Peroxide bleach activation: kinetics and mechanisms of some acyl transfer reactions

Deary, Michael (1990) Peroxide bleach activation: kinetics and mechanisms of some acyl transfer reactions. Doctoral thesis, Newcastle-upon-Tyne Polytechnic.

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Abstract

This research has been concerned with a kinetic and mechanistic study of peroxide bleach activation, which is an important process with regard to detergent formulations. Bleach activation occurs in situ in the laundry liquor and utilises the reaction of alkaline hydrogen peroxide with a peroxyacid precursor (the bleach activator), such as tetraacetylethylenediamine (TAED) or pentaacetylglucose (PAG), to yield an organic peroxyacid - usually peracetic acid. Peroxyacids are more efficient than the traditionally used hydrogen peroxide at stain removal in today's preferred low-temperature washes. The studies contained herein will have hopefully contributed to a greater understanding of the process of bleach activation, facilitating developments in this area. Our approach to this study was to initially work on a bleach activator, p-nitrophenylacetate (PNPA) for which the activation reaction scheme was relatively easy to elucidate. An important feature to arise from the PNPA study was that, in addition to the reaction of hydrogen peroxide with PNPA to yield peracetic acid, peracetic acid itself reacts with PNPA, yielding diacetyl peroxide, which subsequently undergoes hydrolysis and perhydrolysis. It was essential during this study to be able to measure peracetic acid in the presence of excess hydrogen peroxide. This was made possible by the development, by us, of an iodimetric spectrophotometric technique capable of rapidly and accurately measuring peracetic acid concentrations as low as 1x 10-5 M in the presence of up to 1000-fold excess of hydrogen peroxide.

The methods and approach developed in the study of PNPA were successfully applied to the study of two commercially used bleach activators, TAED and PAG. Our studies of the reactivity of various nucleophiles towards TAED revealed enhanced reactivity for the hydrogen peroxide anion compared to other peroxide species. A cyclic intermediate mechanism is proposed by us to explain this observation; the hydrogen peroxide anion acts as both nucleophile and general acid catalyst in its reaction with TAED. From work done on PAG we have shown that, despite the often quoted yield of 2.5 moles of peracetic acid per mole of PAG, all 5 acetyl groups are available for transfer to nucleophiles. Also suggested from the PAG studies is the possibility that rate limiting acetyl migration may be important for these reactions.

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