Polyvalent atoms

He even proposed the existence of polyvalent atoms producing "sponge or net like" molecular mass in 1878. Towards the end of his career, he advanced the hypothesis that natural organic substances-those most closely associated with life, proteins, starch, and cellulose-may consist of very long chains, and derive their special properties from this structure. 

Logically, more bonds between atoms should mean a greater force of attraction and increased carbon density. But doubly bonded alkenes are more reactive, not less reactive, than singly bonded alkanes or triply bonded acetylenic compounds. Further, how could the force associated with a polyvalent atom be divided a priori to react to the atoms encountered, sometimes dividing itself in half, other times by thirds To assume that valences act across empty space, free of atoms, some chemists noted, "is only possible on paper or in a model where there are lines or wires but not forces. "89... 

Cd represents a double-bonded C atom, C, a triple bonded C-atom, Cg a C atom in a benzene ring, and C, an allenic C atom. By convention, group values for C-(XXH)j will always be taken as those for C-(CXH)j when X is any other polyvalent atom such as Cg, C, Cg, O, and S. Cgp represents a carbon atom in a fused ring system such as naphthalene, anthracene, etc. Cgp-(Cgp >3 represents the group in graphite. 

For a polyvalent atom the partial charge builds up every time another highly electronegative substituent is added. Thus the partial charge on tbe carbon atom in carbon telrafluoride is considerably larger than it is in the methyl fluoride molecule, and so all of the C—F bonds shrink, though the effect is not as great for the last fluorine as for (he first ... 

There is a systematic study of the origins of structural theory in organic chemistry, where the emphasis is on the period from about 1830 to 1861.164 The same author has examined Kekule s attempt to account for valency in terms of the internal structure of polyvalent atoms.165 Robinson s work on strychnine has also been studied.166 Examples from structural chemistry have been used for illuminating a number of epistemological issues.167,168... 

The significance of p-bond scission in coal chemistry is limited by the concentration of suitable molecular structures. Rapid p-bond scission generally requires structures containing three or more consecutive, saturated, polyvalent atoms such as carbon, oxygen, nitrogen and sulfur. Examples of coal-related structures susceptible to rapid dissociation by p-bond scission are given in Figure 1. For each structure, once the C-X bond is broken by any means, the radical formed will rapidly dissociate. 

Cl (12), I (13), N (14), Si (9), other polyvalent atoms (15), and of muonium (16). Reviews have also been published on the reactions of recoil atoms with arenes (17), (halo)ethylenes (18), and (halo)-methanes (19). The capture of ir in hydrogenated species is sometimes considered as a part of recoil chemistry (20), and so also are reactions of species formed after decay of multiply labeled (T, 14C) molecules (21-23), for example,... 

Radical transfer reactions involve abstraction of an atom or group B by a radical A from a molecule B-C (reaction 6.24). B is nearly always an atom transfer of a group, which would correspond to substitution at a polyvalent atom, though important in nucleophilic and electrophilic reactions, is very uncommon in radical reactions. The atom transferred is almost always a hydrogen or a halogen atom. 

What Derjaguin considers the central issue of colloidal solutions remains largely unresolved for silica sols. This book mentions the ideas of the proponents of both the kinetic and the thermodynamic approach to the problem of stability of silica sols and is intended to stimulate the continuation of the healthy controversy started at the R. K. Iler Memorial Symposium. In this manner a consensus should eventually be reached that will allow the establishment of common quantitative parameters in the treatment of stability of silica sols and other disperse phase materials composed of polyvalent atoms linked by strong covalent bonds and the explanation of their experimentally observed behavior. 

Recoil Chemistry and Mechanistic Studies with Polyvalent Atoms... 

Contrast Between Chemistry of Polyvalent Atoms and Reactions of Monovalent and Divalent Atoms... 

While such a reaction is in principle possible, it is expected to occur with very low eflBciency. In the recoil chemistry of polyvalent atoms, chemically stable reaction products are likely to be formed in multistep reaction sequences rather than in single reactive collisions. This implies the formation of reactive intermediates, which is the feature that most clearly distinguishes the study of polyvalent atoms from the investigation of monovalent atoms. 

Formation of Reactive Intermediates The Distinctive Feature of Polyvalent Atom Recoil Chemistry. The possibility that reactions of recoiling silicon and germanium atoms would produce novel reactive intermediates originally attracted the author of this chapter to hot atom chemistry. Several such intermediates are shown in a reaction scheme for recoiling silicon atoms that has been under investigation in our laboratory for several years (3,4). 

This reaction schane is perhaps typical, both in its length and in its tentative nature, of those suggested for polyvalent atoms. It is being closely scrutinized, and parts of it are undergoing revision. However, it was the hope of making silylene SiHs via recoil reactions that lured this author into hot atom chemistry. 

Anyone who imdertakes to design a polyvalent atom recoil experiment must keep clearly in mind that the observed reaction products will rqp-... 

Investigation of Systematics of Polyvalent Atom Recoil Reactions... 

The most important mechanistic clue for any chemical reaction is the structure of the reaction products, and a great strength of the recoil technique is that there is no more convenient or effective way to determine what products are formed from a polyvalent atom and a chosen reaction substrate than by examining its recoil chemistry. The harshness of the conditions required to liberate polyvalent atoms chemically severely limits the study of their reactions by other methods (vide infra). 

Identification of Reactive Intermediates. Comparison of Intermediates IN Recoil Reactions with Chemically Generated Species. The reactive intermediates formed from recoiling polyvalent atoms must, for now, be identified by chemical means. Reaction substrates are chosen so that the structures of the products derived from them help us imder-stand the nature of the intermediates. For this purpose it is useful to know the reactivities of the molecules used as reaction substrates toward short-lived intermediates of known structure. The most direct way in which a reactive intermediate in a recoil reaction can be identified is to generate the same intermediate by chemical means and to compare their behavior. 

These examples point to the symbiotic relationship between polyvalent atom recoil chemistry and the study of chemically generated reactive intermediates. Hot atom experiments identify products that suggest the intermediacy and delineate the reactivity of exotic species such as cycloproplyidene, methyne, and silylene. Thus, hot atom experiments have stimulated the search for other ways of generating these species, and in turn these chemical experiments can help interpret the hot atom experiments. 

If a reactive intermediate in a polyvalent atom recoil reaction has been independently characterized, as has methylene or cyclopropyl-methylene, above, then acceptable evidence for its intermediacy in a recoil system is the observation of a product spectrum characteristic of the given intermediate. Enough difiFerent substrates must be used, however, to ascertain that all the major reactions of the suspected intermediate are obtained from the recoil system. 

Cases in Which Postulated Intermediates in Recoil Reactions Cannot Be Chemically Generated. Turning to other approaches for the mechanistic study of recoiling polyvalent atoms, among the most interesting and diflBcult cases arise when observed products and postulated intermediates in the recoil systems cannot be generated by chemical means. Then the resourcefulness of the hot atom chemist is pressed to the utmost. 

Carbon Monoxtoe Formation from Carbon Atoms. In certain cases quite detailed pictures of primary reactions of recoiling polyvalent atoms have been developed. "End-on attack of carbon atoms on oxygen molecules rather than attack normal to the oxygen-oxygen bond was deduced from the formation of carbon monoxide instead of carbon dioxide, even in condensed phases capable of stabilizing excited CO2 (51). 

Diatomic Molecules as Possible Diagnostic Reagents and the Use of Correlation Diagrams. Since the primary reactions of polyvalent atoms are expected to form reactive intermediates, the most general approach to characterizing these primary reactions is to pick substrates whose reactions are most likely to convey information about the electronic states of the reacting atom and the geometry of its attack on the substrate. 

Diatomic molecules offer important advantages that have not been fully utilized for probing the primary reactions of recoiling polyvalent atoms. The primary products of the interaction of diatomic molecules with polyvalent atoms wall, of course, be diatomic and triatomic reactive radicals whose gross structures and electronic states can, in favorable cases, be deduced from their subsequent reactions, as discussed earlier. 

For polyvalent atoms, potential surfaces of suflScient accuracy for trajectory calculations are usually lacking. 

While ab initio calculations of reaction paths for polyvalent atoms are still scarce, the usefulness of semiempirical calculations has recendy been demonstrated. Dewar employed his MINDO/2 molecular orbital method to examine the addition of carbon atoms to ethylene and trans-2-butene (75). An unrealistically large activation energy was foimd for rearrangement of the intermediate cyclopropylidene to allene. 

Conclusion—A Positive View of Polyvalent Atom Recoil Chemistry... 

In conclusion, the author would like to reiterate his belief that several factors argue for more rather than less intensive activity in the field of polyvalent atom recoil chemistry. These factors include the sharpening of our perceptions about the problems that face us and about what can be achieved, as well as the availability of new tools to facilitate our work. 

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