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CHEMISTRY such readiness that their complex character and the part played by the catalyst may be entirely overlooked, so that they may come to be regarded as simple intramolecular changes. Much discussion has taken place on the subject of dynamic isomerism during recent years, and the view has even been advocated that substances may exist an fsomerism unsteady state, having at one moment one configuration and an altogether different one at another. Although there is no reason to suppose that such is the case, and the evidence to the contrary is almost conclusive, as a matter of convenience it is well to distinguish isomerides which are so sensitive that they change spontaneously under the influence of necessarily adherent impurities so soon as the conditions are such as to render change possible. Such substances have been called tautomeric; they are more appropriately distinguished as dynamic isomerides, being dynamically equivalent compounds. The conditions Avhich limit the stability of isodynamic compounds have been fully discussed by Lowry (Trans. C/iem. Soc. 1899, p. 211). The point of chief importance to be borne in mind in connexion with such compounds is that in the fluid state they occur in admixture in dynamic equilibrium, and can only exist separately in a stable condition when in the solid state at temperatures below the stability limit, although the proportion of one of the forms present when equilibrium is reached may be infinitesimal, and the substance consequently appear to be homogeneous. The recognition of dynamic isomerides has an important bearing on the determination of structure from chemical considerations. The outcome of the discussion which has arisen on the subject has been to emphasize the need of extreme care in basing final conclusions on chemical considerations alone, as well as to show the value of physical properties as determinants of configuration. What is of even greater importance, it has drawn attention to the fact that what were formerly regarded as direct products of substitution are in reality very frequently, if not always, simply end products of a series of changes involving, in the first instance, addition— not displacement—and attende4 with an alteration in type. Ethylic acetoacetate may be referred to as a compound of special interest in connexion with the question under discussion. Under most conditions its behaviour is such as to require its representation as a ketonic compound, CH3.CO. CH2.C02Et, but in some cases compounds are obtained from it which are clearly derivatives of an enolic form, CH3.C(OH):CH.CO._,Et, i.e., a form which is both an ethenoid and an ol or alcohol. It is easy to understand how the one form may pass into the other—in the case of the action of sodium ethylate, for example. Instead of hydrogen being displaced by sodium, the ethylate is attracted to the ketonic oxygen—a compound being formed from which alcohol separates, leaving the sodium in combination with the oxygen. The oxygen therefore ceases to be ketonic and becomes “ hydroxy lie ”; thus MeCO. CH2. C02Et + NaOEt=MeC(ONa) :CH. C02Et + EtOH. When the product is subjected to the action of an alky lie iodide the operation is reversed, and the ketonic type is reverted to; thus MeC(ONa) :CH. C02Et + Mel = MeCO. CHMe. C02Et + Nal. In the year 1875 chemists had prominently under their notice only two types of hydrocarbons—open chain hydrocarbons, such as those of the paraffin, Vf^ydro* ethylene and acetylene series, and cycloid or carbons, closed chain hydrocarbons, such as benzene, naphthalene and anthracene. Hot only has our knowledge of these types been considerably extended in the interval, particularly in the case of benzenoid hydrocarbons,

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but other distinctly different types have been discovered. The chief interest attaching to these discoveries has lain in the fact that they have disclosed remarkable differences in behaviour depending on the size and structure of the cycloids. The study of the dimensions and characters of such compounds has in consequence assumed considerable importance. It has gradually been placed beyond doubt that the formation of cycloids can only occur in many cases within certain narrow limits, and many facts have been put on record which tend to show that the “ affinity ” exercised by carbon and other elements in cycloids is a distinctly directed or directive effect. It appears probable, indeed, that considerations inherent, perhaps, in the van’t Hoff-Le Bel doctrine of “ tetrahedral ” carbon may find appheation, although in quite a different manner, in this field as well as in that of stereoisomerism. It was long supposed that the simplest ring obtainable contained six atoms of carbon, and the discovery of trimethylene by Freund in 1882 came somewhat Po/ym as a surprise, especially in view of its behaviour with bromine and hydrogen bromide. Trimethylene is formed on withdrawing the bromine from trimethylene bromide. CEL BrCH0.CH0.CH0Br - 2Br = / H,C CH, In comparison with the isomeric ethenoid (ethylene or ethene-like) hydrocarbon propylene, CH3.HC:CIL, it is remarkably inert, being only very slowly attacked by bromine, which readily combines with propylene. But on the other hand, when digested with a saturated solution of hydrogen bromide it is readily converted into normal propyl bromide, CHg.CEL.CILBr. The separation of carbon atoms united by single affinities in this manner at the time the observation was made was altogether without precedent. A similar behaviour has since been noticed in other trimethylene derivatives, but the fact that bromine, which usually acts so much more readily than hydrogen bromide on unsaturated compounds, should be so inert when hydrogen bromide acts readily is one still needing a satisfactory explanation. A great impetus was given to the study of poly-methylene derivatives by the important and unexpected observation made by W. H. Perkin, junr. in 1883, that ethylene and trimethylene bromides are capable of acting in such a way on the sodium derivative of ethylic acetoacetate as to form tri- and tetra-methylene rings. Perkin has himself contributed largely to our knowledge of such compounds. Penta- and hexa-methylene derivatives have also received considerable attention, and the presumed presence of rings of this kind in camphor and allied compounds has led to an increased interest being taken in their study. The uniform result of all inquiries up to the present time has been to show that the instability of the trimethylene ring is exceptional. Yon Baeyer has sought to explain the variations in stability manifest in the various poly-methylene rings by a purely mechanical hypothesis (Ter. deut. chem. Baeyer’s Ges. 1885, 2279). Assuming the four valencies strain of the carbon atom to be directed from the centre hypoof a sphere towards the four corners of a regular thesistetrahedron inscribed within the sphere, the angle at which they meet is 109° 28'. Baeyer supposes that in the formation of carbon “ rings ” the valencies become deflected from their positions, and that the tension thus introduced may be deduced from a comparison of this angle with the angles at which the strained valencies would meet. He regards the amount of deflection as a measure of the stability of the “ring.” The readiness with which ethylene is acted on in comparison with other types of hydro-