Classifying Reactions

Before we can proceed with the choice of reactor and operating conditions, some general classifications must be made regarding the types of reaction systems likely to be encountered. We can classify reaction systems into five broad types ... 

The first step in an inductive learning process is always to order the observations to group those objects together that have essential features in common and to separate objects that are distinctly different. Thus, in learning from individual reactions we have to classify reactions - we have to define reaction types that encompass a series of reactions with essential common characteristics. Clearly, the definition of what are essential common features is subjective and thus a variety of different classification schemes have been proposed. 

Chemists usually learn about reactions according to fiinctional groups for example, How can I make an aldehyde and what reactions are known for aldehydes " This is clearly not a very good starting point for classifying reactions. The poor state of affairs in the definition of reaction types is further quite vividly illustrated by the fact that many chemical reactions are identified by being named after their inventor Diels-Alder reaction, Michael addition, Lobry-de Bruyn-van Ekenstein rear-... 

Model-driven approaches classify reactions according to a preconceived model, a conceptual framework. 

In type A, the adduct loses water (or, in the case of addition to C=NH, ammonia, etc.), and the net result of the reaction is the substitution of C=Y for C=0 (or C= NH, etc.). In type B, there is a rapid substitution, and the OH (or NH2, etc.) is replaced by another group Z, which is often another YH moiety. This substitution is in most cases nucleophilic, since Y usually has an unshared pair and SnI reactions occur very well on this type of compound (see p. 434), even when the leaving group is as poor as OH or NH2. In this chapter, we shall classify reactions according to what is initially adding to the carbon-hetero multiple bond, even if subsequent reactions take place so rapidly that it is not possible to isolate the initial adduct. 

Of course, there is no oxidation without a concurrent reduction. However, we classify reactions as oxidations or reductions depending on whether the organic compound is oxidized or reduced. In some cases both the oxidant and reductant are organic those reactions are treated separately at the end of the chapter. 

Classify reactions as single-replacement chemical reactions. 

The order of reaction provides the basis for classifying reactions. Generally, the order of reaction can be anywhere between zero and three. Reactions having order three and above are very rare and can be easily counted. 

A valid mechanism for a reaction should be based on sound experimental evidence acquired through a study of the way in which the various intermediates involved affect the kinetics of the reaction. Experimentally-determined rate laws provide us with a means of classifying reactions according to their kinetics. For example, for a reaction X + Y —> Z, if the experimentally-determined rate law allows us to determine the order of the reaction ... 

In this section, you learned how to relate the rate of a chemical reaction to the concentrations of the reactants using the rate law. You classified reactions based on their reaction order. You determined the rate law equation from empirical data. Then you learned about the half-life of a first-order reaction. As you worked through sections 6.1 and 6.2, you may have wondered why factors such as concentration and temperature affect the rates of chemical reactions. In the following section, you will learn about some theories that have been developed to explain the effects of these factors. 

In your previous chemistry course, you classified reactions into four main types synthesis, decomposition, single displacement, and double displacement. You also learned to recognize combustion reactions and neutralization reactions. You have now learned to classify redox reactions. In addition, you have also learned about a special type of redox reaction known as a disproportionation reaction. 

A methodology that can classify reactions by using similarity measures has recently been introduced and has now been extended to include a steric similarity index. Both substitution and elimination reactions are included. 

It would be useful to classify reactions as to their type, heterolytic (ionic) or homolytic (free radical), radical cation, or radical anion. Below are a number of examples of these types of reaction. 

A variety of terms or concepts have been used to classify reactions or sequences of reactions leading to enantiomerically enriched or reasonably pure compounds. During the preparation of this section, it was "discovered that very few terms are precisely defined. The typical ways of introduction are casual footnotes and references to actual examples. 

In short, photochemical reactions should be classified as photoisomerization, photodissociation, photosubstitution, etc. Definite efforts should be made to use the terms already developed by inorganic and organic chemists to describe transformations involving the particular stoichiometric changes. There already exists enough descriptive names to classify reactions and it would be a pity to allow photochemistry to spawn another, independent family of reaction names. ... 

The previous sections have been largely concerned with classifying reactions involving highly reactive intermediates into those that provide evidence for diffusion control and those that appear to occur through pre-association. In conclusion, it is useful to look at the pattern that emerges and at some possibly unjustified simplifications in the arguments used. 

Rate laws are determined by experimentation and cannot be inferred only by examining the overall chemical reaction equation (Sparks, 1986). Rate laws serve three primary purposes (1) they permit the prediction of the rate, given the composition of the mixture and the experimental value of the rate constant or coefficient (2) they enable one to propose a mechanism for the reaction and (3) they provide a means for classifying reactions into various orders. 

Reactions within organic chemistry were studied as a function of hydrostatic pressure historically and attempts to classify reaction types according to the ionic nature or not of the reacting species and thus to the sign and magnitude of the volume of activation were made.54 Later investigators have tended to treat each system studied based upon individual characteristics and properties for interpretation purposes. Parallel efforts were made to examine whether satisfactory correlations could be made between the volume of activation and the entropy of activation.55... 

These reactions may contain components and properties that could place them in earlier classified reaction sections, but are treated here separately as they have specific features and properties warranting separate coverage. 

We would classify reaction (1) in Class C. Reaction (2) on the other hand may be of a polar nature (Class A), if the H—H bond is ruptured heterolytically ... 

In their excellent review of multicenter photochemical reactions, Warrener and Bremner (1966) classify reactions by the number of participating centers, e.g. nC. When orbital overlap of nonadjacent centers is involved, the label XnC refers to orbital overlap in alternate centers. In equation (48) or (49) (Srinivasan and Sonntag, 1965) there... 

Classify reactions as electrocyclic reactions, [x + y] cycloadditions, or [/, /] sigma-tropic rearrangements. (Problem 22.30)... 

Ross and Shu [38], discussing the computer modelling of hydrocarbon pyrolysis for olefin production, classify reaction models in four categories in order of increasing sophistication empirical, semi-kinetic, stoichiometric and mechanistic. Most concepts of this classification are included in Table 3 with, however, a more classical meaning of the word stoichiometry. 

There are a few different ways to classify chemical reactions. We will look at the most traditional method of classifying reactions, which divides reactions into five categories, based on the behavior of the chemicals during the reactions ... 

For our purposes the chemical reactions which occur between gases and metals, gases and solids, and solids and solids can be classified (1) in four groups (a) reactions in which a gas is formed as one of the products, (b) reactions in which one of the reactants is a gas and is therefore removed from the atmosphere or from the vacuum, (c) reactions in which one gas is exchanged for another gas as a result of reaction, and (d) reactions between solids in which a new phase or phases are formed but no gas is involved. We are not classifying reactions in which one of the phases involved is a liquid. This classification may be rather arbitrary, but we have found it very useful. 

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