A selective history of the benzene problem is presented, starting with August Kekulé's proposal of a hexagonal structure in 1865 and his hypothesis of 1872 that the carbon–carbon bonds oscillate between single and double. Only those theories are included that were accepted or at least discussed by a significant number of chemists. Special attention is given to predictions, their empirical tests, and the effect of the outcomes of those tests on the reception of the theories. At the end of the (...) period covered by this article, chemists generally accepted the valence bond theory proposed by Linus Pauling; some of them considered this a more sophisticated version of Kekulé's oscillation hypothesis. The sequel describes the replacement of the valence bond theory by the molecular orbital theory in the period ending around 1980. (shrink)

After briefly presenting the first formulation of the equilibrium constant stated by Guldberg and Waage, this study examines the early thermodynamic and kinetic deductions of the equilibrium law. Firstly, it is discussed how Horstmann applied the concept of entropy to chemical equilibrium reactions, which meant the first thermodynamic explanation of the Guldberg-Waage law of mass action proposed in 1864. A different theoretical derivation of the equilibrium constant came from the works of van’t Hoff. This study analyses the first accurate kinetic (...) treatment of equilibrium reactions as well as van’t Hoff’s imaginative thermodynamic foundation of the constant ratios in chemical equilibrium systems. (shrink)

Summary The first systematic studies on the velocity of chemical reactions (now called reaction rates) were published in the 1850s and 1860s. Inquiring about the course of chemical change, their authors established empirical equations on the basis of their measurement results. But these laws, which represented reaction velocities as proportional to the actual concentration of the reagents, could not be given a physical foundation. The chemists themselves regarded their propositions as mere ad hoc hypotheses. In 1867 Leopold Pfaundler formulated a (...) qualitative theory of chemical processes based on Clausius's version of the kinetic gas theory (and more specifically on his theory of evaporation), and on Saint-Claire Deville's investigations of dissociation processes. Pfaundler's theory was based on farreaching analogies: between evaporation and dissociation; between the gaseous state and the activated state; and between evaporation- and chemical-equilibrium. Four points of Pfaundler's theory must be regarded as essential: (1) the reduction of chemical change to randomly occurring molecular collisions, only showing regularities in great numbers according to the laws of probability; (2) the idea that molecules are in different states of internal and external motion, which determines whether a collision results in a reaction; (3) the view of the reaction step as a transition from internal to external motion and vice versa; and (4) the introduction of a new molecular-kinetic definition of chemical affinity as the maximum of internal motion. With these assumptions, Pfaundler provided the empirical rate equations with a new statistical interpretation and a physical Justification. (shrink)