Unimolecular reactions classification

Intramolecular general base catalysed reactions (Section II, Tables E-G) present less difficulty. A classification similar to that of Table I is used, but since the electrophilic centre of interest is always a proton substantial differences between different general bases are not expected. This section (unlike Section I, which contains exclusively unimolecular reactions) contains mostly bimolecular reactions (e.g. the hydrolysis of aspirin [4]). Where these are hydrolysis reactions, calculation of the EM still involves comparison of a first order with a second order rate constant, because the order with respect to solvent is not measurable. The intermolecular processes involved are in fact termolecular reactions (e.g. [5]), and in those cases where solvent is not involved directly in the reaction, as in the general base catalysed aminolysis of esters, the calculation of the EM requires the comparison of second and third order rate constants. 

The discovery of the unimolecular reactions which depend upon collisions blurred the classification in terms of orders, and the complex kinetics of chain reactions stiU further lessened its utility as a... 

Most elementary chemical reactions can be categorized as unimolecular or bimolecular events. However, further phenomenological classification is useful for the development of detailed chemical kinetic models. This way, rate parameters for new reactions can be estimated rapidly and reliably by analogy to similar reactions in the same phenomenological class. In addition, the number of different elementary reactions that must separately be treated is reduced. It must be recognized, however, that exceptional cases... 

Reaction (2.3a) is called unimolecular, (2.3b) bimolecular and (2.3 c) termole-cular. This classification was proposed by van t Hoff [499]. 

The term photosubstitution reactions has been applied to a large variety of reactions, which involve quite umelated mechanisms. In the present context, it will be referred to every photoinitiated reaction between an aromatic and a nucleophile, which occurs via a polar mechanism. The classification is based on the key intermediates involved. A first distinction depends on whether the initiating step is unimolecular or bimolecular. In the first case, path a involving heterolytic fragmentation and path b involving ionization can be considered (see Scheme 4.2). 

AG° tells us where a reaction equilibrium lies, in which direction it will proceed, and AG tells us how fast we will get there. The free energy of activation also has enthalpic and entropic components (8.7). We can describe the kinetics of organic reactions, just like any other, in terms of a rate equation. For example, the hydrolysis of iodomethane (8.8) shows the rate (Equation 8.9). Measured kinetics of this type tell us about the whole reaction process, whether one or many steps are involved. Another useful classification is the molecularity of processes, since this is applicable to individual steps in reaction mechanisms. Figure 8.8 shows typical unimolecular and bimolecular reaction steps. In the unimolecular process, a single molecule is involved in the RDS, and in many (but not all) cases, this will lead to the rate depending only on the concentration of substrate. In a bimolecular reaction, two molecules are involved in the RDS, and the rate depends on the concentration of each of them. 

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