I= THE KINETICS OF PHOTOCHEMICAL REACTION OF METHYLENE GREEN WITH REDUCTANTS
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Title of Thesis
THE KINETICS OF PHOTOCHEMICAL REACTION OF METHYLENE GREEN WITH REDUCTANTS

Author(s)
Rehana Saeed
Institute/University/Department Details
University of Karachi/ Department of Chemistry
Session
2001
Subject
Chemistry
Number of Pages
248
Keywords (Extracted from title, table of contents and abstract of thesis)
methylene green, thiazine dyes, methyl amine, dimethy1 atnine, trimethylamine, ethylamine, diethylamine, triethylamine, photoreduction

Abstract
Thiazine dyes provide a vast field for the studies of photochemical kinetics in the presence of reductants. Methylene green is a member of thiazine dye which undergoes reduction by the visible light with reductant inpresence of buffer solutions. Acidity plays a significant role in the reduction of the dye. The photochemical reduction of methylene green with organic reductant such as methyl amine, dimethy1 atnine, trimethyl amine, ethylamine, diethylamine and triethylamine in 50% aqueous isopropanol was studied. The photochemical reduction was carried out in deoxygenated system with known intensities of typical light in a special type of optical processor. The optical processor is the combination of several components especially designed apparatus arrangements associated with a temperature controlled unit and a magnetically stirring unit. A monochromatic light of 657 nm mercury red line wavelength was used for irradiation of oxygen free reaction solutions in a double walled temperature controlled reaction cell. The transmitted light was measured and recorded in terms of electrical signals with the help of calibrated galvanometer. Acidities of the solutions were maintained b using acetate buffers which was measured with the help of spectrophotometer based on Hammet acidity function [Ho]. The quantum yield (φ) for photochemical reaction of methylene green with reductants such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine and triethylamine were determined as a function of various factors such as: (i) effect of concentratuions of methylene green, (ii) effect of concentrations of reductants (AH2), (iii) effect of acidity (Ho) (iv) effect of temperature, (v) effect of nitrogen passing time, (vi) effect of alkyl group associated with amine or the nature of reductant.

It was observed that the quantum yield (φ) at constant concentration of methylene green and reductants was found to be a function of acidity (Ho) and temperature. It was also observed that the quantum yield is a function of reductant concentration (AH2) and independent of the concentration of methylene green at constant acidity and temperature. The effect of nitrogen passing through the solution was studied for making complete oxygen free solution system. The results have been interpreted in terms of reaction mechanism. With the help of the proposed reaction mechanism, specific rate constants and the ratios of certain rate constants were determined. The values of specific rate constants for ethylamines are found relatively higher than methylamines, it was concluded that ethylamines are more reactive than methylamines. The protonated tripet excited state of methylene green(MGH++2T) which was quickly associated with hydrogen ion and form complex (MGH++2TAH2). The triplet excited state of methylene green played a major contribution for the determination of quantum yield (φ). The ratios of certain rate constants for the suggested mechanism have been evaluated. It was found that an equilibrium is established between associated complex (MGH++2TAH2) and one form of triplet state of methylene green with reductant. The complex [MGH++2T AH2) is the rate controlling species and quantum yield is controlled by the equilibrium between one form of the triplet state of methylene green with reductant.It was also observed that reactivity of amines increased as the alkyl group increased in amines, the reactivity in terms of quantum yield was observed that.

Trimethylamine > Dimethylamine>Methylamine

Triethylamine>Diethylamine>Ethylamine

And it was also found that ethyl grouped amines are more reactive than methyl grouped amines.

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S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents
99.72 KB
2 1 Introduction 1
452.08 KB
  1.1 Photochemical reactions 2
  1.2 Photochemical excited species of methylene green 3
  1.3 Thermodynamic studies of photochemical reactions 6
  1.4 Literature survey 8
  1.5 Absorption spectra of dye 12
  1.6 Photoreduction of dyes 16
  1.7 Photochemical reduction with organic reductant 19
  1.8 Photochemical reduction with inorganic reductants 32
  1.9 Effect of inhibitors on the photochemical reactions 40
  1.10 Flash photolysis of thiazine dye 42
  1.11 Fluorescence of the thiazine dye 46
  1.12 Photopolymerization of thiazine dyes 50
  1.13 Electron transfer reactions 52
  1.14 Objects of present work 54
3 2 Experimental 55
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  2.1 Optical processor 55
  2.2 Apparatus 58
  2.3 Optical processor arrangements 58
  2.4 Stopwatch 67
  2.5 Chemicals 67
  2.6 Preparation of stock solutions 69
  2.7 Measurement of light intensity 71
  2.8 Measurement of acidity of solution 72
  2.9 Measurement of molar extinction coefficient 75
  2.10 Procedure 76
4 3 Results 78
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  3.1 Absorption spectra of methylene green 78
  3.2 Determination of quantum yield 80
  3.3 Effect of nitrogen on the quantum yield of photochemical reduction of methylene green 94
  3.4 Effect of solvent (medium) on quantum yield of photochemical reduction of methylene green 96
  3.5 Study of the effect of acidity on the quantum yield of the photochemical reduction of methylene green 98
  3.6 Study of the effect of concentration of reductants on the quantum yield of photochemical reduction of methylene green 105
  3.7 Study of the effect of concentration of methylene green on the quantum yield of photochemical reduction 113
  3.8 Study of the effect of temperature on the quantum yield of photochemical reduction of methylene green 117
  3.9 Dark back reaction 124
  3.10 Effect of formation of the coloured reaction products 124
  3.11 Ultraviolet spectrum analysis 125
  3.12 Aalysis of the nitro group in the product to support the protonated states of methylene green 129
5 4 Discussion 131
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  4.1 Introduction 131
  4.2 Nature of reaction 132
  4.3 Time course of the bleaching of methylene green 134
  4.4 Influence of acidity on the photochemical reduction of methylene green 135
  4.5 Effect of solvent on photochemical reduction 138
  4.6 Photochemical reduction of methylene green by methyl amines 139
  4.7 Photochemical reduction of methylene green by methyl amines 139
  4.8 Photochemical reduction of methylene green by ethyl amines 142
  4.9 General mechanism proposed for the photochemical reduction of methylene green by amines 143
  4.10 The mechanism steps of photochemical reduction of methylene green 150
  4.11 Derivation of rate equations 152
  4.12 Determination of ratios of the rate constants of photochemical reduction of Methylene green with amines 160
  4.13 Thermodynamic study of the photochemical reduction of methyiene green With amines 178
  4.14 Comparison with earlier work 183
  4.15 Conclusion 201
  4.16 Proposed works 203
6 5 Figures 204
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7 6 References 237
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