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.
Perfumes and Smelling
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!
Making esters
This activity can be used as a class practical where students can make a variety of esters and identify their smell/perfume. It may be possible to compare the manufactured ester with ones extracted from plant materials - depending on the availability of resources.
Esters are are widely used in perfumes and food flavourings because they have sweet and fruity smells. They are also used to make some medicines such as paracetamol and aspirin.
Esters have other uses, they make good solvents and other useful polymer materials. Polyester is a polymer with lots of ester links. Under acid catalysed conditions, ethanol reacts with carboxylic acids to form an esters and water (Sulphuric acid is generally used as a catalyst for making esters).
e.g. ethanoic acid + methanol ----> methyl ethanoate + water
Esters’ systematic names are derived from the alcohol and the acid from which they were formed. Notice, however, that the ester is named the opposite way around to the way the formula is written. The "ethanoate" bit comes from ethanoic acid. The "ethyl" bit comes from the ethyl group on the end. This is where students get more confused about writing names and formulae for esters than for almost anything else - particularly when it comes to less frequently met esters. Give them plenty of time and practice to make sure they understand.
There is a 'Quick Guide to Esters' produced by the Triple Science Support Programme which has some useful background and web links. students could carry out some research into the various esters used in cosmetic and perfume manufacturing.
Fats, Oils and Soaps
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.
