The gas-phase alkylation of xylenes by t-C4H9+ ions from the radiolysis of neopentane has been investigated in the pressure range from 20 to 771 Torr. The gaseous cation is entirely unreactive toward p-xylene and alkylates o-xylene, yielding exclusively 1,2-dimethyl-4-tert-butylbenzene.R eaction with m-xylene yields a mixture of 1,3-dimethyl-4-tert-butylbenzene and 1,3-dimethyl-5-tert-butylbenzene,w hose isomeric ratio depends on the experimental conditions, ranging from the 8:92 value observed at 20 Torr in the absence of added bases to the 89: I 1 value measured at 770 Torr in the presence of NH3. The substrate selectivity of the gaseous electrophile has been studied by competition experiments with respect to toluene. The observed ko.xylene:ktoluenrea tio of 2.0 & 0.2 appears fairly independent of the experimental conditions, while the apparent km-xylene:ktoluenrae tio is considerably affected by the nature and the concentration of gaseous bases, ranging from 0.65 & 0.1 in the absence of NH3 to 1.9 f 0.2 in the presence of excess NH3. These results are interpreted assuming the reversible tert-butylation at the ring positions ortho to one methyl group of m-xylene in order to explain both the effect of the pressure and of NH3 on the isomeric composition of products and the apparent changes of the reactivity of m-xylene with respect to that of toluene. The reaction provides an example of an aromatic alkylation by a gaseous, unsolvated carbenium ion and allows the direct evaluation of the reactivity and the steric requirements of the electrophile, unaffected by the complicating effects of the solvent and the counterion invariably present in solution. The unusual substrate and positional selectivity of the gaseous t-C4H9+ reagent, in particular the remarkable extent of the substitution ortho to one methyl group of m-xylene, are discussed in comparison with the conventional Friedel-Crafts alkylation.
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|Titolo:||Gas-phase alkylation of xylenes by tert-butyl(1+) ions.|
|Data di pubblicazione:||1976|
|Appartiene alla tipologia:||01a Articolo in rivista|