FRIDAY, FEBRUARY 21, 1919

VALENCE1

THE theory of valence is one of the most important theories of chemistry. Scarcely any other except the atomic theory, with which it is inseparably connected, has been so fruitful in results which have led to industrial applications and also to the development of chemical knowledge. In spite of these results, which no one can dispute, the theory is more or less in disrepute, especially among physical chemists and students and teachers of inorganic chemistry. In many of our elementary text-books structural formulas are used so sparingly that they make no impression on the student and in some of them they are not even mentioned.

This attitude is due, in part, to a reaction from the overemphasis given to the subject at a time when nearly all chemists were working on the structure of organic compounds. It is due, also, to confused and conflicting ideas about the philosophy of science.

Some have gone so far as to claim that speculations and hypotheses form no part of genuine science. To such persons science is only an orderly description of phenomena which we can see and handle, which we can weigh and measure and connect by mathematical processes. An attempt to acquire knowledge about atoms and electrons and molecules, so long as they remain beyond the direct cognizance of our senses, may be interesting but to followers of this school such attempts form no part of science.

To an organic chemist the achievements in the determination of the structure of carbon compounds demonstrate the falsity of such a claim. It may be remarked, in passing, that

1 Address of the chairman and retiring vicepresident of Section C-Chemistry, American Association for the Advancement of Science, Baltimore, December 27, 1918.

the philosophy of science referred to easily leads to the conclusion that the discovery of new facts is of supreme value in science and that one is doing good scientific work when he adds a few facts to our already unwieldy accumulation of knowledge, whether the facts discovered have any valuable relation to fundamental principles or not.

Another school of philosophers contends that the number of explanations which will fit any given set of natural phenomena is infinite, and that, for this reason, any explanation which we use, as for instance, the Copernican system or the atomic theory, is purely a product of our imagination and that it is hopeless ever to arrive at a system which shall actually correspond to the realities of the universe. This, if I understand him correctly, was the point of view held by Poincaré. It is only a step from this to the conclusion that there is no reality outside of our own minds, for, surely, if we can never attain to a knowledge of realities outside ourselves, for all practical purposes such realities do not exist.

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A more true philosophy of science, as it seems to me, recognizes the intimate connection between speculation, hypothesis and theory on the one hand and the accurate study of phenomena on the other. Neither is complete or sufficient alone. Science advances most rapidly by what may be called a cutand-try" method. Speculation alone led to the useless dialectics of the school-men. A study of phenomena alone leads to an almost equally barren accumulation of facts for which we have no earthly use. It is inconceivable that chemistry, or indeed, that science as a whole could have made the progress that it has during the last century if Dalton, or some one else, had not given us the atomic hypothesis as a key for the study of chemical phenomena.

The subject of valence furnishes a particularly good illustration of the methods by which science advances. The positive achievements of the theory are so great that no one can doubt that there is some reality in the

relations of atoms which corresponds to the theory. At the same time our knowledge is so vague and indefinite at many points that we must consider the theory as still very unsatisfactory and in need of further develop

ment.

Ernst von Meyer has pointed out with some truth that the theory of valence is implied in the Law of Multiple Proportions. A somewhat more definite approach was made when Graham demonstrated the polybasic character of phosphoric acid. His results were expressed, however, in the old dualistic formulas in which one, two or three molecules of water of hydration in the acid were considered as replaced by one, two or three molecules of a metallic oxide. When Liebig introduced the idea that acids are compounds of hydrogen the notion of polybasic acids became still more definite and the fact that an atom of antimony may replace three atoms of hydrogen while an atom of potassium replaces only one was given a clear statement. During the same period the discovery of the chloroacetics acids by Dumas and the development of the theory of types by Gerhardt and others gave greater precision to our knowledge of the replacement of one atom by another and it became evident that in such replacements one atom of oxygen may take the place of two atoms of chlorine.

Thus far the rudiments of the idea of valence had been developed only on the basis of the replacing power of different atoms. In 1852 Frankland went a step further and introduced the more exact conception of a definite, though variable, combining power for different atoms. Using the atomic weight 8 for oxygen he gives the formulas NO,, NH,, NI, NS,; PO, PH, PCl; SbO, Sbн ̧, SbCl; AsO, ASH,, and NO, NHO, NHI; PО,, PHI, etc., to show that the elements nitrogen, phosphorus, arsenic and antimony combine with either three or five atoms of other elements. He also pointed out that when an atom of tin is combined with two ethyl groups in tin ethyl, Sn(CH),, it will take up only one atom of oxygen, giving the

compound, Sn(C,H,),O, while an atom of tín alone will combine with two atoms of oxygen to form stannic oxide, SnO,.

A few years later Couper and Kekulé, quite independently of each other, developed clearly the idea that carbon compounds are held together in chains by attractions between the atoms and that the structure of the molecules of such compounds is directly dependent on the valence of the atoms of which they are composed.

In the same year, 1858, at Genoa, Cannizzaro revived the hypothesis of Avogadro and Ampère and gave such convincing evidence of its truth that it was soon accepted by the leading chemists of the world. This introduction of a correct system of atomic and molecular weights aided greatly in the very rapid development of structural organic chemistry. We can still imagine with what enthusiasm the chemists of that day seized the key to nature's mysteries which the doctrines of valence and of the linking of atoms had given them and applied them to the solution of problems of structure and of synthesis. Only a few years before the thought of definite, accurate knowledge of this kind would have seemed the dream of a hair-brained visionary.

Chemistry is primarily an experimental science. New theories make their way slowly and speculations which are not forced upon us by incontrovertible facts have met with little favor. At the time when the theory of valence made itself indispensable as a guide to the investigation of carbon compounds the older electrochemical theory had practically disappeared and no theory for the cause of the attraction between atoms received⚫ more than passing attention. It was tacitly assumed that some sort of attraction between atoms held them bound together but even such necessary terms as "single bonds" and "double bonds" or "linkages" were used with reserve by many chemists.

During the forty years following the publication of the papers by Couper and Kekulé the theory was amplified in only one important detail. The original theory considered only the sequence of atoms in compounds.

While there may have been occasional thoughts about arrangements in space, chemists were very reticent in expressing them. In 1874, however, van't Hoff proposed an explanation of the relation between the structure of optically active compounds and the arrangement of their atoms, based on the fundamental proposition that four univalent atoms or groups combined with a given carbon atom are arranged symmetrically about the center of the atom. From the same starting point he postulated the supposition that two carbon atoms connected by a double union can not rotate independently about the points of union. The first hypothesis gave a satisfactory explanation for optically active compounds and it is impossible now for any one to question the fact that a compound which is optically active in solution must contain a central atom or group around which four or more atoms or groups are arranged in an asymmetric fashion. Incidentally it may be remarked that the discovery of compounds in which the asymmetric atom is nitrogen or sulfur or tin demonstrates that the principle of valence is general in its application and is not simply of value for carbon compounds.

The use of van't Hoff's principle in the explanation of the isomerism of such compounds as fumaric and maleic acids was equally successful.

In 1885 Bayer gave the following statement of the well-established principles used in explaining the structure of carbon compounds 1. Carbon is usually quadrivalent.

2. The four valences are alike.

3. The valences are symmetrically directed in space from the center of the carbon atom. 4. Atoms attached to the four valences do not easily exchange places-van't Hoff's principle.

5. Carbon atoms may be united with one, two or three valences.

6. The compounds may be either open chains or rings.

Baeyer proposed a seventh principle:

7. The directions in which the valences are exerted may be diverted from the normal angle of the tetrahedron, which is 109° 28',

but the tension which results renders the compound less stable.

He explained in this manner the extreme instability of acetylene compounds and the ease with which additions are made to double unions; also, the instability of rings of three carbon atoms in comparison with those of four or five atoms. This so-called "tension theory" has been very suggestive and useful. An interesting confirmation of the ease with which rings containing six atoms are formed and evidence that rings containing seven atoms are not so natural was discovered, almost by accident, by Mr. Potter and myself a few years ago. Aminocamphonanic acid, usually written,

on the other hand, is a dextro compound and both its hydrochloride and its sodium salt are right-handed, indicating that the free acid, which would require the formation of a ring of seven atoms to form a cyclic salt, does not form such a salt.

These relations seem to establish, also, the quadrivalence of nitrogen in ammonium salts. It seems impossible to reconcile this with Werner's idea that the fourth hydrogen of the ammonium group remains combined with the acid radical in such salts.

In 1899 Thiele proposed his theory of partial valences to account for the addition of bromine, hydrogen, or two other atoms or groups to the end carbon atoms in conjugated double unions, which have the structure >C=CH-CH=C<. The compound formed has the structure >CX-CH=CH-CX<. This relation always reminds me of two bar magnets in which the north pole of one is in contact with the south pole of the other N SIN S. At the center of such a system no attraction will be exerted but the free ends will exert their usual attraction. While this is an analogy rather than an explanation, and is also rather closely related to Thiele's idea, it seems to me better than that which he has expressed. If we attach any definite meaning of localized attraction to the term valence it is difficult to conceive of the valence as being divided, as seems to be implied in the term, "partial valence."