Friedel-Crafts reactions 554 INDEX

Friedel and Crafts themselves observed that aluminum chloride is by no means the only specific catalyst in the Friedel-Crafts reaction. A number of other acidic metal halides could also be employed however, these were less reactive. The strength or coordinating power of different Lewis acids can vary widely against different Lewis bases. Hence it is extremely difficult to establish a scale of strength of Lewis acids in a manner analogous to that used for Brpnsted acids. Despite the difficulties, a number of qualitative orders of reactivity have been proposed. A comparative study of the activity of various Friedel-Crafts catalysts was performed by Olah and coworkers. Thus the activity index (the lowest temperature at which reaction occurs) of a large number of Lewis acid halides was measured using the benzylation reaction as the probe. 

The electrophilicity index also accounts for the electrophilic activation/deactivation effects promoted by EW and electron-releasing substituents even beyond the case of cycloaddition processes. These effects are assessed as responses at the active site of the molecules. The empirical Hammett-like relationships found between the global and local electrophilicity indexes and the reaction rate coefficients correctly account for the substrate selectivity in Friedel-Crafts reactions, the reactivity of carbenium ions, the hydrolysis of esters, the reactivity at the carbon-carbon double bonds in conjugated Michael additions, the philicity pattern of carbenes and the superelectrophilicity of nitronium, oxonium and carboxonium ions. This last application is a very promising area of application. The enhanced electrophilicity pattern in these series results from... 

An attempt has been made to analyse whether the electrophilicity index is a reliable descriptor of the kinetic behaviour. Relative experimental rates of Friedel-Crafts benzylation, acetylation, and benzoylation reactions were found to correlate well with the corresponding calculated electrophilicity values. In the case of chlorination of various substituted ethylenes and nitration of toluene and chlorobenzene, the correlation was generally poor but somewhat better in the case of the experimental and the calculated activation energies for selected Markovnikov and anti-Markovnikov addition reactions. Reaction electrophilicity, local electrophilicity, and activation hardness were used together to provide a transparent picture of reaction rates and also the orientation of aromatic electrophilic substitution reactions. Ambiguity in the definition of the electrophilicity was highlighted.15... 

Olah, G. A. Miscellaneous Reactions, Cumulative Indexes, in Friedel-Crafts and Related Reactions 4, 1191 pp. (Interscience Publishers,... 

Olah, G.A. Friedel-Crafts and Related Reactions, IV-Miscellaneous Reactions - Cumulative Indexes Intersci. Publ., New York/London/Sydney (1965)... 

---