Isomerism

“ This connexion of isomerism with resistant linking, and of this with high valency, explains, in considerable measure, why inorganic compounds afforded, as a rule, no phenomena of this kind until the systematic investigation of metallic compounds by Werner brought to light many instances of isomerism in inorganic compounds. Whereas carbon renders isomerism possible in organic compounds, cobalt and platinum are the determining elements in inorganic chemistry, the phenomena being exhibited especially by complex ammoniacal derivatives. ”

“ Since this hypothesis was proposed, the number of cases of "abnormal isomerism," that is to say, of cases of isomerism which cannot be accounted for by the commonly accepted method of explaining structure, has increased to a considerable extent, and the necessity for some new hypothesis, or for some modification of the old ones, has come to be pretty generally recognized. Among the cases of isomerism which it is at least difficult to explain by the aid of the prevailing views are those of maleic and fumaric acids ”

“ These diatomic compounds, like their elements, are to a considerable degree characterized by the prevalence among them of allotropism; or, as it is more usually called when displayed by compound bodies—isomerism. Professor Graham finds reason for thinking that a change in atomic arrangements of this nature, takes place in water, at or near the melting point of ice. In the various series of hydro-carbons, differing from each other only in the ratios in which the elements are united, we find not simply isomerism but polymerism occurring to an almost infinite extent. ”

“ The conception of metamerism, or isomerism in restricted sense, has been of the highest value for the development of our notions concerning molecular structure, i.e. the conception as to the order in which the atoms composing a molecule are linked together. In this article we shall confine ourselves to the fatty compounds, from which the fundamental notions were first obtained; reference may be made to the article Chemistry: Organic, for the general structural relations of organic compounds, both fatty and aromatic.
A general philosophical interest is attached to the phenomena of isomerism. ”

“ In my opinion the progress of chemistry, especially with regard to solutions, should make rapid strides when the cause of the isomerism of solutions, for instance, of ortho- and pyrophosphoric acids, has become as clear to us as the cause of many well-studied instances of the isomerism, polymerism, and metamerism of organic compounds. ”

“ Isomerism is of very frequent occurrence among organic compounds, owing, no doubt, to their unusually large atomic weights, since the numerous atoms of the elements afford much scope for isomeric modifications ”

“ Cases of isomerism in the restricted sense of the word—that is, when with an identity of composition and of molecular weight, the properties of the substances are different—are very numerous among the hydrocarbons and their derivatives. Such cases are particularly important for the comprehension of molecular structure and they also, like the polymerides, may be predicted from the above-mentioned conceptions, expressing the principles of the structure of the carbon compounds [42] based on the law of substitution. ”

“ Physical isomerism (Chem.) , the condition or relation of certain (metameric) substances, which, while chemically identical (in that they have the same composition, the same molecular weights, and the same ultimate constitution) , are yet physically different, as in their action on polarized light, as dextro- and lævo-tartaric acids. In such compounds there is usually at least one unsymmetrical carbon atom. See Unsymmetrical.
I`so*mer`o*mor"phism (?) , n. [Isomer + Gr. &?; form + -ism.] (Crystallog.) Isomorphism between substances that are isomeric. ”

“ ALLOTROPY (Gr. allos, other, and tropos, manner) , a name applied by J. J. Berzelius to the property possessed by certain substances of existing in different modifications. Custom has to some extent restricted its use to inorganic chemistry; the corresponding property of organic compounds being generally termed isomerism (q.v..) Conspicuous examples are afforded by oxygen, carbon, boron, silicon, phosphorus, mercuric oxide and iodide. ”

“ An account of his experiments on ‘Chemical Isomerism' was published in the ‘Transactions of the Royal Society of Edinburgh, 1840-1,’ and during the same winter he delivered, along with Edward Forbes, a course of lectures on the philosophy of the sciences. In 1843 he was a candidate for the chair of chemistry in the university of Edinburgh, but on account of his failure to establish the proposition of the isomerism of carbon and silicon, his other high qualifications were disregarded. ”

“ The crystallographic relationships of organic substances, however, are very much complicated by the possibilities of isomerism, the ortho, meta, and para compounds—corresponding to the replacement of the two hydrogen atoms attached to two adjacent, alternate, or opposite carbon atoms respectively, of the six forming the benzene ring—generally differing extensively and sometimes completely in crystalline form. Consequently, the phenomenon of morphotropy is best considered quite independently of isomorphism. ”

“ But the crystalline form, which may show enantiomorphism, indeed shows this difference in the isomers in question; and especially the behaviour (in the amorphous state) towards polarized light differs in the sense that the plane of polarization is turned to the left by the one isomer, and exactly as much to the right by the other, so that they may be termed " optical antipodes." All these differences disappear with the asymmetric carbon, and the succinic acid, HO2C·CH2·CH2·CO2H, from tartaric acid is optically inactive and shows no stereo-isomerism. ”

“ It is, however, in the mysterious facts to which chemists have given the names of Isomorphism, Isomerism, and Allotropism, that we perceive the most direct and remarkable contribution of modern scientific research to the defence of Catholic revelation. Chemistry enables us to penetrate further than any other science into the secret operations of Nature ”

“ We may conclude, therefore, that an absorption band is not to be attributed to either the one or the other form of a tautomeric substance, but rather to the changing of one form into the other—a dynamical isomerism. If this intramolecular transformation is the source of the disturbance which produces the absorption bands, then an acceleration of this transformation should show itself in the increased persistence of the band, while retardation of the same should diminish this persistence. ”

“ But no inkling of an explanation of these strange variations of molecular structure came until the discovery of the law of valency. Then much of the mystery was cleared away; for it was plain that since each atom in a molecule can hold to itself only a fixed number of other atoms, complex molecules must have their atoms linked in definite chains or groups. And it is equally plain that where the atoms are numerous, the exact plan of grouping may sometimes be susceptible of change without doing violence to the law of valency. It is in such cases that isomerism is observed to occur. ”

“ To the statement of the vitalist that there is no evidence that life is a property of matter, we may reply with emphasis that there is not the slightest proof that it is not.
Chemistry tells us that complexity of composition involves complexity of properties. The grand progress which organic chemistry has made in recent times has been owing to the distinct recognition of the influence of structure upon properties. Isomerism is one of its most significant developments. The number of possible isomers increases enormously with the complexity of the molecule. ”

“ The converse of this phenomenon is also included among the discoveries of modern science; the same substance is sometimes observed to crystallize in two different forms not geometrically allied; and the occurrence of this new exception to the received law of crystallization is called Dimorphism.
Isomerism is the term employed to represent another exceptional class of facts, observed by later chemists to interfere with the general rule, that analogy or similarity of composition implies analogy in form and external properties. ”

“ As we cannot now lean on the views above stated, it is time to abandon the structural theory. They will all be united in chemical mechanics, and the principle of substitution must be looked on only as a preparation for the coming epoch in chemistry, where such cases as the isomerism of fumaric and maleic acids, when explained dynamically, as proposed by Le Bel and Van't Hoff, may yield points of departure. ”

“ It should be noted, too, of these bodies, that though they exhibit in the lowest degree that kind of molecular mobility which implies facile vibration of the molecules as wholes, they exhibit in high degrees that kind of molecular mobility resulting in isomerism, which implies permanent changes in the positions of adjacent atoms with respect to each other. ”

“ The fact that under higher pressures than are generally at our disposal different forms of ice have been proved to exist, and the application of the phase-rule to such cases, will greatly enlarge our knowledge of molecular isomerism. ”

“ Sunlight, or a particular solvent, or the presence of some substance which acts as a catalyst, may effect the inversion. Cis and trans isomerism is also met with among cyclic compounds; it occurs among the terpenes; and certain alkaloids, as, for example, cocaïne, exhibit it.
Although the doctrine of stereo-chemistry was first enunciated in the case of carbon, and was, indeed, for a time solely confined to compounds in which carbon was the nucleal element, there is no a priori reason why the phenomenon should be so restricted. ”

“ The easy transitions of the ortho-, meta-, and pyrophosphoric acids, by means of the hydrogen of ammonia, into the lower acids, and conversely, tend to complicate the study of this very large class of compounds, and it is rarely that the nature of a product thus obtained can be judged from its composition; and this all the more that instances of isomerism and polymerism, of mixture between water of crystallisation and of constitution, &c., are here possible. Many data are yet needed to enable us to form a true judgment as to the composition and structure of such compounds. ”

“ It is often convenient to regard compounds as formed upon certain types; alcohol, for example, may be said to be a compound formed upon the water type, that is to say, a compound formed from water by displacing one of the atoms of hydrogen by the group of elements C2H5, thus—
Constitutional formulae become of preponderating importance when we consider the more complicated inorganic and especially organic compounds. Their full significance is treated in the section of this article dealing with organic chemistry, and in the articles Isomerism and Stereo-isomerism. ”

“ To summarize, we have to deal with polymerism, metamerism, stereoisomerism, polymorphism; whereas phenomena denominated tautomerism, pseudomerism and desmotropism form different particular features of metamerism, as well as the phenomena of allotropy, which is merely the difference of properties which an element may show, and can be due to polymerism, as in oxygen, where by the side of the ordinary form with molecules O2 we have the more active ozone with O3. ”

“ Thus for the plain formulæ we have substituted, in the chemistry of carbon compounds, structural formulæ, which come to represent, thanks to the disposition or grouping of atoms in a molecule, structural relations of the second degree, that is to say, relations inherent in certain chemical transformations with respect to which some groups of elements have in some way an invariant character. And here, because the image of a simple molecule upon a plane does not suffice to explain, for example, the facts of isomerism, we must resort to the stereo-chemical representation of Van't Hoff. ”

“ All kinds of chemical reactions take place in solutions. Some molecules of the dissolved body simply exchange their atoms in succession, while maintaining the same grouping of atoms, and consequently the same chemical composition. In other molecules the grouping only of the same atoms is changed, and we have reactions of replacement, or isomerism. But, at the same time, new and more or less stable combinations between the atoms of both solvent and dissolved body take place in various proportions ”

“ Stereo-chemical Configurations.—Simultaneously with the discussions of Kekulé, Ladenburg, Claus, Baeyer and others as to the merits of various plane formulae of the benzene complex, there were published many suggestions with regard to the arrangement of the atoms in space, all of which attempted to explain the number of isomers and the equivalence of the hydrogen atoms. The development of stereo-isomerism at the hands of J. Wislicenus, Le Bel and van ’t Hoff has resulted in the introduction of another condition which formulae for the benzene complex must satisfy, viz. ”

“ There are subtle and more subtle differences causing isomerism. In the case of metamerism we can imagine that the atoms are differently linked, say in the case of butylene that the atoms of carbon are joined together as a continuous chain, expressed by —C—C—C—C—, normally as it is called, whereas in isobutylene the fourth atom of carbon is not attached to the third but to the second carbon atom, i.e. ”