Mechanism nonprotonic acids

If the mechanism in acid and without acid are the same, one might have expected 4-alkylation under both conditions, and the failure to observe any 4-alkylation when acid is not present is as yet unexplained. Possibly with nonprotonated bases the hydroxyalkylation occurs according to Scheme 11, in which dimerization of two radicals within the solvent cage would lead to attack only at position 2, while in acid the attack could take place, at least in part, according to Scheme 12 but with protonated base, leading to both the isomers (2 and 4), as in the hydroxyalkylation by oxidation of alcohols. The much higher affinity of alkyl radicals toward protonated heteroaromatic bases in comparison with nonprotonated bases would support this interpretation. 

A kinetic study has been carried out in order to elucidate the mechanism by which the cr-complex becomes dehydrogenated to the alkyl heteroaromatic derivative for the alkylation of quinoline by decanoyl peroxide in acetic acid. The decomposition rates in the presence of increasing amounts of quinoline were determined. At low quinoline concentrations the kinetic course is shown in Fig. 1. The first-order rate constants were calculated from the initial slopes of the graphs and refer to reaction with a quinoline molecule still possessing free 2- and 4-positions. At high quinoline concentration a great increase of reaction rate occurs and both the kinetic course and the composition of the products are simplified. The decomposition rate is first order in peroxide and the nonyl radicals are almost completely trapped by quinoline. The proportion of the nonyl radicals which dimerize to octadecane falls rapidly with increase in quinoline concentration. The decomposition rate in nonprotonated quinoline is much lower than that observed in quinoline in acetic acid. 

Photochemical a-hydroxyalkylation by alcohols is strictly connected with photochemical alkylation. With quinoline an acidic medium leads to alkylation hydroxy alkylation takes place with nonprotonated bases. The suggested mechanism is substantially identical for both processes. A IT -> 7T excited state would cause the initial hydrogen abstraction (Scheme 11). 

The rate equation with strongly acidic catalysts is also second order in silanol and first order in catalyst (75). The mechanism is proposed to proceed via protonation of silanol, followed by an electrophilic attack of the conjugate acid on nonprotonated silanol. The condensation processes outlined in reactions 16a and 16b for sulfonic acids is also an applicable mechanism for the acid catalysis. The condensation polymerization in emulsion catalyzed by dodecylbenzenesulfonic acid is second order in silanol, but the rate has a complex dependence on sulfonic acid concentration (69). This process was most likely a surface catalysis of the oil-water interface and was complicated by self-associations of the catalyst-surfactant. 

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