Chemical Insights Seeing Reaction Mechanisms

The single most important factor that determines the rate of a reaction is concentration — primarily the concentrations of the reactants, but sometimes of other species that may not even appear in the reaction equation. The relation between the rate of a reaction and the concentration of chemical species is termed the rate law it is the cornerstone of reaction mechanisms. The rate law alone allows much insight into the mechanism. This is usually supplemented by an examination of other factors which can also be revealing. (For these, see Chap. 2)... 

It is interesting to compare early experimental work with the facilities available in a modem laboratory. Von Auwers was the first to separate the chloro ketones from the normal phenolic aldehydes of the Reimer-Tiemann reaction. Identirication and strocture proof by chemical means followed. It is a tribute to von Auwers skill and insight that his identification procedures have withstood the onslaught of modem spectroscopic means. Although the chloro ketones (equations 8-10) have been called the abnormal products of the reaction, in cases where the ortho and para positions are occupied (see equation 10) only ketonic material is obtained.In fact, the presence of the dichloro ketones provides one of the better arguments for the intermediacy of dichlorocarbene in the reaction mechanism. ... 

There are two central questions in chemical kinetics (1) How fast can the fastest chemical reactions be (2) Why are many chemical reactions slow We will try to provide some elementary insights when answering these questions. Kinetics has several levels. First, there is a level of correct stoichiometry for a reaction. Second, there is a level of energetic characterization of a reaction, that is, free energy, enthalpy, entropy, and volume changes of reaction (see Section 2.4 and Table 2.6). Third, there is a level of experimental study of reaction rates and the formulation of rate laws that correctly describe the observed rates. Finally, there is the level of mechanism, where elementary reaction steps are proposed, verified experimentally, and used to predict rate expressions, which are then compared with observation. 

Using concentration or the number of moles as the dependent variable allows better physical insight than using reaction coordinates. A substantial objection to the reaction coordinate method is that the minimal set of M independent chemical reactions is likely to be nonsense in terms of mechanism. See Problem 2.24. 

Although on the basis of current-potential relationships important conclusions can be drawn regarding the mechanism of the electrode processes - especially if the experimental parameters are varied over a wide range - the use of combined electrochemical and non-electrochemical methods is inevitable to elucidate the mechanism of the complex electrode processes. As we will see later in this volume, a great variety of advanced electrochemical and in situ probes are available which give different types of information and therefore provide a better insight into the nature of the chemical events that occur during electrochemical reactions. The solid theoretical foundations and the relative simplicity of the final formulae and techniques make electroanalysis an attractive and powerful tool to obtain fast and reliable information on chemical systems. 

Classical dynamics has been used extensively over the past twenty years to aid in our microscopic understanding of chemical reactions and properites of matter. As our experience has grown, the complexity of the systems studied has expanded from simple atom-diatom collisions and hard sphere liquids to more complicated gas-phase reactants (see the chapters by Schatz and Eigersma in this book) and more realistic liquids. Dynamics calculations allow the determination of average experimental quantities, and at the same time, they give physical insight into the microscopic mechanisms. Results of the calculations are very visual, allowing one to picture the motion of particles. The variety of applications of classical dynamics in chemistry is evidenced by the contributions to this volume. 

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