OXIME DERIVATIVES OF ACETOPHENONES AS CHELATING AND ANALYTICAL REAGENTS

Abstract

The importance of organic compounds as analytical reagents, is well recognised. Of surpassing value as analytical reagents, are the compounds which are capable of forming chelates particularly inner-complex salts or chelate non-electrolytes, with metals. Certain oximes,e.g., orthodioximes, acyloin-oximes, ortho-hydroxy aromatic oximes, monoximes of diketones etc. belong to this class of organic reagents, Ortho-dioximes : The reaction between nickel salts and dimethyl - glyoxime1, CH3- C(=NOH)-C(= NOH)-CH3, the often quoted example of ortho-dioximes, takes place through the central group, -C( = NOH)-C(» NOH)-, resulting in the formation of brilliant red, insoluble nickel chel?te. The replacement of the methyl groups by other aliphatic, aromatic or hetero - cyclic groups, does not prevent the formation of similar nickel compounds. The composition (C4H702N2)2Ni of the nickel-dimethylglyoxime indicates that one of the hydrogen atoms of each of the two molecules of dimethylglyoxime, C4H„02N9, has been replaced by the bivalent nickel atom. Following the demonstration of the existence of 2 two tautomeric forms of the oxime group , c— —c— • A \ / \ OH OH Oxime form Nitrone form * Pfeiffer » proposed that the nitrone form is involved in the formation of the nickel derivative and a five-membered ring is formed in which the nickel is attached directly to the nitrogen atom. R C C R 1! i' N N / N/\ 0 Mi OH That the hydroxyl radical of the second oxime group Is not involved in the formation of the metal complex, is further demonstrated by the fact that mono-O-ether of benzildioxime forms an exactly similar compound with nickel * O6^H5r C| C(| C6^H5e N N A-/\ 0 Ni 0— CH0 2 J Brady and Meurs have proposed the following formula for nickel dimethylglyoxime, H—C C C Q"L» 0 « N /N 0V \ -S \ H Ni H " T i H3C -— C C -—CH3 The postulated hydrogen bonding eliminates the possibility of cis-trans isomerism and also explains the lack of reactivity of the hydroxyl group. o - -» o" * Acyloin oximes t The mode of reaction of acyloin oximes containing the group >C(0H)- C(= NOH)-, with metals is different. Available evidence seems to indicate that^-acyloin oximes react as dibasic acids towards cupric salts to form compounds of the type, R C C R 0 N > 0 Cu Under special conditions^-benzoinoxime forms compounds with cobalt, nickel and with bivalent platinum and palladium, but in the formation of these compounds, the oxime functions as monobasic acid . Ortho-hydroxy aromatic oximes : Ortho hydroxyaromcrticoximes,however,differ from the ortho-dioximes and acyloinoximes, in their reactions with metals. In these phenolic compounds of unusual analytical importance, the hydroxyl group is so located with respect to the nitrogen atom of the oxime group that six-membered rings instead of five-membered (as in the case of ortho - dioximes and acyloinoximes), are formed with metals. The structures of the metal compounds of hydroxyarometicoximes have, however, not yet be^n systematically Investigated, although quite a few references dealing with their applications in analytical chemistry, are available in the literature. The need of investigating the structure u 4 of these metal complexes as well as the desirability of finding new reactions of analytical importance,exist. Before proceeding further with this aspect, a brief survey of the existing literature is desirable. Most of the salts of hydroxyaldoximes are insoluble In water and consequently adaptable to the detection and determination of numerous cations. In order to Increase the selectivity and sensitivity of these reactions, many derivatives have been prepared and studied. The most readily available compound of this class is selicylaldoxime. Ephraim7, for the first time, used this compound for the detection and gravimetric determination of copper. The following structures for the copper-salicylaldoxime, based on the replacement of the hydrogen atom of either acidic phenolic or the oxime group by the metal and coordina tion of the metal to the other group resulting in the formation of typical inner-complex were proposed, I H Feigl and Bondi8 while investigating the reactions of the isomeric methyl ethers of salicylaldoxime f\-0H \—oCHi with copper, had observed that the compound containing the free hydroxyl group reacted with copper to form a dark a brown salt, while the isomeric phenolic ether did not react and hence reached the conclusion that the phenolic g hydrogen was replaced by the metal. Ephraim had studied the reactions of a number of other compounds which contain the same reactive grouping as salicylaldoxime. The presence of the group HO—Cl -CI^-CI mNOH in o-hydroxyaldoximes, is not in itself sufficient to yield a specific copper reaction. For example, pentanol-2-one-4- oxime, CH~-CH(OH)-CH-—C(* NOH)-CH3and chloral acetophenoneoxirne, CCl3- CH(OH)-CH2—C( = NOH)-C6H&, containing the same grouping,neither yield insoluble copper salts nor character istic colour reactions with copper. In fact, the enhanced acidity of the hydroxyl group because of its linkage to the aromatic ring and lack of any steric hinderance between the -HOH an^ the-OH group,are responsible for the copper specific action of the group HO-C^- C(= NOH)- . Flagg and Furman had made a detailed study of the reactions of many ions with salicylaldoxime and found that positive reactions were obtained in many cases. The same authors had also studied the effect of introducing a halogen atom or nitro-group into the salicylaldoxime and had found that the resulting derivatives offered no special advantages as analytical reagents, over the parent compound. The colours of certain metal salts of the 5-nitro-oxime, however, were more vivid than those of the corresponding metal compounds with salicylaldoxime. *. 6 Salicylaldoxime has also been found to be useful 11 12 13,14 for the quantitative determination of lead ' .palladium , iron15, nickel16 and bismuth17. Resorcylaldoxime (2,4-dlhydroxybenzaldoxime) yields a purple colour with ferric iron in slightly acidic solution and this reaction may be used for the detection and colorimetric determination of ferric iron. Musante18, for the first time, introduced OH salicylhydroxamic acid, C6H4(0H).C =NOH, as an analytical reagent and studied its nickel and cobalt salts. Subsequent ly Bhaduri19 had studied the qualitative reactions of this compound with various other metallic ions. Bhaduri and Ray20'21 had also suggested the application of this compound for the colorimetric estimation of uranium, molybdenum, vanadium and ferric iron. Bandvopadhayay and Ray22 had investigated the NH2 reactions of salicylamidoxime, C6H4.(0H).C =NOH, with a number of metallic ions. They observed that salicylami doxime resembled salicylaldoxime, in its general behaviour, with respect to the colour, solubility etc. of its metallic derivatives but the pHs of incipient precipitation of metals with salicylamidoxime are somewhat higher. Poddar23 studied the reactions of o-hydroxyacetoohenoneoxime, an aryl hydroxyketoxime, with a number of metallic ions such as Ag+, Hg , Pb , Cu , Cd , Zn++, Fe*+, Fe+++, Ce+++, Ni++, *** Ti++++, Co++,Uo*+, v'o; and MoO~". He also studied the application of this compound as an analytical reagent and employed it success- 24 24 fully for the gravimetric estimation of copper , nickel , palladium25, vanadium26, titanium27 and for colorimetric estimation of uranium28, vanadium29 and ferric iron . Appala and Neelkantam31 employed successfully resacetophenoneoxime as an analytical reagent for qwantita- 32 tive separation of copper and cadmium. Bhatki 'employed this reagent for the gravimetric estimation of nickel. Reddy33 used this reagent for the amperometric determination of copper in the presence of cadmium and nickel. Singh and Sharna34 studied the reactions of 2-hydroxy-5-methyl acetophenoneoxime with various metallic ions and also successfully carried out the quantitative estimations of nickel and copper and their separation when present in the sams solution, with this reagent. The Problem : The above survey of the existing literature supports the view point that much remains to be investigated so far as the structural and analytical aspects of the chemistry of metal-dihydroxy aldoximes, is concerned. The present studies were planned with an aim to study the structure and analytical uses of some soluble and insoluble metal complexes o* hydroxy acetophenoneoxlmes. The soluble complexes were exploited for colorimetric estimation xvhile insoluble complexes were made the basis for the amperometric estimation of these metals. «> 8 The investigations may roughly be divided under the following heads ;- (1) Reactions of Mo(VI) with the oximes of o-hydroxyacetophenone and its derivatives namely 2-hydroxy- 5-methyl acetophenone and 2,5-dihydroxyacetophenone. (2) Colorimetric estimation of molybdenum using 2,5-dihydroxyacetophenoneoxime as a colorimetric reagent, (3) Reactions of copper(II), palladium(II) and nickel(II) with the oximes of hydroxyacetophenones mentioned above, (4) Amoerometric determination of copper, palladium and nickel with 2,5-dihydroxyacetophenoneoxime. (5) Reactions of uranium and thorium(IV) with 2-hydroxy-5-methyl acetophenone. The composition and structure of the chelates formed in the above reactions, were determined using spectrophotometry (in the case of soluble complexes), conductometric, amperometric, pH metric methods, chemical analysis and I.R. studies.