The role of ion-molecule pair intermediates in acid-catalyzed solvolysis. General base-catalyzed formation of 4-methylbenzyl carbocation and its trapping by nucleophiles

Article abstract of DOI:10.1021/jo00078a022

The specific acid-catalyzed hydrolysis of 3,6-dimethoxy-3,6-dimethyl-1,4- cyclohexadiene (1) in 25 vol % acetonitrile in water at 25 °C provides 2,4- dimethylphenol (2a), 2,5-dimethylphenol (2b), 2,4-dimethylanisole (3a), 4- methylbenzyl alcohol (4), and small amounts of 2,5-dimethylanisole (3b) and 4-methylbenzyl methyl ether (5). The formation of 3b and 5 is proposed to involve intramolecular rearrangement of carbocation-molecule pair intermediates. The formation of 4-methylbenzyl alcohol (4) is catalyzed by general bases, indicating that dehydronation of the cyclohexadienyl carbocation is rate-limiting in its reaction to benzylic products. The Bronsted parameter was measured with carboxylate anions as β = 0.28. The 4- methylbenzyl carbocation, which is postulated to be an intermediate in this reaction, discriminates between added nucleophiles and solvent water: k(SCN)/k(HOH) = 37, k(Cl)/K(HOH) = 9.8, k(EtOH)/k(HOH) = 1.9, and k(MeOH)/k(HOH) = 2.1 (ratios of second-order rate constants). The rate constant for reaction of the carbocation with solvent water is estimated as k(HOH) = 1.4 x 10⁸ M^-1s^-1 (5.6 x 10⁹ s^-1), assuming a diffusion- controlled rate constant of 5 x 10⁹ M^-1s^-1 for its reaction with thiocyanate anion. The nucleophilic selectivity values are considerably smaller than those exhibited by 4-methylbenzyl bromide under the same reaction conditions: k(Cl)/k(HOH) = 210, k(EtOH)/k(HOH) = 3.8, and k(MeOH)/k(HOH) = 5.3. These differences are concluded to be due to different mechanisms. The bromide reacts by a concerted bimolecular S(N)2 mechanism with chloride anion. With alcohols and water, the reactions occur either by a concerted uncoupled mechanism or via the ion pairs.