NEWS NOW READY. Cloth, 3/6; Paper covers, 2/6. (Postage, 4d. extra). THE WHEAT PROBLEM: Based on Remarks made in the Presidential Address to the British Association at Bristol in 1898. REVISED WITH AN ANSWER TO VARIOUS CRITICS By SIR WILLIAM CROOKES, F.R.S. SECOND EDITION. VITH PREFACE AND ADDITIONAL CHAPTER, BRINGING THE STATISTICAL INFORMATION UP TO DATE. With Two Chapters on the Future Wheat Supply of the United States, by MR. C. WOOD DAVIS, of Peotone, Kansas, and the HON. JOHN HYDE, Chief Statistician to the Department of Agriculture, Washington. OPINIONS of the PRESS. "Sir William discusses at length the criticisms passed upon his address, and he appends valuable papers sup porting his arguments on the future wheat supply of the United States."-Globe. "Sir William Crookes . . has propounded a problem which in the next century [written in 1899] is bound to engage the close attention not merely of agricultural experts, but of economists and statesmen."-Speaker. 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Medium 8vo, gilt tops. [Shortly. NATURE. "The standard English book of reference on the subject." Laboratory Companion to Fats and Oils Industries. A Dictionary of Chemical Solubilities, Inorganic. By A. M. An Experimental Study of Gases. By Prof. M. W. TRAVERS. 8vo. IOS. net. Practical Chemistry. By CARAMELS and COLORINGS Blowpipe for all purposes. A. BOAKE, ROBERTS, & CO. (LIMITED), STRATFORD, LONDON, E. IOS. net. Translated by J. LANDAUER. Third Edition. Translated by J. TAYLOR. Globe 8vo. 45. 6d. MACMILLAN & CO., Ltd., LONDON. CHEMICAL NEWS, March 12, 1909 İsolation and Synthesis of Urea. THE CHEMICAL NEWS. VOL XCIX., No. 2572. THE ISOLATION AND SYNTHESIS OF UREA. By F. D. CHATTAWAY, F.R.S. AMONG the almost bewildering number of compounds known to chemists there are few, if any, which surpass urea in interest. This is obvious whether one considers the part it has played in the development of organic chemistry and the number of well-known compounds which can be made from it, or the circumstance that it is the form in which nitrogen is mainly excreted. There is no animal matter which has been subjected to a more rigorous examination than urine, and it is somewhat of a surprise to find that urea, which is its most important constituent, was not properly isolated till the close of the eighteenth century. During the preceding century, chemists from the time of Boerhaave (1668-1738) to that of Scheele (1742-1786) almost exclusively occupied themselves with the phosphates which are contained in it. This great attention to the phosphates arose from the interest inspired by the discovery of phosphorus, which was obtained originally by strongly heating with white sand the residue left after evaporating urine. Scheele even failed to notice the presence of urea in urine. He, how ever, was chiefly concerned with the uric acid contained in it. It is worth noting that it is not necessarily the substance present in largest amount in any natural product which is first discovered, but the one which is most easily isolated, and hence generally the insoluble or least soluble part. Urea appears to have been noticed first in 1773 by Rouelle the younger, and styled the "saponaceous extract of urine." It was, however, first definitely isolated by Fourcroy (1755-1809) and Vauquelin (1763-1829), who in 1799 and 1800 published two memoirs upon the constituents of urine (Annales de Chimie, 1799, xxxi., 49; 1800, xxxii., 80). They state that, although they have searched through the works of all authors who have written upon urine, they cannot find any precise information which could refer to this body before 1773. Boerhaave, of Leyden, in speaking of the evaporated residue, states that it resembles honey in appearance, but Rouelle was the first to make any express mention of a distinct substance and to give a few of its characters, publishing his observations in November, 1773, in the Journal de Medecin. Fourcroy and Vauquelin obtained urea in a state of comparative purity by evaporating urine and extracting the syrupy residue by alcohol. The alcohol was distilled off, and the solution left to cool, when crystals separated; these were purified by repeated crystallisation from water, or better from alcohol. They recognised their product as a definite new compound, and described many of its more obvious properties They gave it the name of "urée," which they say connects it in the mind with urine from which they had obtained it. During the next twenty years urea was prepared by a number of chemists, but evidently their methods still left something to be desired, for Dr. William Prout, in 1818, devised an improved process. His paper was published in the eighth volume of the Med. Chir. Transactions, and the part dealing with urea is reprinted in Thomson's "Annals of Philosophy," 1818, xi., 352. Hilaire Marin Rouelle (1718-1779), brother of the more celebrated Guillaume Francois Rouelle (1703-1770) who was one of the most eminent of the French chemists of the eighteenth century, and the teacher of Lavoisier. He obtained urea by evaporating urine to the consistency of syrup and adding nitric acid till the whole was converted into a crystalline mass. This was washed slightly with cold water, and the nitric acid then neutralised by a solution of potassium or sodium carbonate. The greater part of the potassium or sodium nitrate was then removed by crystallisation, the mother-liquor was made into a paste with animal charcoal, and the urea dissolved out from this by cold water. This aqueous solution was next evaporated to dryness and the urea extracted by alcohol. To obtain it perfectly pure it was necessary to re-crystallise it several times from alcohol. Having thus prepared pure urea Prout described its properties very carefully and analysed it. He does not give any details of the method used, but says "The substance under examination was heated with oxide of copper in an apparatus so contrived that the amount of water and carbonic acid formed might be accurately ascertained and the carbon and hydrogen thus estimated while the azote remained uncombined." The accuracy of his results, considering the time at which the analysis was made, is remarkable. He determined the percentage composition to be-H = 6·66, C = 19′99, N = 46.66, O (by difference) 26.66; the percentage composition calculated from the formula, using the most recent international atomic weights, beingH-671, C = 1996, N = 46·71, O=26 62. Some years later large quantities of urea were required in the hospital of St. Antoine in Paris for making experiments (which were unsuccessful) as to its efficacy as a therapeutic agent in cases of diabetes, and M. Henry, who was charged with making it in the Central Pharmacy of the Civil Hospitals of Paris, devised a somewhat less troublesome method of obtaining it in quantity (Journal de Pharmacie, 1829, xv., 161). Henry suggested that a slight excess of subacetate of lead or hydrate of lead, either of which precipitates the acids of the salts contained in it and much of the mucous and animal matter, should be added to fresh urine. The precipitate having been filtered off and any dissolved lead removed by adding dilute sulphuric acid, the liquid was concentrated, animal charcoal being added during ebullition, and filtered; the filtrate was then further evaporated till crystals separated; these were mixed with a little dry sodium carbonate and the urea extracted by alcohol. The product was finally re-crystalllsed from water. This practically concludes the important work on the preparation of urea from urine, the problem having been to separate it from the uric acid, phosphates, and animal matters contained in this fluid. It is still always prepared by processes not essentially differing from those of Prout and Henry, either by removing the urea from the concen. trated urine as a sparingly soluble salt such as the nitrate or oxalate, the other constituents remaining behind in the mother-liquor, or by precipitating the other constituents by oxide of lead, thus leaving the urea and little else in the filtrate. This carries the history of the compound to the period of one of those great discoveries which from time to time alter the whole outlook of chemistry. Organic chemistry dates from it in two senses; it broke down the barrier between organic and inorganic chemistry, and was the first example of organic synthesis. At this time, 1828, a very large number of well-defined substances obtained from animal and vegetable sources had become thoroughly well known; they had been analys: d and the fact established that in spite of a great diversity in properties they were composed of a very few elements. The more these substances were studied the more evident it became that they possessed no peculiar characteristics which distinguished them from compounds of mineral origin; they had the same constancy of composition as inorganic compounds, and their constituent elements were present in the proportions required by their atomic weights. Almost all chemical and physical properties were common to both classes; only in one particular did substances of animal or vegetable origin differ from substances obtained from non-organic sources-they could not be built up from their constituent elements in the way most inorganic substances could. This is, however, such a fundamental difference that one is not surprised that many chemists of the time thought that these bodies must be elaborated in animals and plants under conditions which could not be realised outside such living organisms. The processes by which they were formed appeared so mysterious that a peculiar force, the "vital force" or "lebenskraft" was called into existence to explain them. It is difficult to trace the origin of this conception; it probably arose from the idea that some particular substance active only during life was common to all living matter, and that this living material was necessary to the formation of all organic compounds. The idea that there exists an essential difference ir. nature between organic and inorganic compounds was shown to be untenable when Wöhler effected the synthesis of urea. This discovery so changed the direction of chemical thought and opened up such wide possibilities that the paper in which it is described, in spite of its confused style and frequent repetition, takes rank as one of the most notable ever written. It was published in 1828 in Poggendorf's Annalen der Physik und Chemie, 1828, xii., 253, under the title" Ueber Künstliche Bildung des Harnstoffs," and no apology is needed for quoting from it a free rendering of Wöhler's own words : "A crystalline white substance is invariably obtained whenever one attempts, for example, by socalled double decomposition to combine cyanic acid with ammonia. The circumstance that on combination these substances appear to alter their nature and that a new compound results has turned afresh my attention to the subject. The investigation has led to the unexpected conclusion that urea is formed by the union of cyanic acid and ammonia. This observation is especially noteworthy, as it affords an example of the artificial production of an organic and indeed of a so-called animal substance from inorganic material. I have further noticed that the above mentioned white crystalline body is best obtained either by decomposing cyanate of silver by a solution of salammoniac or lead cyanate by liquid ammonia. I prepared the considerable quantity required for its investigation in the latter manner. I thus obtained it in clear colourless crystals often more than an inch long, which formed slender rectangular four-sided prisms without definitely pointed ends. When this substance was heated with potash or with lime no trace of ammonia was liberated. With acids it did not show the characteristic behaviour of cyanates which when thus treated yield carbonic and cyanic acids. Further, it did not, as true salts of cyanic acid do, give a precipitate with salts of silver and lead. It could, therefore, contain neither cyanic acid nor ammonia as such. "As I found that in the last named method of preparation nothing was formed beside it except pure lead oxide, I drew the conclusion that by the union of cyanic acid and ammonia an organic substance was produced perhaps similar in nature to the vegetable salt-forming bases. With this idea in my mind, therefore, I set on foot an investigation into the behaviour of the crystalline body towards acids. It, however, showed itself indifferent to these except to nitric acid, which when added to a concentrated solution at once produced a precipitate consisting of glittering scales. These after being purified by frequent re-crystallisation showed such a marked acid character that I was inclined to regard them as a peculiar acid when I found that on neutralising them by bases they yielded salts of nitric acid from which the crystalline material in question could again be extracted by alcohol, with all the characteristics it possessed before being acted upon by the nitric acid. This behaviour, so strikingly similar to that of urea, induced me to compare it with perfectly pure urea separated from urine. This comparison established beyond doubt that the crystalline body or cyanate of ammonia, if | one may call it so, and urea are one and the same substance." He adds in conclusion, "I refrain from putting forward here the reflections which naturally present themselves as a consequence of the fact here established that compounds of the same elements having the same composition may possess very different properties." Urea, Berzelius, referring to Wöhler's discovery in the ninth issue of the Jahresbericht, is much clearer. He states:"One of the most unexpected, and on that account most interesting, of recent discoveries in the department of animal chemistry is incontestably Wöhler's artificial production of urea. This chemist finds that when silver cyanate is treated with a solution of ammonium chloride or lead cyanate with ammonia, a crystalline substance is formed, which possesses to the slightest detail all the properties of pure urea, and is, in fact, urea itself. however, is not on that account to be regarded as ammonium cyanate; the elements are united in it in a different way, so that the strongest bases can no longer liberate ammonia from it, nor the strongest acids set free cyanic acid. One can only say that this substance has changed from a compound inorganic atom of the second order to a compound organic atom of the first order. This fact affords a key to many difficulties, and shows that the same number of simple atoms distributed among themselves in dissimilar ways in compound atoms can give rise to bodies with different properties." The course of the reaction which takes place when ammonium cyanate passes into urea has never been explained. Liebig and Wöhler later showed that the reverse change could be effected, and that when urea is mixed with an aqueous solution of AgNO3 and evaporated to dryness, it is completely resolved into ammonium nitrate and cyanate of silver. Walker has recently shown that the transformation of ammonium cyanate into urea is a reversible action, and that equilibrium is reached at 100° when about 5 per cent of the urea is changed. Although the fact that urea is easily broken down into carbon dioxide and ammonia when heated with acids or with alkalis led chemists, as soon as molecular structure began to be considered, to regard it as the amide of carbonic acid, this view was not supported by any direct synthesis of the compound from derivatives of the acid until Natanson, nearly thirty years after Wöhler's work, obtained it by the interaction both of the chloride and of the ethyl ester of carbonic acid with ammonia. Natanson in his paper ("Ueber zwei Künstliche Bildungsweisen des Harnstoffs, Ann., 1856, xcviii., 287) describes his experiments as follows:-" If ethyl carbonate is heated with ammonia in a sealed tube at 100° urethane only is formed, but if the temperature is raised to about 180°, the latter through the action of the excess of ammonia is converted into urea. In the part of the tube not filled by the liquid, a sublimate of undecomposed urethane is formed, while the aqueous solution contains the urea. When this solution is evaporated to dryness and the residue heated for some time to 100°, most of the urethane remaining volatilises, and urea, which can be immediately recognised by its reaction with nitric acid, remains behind." Completely to remove the last traces of urethane the urea was washed with ether, in which the former compound is easily soluble. The urea thus obtained was analysed and compared with natural urea, with which it was found to correspond in every respect. Much earlier, Regnault (Ann. Chim. Phys., 1838, lxix., 180) had investigated the white neutral substance produced when dry phosgene and ammonia are brought together, and had shown that it behaved in some of its reactions as a mixture of sal-ammoniac and the amide of carbonic acid. He, however, concluded that the carbamide which he believed to be present was not identical with urea, since, on adding strong nitric acid to a saturated solution of the white mixture, nothing separated, while urea nitrate was at once precipitated on similarly treating a solution of urea. CHEMICAL NEWS, March 12, 1909 Isolation and Synthesis of Urea. Natanson re-investigated the reaction between carbonyl | chloride and ammonia, and found urea in the white solid product which he obtained. He showed that if the gases were not thoroughly dried before being mixed, the amount of urea formed was so much diminished relatively to the amount of sal-ammoniac that it could not be recognised by adding nitric acid, which only throws down urea nitrate from a sufficiently concentrated solution of urea. Natanson describes his procedure as follows:-"I prepared the phosgene gas required for the investigation by leading carbon monoxide through boiling antimony perchloride. This method, recommended by Hofmann (Ann., 1849, Ixx., 139), I found very convenient. The two gases were led into a spacious dry glass balloon. The presence of urea in the white solid obtained could be shown, even by extracting it with absolute alcohol, evaporating the alcoholic extract to dryness, dissolving the residue in a little cold water, and adding strong nitric acid; urea nitrate separated at the latest in a few hours. All the urea formed can be obtained by adding a solution of baryta to the white solid product in sufficient excess to decompose the whole of the sal-ammoniac, evaporating the resulting liquid to dryness over sulphuric acid under the receiver of an air pump, and extracting the residue by absolute alcohol. The alcoholic extract having been evaporated to dryness and dissolved in a little water, sufficient ammonium carbonate must be added to precipitate any barium salt present. By adding strong nitric acid the urea can then be separated as its nitrate after sufficiently concentrating the filtrate. The treatment with ammonium carbonate cannot be omitted, because barium nitrate, which would be formed if the barium salts were not removed, is only very sparingly soluble in excess of nitric acid. The excess of ammonium carbonate used is mainly volatilised during the concentration, any small quantity which may remain in no way interferes with the reaction, as ammonium nitrate is very easily soluble in nitric acid." The precipitate thus obtained Natanson showed to be urea nitrate by analysis and by comparing its properties and behaviour with the salt otherwise prepared. He also isolated urea from it by decomposing it with potassium carbonate, and showed this to be identical with that obtained from urine. Although the exact constitution of urea gave rise to some discussion at the time when opinions regarding molecular structure were settling down into their present form, the view now universally held, that urea is the amide of carbonic acid and of symmetrical structure, was established by Natanson's work, and no objection deserving of serious attention has ever been made to it. The arguments which have from time to time been brought forward in favour of an unsymmetrical structure have been based upon a want of sufficient recognition of the fact that a complex molecule is not a rigid structure, and that every part affects the affinities and relationships of every other part. The two NH2 groups of urea ought not therefore to be expected to behave alike in all reactions, for since, in any given reaction, one must be first attacked, from this moment the properties of the other are modified, and consequently might not, indeed would not, behave exactly as the first had done. Wöhler's synthesis could not be immediately employed for the preparation of substituted ureas, as at the time, and for more than twenty years afterward, the compound ammonias were unknown; indeed, it is interesting to recall that it was from the compound ureas that the first substituted ammonias were prepared by Wurtz. Wurtz had synthesised methyl and ethyl cyanate, and tried, among other things, the action of a concentrated aqueous solution of ammonia upon them. By this treatment he obtained crystalline bodies which on investigation proved to be substituted ureas containing a methyl or an ethyl group in place of an atom of hydrogen. By heating these, which were solid substances easily isolated, with caustic potash, he obtained methylamine and ethylamine, another very notable discovery. In a note published fourteen years afterwards (Repertoire de Chimie | 123 Pure, 1862, iv., 199), Wurtz definitely states:-"It was in studying the decomposition of these ureas that I discovered the compound ammonias." Wurtz soon found that these and many similar primary amines could be formed directly by the action of potash on the cyanic esters. Having obtained the amines, Wurtz at once substituted them for ammonia in the Wohler synthesis, and thus obtained a whole series of substituted ureas. He states the case very clearly (Comptes Rendus, 1851, xxxii., 414): -"The different terms of this series are formed, like urea itself, by the reciprocal action of the elements of cyanic acid and the elements of any ammoniacal base whatever. If, for example, one allows cyanic acid to react upon methylamine, there is formed a substance which stands in the same relation to ordinary urea that methylamine does to ammonia; that is, it is urea in which an equivalent of hydrogen has been replaced by an equivalent of methyl. It is methyl urea. To prepare it, it is sufficient to evaporate to dryness a solution of equivalent quantities of sulphate of methylamine and of potassium cyanate, and to take up the residue by alcohol. The cyanate of methylamine which is formed by double decomposition undergoes by the action of heat a metamorphosis analogous to that which ammonium cyanate undergoes. It becomes transformed into a true urea, which only differs from ordinary urea by the elements of CH2." Wurtz had found that the action of ammonia on the cyanic esters which gave rise to the first substituted ureas was a general reaction, and continues:-"Nothing would be more easy thin to substitute for methylamine another ammonia, and to prepare by this method a series of bodies similar to urea in constitution and properties. It is more convenient, however, to obtain these bodies by another procedure which I indicated some years ago, and which consists in substituting for cyanic acid the cyanic ethers, and treating them directly with ammonia. Not only ordinary ammonia, but even the compound ammonias and certain soluble natural alkalis, react energetically with the cyanic ethers. The numerous compounds which one is thus able to obtain resemble urea closely in their more obvious characteristics; thus they are neutral to litmus ; they combine more or less easily with nitric acid, and under the influence of potash they split up into carbonic acid and ammonia. I call them compound ureas, because they are to be looked upon as ordinary urea in which one or more equivalents of hydrogen are replaced by one or more complex molecular groups.' In spite of various attempts, ammonium thiocyanate was not transformed into the sulphur analogue of urea until 1869, when Emerson Reynolds (Trans. Chem. Soc., 1869, xxii., 2) showed that although, owing to the greater stability of ammonium thiocyanate, intramolecular rearrangement did not take place at 100° C., it took place at temperatures above 180° C. With this synthesis of thiourea one important chapter in the history of the urea group was concluded. That much remains to be discovered, even regarding urea itself, is shown by the recent synthesis of its dichloro derivative. Substitution of Magnesium for Zinc in the Synthesis of Unsaturated Alcohols.-W. Jaworsky.Magnesium can conveniently be used in place of zinc for the preparation of unsaturated alcohols by Saytzew's method under certain conditions. The reaction is performed in one phase, without previous preparation of the organo-metallic compound, and takes place in absolute The mixture of allyl haloid and carbonyl compound must be added drop by drop at a rate regulated by the energy of the reaction, and the mixture may be dissolved in ether if the reaction is very rapid.-Berichte, xlii., No. 2. ether. * Hofmann had somewhat earlier (Ann., 1845, liii., 57; and 1846, Ivii., 265) prepared phenyl urea by the interaction of aniline sulphate and potassium cyanate, A SIMPLE NOTATION FOR INDICATING THE CONFIGURATION OF THE SUGARS AND ALLIED SUBSTANCES. and By T. S. PATTERSON. purpose = the configuration of substances belonging to the sugar It is common knowledge that for the of indicating group, Emil Fischer has suggested the use of the signs + in a given molecule; thus, d-glucose to represent the position of the OH groups hexapentolal +-++; d-saccharic acid = hexantetroldiacid +++ (or +-); d-fructose = hexanpentol-2-on ++, &c. (E. Fischer, Ber., 1894, xxvii., 3222); and that an attempt has been made by Lespieau (Bull. Soc. Chim., 1895, [3], xiii., 105), and another by Maquenne ("Les Sucres et leur Principaux Dérivés," Paris, 1900), to introduce an even more comprehensive nomenclature. None of these methods, however, is of very great assistance to the memory. The two latter fail, perhaps, on account of their very comprehensiveness, and the first is not fully satisfactory, possibly because our minds are not trained to memorise a sequence of and signs. We have, however, become accustomed to register the positions of radicles by associating them with a sequence of letters or of figures, and in the following suggestion an attempt is made to translate, in as simple a way as possible, Emil Fischer's method into figures, somewhat after the manner of Lespieau and Maquenne. For this purpose the following conventions, which for the most part are already in use, are adopted. 1. The empirical names of the compounds under consideration are retained. The present scheme makes no attempt, therefore, to indicate the chemical constitution of a given compound, and it thus remains necessary to remember, e.g., that arabinose and lyxose are aldehydes with a chain of five carbon atoms; that erythulose, fructose, sorbose, tagatose are ketones; that dulcite is entirely an alcohol; that mucic acid is a dibasic acid, and so on. 2. It is clear that it is only necessary to remember the configuration of one of any pair of antimeres. We may therefore fix our attention on the d-compounds. 3. Writing, e.g., the formula for d-arabinose in Emil Fischer's manner, it is obviously only necessary to consider the distribution of the H- and -OH groups on one side of the formula. We may therefore select the right-hand side. hot vapours in passing through D thoroughly extract the substance which it contains, and are then condensed in the double surfaced condenser E, and thence return to the flask. For safety purposes a small side arm, G, open to the air, is fused into the adapter F, and thus prevents the generation of pressure in the apparatus. If the positions of D and E are interchanged, the solid in D can then be extracted by percolation by the cold liquid condensed in E. The whole is very simple, and has been found extremely useful in many cases. Chemical Laboratory, The Technical College, Derby. 4. Since only two radicals, H- and -OH, come into consideration, we need only take account of one of these, and, following Emil Fischer, we choose the OH groups. 5. If, then, the most highly oxidised end of the chain is always placed uppermost, the configuration of a compound may be accurately defined if we indicate by a symbol the * The reason for so doing has been expressed by Emil Fischer (Ber., 1907, xl., 103) as follows:-"Meine erwartung das diese rationellen Namen bald die alten empirischen Ausdrücke in den Hintergrund drängen würden, ist aber nicht eingetroffen. Im Gegenteil, für neu entdeckte Zucker mit geringerem Kohlenstoffgehalt sind seitdem immer noch empirische Namen gewählt worden, und ich ziehe daraus den Schluss, dass diese der Mehrzahl der Fachgenossen sympathischer sind, als die rationellen Ausdrücke, die man sich zum Verstandniss immer erst in die konfigurationsformel übersetzen muss." |