Linolenic acid was first isolated from linseed oil (I presume) by bromination to give a hexabromostearic which was crystallised until it had a sharp melting point and was assumed to be pure (mp 181º, 25% yield). This was debrominated by reaction with zinc to give linolenic acid and shown to be the Δ-9,12,15 triene acid which was liquid. Crystallisation of the hexabromostearic gave other fractions of solid products which could also be debrominated to give an 18:3 acid. To differentiate these two products they were designated α and β linolenic acids respectively. Therefore when a Δ-6,9,12 isomer was isolated (by a similar process) it was called the γ-isomer.
At that time reaction mechanisms and stereochemistry was not well understood. We now know that bromination is a trans addition and debromination a trans elimination but the latter reaction is probably not a clean reaction. Bromination will give a threo dibromide but with six CBr groups there will be several stereoisomers. Each threo dibromide should give a cis double bond when debrominated but if this reaction is not entirely sterospecific some trans isomers may also be formed. The β acid has never been isolated and identified. It probably consists of a mixture of α-linolenic acid and several impurities which are trans isomers. There is no β acid – just an unidentified mixture of compounds to which the name was assigned. All this was achieved without any chromatography and without any spectroscopy.
© Frank Gunstone 2005
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