In the presence of COOH -656 β -649 Differences from -645 Cal., the "normal" Value α In the presence of COOH Y -646 γ It is apparent from a consideration of the above tables that the effect upon the heat of reaction of a molecule exerted by a group which is itself not directly concerned in the reaction varies in a regular way with variation in the position of the group with respect to the reacting portion of the molecule. This is most clearly apparent in the reaction CH3COOH in the presence of the group COOH, probably because here the effect is great and the data abundant and accurate. In this case, when the carboxyl group is a to the reacting group the heat of reaction is diminished by 13 Cal., compared with its "normal" value in the absence of the carboxyl group; when the carboxyl group is in the ẞ position the heat of reaction is diminished by about 3 Cal.; when the carboxyl group is in the y, 8 ore position it is increased by about 2.5 Cal., returning finally to the "normal" value when the distance between the two groups is greater. The effect may be represented by the accompanying curve. 15 15 The nature of this curve is apparently inconsistent with the idea developed by Flürscheim (1. c.) and by Biach (Zeits. f. physikalische Chemie, 50, 43, 1904) that valence energy alternately increases and decreases from one carbon atom to the next, in a chain or ring; the period of alternation being, in the present matter, sometimes one carbon atom, sometimes more, according to the nature of the case. It is clear in all the above cases that the effect of a group upon the heat of reaction is greatest when it occupies the a position, and that when it is sufficiently distant from the reacting group, this necessary distance varying with the nature of the case, it has no appreciable effect. Evidently, too, in most cases, before the effect disappears it is reversed, as in the y, 8, and positions on the above curve and in the B position in those cases where the effects of the groups COOH, CONH2, etc., upon the reaction CH-C CH, are considered. In most cases an atom or atom group does not appreciably influence the heat of a reaction when the reacting group is further away from it than the ẞ or y position, and it may be noticeable only when it occupies the a position, as in the case represented by the accompanying In the case represented by the first curve, however, an effect is still notable when the two groups are in the position to each other, provided the data are nearly correct, and it is important to consider that the data for this series of compounds are perhaps the best of all Stohmann's admirable determinations. Accordingly it is not impossible that curve I represents at least one general case of variation in heat of reaction with variation in position of an inactive group; at any rate the data here represented is not inconsistent with such a conclusion, and for the most part the curves would be of similar shape. These regularities in the variation of heats of reaction which have been here considered, as well as those previously pointed out, lead naturally to the conclusion that the valence energy of a tie between two atoms must be capable of continuous variation 16 within certain limits, not yet to be defined; otherwise we must assume the existence of large forces of unknown nature within the molecule. According to this conclusion and the above considerations the valence energies of two ties between like atoms will under like conditions be equal, under unlike conditions different. Thus in the compound CR, 16 Or, what is less probable, through numerous distinct magnitudes discontinuously. the four valence energies of the central carbon atom will be equal, in the compound CR'R", three will be equal and one different from these three, and in the compound CR'R"R"R""" all four will be different, even when the central carbon atom is tied to like atoms by all four of its valences as in methylethylacetacetic ester because every one of the four carbon atoms which are tied to the central carbon atom is different in position from every other and consequently subjected to different influences from at least some of the atoms of the molecule. It cannot be too strongly emphasized that this conclusion is a logical deduction from established experimental data with the aid of but two postulates, the current theory of chemical constitution and the current theory that the interatomic forces within the molecule are exclusively (or almost exclusively) valence energies. It remains to be pointed out that through the consideration of many cases similar to those here studied, one may hope to obtain accurate quantitative information regarding the phenomena of orientation of substituting groups 17 and the countless other cases familiar to every organic chemist of the influencing of reaction by substituted groups, and thus to replace the existing empirical rules for such cases by accurately formulated principles. SUMMARY. In development of the conception of varying valence energy it is shown that the effect of an inactive atom group upon a reaction of another atom group of the same molecule, measured by the heat of reaction, varies in a regular way according to the relative positions of the two groups. Curves are presented representing the nature of this variation in one case of special importance because of the magnitude of the effect and the accuracy of the data, and in a case where the effect is slight. It is shown that, in accordance with this conclusion and the previous considerations, the valence energies of two ties between like atoms are under like conditions equal, under unlike conditions different. The importance of these considerations for the understanding of the influencing of reactions by substituted groups is pointed out. 17 See for instance Flurscheim, 1. c. |