Practical investigations
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I'm doing an experiment on how to identify a drug as aspirin, paracetamol or ibuprofen by reacting them with neutral iron (III) chloride, noting colours and then heating it and noting any other colour changes. Ibuprofen doesn't have an OH group in it, so it doesn't change colour. However, paracetamol does, and so does aspirin after heating.
I'm having trouble explaining how the Fe3+ ion reacts with the enol group - what exactly does it do to make it change colour? (I've read the chemguide tutorial and didn't understand a word of it ... ) Help much appreciated!
Igloo says ...
An enol contains a -C=C-OH linkage, which is not relevant to any of these drugs. However the presence of a hydroxyl group, –OH, is the key to your answer. Fe3+ ions can form darkly-coloured complex ions with hydroxyl compounds and this is why you are observing colour changes in your experiments.
Fe3+ ions normally form complexes in which the co-ordination number is six, e.g. [Fe(H2O)6]3+. But the number of ligands actually surrounding the ion will depend on whether the ligand is monodentate, e.g. H2O, or bidentate, such as salicylic acid (derived from aspirin), HO.C6H4.COOH. If you understand what I’m getting at, all well and good. If not, I suggest you use the web to find a good article on the formation of metal complexes. Please give Jim Clark’s chemguide another look. It really has an excellent presentation and is strongly recommended. This topic is bound to form part of your A2 course, but perhaps you haven’t covered it yet. Why not try looking at your textbook too?
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updated: 18 December 2006
