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THE GENERAL INDEX TO VOLS. 1 to 100 can stil; be purchased at £1 1s. All communications should be addressed to the MANAGER, CHEMICAL NEWS. THE CHEMICAL NEWS, VOL. CXXXII. No. 3440. MERTON HOUSE, SALISBURY SQUARE, TELEPHONES : RING CHAIN TAUTOMERISM, PART II. THE OCCURRENCE OF TAUTOMERISM OF THE THREE CARBON TYPE BETWEEN A HOMOCYCLIC COMPOUND AND ITS UNSATURATED ISOMERIDE. By R. F. HUNTER, M.Sc., D.I.C., B.Sc., A.R.C.S. It was pointed out, it will be recalled, by Ingold and Perren (J.C.S., 1921, CXIX., 1582) and Ingold and Powell (J.C.S., 1921, CXIX, 1976) that the condensation usually termed the Michael reaction between a В unsaturated esters and compounds such as ethyl cyanoacetate which contain the reactive hydrogen atom is a reversible process, the condensation product undergoing fission into its components under the conditions of formation. In general, therefore, the result is a balanced action, for any particular set of conditions the ultimate product is an equilibrium mixture, the composition of which is the same no matter whether the starting material is the condensation product or a mixture of the two esters which have to be condensed to give this condensation product. The proof of the reversibility of this reaction is of interest because it completes the analogy with three carbon tautomerism. For example the equilibrium which we have indicated and which we will term the equilibrium (A) is strictly comparable with the equilibrium, the existence of which is clearly proved, if further proof is necessary by the recent researches of Feist and his collaborators (Annalen, 1922, CCCCXXVIII 25, 40, 59 and 68) between the labile glutaconic esters, which we may term the equilibrium (B). Both reactions are reversible but the reaction (A) is intermolecular whilst the reaction (B) is intramolecular, this would appear to be the whole formal difference between them. It is precisely this difference, however, which renders the second reaction alone capable of being brought within the philosophical definition of tautomerism as reversible change between isomerides. From this point of view, then, the glutaconic acid change is an intramolecular Michael condensation. Clearly, however, it is but one of a large number of theoretically possible intramolecular changes of which the reaction (A) that is the reversible Michael reaction is the intermolecular prototype for the two carbon atoms which become joined by a single bond in the reaction (A) and by the double bond in the corresponding change expressed in the change (B) might one may suppose become involved in a cyclopropane or cyclobutane ring or in larger rings as a result of intramolecular condensation. Since this internal condensation would be of the same nature as a Michael reaction we should expect it to be reversible. It is obvious that the case (B) is the case where n=0. The unsaturated open chain compound (I) and the saturated cyclic compound (II) are isomerides and the equation (C) must therefore be regarded as expressing a kind of tautomerism, and clearly moreover, the relationship represented belongs to the group of phenomena to which the name ring chain tautomerism has been applied (loc. cit.). All the examples of ring chain tautomerism hitherto discussed in this journal have been of the keto-enol type the simplest of which is the example of Deshapande and Thorpe on the occurrence of ring chain tautomerism in the gem diethyl series (J.C.S., 1922, CXXI., 1430). Keto enol tautomerism may be formulated in general terms as follows: and there is ample evidence at the present time of its occurence. Equations (C) and (D) are precisely similar, and the former is therefore to be regarded not only as the general expression for an internal Michael condensation, but also on broader lines, as the fundamental formula for three carbon ring chain tautomerism. From what has been written with regards the relationship of three carbon carbon ring chain tautomerism to the ordinary direct and retrograde Michael reaction (A), to the tautomerism of the glutaconic esters (B) and the keto-enol ring chain tautomerism (D) there seems every reason to believe in the possibility of both the occurrence and reversibility of the reactions expressed in equation (C), and indeed in the reality of the general phenomenon which this equation represents. It will be recalled that at an early period Guthzeit found that ethyl a carboxyglutaconate (III) on keeping at the ordinary temperature for a year or on treating with a small amount of piperidine for a much shorter period became transformed into a liquid bimeric ester to which the formula (IV) was assigned (Berichte, 1898, XXXI., 2753; J. pr. Chem. 1909, (ii) LXXX., 435). (CO,Et),CH.CH : CH.CO,Et Ethyl dicarboxyglutaconate (V) appeared to undergo a similar change, but more slowly. In this case, however, the condensation product represented by formula (VI) was a solid, melting at 103°, from which a stereoisomeride melting at 87° could be obtained. In the absence of piperidine, the conversion was appreciable after one year, whilst when piperidine was added the change proceeded to the extent of 75 to 80 per cent. in seven weeks (J. pr. Chem., 1909 (ii) LXXX, 412) and (Berichte, 1901, XXXIV., 675). (COEt) CH.CH: C(COEt)2 CO,ETCH-CH.CH(CN)CO,Et In none of these cases has any inquiry been made into the cause or mechanism of the changes observed previous to the work of Ingold, Perren and Thorpe (J.C.S., 1922., CXXI., 1765). The term "polymerism " seems to have been considered sufficiently self-explanatory, and no attempts had been made to show why some glutaconic esters 66 polymerise" and others do not, or why the rapidity and completeness of the process vary in the way they do in different instances. Neither had any one previously endeavoured to demonstrate the ordered solidarity of the reactions under discussion with any large and well worked out group of phenomena. The fundamental circumstance is that a glutaconic ester contains within its molecule both the residues by the interaction of which a Michael condensation can take place. For instance, a carboxyglutaconic ester contains a malonic ester residue and an acrylic ester residue, if the reaction between these is intramolecular, three carbon tautomerism in the sense of (B) is the result, but if the malonic ester residue of one molecule reacts with the acrylic ester residue of another molecule, the product (IX) is a substance capable of exhibiting ring chain three carbon tautomerism (Equation C) with its cyclic isomeride (X) (COEt),C[H].CH: CHCO2Et CO.Et.CH: CH.CH(CO,Et), (CO,Et),C.CH:CH.CO.Et CO.EACH(H)CH.C[H](CO,Et), < (IX) (CO,Et),C- CHCH [H] CO.Et CO,EtCH[H]CH–C(CO,Et). (X) |