Ingold mechanism classification

But Ingold s triumph came in finally seeing the advantages of Robinson s explanation system, revising it, and substituting a new and clearer language and classification of types of reaction mechanisms. Lapworth, Robinson, and their collaborators referred to Ingold s "conversion" experience, a conversion in which Paul eventually helped create the myth of his role not as saint but as savior. 

The discussion and classification of reagents is masterful in identifying Ingold s new nomenclature and principles with more widely known oxidation-reduction and acid-base theory. The 1953 lectures at Cornell University, published as Structure and Mechanism in Organic Chemistry, follow this same strategy, showing how old classification schemes overlap with each other and how apparent inconsistencies disappear as old schemes are incorporated into the new one. Nineteenth-century Berzelian electrochemical dualism, revived by Lapworth and Robinson in the cationic/anionic schema, disappears into the electrophilic/nucleophilic language. 

Other terms that he invented include the system of classification for mechanisms of aromatic and aliphatic substitution and elimination reactions, designated SN1, SN2, El, and E2. "S" and "E" refer to substitution and elimination, respectively, "N" to nucleophilic, and "1" and "2" to "molecularity," or the number of molecules involved in a reaction step (not kinetic order, having to do with the equation for reaction rate and the concentration of reactants). Ingold first introduced some of these ideas in 1928 in a... 

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. 

Shiner has studied the effect of ft deuterium substitution on the solvolysis of tertiary amyl chlorides. The mechanism and classification of the reaction in the Ingold scheme is... 

Organic chemists refer to Lewis acids as electrophiles because they are attracted to electron-rich sites on other molecules, and they refer to Lewis bases as nucleophiles because they are attracted to electron-deficient molecular sites. Ingold (1969) developed a classification of reaction mechanisms based on this idea and this classification is the foundation for modeling the mechanisms of organic reactions (Bruckner, 2002 Grossman, 1999). Casey (2001) and Casey and Swaddle (2003) adapted some of these principles to apply to the dissolution of oxides. The electron-rich, and therefore nucleophilic, sites on molecules are nicely visualized using the electron localization function (Gibbs et al, 2005). 

Langford and Gray presented another classification for the mechanism of substitution reactions that is particularly appropriate for metal complexes [5], but also englobes the classification of Ingold and co-workers for organic substitutions. This classification makes a distinction between the stoichiometric mechanism and the activation mechanism. The stoichiometric mechanism concerns the nature of the intermediate. It distinguishes between ... 

Figure 11.2 Relationship between the mechanisms of substitution reactions and their energy profiles, and the classifications of Hughes-Ingold and Langford-Gray.

Figure 11.2 presents the relation between the classification of Ingold-Hughes and that of Langford-Gray. An example of a dissociative mechanism, D, which corresponds to the S,., mechanism in the classification of Ingold-Hughes, is reaction (11. VII) and... 

The classification of chemical substances as electrophilic and electrodotic according to their behavior in their reactions with other substances can be extended to radicals in a molecule. When this is done, a flood of light is thrown upon the nature of acid-base catalysis. Ingold,Robinson, and others have already done much to clarify many organic reaction mechanisms, but the electronic theory of acids and bases provides a measure of correlation and insight which so far is unobtainable by any other method. Generalized acid-base catalysis will be considered later. In this chapter we shall deal primarily with the effect of acidic and basic radicals in the benzene ring. 

---