Posted by: Mark Foreman | June 11, 2011

Esters and the soap bar

Back to ‘normal’ esters, the classic way to make these is to react an alcohol with the carboxylic acid using a strong acid (mineral acid) as the catalyst. Some time before christmas I mixed some soap with methanol, excess sulphuric acid and a reagent which removes any water (trimethyl orthoformate). I recall the time in Aberdeen when John Plater introduced me to the joys of trialkyl orthoforates. I think of these trialkyl orthoformates as the “thinking man’s dean stark head”. I will write about dean stark conditions another day.

After leaving it to stand over the holiday, the mixture was boiled for a day evaporated down. Then I extracted the residue with ether, the ether extract was washed with water, saturated sodium bicarbonate and brine (saturated sodium chloride solution). After I dried the ether extract with sodium sulphate, I evaporated off the ether to give me an oil.

This oil was diluted with petrol and injected into a gas chromatography machine. I looked at the GC trace and I saw a nice group of four peaks, the mass spectrum for each GC peak indicated that these compounds were methyl esters of carboxylic acids with even numbers of carbons.

The FAME (Fatty Acid Methyl Ester) profile of the soap tells me something about the soap, but I am running out of time today. Now we have seen how ester formation can be useful (GC on soap or free carboxylic acids is not normally possible) we need to think how the reaction works.

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