Organic synthesis
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I'm doing an investigation on aspirin synthesis and am looking at different catalysts (phosphoric acid and sulphuric acid) why are catalysts used in the reaction between salicylic acid and ethanoic anhydride and would the reaction happen without a catalyst?
I have found that phosphoric acid is a more effective catalyst, why might this be? Thanks for your help
Igloo writes ..........
The mechanism for this esterification reaction involves a nucleophilic attack of the phenol group of salicylic acid on one of the carbonyl carbon atoms of the ethanoic anhydride molecule. However, the lone pair of electrons on the phenol group is to a certain extent “tied up” with the delocalisation in the benzene ring, so is not as “available” as might be.
An acid catalyst will protonate one of the oxygen atoms in ethanoic anhydride, and this protonation will enhance the electrophilic character of the carbonyl carbon atom: >C=O + H+ becomes >C+-OH. This willl in turn increase the chance of a fruitful nucleophilic attack from the salicylic acid. I am sure that the reaction will proceed without the catalyst, but at a somewhat slower rate. Perhaps it will be necessary to reflux the mixture in a hot water bath for 30, rather than 5 minutes in order to achieve reasonable yields.
I should have thought that a few drops of either concentrated phosphoric or sulphuric acid would catalyse this reaction successfully. I cannot provide an explanation of why phosphoric acid should be better at supplying protons. It is possible that concentrated sulphuric acid might react with salicylic acid to form unwanted by-products, but in the trace amounts used I cannot think that this is a serious problem. In a school laboratory, phosphoric acid is certainly the safer choice, and on an industrial scale there may well be good economic reasons for its use.
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updated: 17 April 2007
