LIAISON in ORGANIC- INORGANIC 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 num- ber 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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T HE problem of bridging the gap between inor- ganic 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, ex- cellent as they are. The usual textbook explains by way of introduction the origin of the term "organic chemistry" and the vital force conception. Then fol- lows an account of Wohler's classical experiment on urea, and the conclusion is triumphantly made that with this experiment the barrier between inorganic chemistry and organic chemistry was razed, and that henceforth it was evident that the same laws of inor- ganic chemistry also govern the behavior of organic phenomena. To the student, however, it is not always so apparent how the laws he learned in inorganic chem- istry apply to the carbon compounds. He is apt to feel that his previous study of chemistry is of little use in the organic field with the result that his approach to success in the subject is stymied by a mental hazard.

The present paper attempts to offer another slant on the transition from inorganic to organic chem- istry. It is not presumed that the author has new information for the advanced student, but it is hoped that the Dresent discussion mav serve as reference

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 chem- istry 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 ac- curately as it does in inorganic synthesis or decompo- sition. The law of definite composition of pure chemi- cal compounds likewise holds equally well for or- ganic 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 liquid- in-solid and liquid-in-liquid disperse systems, obeys the same generalizations whether the systems are or- ganic 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 be- tween the two arbitrary divisions of chemistry; and the same thing may be said for Faraday's laws of elec- tricity, the elevation of the boiling point and the de- pression 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.

ORGANIC CHEMISTRY PRESENTS EXTREME CASES

The items just mentioned present no particular difficulty as creating a gap between organic and in- organic chemistry. Indeed, they should be viewed as tending to close that gap which is, as we have said, only a mental hazard. But there are other difficulties which serve to set off organic chemistry rather sharply from general chemistry, as for instance reactions and isomerism.

Concerning these other laws and principles which tend to present difficulties it should be observed that in organic chemistry the laws of general chemistry may mangest themselves i n either gigantic or dwarfish forms. Thus, for examde, the tendency of like kinds of atoms

material for the undergraduate Btudent. to link together to form chains is only weakly de- 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 com- paratively rare, although they do occur to a minor extent, in organic chemistry. Williams (6) has ex- pressed 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 chem- istry. The law may he stated, Whenever gases com- bine, 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 nttmh~rc: .. --. . - - -.

DALTON'S LAW OF MULTIPLE PROPORTIONS. This law, even more than the preceding one, emphasizes the enlarged aspects in organic chemistry of the sim- ple law appropriate to 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 mem- bers 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 hydro- gen 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 chem-

silicon to silicon is endothermic whereas the carhon- to-carbon linkage is exothermic. Silicon does, how- ever, form a chain of six atoms in silicohexane [(lo), p. 9631. Sulfur forms chains containing up to 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 mole- cule. Nitrogen forms chains of two nitrogen atoms in some nitrides and in hydrazine: in hydrazoic (or hydronitric) acid three nitrogen atoms are hound to- gether; four nitrogen atoms are linked together in ammonium azide, and five nitrogen atoms form the chain in hydrazine azide [(lo), p. 6731, while com- pounds 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, to 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 chem- istry is due in no small part to 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 in- organic chemistry, yet one can find in inorganic chem- istry 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 in- tended for students of organic chemistry.

Metamers, such as dimethyl ether and ethyl alco- hol, find their counterparts in inorganic chemistry, as Bailar points out, in a number of examples such as ammonium nitrate and hydroxylamine nitrite. Po- sition 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.

istry may obscure its applications to organic chemis- try. 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 chem- istry.

CHAINING. The capacity for forming long chains of atoms that carhon possesses is very characteristic of

0 organic compounds, but it is not a phenomenon unique with carbon. Other elements such as oxygen, nitro- gen, sulfur, silicon, germanium, and tin are able to Nuclear isomerism, that is, isomerism due to hranch- link together, but the tendency is so slightly mani- ing of chains of like atoms, while abundant in organic fested as to be largely ignored in the inorganic courses. chemistry fails of representation, so far as the author Holleman (9) explains this failure to form chains in is aware, among the inorganic compounds due to their the case of silicon as due to the fact that linkage of slight tendency to form chains.

Isomerism of the multiple bond, such as that be- tween the alkines and the diolefines, likewise is not found among the inorganic compounds for the reason given above, although the existence of unsaturated silicon compounds has been reported.

Linkage isomerism, corresponding to that in nitro- ethane and ethyl nitrite, are suggested by Bailar, al- though in each case one isomer of his pairs is hypo- thetical. Analogous nitro and nitrite compounds, however, do occur among the complex cobalt salts. Thus there are the brownish yellow [Co(NH&- N02]C12 (nitropentamminecobalti chloride) and the red [CO(NH~)~-O-N=O]C~~ (nitritopentamminecobalti chloride) [(13), p. 401 as well as [CO(NH~)~SCN]CI~ (thiocyanatopentamminecobalti chloride) and [Co- (NHs)sNCS]C12 (isothiocyanatopentamminecobalti chloride). If Arbeiter's formulas for pyrite and marcasite (see under cyclic compounds) are correct, then they too may be considered as examples of linkage isomerism.

Clear-cut examples of the various manifestations of tautomerism in inorganic chemistry are not easy to find. Indeed, there is considerable confusion in the definition of terms in this subject, but using the con- cepts set forth in Henrich's "Theories of Organic Chemistry" (14) we may list hydrogen trisulfide as assumed to be allelotropic [(lo), p. 4991; sodium potassium sulfite, sodium bisulfite, and potassium cya- nide are pseudomeric; and hydrocyanic acid, nitrous acid, and phosphorous acid are either pseudomeric or allelotropic. Ammonium cyanate-urea and am- monium thiocyanate-thiourea, especially the latter pair, are desmotropic, although in neither case do we have strictly inorganic pairs of compounds.

When we come to polymerism we are on much more definite experimental ground than in the previous ex- amples. Thus there are dimers, trimers, tetramers, and pentamers of the simple formula [CO(NH&(NO~)~] [(12), p. 461. Such for instance (to give examples of the dimers only), are the compounds [Co(NH& (NO&] [CO(NH~)~(NO~)~] (dinitrotetramminecobalti tetranitrodiamminecobaltiate) and [Co(NH&] [Co- (NO&] (hexamminecobalti hexanitrocobaltiate).

Spacial or physical isomerism is well established in inorganic chemistry. The several examples of cis- trans and optical isomerism mentioned by Bailar can be expanded almost a t will. Thus, for instance, there are an even dozen isomers, including the racemates, of the compound [C~(NH~)~(en)(pn)]Cb where "en" means ethylene diamine and "pn" means propylene diamine. All of these compounds and racemates have been fully characterized [(12), p. 561. There are also 15 more geometric and crystallographic isomers, de- pending on the proximity or remoteness to the plane of the methyl group in propylene diamine in the case of the cis compounds, or a total of 27 isomeric forms of this one formula. It might also be mentioned that one of the optically active complex cobalt compounds, tetraethylenediamine - N-peroxo-mono - iminodicobaltic nitrate

has the highest molecular rotatory power of any known active compound, 68,550. (Something similar might be said for its name!)

Bailar mentions the polymorphic modifications of many inorganic compounds. These may be matched in part by organic compounds such as the crystallo- graphic variations of cinnamic acid.

We have indicated that in inorganic chemistry one may find types of isomerism unknown, or a t least only slightly represented in organic chemistry. Of these, the electromers form an interesting theoretical type, but experimentally there is little evidence that they exist. Ionization isomers do, however, exist in both fields, but only rarely in organic chemistry. Thus, for example, bromine is an ion-former in [CO(NH~)~- SO,]Br, (sulfatopentamminecobalti bromide), but not in [Co(NH3)sBr]SOn (hromopentamminecobalti sul- fate). In the organic field the same is true for p- chloroaniline hydrobromide and 9-bromoaniline hydro- chloride, respectively.

Hydration isomers, such as the hydrated chromic chlorides, find no corresponding examples among the carbon compounds because these do not form coordi- nation compounds. For the same reason organic chemistry can have no coordination isomers such as [CO(SH;)~][C~(CN)~] (hcxmminccobalti hexacyano- chromiate) and [Cr(NH&][Co(CN)sj (hcxamminc- chromi hexacyanocobaltiate).

In this discussion of isomerism no effort has been made to be exhaustive, and the capable instructor or enterprising student can multiply these examples a t will by reference to any good text on complex com- pounds. It has been the intention, however, to sup- port the contention that inorganic isomerism is as extensive and as interesting as organic isomerism and that, in turn, both types are manifestations of one underlying phenomenon of chemistry.

MOLECULAR REARRANGEMENTS. Proof of the struc- tures of compounds, particularly of isomers, must be based on the assumption that atoms occupy definite positions within the molecule. There are, neverthe- less, many examples in organic chemistty of the la- bility, the mobility, or wandering of atoms within the molecule. Tautomerism represents one type of molecu- lar rearrangement. Molecular rearrangements have been observed in a number of cases among the inor- ganic compounds [(13), p. 771. Thus when the trans form of [ C O ( H ~ O ) ~ ( ~ ~ ) ~ ] C I ~ is allowed to stand for some time as the dry salt it gradually passes over almost entirely into the cis form of [C~Cl~(en)~]Cl. This means that a t least one of the nitrogen atoms in one of the ethylene diamine groups has changed its position in the molecule in order that two chlorine atoms should be side by side as they are in the cis compound. And again if the cis form of [CO(H~O)~- (en),]C18 is heated with concentrated hydrochloric acid the trans form of [C~Cl~(en)~]Cl is formed exclusively.

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. Hy- drogen is singly bound to nitrogen in ammonia; oxygen is doubly linked to nitrogen in nitric oxide; and alumi- num must be triply linked to nitrogen in AlN. Link- age 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 five- membered 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 chem- istry.

POLAR AND NON-POLAR COMPOUNDS. The reactions of the ionogens receive extended treatment in all in- organic courses, but little is said of non-ionic reac- tions. There are, however, many non-polar inorganic compounds such as the acid chlorides and the acid anhydrides as well as other compounds. And con- versely 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 chem- istry and organic as non-polar chemistry. This mat- ter 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 char- acteristic of inorganic compounds, for many organic compounds also ionize. Thus, for example, trichloro- acetic 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 or- ganic field.

INSTABILITY OF POLY-HYDROXYL COMPOUNDS. A generalization is made in organic chemistry that com- pounds having more than one hydroxyl group at- tached to a single carbon atom are spontaneously un- stable. Exceptions to this generalization are found in the cases of carbonic acid, chloral hydrate, and gly- collic aldehyde. The same generalization applies with somewhat less rigor to polyhydroxyl inorganic com- pounds. 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 dehy- drated hydroxides; silica is determined quantitatively by the dehydration of silicic acid and, in general, hy- droxyl 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 or- ganic reactions involves inorganic substances either as reagents, catalysts, or by-products. And this fact serves admirably to illustrate and emphasize the inter- connection and interdependence of all chemistry, whether organic or inorganic.

CONCLUSION

Nearly all the organic textbooks, in an early chap- ter, give as one of the reasons for a separate course in the chemistry of the carbon compounds the argu- ment 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 or- ganic chemistry were to emphasize that in carbon chemistry we study some exaggerated phases of chemi- cal phenomena, rather than to emphasize the dif- ferences in organic and inorganic chemistry. Adop- tion of this point of view might give the idea of transi- tion 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 chem- istry. 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 car- bon chemistry, instruction in advanced phases of in- organic chemistry. For it is erroneous to teach a set of phenomena as being unique with the carbon com- pounds.

LITERATURE CITED

(1) KHARASCH AND REINMUTH. "The Electron in Organic Chemistry. I," J. CHEM. EDUC., 5, 404 (Apr., 1928); and "11," ibid.. 8.1703 (Seot.. 1931).

(2j 'KRAUS, "The ~dorganic Side of Organic Chemistry," ibid.. 6.1478 (Scot.. 1929).

(3) 'ADAM;, he lntkductory Course in Organic Chemistry," ibid., 4,1003 (Aug., 1927).

(4) WHITMORE, "HOW Much Organic Chemistry Should Be Included in the General Chemistry Course?" ibid., 4,1006 (Aug., 1477) ,.

(5) Discussion of the previous paper. Ibid., 4, 1007-8 (Aug., 1927).

(6) WILLIAMS. 'Tonization and the Atomic Structure Theorv in Gganic Chemistry," ibid., 4,867 (July, 1927).

(7) SABXPEY, "The Teaching of First-Year Organic Chemis- try," ihd. , 7,2115 (Sept., 1930).

(8) LADENBURG. "History of Chemistrv." Universitv of Chi-

p. 86. (10) MELLOR, "Modern Inorganic Chemistry," Longmans,

Green & Co., New York City. 1925,1104 pp. (11) B A ~ A R , "A Study of Isomerism in General Chemistry,"

J. CHEM. EDUC., 8,310 (Feb., 1931). ,(12) THOMAS, ''Complex Salts," D. van Nostrand, New York

City, 1924, 122 pp. (13) WEINLAND, "Einfiihrung in der Chemie der Kompler-

verbindungen," Verlag von Ferdinand Enke, Stuttgart, 1919, 441 PP.

(14) HENRICH, JOHNSON, AND HAHN, "Theories of Organic Chemistry," John Wiley & Sons, Inc., New York City, 1922, 603 pp.

(15) Auonmre, "A Classification of the Compounds of Nitro- gen," J. CHEM. EDUC., 7,2055 (Sept., 1930).

(16) NEWELL, "The Centenary of Cannizzaro," ibid., 3, 1364 (Dec., 1926).