Valence, partial

This by no means exhausts the list of structures proposed for benzene. The remainder, however, such as the structure of Thiele based upon his theory of partial valence and that of Collie based upon a dynamic model, are usually complicated and cannot be represented in any simple valence scheme. 

A second factor of importance in this connection is found in the emphasis which was placed in the 1890 s and early 1900 s on secondary association of molecules. Coordination complexes, the concept of partial valences, and van der Waals forces attracted wide attention... 

In an important paper published in 1920 Staudinger deplored the prevailing tendency to formulate polymeric substances as association compounds held together by partial valences.He specifically proposed the chain formulas... 

The next example for this rule may be the heterolytic addition of chlorine to the C=C bond. Fig. 4.3b indicates the partial valence-inactive population 60> of the 2pz AO of the /9-carbon in LU, calculated by the extended Htickel method. It is seen that this quantity, (c )2, largely increases according to the approach of the chlorine cation to the carbon atom at which the addition is to take place, so that the reactivity of the /9-position towards the second chlorine atom (anionic species) grows. Also Fig. 4.3a shows the decrease of the LU energy in the direction of the reaction path which has already been mentioned above. 

Thus in 1899, Johannes Thiele extended his valence theory of double bonds to include colloids. Thiele suggested that in such materials as polystyrene the molecules of styrene were bound together merely by association of the double bonds. He referred to this association as "partial valence" (21). In 1901, Rohm concluded that the transformation of acrylic esters into polymers was from an "allotropic alteration" and not a chemical reaction (22). Schroeter, working with salicylides just as Kraut, Schiff, and Klepl before him, concluded that the tetrameric salicylide was formed by "external forces about the monomeric molecules", and that the chemical structures of the monomers were unaltered (23). Thus the association theory rapidly grew in popularity. 

From these results he assigned a cyclooctadiene formula to rubber, and concluded, in the tone of the times, that the rubber molecules combined through the action of "partial valence" into much larger aggregates (39). 

When Herman Mark first evaluated the crystal structure of rubber (with E. A. Hauser) and cellulose (with J. R. Katz) in 1924 and 1925, it was generally accepted that these materials were low molecular weight or monomeric. The unusual properties of these substances, now known to be related to high molecular weight, were then attributed to aggolomeration or "association" of the low molecular weight precursors. A common explanation for the associations were secondary forces such as Johannes Thiele s partial valences. 

If, with Thiele, we indicate the inactivation of adjacent carbon atoms by a bracket, as shown above, we see that in benzene all the partial valencies cancel out. 

Robinson referred in very general terms to electrons, saying that "cohesion via partial valencies" is followed by "movement of electrons along a potential gradient."92... 

For representing the splitting of valence, Robinson used two or more dotted lines not necessarily of equal value but together quantitatively equivalent to the normal unit of valency. Looped dotted lines were used in which the loop could be opened up to produce partial valencies of opposite sign. 97... 

Robinson defined conjugation as the transfer of free partial valency to an adjacent atom or to the end of a chain of atoms "It is the explanation of action at a distance in a molecule."98 By assuming the possibility of subdivision of valency, reactions could be represented as "being almost continuous and with... 

Arthur Lapworth. "Latent Polarities of Atoms," 116 Robert Robinson, "The Conjugation of Partial Valencies," ibid., no. 4 (1920) 114. This paper is reproduced in Robinson s Memoirs. 

In a 1922 paper written with W. O. Kermack, Robinson used the Lewis-Langmuir approach to give physical meaning to partial valency (so that the double bond [=] was equivalent to four-electron sharing to three-... 

W. O. Kermack and Robert Robinson, "An Explanation of the Property of Induced Polarity of Atoms and an Interpretation of the Theory of Partial Valencies on an Electronic Basis, JCS 121 (1922) 427ff quoted in Seddon, "Development of Electronic Theory," 25. W. O. Kermack was a member of the Royal College of Medicine at Edinburgh. 

Stewart clearly stated the dilemma for organic chemists at the turn of the century. Through laboratory experience, they had learned to interpret the chemical bond in different ways. Although they might draw double bonds in formulas for diphenyl-ethylene, ethylene, and fulvene, chemists did not really take the bonds to resemble each other chemically. Chemists "knew" that there is an increase in unsaturation, or reactivity, of the double bond toward bromine or oxygen, from one of these compounds to the next. They "knew" that a bond must be looked at not as a fixed unit but as a sum of an infinite number of small forces or partial valences. This is what Polanyi later called tacit knowl-... 

These points need emphasizing, for the use of free valence as a measure of reactivity was first introduced on intuitive grounds, based on the Thiele theory of partial valence. On this basis there seems no obvious reason why free valence should serve as a measure of reactivity only for compounds of one type. The perturbational treatment given above shows that the validity of the correlation is due merely to a fortuitous coincidence which holds only for alternant hydrocarbons. 

If one organic compound has dominated the historical literature of the last few years, that compound must be benzene. Most probably, this is because its structure in some respects marks a transition from the most austere form of classical organic chemistry, in which carbon was tetravalent and tetrahedral, to a continuing series of changes from oscillating molecules, through partial valencies to MO descriptions, and Huckel s rules of aromaticity. It is the case par excellence of a single substance whose history intersects all major streams of chemical theory - except perhaps the periodic law - and which also has enormous industrial and economic importance. 

It is noteworthy that among the formulae proposed for benzene in the 19 century, only the first one, that of Loschmidt, is not far from being correct. The next acceptable formula only appeared with Thiele s suggestion of fractional carbon-carbon bonds (partial valences) in 1899-1900. This formalism did not explain why cyclooctatetraene is not aromatic, however, as shown experimentally (for historical accounts, see refs. [15, 16]). 

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