A series of seven substituted phenyl sulphates, as model compounds for vat esters, were irradiated with U.V. light, in an attempt to elucidate the mechanism of their photolysis. The rates of photolysis were measured at various pH's in buffer solutions by following the liberation of sulphate, and were found to be first order with respect to the ester. With the exception of the meta-nitro ester, the esters containing electron donating substituents e.g.p-methyl and p-methoxy, reacted much ~aster than the ones containing electron attracting substituents. It was also found that e~ters containing electron donating substituents did not give 100% yields of sulphate at the end of the reaction, whereas the ones with electron attracting substituents did. It is thought that for all esters, except the m-nitro derivative, the reaction proceeds by a free radical mechanism, and this would seem to be confirmed by experiments with the p-chloro and p-methyl esters in the presence of the radical scavenger, acrylonitrile. No free sulphate at all appeared in irradiated solutions of the p-chloro ester, and ohly very small amounts with the p-methyl ester. A mechanism is postulated in which both -0-0- bond fission and -O-S- bond fission may occur, depending on the substituent. The latter mode of fission is thought to be absent in the case of p-nitro and p-chloro ester, but increases with the electron donating power of the substituent. The deviation from 100% yields of sulphate is explained by free radical sulphonation, arising from the formation of sulphite radical ions by -O-S- fission. The m-nitro ester is completely photo-hydrolysed rapidly, and a mechanism of solvolysis from the excited state is postulated. The work does not cast much light on the mechanism of photodevelopment of the vat esters, especially as little information was obtained of the fate of the organic radical species, but evidence of the formation of phenols from the esters was found, and it may be that photohydrolysis initiates the photo-development of vat esters.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:601463 |
| Date | January 1964 |
| Creators | Hill, Ronald |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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