Reaction classification substitution reactions

Numerical values for the structural parameters of QSAR are obtained from an evaluation of the effect of the substituent on the properties (e.g., the rate or equilibrium constants) of a model reaction. The classification of these parameters is therefore model dependent. The model reactions are chosen to represent the most pervasive types of physicochemical phenomena (e.g., dissociation reactions, hydrolysis, substitution reactions, partition between solvents). 

A more detailed classification of chemical reactions will give specifications on the mechanism of a reaction electrophilic aromatic substitution, nucleophilic aliphatic substitution, etc. Details on this mechanism can be included to various degrees thus, nucleophilic aliphatic substitutions can further be classified into Sf l and reactions. However, as reaction conditions such as a change in solvent can shift a mechanism from one type to another, such details are of interest in the discussion of reaction mechanism but less so in reaction classification. 

Classification of Substituents in Electrophilic Aromatic Substitution Reactions... 

A further factor which must also be taken into consideration from the point of view of the analytical applications of complexes and of complex-formation reactions is the rate of reaction to be analytically useful it is usually required that the reaction be rapid. An important classification of complexes is based upon the rate at which they undergo substitution reactions, and leads to the two groups of labile and inert complexes. The term labile complex is applied to those cases where nucleophilic substitution is complete within the time required for mixing the reagents. Thus, for example, when excess of aqueous ammonia is added to an aqueous solution of copper(II) sulphate, the change in colour from pale to deep blue is instantaneous the rapid replacement of water molecules by ammonia indicates that the Cu(II) ion forms kinetically labile complexes. The term inert is applied to those complexes which undergo slow substitution reactions, i.e. reactions with half-times of the order of hours or even days at room temperature. Thus the Cr(III) ion forms kinetically inert complexes, so that the replacement of water molecules coordinated to Cr(III) by other ligands is a very slow process at room temperature. 

In a ligand substitution reaction, two groups must always receive attention. There is a bond to the leaving group to be broken and a bond to the entering group to be formed. The relative importance of these two processes provides a basic dichotomy for the classification of substitutions. If a reaction rate is sensitive to... 

The mechanistic classification generally accepted for ligand substitution reactions was proposed by Langford and Gray in 1965 (19). This classification was often discussed in the literature and its principles are only summarized here for convenience. 

Langford and Gray proposed in 1965 (13) a mechanistic classification for ligand substitution reactions, which is now generally accepted and summarized here for convenience. In their classification they divided ligand substitution reactions into three categories of stoichiometric mechanisms associative (A) where an intermediate of increased coordination number can be detected, dissociative (D) where an intermediate of reduced coordination number can be detected, and interchange (I) where there is no kinetically detectable intermediate [Eqs. (2)-(4)]. In Eqs. (2)-(4), MX -i and... 

The rationale of classification by reaction types is that different functional groups may show the same kinds of reactions. Thus, as we have just seen, alcohols, carboxylic acids, and amines all can accept a proton from a suitably strong acid. Fortunately, there are very few different types of organic reactions — at least as far as the overall result that they produce. The most important are acid-base, substitution, addition, elimination, and rearrangement reactions. Some examples of these are given below, and you should understand that these are descriptive of the overall chemical change and nothing is implied as to how or why the reaction occurs (also see Section 1-11). 

Whereas various reactions of phenotellurazines and particularly of phe-noxatellurine have been thoroughly studied, information on the reactivity of tellurantrene is rather scarce and reactions of phenothiatellurine are practically unstudied. The chemical behavior of heterocyclic compounds 92 is determined by the presence in these tricyclic systems of two reaction centers represented by a tellurium and a second heteroatom M (NR, O, S) and also by a tendency of the activated benzene rings to enter into electrophilic substitution reactions. We shall follow this classification of reactions of tricyclic systems 92. 

Fig. 1. 2. Classification of substitution reactions via radical intermediates, according to the type of reaction(s) involved.

The solution kinetics involves three fundamental types of experiment, namely (i) rate law, (ii) stereochemistry and (iii) variation of rate constant with structure or environment. Each one permits the analysis of slightly different aspects of a reaction, and then develops a scheme of reaction classification which throws light on the behaviour of a reaction in each experimental situation. It is useful to briefly discuss the role of each of the three types of experimental investigation in the study of complex substitution reactions. 

The kinetics and mechanisms of substitution reactions studied in detail have been reviewed elsewhere 1-3). Here we shall summarize some recent data obtained in this field. As far as terminology is concerned, in the majority of cases that of Ingold 4) has been used, in which substitution of one ligand by another is regarded as a nucleophilic (SN) reaction. However, such a classification is rather rigid, and the term nucleophilicity is imprecise if one considers the variety of ligands from the simplest anions to olefins, acetylenes, arenes, etc. 

Taubers review article 64), which provided the first basis for classifying inert and labile complexes, made use of all available observations of rates of formation or substitution reactions. Most of the observations were qualitative, but for the purpose of the classification it was sufficient to know whether the reaction was complete within a couple of minutes or whether a complex was sufficiently inert for geometrical or optical isomers to be isolated. Important qualitative observations came from attempts to resolve complexes, and much of the quantitative data came from studies of rates of racemization or isomerization. 

During the last two decades, the volume of activation has become a recognized criterion to complement traditional investigations of the mechanisms of substitution reactions. At this stage, it may be useful to recall the classification... 

The mechanism classification and the overall transformation classification are orthogonal to each other. For example, substitution reactions can occur by a polar acidic, polar basic, free-radical, pericyclic, or metal-catalyzed mechanism, and a reaction under polar basic conditions can produce an addition, a substitution, an elimination, or a rearrangement. Both classification schemes are important for determining the mechanism of a reaction, because knowing the class of mechanism and the overall transformation rales out certain mechanisms and suggests others. For example, under basic conditions, aromatic substitution reactions take place by one of three mechanisms nucleophilic addition-elimination, elimination-addition, or SrnL If you know the class of the overall transformation and the class of mechanism, your choices are narrowed considerably. 

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