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Art. XII.-Review of the Organic Chemistry of M. CHARLES
This book appeals with peculiar claims to the notice of all interested in the progress of chemical science. Organic chemistry has made great progress during the last few years; but until the publication of the Précis, with the exception of Liebig's excellent Traité de Chimie Organique, no systematic work embracing the results of the last decade had appeared. This is to be ascribed to the great difficulty of classifying the immense array of facts, and harmonizing the various conflicting theories—a task indispensable as a preparation for such a work and at the same time exceedingly delicate.
Liebig in his Traité assumed as the basis of his system, the theory of compound radicals, and commences with the assertion, that "organic chemistry is the chemistry of compound radicals.” This was a most ingenious application of the electrochemical philosophy of Berzelius to the investigation of this class of compounds, and was supported by so many analogies as to render it very probable; at the same time it admitted the application of the received nomenclature to these bodies. These radicals are generally however purely hypothetical, and when we are able to isolate substances having the composition assigned to them, they are found to possess none of the properties which theory would require. Recent experiments have shown that mellon and mellonids have not the composition ascribed to them by Liebig, and that mellon cannot be regarded as a compound radical. Cyanogen and kakodyle must however be excepted, as compounds which comport themselves in many respects like ele
The progress of discovery has shown, that this hypothesis is but poorly adapted to form the basis of a system of classification, for the discovery of nearly every new body requires the assumption of an imaginary compound to explain its reactions in accordance with the theory of radicals; and so uncertain are the principles which are to direct us in the application of this theory, that different chemists often assign very different rational formulas to the same compound. There have been not less than seven different formulas proposed, to express the arrangement of the elements in alcohol; each author seeking by his own to explain some practical relation. Thus Dumas regards it as the bi-hydrate of olefiant gas; Liebig as the hydrated protoxyd of ethyle,
Précis de Chimie Organique ; par M. CHARLES GERHARDT, Professeur à la Faculté des Sciences de Montpellier. 2 vols. 8vo. Paris, (Fortin, Masson et Cie.) 1845.- We are indebted for this review and abstract of M. Gerhardt's valuable work, to Mr. Thomas S. Hunt, lately from the Laboratory of Yale College, and now Chemist to the geological survey of the Canadus.
C,H,; Berzelius as the bin-oxyd of C,H,, and Zeise as a hydruret of C,H,0,. The inconvenience of this system arises not only from the fact that the radicals are hypothetical, but that their very existence in the compounds is alternately claimed and denied, and the elements are arranged and re-arranged like the letters in an anagram, as the case may require. M. Liebig seems to have felt its deficiencies, for after describing in the first volume of his Traité, a number of bodies as derivatives of compound radicals, in the succeeding portions of the work he returns to the old divisions of acids, alkalies, essential oils, etc.
This mode of viewing organic compounds resulted from the idea of dualism in chemical compositions, which had found advocates in the great majority of chemists since the days of Lavoisier, and has been perpetuated by the received system of nomenclature. And although there have been at different times those who have seen the difficulties of the binary system, it is only within a few years that a different philosophy has gained partisans.* This new system is distinguished as that of the French school, and ranks among its adherents the most distinguished chemists of France. It rejects entirely the idea of a binary arrangement in the composition of bodies, and regards their atoms as constituting a system, in which one or more molecules may be exchanged for others without altering the chemical constitution or type of the arrangement.
M. Gerhardt, who has been long known as one of the most distinguished chemists of France, has attempted the task of systematizing the great accumulation of facts which organic chemistry presents, and framing a classification that shall embrace all those substances whose composition is accurately determined, and in the present work he has given us the result of his labors.
Researches in organic chemistry have shown that we can produce artificially many products of the vegetable and animal organisms. Thus sugar yields by different processes, butyric, oxalic and formic acids; the first of these is one of the acids of butter, the second exists in the fluids of many plants, the last is a secretion of ants. Again bee's wax, when fused with caustic potash, forms stearic acid, one of the acids of animal tallow; by The action of nitric acid, it yields a number of new compounds among which is succinic acid, which exists in amber. These products are less complex in their constitution than the original substances; sugar by the action of oxydizing agents yields, besides formic acid, carbonic acid gas and water, and wax when converted into succinic acid, undergoes a similar decomposition.
* Mr. J. D. Whelpley attempted some years since, to show from the electrochemical decomposition of the metallic salts of the mineral acids, that they must be regarded not as binary compounds of an acid with an oxyd, but as ternary combinations of the metal, oxygen, and the other element. This principle was made by him the basis of a beautiful and ingenious classification of all saline compounds.
We cannot retrace this process and bringing together the formic acid, carbonic acid and water, by a process of dexoydation reproduce the sugar. These products were formed by a combustion in which a part of the carbon and hydrogen is converted into carbonic acid and water, and the power of reducing them belongs to the vegetable organism, where the chemical affinities are controlled and directed in a peculiar way by the vital force. It is thus that in these operations, we commence with a complex body and by a process in which its carbon and hydrogen are gradually oxydized, reduce it to simpler and simpler forms.
There are however some exceptions to this law; a few synthetical processes are known by which we can unite the elements of simpler compounds to form one more complex. Two polymeric bodies are known which are formed by a grouping together of several molecules of aldehyde; and many of the essential oils undergo a similar change by action of sulphuric acid. The decomposition of organic substances by heat offers some remarkable instances of this kind; in the dry distillation of wax C,,,,0, we obtain paraffine, which is C, H,..
In view of these relations, observes our author, we may consider all organic substances as the result of the combustion of others more rich in carbon and hydrogen, or reciprocally as the products of the reduction or complication of other bodies containing less carbon and hydrogen."
* In considering from this point of view the whole of organic substances, we observe that they offer successive and almost insensible gradations, in such a manner as to form an immense scale, the two extremities of which are occupied, the one at the summit, by the cerebral substance, albumen, fibrine and other bodies still more complex; and on the other at the bottom by carbonic acid, water and ammonia, preceded by wood-spirit with formic acid and the other bodies derived from it."
• The chemist in applying the agents of combustion to substances, descends the scale, that is to say, he gradually simplifies these substances by burning successively, portions of their carbon and hydrogen. On the contrary, he remounts the scale in applying to organic substances the processes of reduction. These considerations conduct us to an exact appreciation of the principles upon which we may classify all organic substances in a simple and complete manner, which does not have recourse to hypothesis, but confines itself strictly to the limits of experience.”
Pp. 21, 22.
In the examination of organic substances, we observe that those which correspond in their chemical characteristics, present a similarity of relation in the proportions of their constituent elements. The alcohols, embracing wood-spirit, spirit of wine, potato-oil and ethal, are examples; their composition is respectively CH 0,,C,H,O, C,H,,0 and C, H, 0.*
If the single equivalent of oxygen which each of them contains, were united with two equivalents of the hydrogen to form water, the carbon and hydrogen in the residue of each would be in the proportion of 1 to 2. By oxydizing agents the alcohols lose two equivalents of hydrogen and gain one of oxygen, giving rise to the formic, acetic, valerianic and ethalic acids, in each of which the carbon and hydrogen are in the proportion of one to two; and in all the products of the transformation of these bodies, the proportions of these elements still bear a similar relation to each other. Hence if we know the composition of any derivative of spirit of wine, we can at once foresee that of a similar product derived from any other body of the group.
Substances like these having a likeness in characters depending upon a similarity of constitution are denominated homologues ; and are to be carefully distinguished from those which resemble each other merely in physical characters, and which are called analogues. For example, wood-spirit resembles acetone in being inflammable, odorous, very volatile, and soluble in water, while ethal
, is allied to stearine in being solid at ordinary temperatures, insoluble in water and having other properties common to the fatty bodies; but their resemblances are only analogies, and when we examine wood-spirit and ethal in relation to their constitution and the products of their decomposition, we find that they are closely related to each other and are homologues.
In homologous bodies, the combustible elements, carbon and hydrogen vary exceedingly in their proportions, while the oxygen and azote are always atomically the same. Two bodies therefore which contain the one 02 and the other 0,,, or one N and the other N,, cannot be homologues, while bodies containing C,, or C, and H,, or H,,, may very well be so, as in the alcohols already mentioned. M. Gerhardt has adopted some general formulas to express these relations; R, representing the carburets of hydrogen; RO, those bodies which like alcohol, contain one equivalent of oxygen ; while other oxygenized compounds are designated as RO,, RO3, &c. Those containing nitrogen are represented in a similar manner, thus RN, RN,0g.
In order that two or more bodies may be homologues, it is not sufficient that they can be represented by the same general formula; the equivalent ratio between the proportions of carbon and hydrogen must also be identical. Formic acid CH, 0,, acetic acid C, H, 0,, valerianic acid C, H, 0,, and ethalic acid C, H3,0, are designated by the general formula RO,, and in
* In these formulas it will be observed that our author divides the equivalent of hydrogen, representing water by H,0. The equivalent of most of organic compounds is taken at one-half the number usually adopted, for reasons which will be explained farther on.
each of them R represents a compound in which the carbon and hydrogen are in the proportion of 1:2. These bodies are homologues, and the relation of their elements is such that they may evidently be derived from each other by the abstraction of equal equivalents of carbonic acid Co, and water H,0. This is then the most simple ratio, and is selected as the term of comparison. It is not however the most frequent; generally the hydrogen is less than two, and when it exceeds it, the excess is seldom more than two equivalents.
“When homologous bodies are decomposed into other homologues, they lose or fix atomically the same quantities of carbonic acid, water, oxygen, &c." This principle is illustrated by the group of alcohols so often referred to; when converted into hydrocarbons, they give up one equivalent of water, and in the formation of acids they severally lose H, and fix 0. From this it follows that a geometrical ratio between the elements of homologous substances is not necessary; bodies having the following proportions of C and H may be homologues: CH
С H 1: 4= 1:( 2+2)
4: 4= 4:( 8 – 4) 2 : 6 = 2: ( 4+2)
6: 8 6 : (12 – 4) 5 : 12 = 5: (10+2)
8: 12= 8:(16 – 4) 16 : 34= 16 : (32+2)
16 : 28=16 : (32 – 4) and the same principle applies to any other proportions of these elements. In the first group, each compound by losing in equivalents of hydrogen is reduced to the normal ratio, and in the second, the addition of four is required.
To express these relations, the symbol R is preserved for the ratio of 1:2; for those bodies in which the proportion of hydrogen is greater, the number of equivalents is indicated by an exponent preceded by the sign plus (+), and when its proportion is less it is expressed by a similar exponent with the sign minus ( - ).
Wood-spirit CH, O, alcohol C, H, O, potato-oil C, 1,20 and ethal C, H, O, are by this notation, homologues of the form R+2 0, and the acids derived from them by the abstraction of two equivalents of hydrogen and the addition of one of oxygen are expressed by the formula RO.. The acids, oxalic C,H,O,, succinic C, H, 04, pimelic C, H,20, and suberic c, H. o, are homologues of the form R-20,; oxamid C, H, N, 0, and succinamid C, H, N, 0,,, are homologous bodies of the form RN, 0, ; benzene C, H, and cumene C, H, are expressed by R-s, and so on. To determine whether two bodies having the same amount of oxygen, can be homologues, we assume a number of equivalents of hydrogen equal to twice that of the carbon, (this being the proportion of 1:2,) and observe whether the excess or deficiency of hydrogen is the same in both; and consequently whether they can be expressed by the same formula.
SiconD SERIES, Vol. IV, No. 10.-July, 1847.