Representation of Chemical Reactions

Reactions represent the dynamic aspect of chemistry, the interconversion of chemical compounds. Chemical reactions produce the compounds that are sold by industry and that play a big role in maintaining the standard of living of our society they transform the food that we take up in our body into energy and into other compounds and they provide the energy for surviving in a hostile environment and the energy for a large part of our transportation systems. 

Clearly then, the understanding of chemical reactions under such a variety of conditions is still in its infancy and the prediction of the course and products of a chemical reaction poses large problems. The ab initio quantum mechanical calculation of the pathway and outcome of a single chemical reaction can only be 

Chemoinjbrmatics A Textbook. Edited by Johann Gasteiger and Thomas Engel Copyright 2003 Wiley-VCH Verlag GmbH Co. KGaA. 

Nevertheless, chemists have been planning their reactions for more than a century now, and each day they run hundreds of thousands of reactions with high degrees of selectivity and yield. The secret to success lies in the fact that chemists can build on a vast body of experience accumulated over more than a hundred years of performing millions of chemical reactions under carefully controlled conditions. Series of experiments were analyzed for the essential features determining the course of a reaction, and models were built to order the observations into a conceptual framework that could be used to make predictions by analogy. Furthermore, careful experiments were planned to analyze the individual steps of a reaction so as to elucidate its mechanism. 

Let us illustrate this with the example of the bromination of monosubstituted benzene derivatives. Observations on the product distributions and relative reaction rates compared with unsubstituted benzene led chemists to conceive the notion of inductive and resonance effects that made it possible to explain the experimental observations. On an even more quantitative basis, linear free energy relationships of the form of the Hammett equation allowed the estimation of relative rates. It has to be emphasized that inductive and resonance effects were conceived, not from theoretical calculations, but as constructs to order observations. The explanation is built on analogy, not on any theoretical method. 

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First, the objects of investigation, chemical compounds or chemical reactions, have to be represented. Chemical compoimds wUl mostly be represented by their molecular structure in various forms of sophistication. This task is addressed in Chapter 2. The representation of chemical reactions is dealt with in Chapter 3. The vast number of compounds known can only be managed by storing them... 

Representation of Chemical Reactions 1185 Table 3-2. The 16 possible unit exchanges at any skeletal carbon atom. 

It is essential to indicate also the reaction center and the bonds broken and made In a reaction - In essence, to specify how electrons are shifted during a reaction. In this sense, the representation of chemical reactions should consider some essential features of a reaction mechanism. 

Just as mass and energy must be conserved, so also must electrical charge. Yet free electrons are not found stable in nature under the conditions of chentistry on earth, so caimot appear as reactants or products in representations of chemical reactions. Example 11 is a half-equation , something that represents a common pattern in chemical reactions, but only occms when coupled to another suitable half-equation (i.e., this reduction process must be paired with an oxidation process that releases electrons), e.g. 

Hinton, M. E., Nakhleh, M. B. (1999). Students microscopic, macroscopic, and symbolic representations of chemical reactions. The Chemical Educator, 4(4), 1-29. 

The learning process involves many students"activities Students interest increased because they liked playing an active role in the learning process (e.g. hands on experiments, constraction of representations of chemical reactions with the use of models). Those who were not used to such an approach from earlier teaching also improved their experimental skills and gained experiences through the work with models. In the reflective diaries of all the teachers, statements to support this point could be found as follows ... 

We first describe our representation of chemical reactions. Consider the isomerization reaction of cyclopropane to propylene,... 

Internally IGOR uses since 1980 canonical representation of chemical reactions that is based on a linear combination B + kE (k is an integer the maximum entry of any B or E, e.g k = 10). The latter is subjected to a canonical indexing procedure by an extended version of the algorithm CANON (ref. 21,22). 

Gulinska H., Bartoszewicz M. (2006), Multimedia software for representation of chemical reaction mechanism — high school and college level. Journal of Science Education, 7, 1... 

Simultaneous Representation of Chemical Reaction and Phase Equilibrium and the Evaluation of Phase Envelopes of Reactive Mixtures... 

The ACM are not quite adequate for the representation of chemical reactions, since the latter involve not only redistributions of bonds but also unshared valence electrons that do not participate in bonds between atom pairs of the individual molecules. The adequate representation of interconvertible chemical systems requires a device with double bookkeeping capability, i. e., matrices which account for both, bonds and electrons. The Je-matrices are obtained from the ACM by inserting diagonal entries which indicate the number of free, rmshared valence electrons of the atom Ajt. Clearly, each 6e-matrix B is symmetric, so that the entries on the diagonal and in the upper triangle determine the matrix B uniquely. 

In the development of EROS it was decided from the start to work with a representation of chemical reactions of the third level, to consider explicitly the breaking and making of bonds in chemical reactions. The early versions of EROS worked with bond rearrangement schemes representing overall reactions. One very important scheme is the one breaking two bonds and making two bonds (Figure 4). 

Chemical equations Representations of chemical reactions by the formulas of reactants and products... 

R.5 Symbolic Representations of Chemical Reactions Using Curved Arrows... 

Muller, S. L., Plesser, T. Hess, b. (1986). Two-dimensional spectrophotometry and pseudocolor representation of chemical reaction patterns. Naturwissenschaften, 73, 165-79. 

Let us consider the interconnection of the steric factor p value with the structure of the reaction products, which can be characterised by its fractal dimension D [37]. Within the frameworks of model representations of chemical reactions the reaction rate constant can be expressed as follows [73] ... 

The representation of chemical reactions in solution in the thermodynamic models [21, 26, 27] necessitates the knowledge of the equilibrium constants of CO2 dissociations, water dissociation, amine protonation and carbamate formation. For original amines the protonation or carbamate formation equilibrium constants are usually not available and must be measured. In order to derive enthalpy properties using Van t Hoff equations, these equilibrium constants must be determined as function of temperature. Such data can be obtained from a protonation constant determined at a... 

O The illustrations and photos provide visual representation of chemical reactions, phenomena, and processes to help students understand chemistry and to make it more inviting to them. 

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