The Oxidation of Azo Dyes and Its Relation to Light Fading

Desai, N. F (1950) The Oxidation of Azo Dyes and Its Relation to Light Fading. PhD thesis, University of Glasgow.

Abstract

The action of light on dyed fibres in air appears in many cases to involve an oxidation of the dye. The present work has been a preliminary to the fuller investigation of the photochemistry of azo dyes several series of simple monoazo dyes have been examined. A study of thermal oxidation in solution by common oxidising agents was followed by comparison with a series of measurements of relative light fastness in an attempt to elucidate the mechanism of the light fading process. The work has confirmed that oxidation of azo dyes in aqueous media, with several common reagents, e.g., ceric sulphate, potassium dichromate or hydrogen peroxide, disrupts the azo group and leads to the formation of a diazo compound and a quinone. subsequently, these further decompose to give, respectively, a phenol and nitrogen (in acid media); and phthalic acid. The reaction thus appears to be common to many types of oxidising agent, since previous workers have observed similar effects with, e.g., sodium hypochlorite, lead dioxide and ozone. An acceptor has been used to combine with the quinone formed and thus enable it to be identified without danger of its further oxidation. By boiling the acid solution after oxidising the dye a quantitative yield of nitrogen from the azo group is obtained, except when free amino groups are also present. This procedure could be used in quantitative analysis. An hypothesis is put forward suggesting that the initial step in such chemical oxidation is one of hydrolytic attack on the -C= N- bond of the dye, when it is present as the hydrazone and not the azo tautomer. In order to confirm or disprove the postulated hypothesis, several dyes containing nitro, chloro, methyl, methoxy or sulphonic and carboxylic acid groups in various positions in the phenyl nucleus in the following parent dyes, were prepared. The rates of oxidation of the water-soluble dyes in presence of various oxidising agents, such as neutral and alkaline hydrogen peroxide, potassium persulphate and dilute nitric acid were studied quantitatively. In the case of the insoluble dyes the rate of nitrogen evolution was examined as a measure of ease of oxidation. To correlate the oxidation of azo dyes with their relative light fastness, the above dyes were exposed in graded depths on substrates of wool, cellulose (cotton and filter paper), unglazed tile, and anodised aluminium. The last two materials were chosen in the hope they might he inert substrates. Fading and oxidation data were plotted against Hammett sigma values to determine the effect of substituents in the phenyl nucleus. The results appear to show that fading on wool is caused by hydrogen peroxide, which it is suggested may be produced by photolysis of the fibre itself, whereas on other substrates some other disruption process of the dye molecule occurs, perhaps by attack of atmospheric oxygen. The results also signify that the initial step in fading is a simple reaction involving attack at one of the unsaturated centres of the dye molecule.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Organic chemistry, Physical chemistry
• Date of Award: 1950
• Depositing User: Enlighten Team
• Unique ID: glathesis:1950-78835
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 30 Jan 2020 14:49
• Last Modified: 30 Jan 2020 14:49
• URI: https://theses.gla.ac.uk/id/eprint/78835