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Authors: Pal, Raj
Issue Date: 1975
Abstract: The problem of the oxidation of phenols is a very complex one and poses many difficulties both in terms of experimentation and interpretation of data,which defy easy solution. Two factors markedly influence the oxidation process,viz., their stability and proper choice of solvent to bring there into solution. Introduction of various substituents in phenol ring brings about a radical change in its acidity and so also in its solubility in water. A similar behaviour Is observed in alkaline solutions. Substitution of hydroxyl group or other electron donating groups increase the solubility in that medium while electron withdrawing groups make them less soluble. Instability of these compounds, in many cases, is attributed to their deoomr osi tion in the aqueous medium. The most appropriate approach to investigate their oxidation,would therefore,be by either carrying out studies in non-aqueous media or by choosing c mpounds which can exist in fairly stable state in their aqueous solutions. Taking into consideration the above mentioned facts, preliminary studies were carried out to find a suitable phenol as well as a suitable oxidant which could be employed in order to get precise information regarding the mechanism of the oxidation process. It was round that thymol, which is a naturally occurlng phenol, gives str.ble solutions both in slightly aqueous alcoholic as well as highly alcoholic media. It was also round to possess great solubility fornonwpolar solvent like ether. ?rom the aprlied point of view its oxidation products are of industrial and technological importance and find much use in stabilizing motor fuels,edible fats,oils,rubber greases etc. Literature survey has show that,although thymol undergoes oxidation by a number of oxidents,e»g., nitric acid1,potassium ferri cyanide*, ferric chloride* etc., the products obtained are either resinous in nature or difficultly Identifiable. In a few cases where some identified products are obtained, the yield is very low. It wrs therefore considered to cnrry out synthetic studies on the oxidation of thymol in purely non-aqueous medium, for this purpose etheral solution of thymol wif taken and was subjected to oxidation by an oxidant which itself was ether soluble. This apr roach provided a new method for obtaining a high yield of the oxidation product,namely, thymoquinone. The method thus developed not only gave much higher yield (88 f ) than that so far reported (73-80 f ) but was also found more convenient and less cumbersome. Since some resinous matter was also obtained together with thymoquinone in this reaction, chromatographic and polarographic techniques respectively were employed to separate and Identify them. The oxidant used for this purpose was chromium peroxy perchromate abbreviated as blue perchromate. Since this oxidant had not so far been used for the oxidation of phenols,in general, and thymol in particular, it was considered highly desirable to study its kinetics comprehensively taking into consideration the effect of variables like concentration, temperature,etc. Evaluations of order of reactions as well as thermodynamical parameters provided important information regarding the role of this oxidant in obtaining thymoquinone from thymol. From various evidences it was concluded that peroxy oxygen and Cr( VI) , Cr(III) couple both play an important role in this oxidation reaction. Separation of thymol from the oxidation products revealed that thin-layer chromatography should be preferred to other chromatographic techniques for these compounds. Tha separation technique used in the synthetic pert was, therefore,extended to other compounds of this group,viz., substituted li4-quinones,ni troso thymol, amino thymol, thymo hydro quinone etc. Various adsorbent-solvent systems were tried and useful data on TLC coatings and R^-vsIues were provided. riven quantitative estimations of some of these compounds were possible in the range 3ug to 5 ug by this technique. H aectromatric studies on the redox potential of phenols have been reported in quite a faw cases. Interesting data depicting the effect of substituents like-CHa? in bensoqulnone, or the methoxy or the S08R groups,heve been reported. Thus It has been found that electronegative substituents in the quinone nucleus Increases the redox potential in the following order: SOaR> N0«> CN > COOH> Ur > Gl !?urprislngly enough redox potentlal studios on thymol and its Mated compounds are lacking, rince it was found that the oxidation products of thymol undergo polarographic reduction easily, giving well defined reversible diffusion controlled waves, it was considered worthwhile It carry out coEprehensive polarogrpphic studies on these compounds from two angles. Firstly, from the point of view of currentvoltage relationship and secondly to determine the ^ by this very rcethod choosing thymoquinone as the representative compoun d. Folarography of 13,li4 substituted quinonest (1) Thymoquinone. (ii) 3,6-dlbromothymoquinone. (ill) Thymo qui none-( mono) -semicarbazone. (iv) Thymoquinone-(bis) -semicarbazone. (v) Thymoquinone oxime«l (vi) Thymoquinone-(mono) -hydrazone. 0 (vii) Thymoquinon e-(mono) -phenyl hydrazone. (vlii) Thymo quinon e-(mono)-p-nitrop hen yl hydrazone. (ix) Thymo quinon e-( mono) -2,4-dini trophenyl hydrazone. (x) Thymohydro quinon a, (xi) Thymoquinhydrone, (xli) Nitro so thymol, (xili) iimino thymol. vera studied anl data on E^y2 and diffusion current constants etc.were provided. The standard oxidation potential was studied by carrying out the oxidation of thymo hydro quinone by various oxld&nts, viz.,KaCr10?,IWn04,K»?,e(CN) a etc. The & values obtained pol aro graphically was found to be -0.00 V.vs.-CE. Potentiometric titrations were further performed in the hope to obtain confirmation for the E0 value evaluated polarographic-11 y. Interestingly enough the v?.JLue determined by potential mediator method did not concord with the value obtained from polarographic data. On the other hand,the & value by the po ten tiometric method was almost twice that of the value determined pol aro graphically. On the basis of above observations,!t is suggested that redox Jotential studies on phenols can be more informative if the polarographic method is used in conjunction with the po ten tiometri c method in such cases.
Other Identifiers: Ph.D
Appears in Collections:DOCTORAL THESES (chemistry)

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