Friedel-Crafts catalysts reactivity order

Disproportionation (transalkylation) and positional isomerization usually take place simultaneously when either linear or branched alkylbenzenes are treated with conventional Friedel-Crafts catalysts or with Nafion-H. The reactivity of alkyl groups to participate in transalkylation increases in the order ethyl, propyl < isopropyl < tert-butyl.117 207 217... 

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. 

With Friedel-Crafts halides (usually A1C13 and BF3) it is necessary to use equimolar or excess catalyst when alcohols are the alkylating agents. With primary alcohols usually 2 mol of catalyst per mole of alcohol must be used. Complexing the alcohol, as well as binding the water formed in the reaction explains the experimental findings. With secondary and tertiary alcohols, lesser amounts of catalyst are needed. The reactivity of different alcohols follows the order methyl < primary < secondary < tertiary, allyl, benzyl. The ease of carbocation formation according to Eq. (5.51) is most probably responsible for this reactivity order ... 

Olah and Kuhn S found that fluorochloro-, fluorobromo- and fluoroiodo-alkanes are effective chloro-, bromo- and iodo-aikylating agents, respectively, in Friedel-Crafts alkylations of arenes in the presence of boron halide catalysts (equation 86). Boron trihalides catalyzq reactions of only the C—F bonds, but not of the C—Cl, C—Br or C—I bonds. The order of reactivity of the catalysts was found to be BI3 > BBr3 > BCI3 > BF3, and that of the carbon-halogen bonds C—F > C—Cl > C—Br > C—I. 

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