a formula with COOH rather than CO2H (although this is not wrong) as it conveys structural information.
The importance of carboxylic acids is related to their reactivity and their use in making other useful compounds used in a wide range of applications e.g. making esters, fats and oils. Carboxylic acids are weak acids because the dissociation is not complete and the position of the equilibrium lies more to the left. Weak acids are sometimes preferable to strong acids in order to limit the possibility of corrosion or other damage which a strong acid may produce. Hence ethanoic acid is used in household descaling products.
In the food industry there are many carboxylic acids, and compounds containing –COOH groups. Citric acid, malic acid and tartaric acid are widely used examples. In the pharmaceutical industry many carboxylic acids are used in the synthesis of complex drug molecules. There are many polymers whose manufacture requires carboxylic acids. Nylon is one example, using a dicarboxylic acid.
Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS, SSERC or recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our Health and Safety guidance.
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Some examination boards will require the students to know about the common properties of ethanoic acid.This activity allows students to see them first hand - including its action as a weak acid. This will also provide the opportunity for students to practice writing equations using these unfamiliar formulae. Sometime molymod models can help to explain certain organic reactions.
When ethanol e.g. in wine, is exposed to air it is oxidised to ethanoic acid which is the main constituent in vinegar (oxidation of beer results in malt vinegar).
Ethanoic acid is one of the family of carboxylic acids – other members include methanoic acid, propanoic acid and butanoic acid. Their systematic naming follows the same rules as the alkanes and alcohols, however students may not be clear that they have to count from the carbon which is part of the carboxyl group itself. You will need to give them plenty of opportunities to practice so that they get the hang of it.
Students also need to realise that the carbonyl functional group consists of a carbon atom with one oxygen atom double bonded to it and the other oxygen atom singly bonded to it (it is this oxygen atom that has the hydrogen bonded to it).
Many natural oils contain esters and these can be found in many plant materials. They tend to be pleaseant smelling. In this activity students revisit and use a variety of separation techniques to extract natural oils. This provides an opportunity to carry out a variety of separation techniques which should be familair to students.
Activity A1 in the SEP Biofuels booklet (page 25, 32 & 33) provides instructions for extracting Lavender oil which contains the ester linalyl acetate (commn name) or 3,7-dimethylocta-1,6-dien-3-yl acetate (IUPAC name). Students do not need to worry about the chemical name or structure!
If raw materials are in short supply this microscale preparation of an ester may be a useful alternative to the previous activity, especially if you would like a whole class of students to have the experience of making an ester and observing the characteristic smell.
This activity can be used to demonstrate to students the application of organic chemistry in the pharmaceutical industry. This may be an opportunity to highlight a possible career option.
Preparing a larger scale sample of aspirin from salicylic acid (or preparing an ester) and calculating percentage yield would provide an opportunity for students to demonstrate practical skills and analysis of results. The preparation of aspirin uses phosphoric acid as a catalyst (in contrast the making soap which uses sulphuric acid as a catalyst).
As a summary, students could revisit the general formula for ester synthesis from alcohols and carboxylic acids. This information together with the practical experience of producing a synthetic drug could be applied to the synthesis of fats and oils.
As an extension activity students could research processes involved in chemical synthesis that are used by chemists in designing specific molecules such as drugs. They could also explore the structures of esters that are used as drugs and how these affect the drug function.
This article can be used as a starter to introduce to students the range of materials that can be made from naturally occuring esters and oils. Students can carry out an experiment to make soap. This is a hazardous procedure as it involves heating 5M sodium hydroxide which is corrosive to near boiling point. The use of a hot water bath may reduce the health and safety risks of this activity.
Soaps can be made from fats and oils. They are the sodium or potassium salts of fatty acids and are created by hydrolysing esters. This is very slow unless there is a catalyst present, like sodium hydroxide.
Ester + Water --------> Acid + Alcohol
Many molecules have more than one ester link in their structure. Fats and oils are examples of these. The alcohol in fats and oils is glycerol and it has three –OH groups. The carboxylic acids in fats and oils have long hydrocarbon chains and are called fatty acids. An ester link forms from each of the three –OH groups to a fatty acid molecule.
RSC Contemporary Chemistry for schools and colleges Chapter 9 has some ideas and resources on Cleaning Chemistry.
This experiment involves titrating bromine water from burettes into samples of fats or oils in a solvent using potassium manganate as the indicator. This will need to be carried out in a fume cupboard and so you may want to do this as a demonstration.
This activity would be a good alternative to the activity described above as it's safer, very visual and can be used to test a wider variety of fats and oils that otherwise would be difficult to mix with a solvent.
This is an alternative test for unsaturation
Students will be familiar with alkenes which are organic compounds containing have double bonds.They may also have come across the common test for this which is decolourisation of bromine.
Students need to appreciate that fats and oils made from alkenes (i.e. containing a double bond somewhere along its length) are called 'unsaturated'. Therefore the same test with bromine can be carried out to test for unsaturation.
There have been many reported cases of oil spillages and their effects on the environment and wild-life. This activity provides an opportunity for a discussion on applications of chemistry. The experiment demonstrates a modern application of polymers for cleaning up oil pollution and in it's control.
Students could follow up their discussions with evaluating the use of contemporary scientific and technological developments - their benefits, drawbacks and risks.