LIAISON in ORGANICINORGANIC CHEMISTRY GEORGE W. BENNETT Grove City College, Grove City, Pennsylvania
This paper i s a contribution to the effort to bridge the gap between inorgallic and organic chemistry. A number of laws and principles applicable with equul farce to both branches of chemistry are cited. The idea i s then presented that in organic chemistry some of the laws learned in gsneral chemistry manifest themselves in a greatly exaggerated manner assuming either gigantic or dwarfish dimensions. Examples are offered to illustrate this fact, and the conclusion i s drawn that such information may be used to bridge the gap between inorgawic and organic chembtry.
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CONCORDANCE IN ORGANIC-INORGANIC CHEMISTRY
The instructor may lay the basis for some confidence in the dictum that inorganic and organic behaviors obey the same laws if he will point out that many of the laws and principles learned in inorganic chemistry apply with approximately the same exactitude in regard to organic chemistry. Thus, for example, the conservation of organic matter is as true as the conservation of inorganic matter, and the law of the conservation of energy holds when carbon compounds burn or undergo other transformations just as accurately as it does in inorganic synthesis or decomposition. The law of definite composition of pure chemical compounds likewise holds equally well for organic and inorganic compounds. So also does the mass action principle, the kinetic theory, and the gas laws of Boyle, Charles, Gay-Lussac, Avogadro, and Graham. Colloidal behavior, particularly of the liquidin-solid and liquid-in-liquid disperse systems, obeys the same generalizations whether the systems are organic or inorganic in whole or in part. Modern theories of valence, atomic linking, and crystal structure are also intended to hold for all classes of compounds. And again, the Phase Rule does not differentiate between the two arbitrary divisions of chemistry; and the same thing may be said for Faraday's laws of electricity, the elevation of the boiling point and the depression of the freezing point of solvents by solutes, and other physico-chemical generalizations that may not be included in the beginning chemistry course. It is apparent, then, that many of the generalizations learned in beginning chemistry may be carried over bodily, and with confidence, as the groundwork for organic chemistry.
HE problem of bridging the gap between inorganic and organic chemistry is one of perennial interest because the literature and the pedagogy of chemistry have not yet discovered a happy means to this end. Neophytes in organic chemistry each autumn, however, are face to face with the problem of passing from two years of inorganic chemistry over into the subject of carbon chemistry and, more often than not, the passage requires a tremendous act of faith. Many instructors, doubtless, are easing the way for their charges as is evidenced by a considerable reference to such efforts in the journal literature (I), (21, (31, (4h (5h (61, (7). Granted that instructors are bridging the gap into organic chemistry, the same cannot be said with much enthusiasm for modem textbooks in the subject, excellent as they are. The usual textbook explains by way of introduction the origin of the term "organic chemistry" and the vital force conception. Then follows an account of Wohler's classical experiment on urea, and the conclusion is triumphantly made that with this experiment the barrier between inorganic ORGANIC CHEMISTRY PRESENTS EXTREME CASES chemistry and organic chemistry was razed, and that henceforth it was evident that the same laws of inorThe items just mentioned present no particular ganic chemistry also govern the behavior of organic difficulty as creating a gap between organic and inphenomena. To the student, however, it is not always organic chemistry. Indeed, they should be viewed so apparent how the laws he learned in inorganic chem- as tending to close that gap which is, as we have said, istry apply to the carbon compounds. He is apt to only a mental hazard. But there are other difficulties feel that his previous study of chemistry is of little use which serve to set off organic chemistry rather sharply in the organic field with the result that his approach from general chemistry, as for instance reactions and to success in the subject is stymied by a mental hazard. isomerism. Concerning these other laws and principles which The present paper attempts to offer another slant on the transition from inorganic to organic chem- tend to present difficulties it should be observed that istry. It is not presumed that the author has new in organic chemistry the laws of general chemistry may information for the advanced student, but it is hoped mangest themselves i n either gigantic or dwarfish forms. that the Dresent discussion mav serve as reference Thus, for examde, the tendency of like kinds of atoms to link together to form chains is only weakly dematerial for the undergraduate Btudent. 20
veloped in inorganic compounds, but i t is developed to an enormous extent in organic compounds. And again, on the other hand, instantaneous ionic reactions which are common in inorganic chemistry are comparatively rare, although they do occur to a minor extent, in organic chemistry. Williams (6) has expressed this idea somewhat differently by pointing out that organic chemistry is replete with many cases of extreme behavior, such as complexity of molecular structure and unreactivity of compounds. Some of these extreme cases are well worth examining. SOME ANALOGOUS ORGANIC AND INORGANIC PHENOMENA
GAY-~ussnc's LAW OF COMBINING VOLUMES. This law will serve to illustrate how the laws of general chemistry assume large dimensions in organic chemistry. The law may he stated, Whenever gases combine, the volumes of the reactants and of the products bear the relation of small whole numbers to each other. But in the combustion of nonane, for example, we have
and it follows that for organic reactions the ratio of volumes may need to be expressed by large whole
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nttmh~rc: . .--. .- -. LAW OF MULTIPLE PROPORTIONS.This law, even more than the preceding one, emphasizes the enlarged aspects in organic chemistry of the simple law appropriate t o general chemistry. The law states, Whenever two elements combine to form more than one compound the weights of one, which combine with afixed weight of the other, bear the relation of small whole numbers to each other. In an homologous series of organic compounds, however, as the paraffins, the numbers are certainly not small numbers, or else they are not whole numbers. Thus for the first ten members of this series the ratio of the weights of carhon per 1.008 grams of hydrogen are (with rounding off in two instances): 60, 80, 90, 96, 100, 103, 105, 107, 108, and 109. If one calculates the weights of hydrogen per 12 grams of carbon the ratios of the weights are very large, but they may he rounded into: 403, 302, 269, 252, 242, 235, 229, 227, 224, and 222. Here again the wording of the law as used in general chemistry may obscure its applications to organic chemistry. Even the great Berzelius (8) for a while was not sure whether or not this law held in organic chemistry. And Dumas, we are told (16) insisted that the same laws do not govern both organic and inorganic chemistry. CHAINING. The capacity for forming long chains of atoms that carhon possesses is very characteristic of organic compounds, but it is not a phenomenon unique with carbon. Other elements such as oxygen, nitrogen, sulfur, silicon, germanium, and tin are able to link together, but the tendency is so slightly manifested as to be largely ignored in the inorganic courses. Holleman (9) explains this failure to form chains in the case of silicon as due to the fact that linkage of
DALTON'S
silicon to silicon is endothermic whereas the carhonto-carbon linkage is exothermic. Silicon does, however, form a chain of six atoms in silicohexane [(lo), p. 9631. Sulfur forms chains containing up t o six atoms of sulfur in the polythionic acids [(lo), p. 5491, and the polysulfides may have as many as nine atoms of sulfur in the molecule [(lo), p. 4891, although there is no agreement as to the atomic linkages in the molecule. Nitrogen forms chains of two nitrogen atoms in some nitrides and in hydrazine: in hydrazoic (or hydronitric) acid three nitrogen atoms are hound together; four nitrogen atoms are linked together in ammonium azide, and five nitrogen atoms form the chain in hydrazine azide [(lo), p. 6731, while compounds containing eight nitrogen atoms in one chain are also known (15). Kraus, in a thought-provoking article (2), describes a compound in which five atoms of tin form a chain. These illustrations will serve, then, t o show once again how a general principle of inorganic chemistry has grown to gigantic dimensions in the organic field. ISOMERISM. The enormous extent of organic chemistry is due in no small part t o the phenomenon of isomerism. Isomerism in turn has its roots in the capacity of carbon atoms to form chains. Isomerism is rarely even so much as mentioned in courses in inorganic chemistry, yet one can find in inorganic chemistry examples of practically every type of isomerism that carbon compounds exhibit and even a few other types unknown to organic chemistry. Bailar (11) has discussed isomerism among inorganic compounds in an excellent article which we wish to supplement by additional examples, since that paper was intended for inorganic students while the present paper is intended for students of organic chemistry. Metamers, such as dimethyl ether and ethyl alcohol, find their counterparts in inorganic chemistry, as Bailar points out, in a number of examples such as ammonium nitrate and hydroxylamine nitrite. Position isomerism is illustrated by the two forms of dichlorodiammine platinum. These latter compounds might also be viewed as cis-trans isomers. The two forms of the complex salt of cobalt and glycine [(12), p. 431 are, however. position isomers.
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Nuclear isomerism, that is, isomerism due to hranching of chains of like atoms, while abundant in organic chemistry fails of representation, so far as the author is aware, among the inorganic compounds due to their slight tendency to form chains.
Isomerism of the multiple bond, such as that between the alkines and the diolefines, likewise is not found among the inorganic compounds for the reason given above, although the existence of unsaturated has the highest molecular rotatory power of any known active compound, 68,550. (Something similar might silicon compounds has been reported. Linkage isomerism, corresponding to that in nitro- be said for its name!) Bailar mentions the polymorphic modifications of ethane and ethyl nitrite, are suggested by Bailar, although in each case one isomer of his pairs is hypo- many inorganic compounds. These may be matched thetical. Analogous nitro and nitrite compounds, in part by organic compounds such as the crystallohowever, do occur among the complex cobalt salts. graphic variations of cinnamic acid. We have indicated that in inorganic chemistry one Thus there are the brownish yellow [Co(NH&N02]C12(nitropentamminecobalti chloride) and the red may find types of isomerism unknown, or a t least only [CO(NH~)~-O-N=O]C~~ (nitritopentamminecobalti slightly represented in organic chemistry. Of these, the electromers form an interesting theoretical type, chloride) [(13), p. 401 as well as [CO(NH~)~SCN]CI~ (thiocyanatopentamminecobalti chloride) and [Co- but experimentally there is little evidence that they (NHs)sNCS]C12 (isothiocyanatopentamminecobalti exist. Ionization isomers do, however, exist in both chloride). If Arbeiter's formulas for pyrite and fields, but only rarely in organic chemistry. Thus, marcasite (see under cyclic compounds) are correct, for example, bromine is an ion-former in [CO(NH~)~then they too may be considered as examples of linkage SO,]Br, (sulfatopentamminecobalti bromide), but not in [Co(NH3)sBr]SOn (hromopentamminecobalti sulisomerism. Clear-cut examples of the various manifestations of fate). In the organic field the same is true for ptautomerism in inorganic chemistry are not easy to chloroaniline hydrobromide and 9-bromoaniline hydrofind. Indeed, there is considerable confusion in the chloride, respectively. Hydration isomers, such as the hydrated chromic definition of terms in this subject, but using the concepts set forth in Henrich's "Theories of Organic chlorides, find no corresponding examples among the Chemistry" (14) we may list hydrogen trisulfide as carbon compounds because these do not form coordiassumed to be allelotropic [(lo), p. 4991; sodium nation compounds. For the same reason organic potassium sulfite, sodium bisulfite, and potassium cya- chemistry can have no coordination isomers such as )~] hexacyanonide are pseudomeric; and hydrocyanic acid, nitrous [ C O ( S H ; ) ~ ] [ C ~ ( C N(hcxmminccobalti acid, and phosphorous acid are either pseudomeric chromiate) and [Cr(NH&][Co(CN)sj (hcxammincor allelotropic. Ammonium cyanate-urea and am- chromi hexacyanocobaltiate). In this discussion of isomerism no effort has been monium thiocyanate-thiourea, especially the latter pair, are desmotropic, although in neither case do we have made to be exhaustive, and the capable instructor or enterprising student can multiply these examples a t strictly inorganic pairs of compounds. When we come to polymerism we are on much more will by reference to any good text on complex comdefinite experimental ground than in the previous ex- pounds. It has been the intention, however, to supamples. Thus there are dimers, trimers, tetramers, port the contention that inorganic isomerism is as and pentamers of the simple formula [CO(NH&(NO~)~]extensive and as interesting as organic isomerism [(12), p. 461. Such for instance (to give examples and that, in turn, both types are manifestations of one of the dimers only), are the compounds [Co(NH& underlying phenomenon of chemistry. MOLECULAR REARRANGEMENTS. Proof of the struc(NO&] [ C O ( N H ~ ) ~ ( N O ~(dinitrotetramminecobalti )~] tetranitrodiamminecobaltiate) and [Co(NH&] [Co- tures of compounds, particularly of isomers, must be based on the assumption that atoms occupy definite (NO&] (hexamminecobalti hexanitrocobaltiate). Spacial or physical isomerism is well established in positions within the molecule. There are, nevertheinorganic chemistry. The several examples of cis- less, many examples in organic chemistty of the latrans and optical isomerism mentioned by Bailar can bility, the mobility, or wandering of atoms within the be expanded almost a t will. Thus, for instance, there molecule. Tautomerism represents one type of molecuare an even dozen isomers, including the racemates, lar rearrangement. Molecular rearrangements have of the compound [C~(NH~)~(en)(pn)]Cb where "en" been observed in a number of cases among the inormeans ethylene diamine and "pn" means propylene ganic compounds [(13), p. 771. Thus when the trans diamine. All of these compounds and racemates have form of [ C O ( H ~ O ) ~ ( ~ ~is) ~allowed ] C I ~ to stand for been fully characterized [(12), p. 561. There are also some time as the dry salt it gradually passes over 15 more geometric and crystallographic isomers, de- almost entirely into the cis form of [C~Cl~(en)~]Cl. pending on the proximity or remoteness to the plane This means that a t least one of the nitrogen atoms in of the methyl group in propylene diamine in the case one of the ethylene diamine groups has changed its of the cis compounds, or a total of 27 isomeric forms position in the molecule in order that two chlorine of this one formula. It might also be mentioned that atoms should be side by side as they are in the cis one of the optically active complex cobalt compounds, compound. And again if the cis form of [CO(H~O)~tetraethylenediamine - N-peroxo-mono iminodicobaltic (en),]C18 is heated with concentrated hydrochloric acid the trans form of [C~Cl~(en)~]Cl is formed exclusively. nitrate
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These examples can be extended by additional cases. SINGLE AND MULTIPLE VALENCE LINKING. Carbon atoms , m y link (according to the conventional Kekule representation) by single, double, or triple bonds. Whatever convention of valence is used in discussing carbon compounds, counterparts of multiple linking may be found among inorganic compounds also. Hydrogen is singly bound to nitrogen in ammonia; oxygen is doubly linked to nitrogen in nitric oxide; and aluminum must be triply linked to nitrogen in AlN. Linkage by single or multiple bonds is a phenomenon of minor importance which is not particularly over- or under-developed in either branch of chemistry. CYCLIC COMPOUNDS. Closed chains of atoms or cyclic compounds abound in organic chemistry. These cycles or rings may be made up of like or unlike atoms. They have their inorganic counterparts, perhaps not very numerous, but they exist. Ozone probably is a ring of three atoms of oxygen; nitrogen may perhaps form a cycle of three atoms of nitrogen in hydrazoic acid; nitrous oxide is a three-membered heterocyclic ring; sulfur heptoxide is probably a heterocyclic fivemembered ring; while trisulfimide and, according to Arbeiter [(lo), p. 495, p. 6671, certain sulfide minerals probably have the following atomic arrangements:
These examples will, perhaps, suftice to indicate that cyclic formation is also a general phenomenon of chemistry. POLAR AND NON-POLAR COMPOUNDS. The reactions of the ionogens receive extended treatment in all inorganic courses, but little is said of non-ionic reactions. There are, however, many non-polar inorganic compounds such as the acid chlorides and the acid anhydrides as well as other compounds. And conversely there are in organic chemistry many polar compounds as well as the non-polar. It is therefore incorrect to refer to inorganic chemistry as polar chemistry and organic as non-polar chemistry. This matter is discussed at length by Kharasch and Reinmuth (1) and briefly by Williams (6). IONIZATION. Most inorganic compounds ionize to a greater or less degree, but this is not a unique characteristic of inorganic compounds, for many organic compounds also ionize. Thus, for example, trichloroacetic acid is more highly ionized than most of the weak inorganic acids. And again the quaternary alkyl ammoniun bases are as highly ionized as sodium or potassium hydroxide. Ionic reactions may well serve to illustrate a principle of inorganic chemistry that has dwindled to dwarfish dimensions in the organic field.
INSTABILITY OF POLY-HYDROXYL COMPOUNDS. A generalization is made in organic chemistry that compounds having more than one hydroxyl group attached to a single carbon atom are spontaneously unstable. Exceptions to this generalization are found in the cases of carbonic acid, chloral hydrate, and glycollic aldehyde. The same generalization applies with somewhat less rigor to polyhydroxyl inorganic compounds. Thus blue cupric hydroxide passes over into black cupric oxide at the temperature of boiling water; the trivalent metal hydroxides are generally considered to be simply hydrated oxides, that is, partially dehydrated hydroxides; silica is determined quantitatively by the dehydration of silicic acid and, in general, hydroxyl compounds more or less readily lose water to form oxides. INORGANIC REAGENTS IN ORGANIC REACTIONS. AS a final item we may point out that the great bulk of organic reactions involves inorganic substances either as reagents, catalysts, or by-products. And this fact serves admirably to illustrate and emphasize the interconnection and interdependence of all chemistry, whether organic or inorganic. CONCLUSION
Nearly all the organic textbooks, in an early chapter, give as one of the reasons for a separate course in the chemistry of the carbon compounds the argument that carbon chemistry is very different from other chemistry. There follows then, usually, an enumeration of the outstanding differences, and the student often gets the idea that these variations are absolute rather than relative. Some of the textbooks mention that these differences are only of degree and not of kind, but few emphasize this fact. I t would seem to be a more rational approach, and perhaps better pedagogy, if textbooks and instructors in organic chemistry were to emphasize that in carbon chemistry we study some exaggerated phases of chemical phenomena, rather than to emphasize the differences in organic and inorganic chemistry. Adoption of this point of view might give the idea of transition from inorganic to organic chemistry rather than an abrupt jump over a wide gap. It may perhaps be objected that the examples used to illustrate the various arguments are taken from obscure and uncommon chapters of inorganic chemistry. Alert instructors, doubtless, can improve on the illustrations selected. At best, however, many of the examples will have to come from paragraphs little studied by the average inorganic student. Are we, then, to suggest that sufficient of this, at present, obscure inorganic chemistry should be included in the general course so that the organic instructor might refer to it when developing his course in the carbon chemistry? It is devoutly to be wished! Heavily burdened instructors in general chemistry may groan a protest at this suggestion, pointing out, as they groan, that the course is voluminous enough as it stands. Long-suffering "organischers" will be sympa-
thetic with their inorganic brethren, but if these will not assume the burden of laying the groundwork, then the organic instructor will have to add to his load of carbon chemistry, instruction in advanced phases of inorganic chemistry. For it is erroneous to teach a set of phenomena as being unique with the carbon compounds. LITERATURE CITED AND REINMUTH. "The Electron in Organic (1) KHARASCH Chemistry. I," J. CHEM.EDUC.,5, 404 (Apr., 1928); and "11," ibid.. 8.1703 (Seot.. 1931). (2j 'KRAUS,"The ~dorganicSide of Organic Chemistry," ibid.. 6.1478 (Scot.. 1929). (3) 'ADAM;, he lntkductory Course in Organic Chemistry," ibid., 4,1003 (Aug., 1927). "HOW Much Organic Chemistry Should Be (4) WHITMORE, Included in the General Chemistry Course?" ibid., 4,1006 (Aug.,
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(5) Discussion of the previous paper. Ibid.,4, 1007-8 (Aug., 1927).
'Tonization and the Atomic Structure Theorv (6) WILLIAMS. in Gganic Chemistry," ibid., 4,867 (July, 1927). (7) SABXPEY, "The Teaching of First-Year Organic Chemistry," ihd., 7,2115 (Sept., 1930). "History of Chemistrv." Universitv of Chi(8) LADENBURG.
p. 86. (10) MELLOR,"Modern Inorganic Chemistry," Longmans, Green & Co., New York City. 1925,1104 pp. , Study of Isomerism in General Chemistry," (11) B A ~ A R"A J. CHEM.EDUC.,8,310 (Feb., 1931). ''Complex Salts," D. van Nostrand, New York ,(12) THOMAS, City, 1924, 122 pp. "Einfiihrung in der Chemie der Kompler(13) WEINLAND, verbindungen," Verlag von Ferdinand Enke, Stuttgart, 1919, 441 PP. AND HAHN,"Theories of Organic (14) HENRICH,JOHNSON, Chemistry," John Wiley & Sons, Inc., New York City, 1922, 603 pp. (15) Auonmre, "A Classification of the Compounds of Nitrogen," J. CHEM.EDUC.,7,2055 (Sept., 1930). (16) NEWELL, "The Centenary of Cannizzaro," ibid., 3, 1364 (Dec., 1926).
