CHROMATOGRAPHIC STUDIES ON AMINES AND PHENOLS AND CHROMATOGRAPHIC SEPARATION OF OXIDATION PRODUCTS OF PHENOLS

Abstract

Since the very beginning of the science of chemistry, its practitioners have been plagued by two technical problems. The first is to learn the purity of a given preparation or, conversely, how many components are in a given system. This information is necessary regardless of whether the system is a mineral mixture, an extract of some plant or animal, or the result of a chemical reaction. The second problem is to resolve the system into its pure components so that they can be characterized and studied. The earlier chemists performed phenomenal feats by fractional crystallization and distillation, but it has only been in the last fifty years or so that the various techniques of chromatography have promised true solutions of these problems. It follows from the stated problems that chromatography can be considered from two view points. One is diagnostic or qualitative and the second is preparative. The object of the former is to determine the number of components in a system and to learn, if possible, what they are without actually isolating them. The latter view point involves the separation of a mixture into its components in such a way that reasonable amounts can be isolated and studied . Thin-layer chromatography was developed because of a specific need for a -vapid method which would separate small amounts of compounds. Soon after its development it was apparent that the method was more than a micro method for separating compounds. It was a means of (i) investigating adsorbents and solvents for column work (ii) following the cource of elution chromatography of colourless compounds (iii) checking the course of reaction and (iv) carrying out certain reactions such as oxidation, reduction, dehydration and so forth, directly on the strip or plate. By these means and other reactions applied directly to the unknowns, an insight into the type of compound could be gained. All these functions of thin-layer chromatography are enhanced by the speed of the method and the fact that only minute amounts of materials are needed. All techniques of chromatography are based upon the same simple principle. They involve a moving system of some type (liquid or gas) which is in equilibrium with a stationary phase. These phases are so designed that the mixture to be separated will be distributed between the two. When the stationary phase is a solid and the forces acting between it and the mixture are adsorptive in nature, the technique is called adsorption chromatography. When the stationary phase is a simple liquid or a liquid held on some type of support, the chromatography is considered to be partition chromato graphy. In essence TLC is a type of adsorption chromatography where the adsorbent is a thin layer of some solid deposited on a glass plate support. 61 operation, it is analogous to paper chromatography, that is, the substance to be separated is placed a short distance from one end of the layer and is resolved by a solvent passing through the layer by capillary action. The development is carried out in a simple closed system as in paper chromatography, but is much more rapid. When the proper solvent mixtures are used, the method can become a partition technique. In chromatography, the behaviour of the compound depends upon both the adsorbent and the solvent. The choice of solvent or the mixture of solvents depends upon the nature of the process- whether the process is a case of adsorption or of partition chromatography. The separation of substances depends upon the polarity of a particular substance. Generally mixtures of two or three solvents of different polarity often give better separation than mixtures of solvents of similar .polarity. The adsorbents most commonly used in thm l*yer chromatography are silica gel, alumina, kieselguhr and powdered cellulose, while those which have been used to a lesser extent are polyamide powder, ion-exchange powders, florisil (mixed silica gel and magnesia), calcium sulphate, polyethylene, magnesol (adsorptive magnesium silicate), zinc carbonate, calcium silicate and hydroxyl 2 3 apatite . Carton and Bradbury have studied the use of mixed adsorbents in chromatography. Their data show that the mixture behaves as one of the two adsorbents in the pure form, the second acting simply as diluent or adsorption is shared between the two adsorbents, in which case adsorption varies almost linearly with percentage 4 composition of the mixture. Wang Cheng Hsia and coworker studied the effect of impregnation of silica gel G with D-tartaric acid and of silica gel G with KOH on the TLC of aromatic amines and phenols respectively. They found that in each case there was a change in the Upvalue; the Revalue also depending on the amount of impregnant. These results were explained by them on the basis of salt formation between the analyte and the impregnating substance. On the other hand, K. Yasuda have reported the TLC separation of aromatic amines on silica gel thin layers impregnated with cadmium sulphate , cadmium acetate , zinc salts and the manganese salts . He explained that complex formation alone can not account for the separation of amines on impregnated layers and suggested that much weaker bonding, probably by the simple electron donation from nitrogen to metal atom is essential for good separation. Much of the chromatographic work carried out earlier has been rather of an empirical nature and was aimed at working out the most suitable system for a particular separation. However, lately there have been good attempts at correlating chromatographic behaviour with chemical structure. The theoretical basis for the relation between Rp values in partition chromatography and chemical structure was first proposed by Consden, 9 10 Gordon and Martin and later by Martin who deduced the expression _ m, Al / _L -1) = A AiA + n ^v # + m^i xu. + etc. RTln A7 ( RF ^ where Al = cross-sectional area of the mobile phase Ae • cross-sectional area of the stationary phase for the change in partition coefficient of a substance between two phases on addition of a group or groups to the parent substance. Bate-Smith and Westall introduced the term RM, given by RM - ^(-—-1) and showed experimentally that the relationship predicted by Martin was followed for a number of flavones, anthocyanins and some related compounds. Since then a good deal of work has been done to demonstrate the experimental validity of Martin's postulates or otherwise, Green and Marcinkiewicz12 have rightly observed that the accurate determination of Revalues is at the very centre of all attempts to correlate chromatographic behaviour with chemical structure. They suggested that by using reversed phase chromatography and 'tankless' conditions the experimental 13-17 obstacles could probably be overcome. Green and coworkers then carried out the paper chromatography of a large number of phenolic compounds and demonstrated that Martin's equation is probably obeyed for all groups and deviations can be accounted for by constitutive effects in molecules. Significant advancement was made in the field of chromatography on polyamide surface by the systematic work of Bark and Graham18"20, who carried out an extensive work on the TLC separation of phenols on cellulose layers impregnated with polyamide. They showed that the dual behaviour of polyamide, as proposed earlier, is not correct and that hydrogen bonding between a phenolic group and the polyamide surface is the main mechanism of adsorption and that the removal of the phenol from the surface is the result of breaking of the hydrogen bond, while the migration of the phenol with the eluent is a result of solvation of either the hydrophobic part of the molecule for non aqeous eluents or the phenolic group by aqueous eluents. This work was later on extended . 21 by these workers on thin layers of cellulose impregnated with formamide , 22 on alumina impregnated papers and on thin layers of alumina , on thin 23 layers of cellulose impregnated with N-methylated formamides . The RM function and its use in the structural analysis of organic compounds was critically examined by Bark24 recently. The above quoted investigations may be considered as significant because they were aimed at correlating chromatographic behaviour with molecular structure. A review of the literature given under Chapters II and HI shows that although some adsorbent system have been proposed for the TLC separation of amines and phenols^however, these methods still appear to be inadequate to yield good separation of one isomer from the other. I have, therefore, tried a number of single as well as mixed adsorbents for these separations in order to find out the most suitable adsorbent system for these types of compounds, the results of which are given in subsequent chapters. The oxidation of phenols by different oxidants has been the subject of study by various workers. It has engaged attention for long because of the difference in the nature of the products formed with different oxidants and because of the formation of polymeric products, many of which find application in the drug and dye stuff industries. The oxidation of numerous phenols with lead tetra acetate pe (LTA) have been studied in detail by Wessely and reviewed by p C Loudon . LTA is used in cold glacial acetic acid and tends to introduce acetoxy groups predominantly into the position ortho to the original hydroxyl- yielding quinone or dienone acetates. The ortho attack may occur even if an alkyl substituent is present. For o-cresol gives(l,) but para attack can also occur, for p-cresol yields both(ll)and O (I) QJLo,cCoH3ch. Q°W0COCH^3 Qg H3 C *" OCOCH, am 8 27 Cosgrove and Waters found that when p-cresol is refluxed with benzoyl peroxide in chloroform it yields 4-benzoyloxy-3-hydroxytoluene( lV)and the same product is obtained, though in poorer yield, from m-cresol. With other phenols, too a benzoate group is introduced frequently into a position ortho to the original hydroxyl group. Since chloroform is a non-ionizing solvent it was thought that benzoyloxy radical (Ph-CO-O) might be involved but a kinetic sttidy by WaUing and 28 Hodgdon has shown that the reaction is a simple bimolecular process involving the OH group of the phenol. CH, CH, OH m (IV) The fact that both para-and meta-cresobgive the same reaction product can be explained by supposing that a trans esterification through a common intermediate(V)occurs. When the ortho positions of a phenol are blocked the following 29 oxidation occurs : OH H3C^ J^CH3 HoC CH3 OH CHCH •» •*»'• > o- >o r ~\ ss. o CHc Cosgrove and Waters30 studied the oxidation of phenols with H202. p-Cresol, m-4-xylenol, m-2-xylenol and mesitol have been oxidized with H202 in the presence of acidified ferrous sulphate in water or 20 %acetic acid medium. p-Ct*»«1 gave dimeric products identical with those obtained by Pummerer by alkaline K3Fe(CN)6 oxidation, and m-4-xylenol oxidises similarly. wv»2-XyleHolgave 3:5:3«:5! - tetra methyl-4:4' - diphenoquinone, the corresponding 4:4' di-phenol and also 2:6- dimethylquinol; mesitol gave two diphenols, one of which was identified as 4:4'- dihydroxy-3:5:3':5'-tetramethyldiphenylmethane. The phenols on oxidation with alkaline ferricyanide gave 31-34 dimeric products. The oxidation was carried out by Pummerer et.al. They suggested that in the process of oxidation,mesomeric free phenolic radicals are formed. Since such free radicals can be obtained quite simply by treating aromatic compounds in aqueous solution with hydrogen OC 36 peroxide and a ferrous salt00 (Merz and Waters), Goldhammer used this reagent for phenol oxidation and obtained catechol, pyrogallol and purpuroqgallol from phenol by this oxidation and Chwala and Pailer37 obtained about equal yields of quinol and catechol. . ,38 The oxidation of phenols with Fermy's radical represents an excellent synthetic method for the preparation of either o- or pbenzoquinones under mild conditions and usually in good yield. The presence (or absence) of substituent on the aromatic ring, para to the hydroxyl group appears to control the kind of benzoquinone that will be formed.