Carbonium ions ethynyl

Richey [8], and then Pittman and Olah [9], observed the NMR spectra of acidic solutions of tertiary ethynyl carbinols, and their data support the idea that allenyl carbonium ions contribute significantly to the following ion structure ... 

Connor, R. E., Nicholas, K. M. Isolation, characterization, and stability of a-[(ethynyl)dicobalt hexacarbonyl] carbonium ions. J. Organomet. Chem. 1977,125, C45-C48. 

Braude ei al. studied the dependence of rates of acid-catalyzed isomerization of of-ethynyl-y-methylallyl alcohoF and a-phenyl-y-methylallyl alcohol on the composition of mixed solvents such as aqueous ethanol and aqueous dioxane. If allowance is made for the effect of solvent on the concentration of the reactive conjugate acid of the alcohol, these data show that the rate of isomerization of the conjugate acid is nearly independent of solvent composi-tion as would be expected for a unimolecular rate-limiting dissociation of the conjugate acid to a solvated carbonium ion, Kwart and Herbig reached a... 

In protic solvents, the allylic carbonium ion formed by acid-catalyzed alkyl carbon-oxygen bond fission can recombine either with the carboxylic acid molecule or with a solvent molecule. The electrostatic attraction between the carbonium and carboxylate ions, which is a major factor in isomerization of allylic esters by ion-pair internal return during solvolysis, is absent in the acid-catalyzed reaction. The more numerous, usually more nucleophilic, solvent molecules in the solvation shell of the carbonium ion should compete effectively with the departed carboxylic acid molecule and solvolysis rather than isomerization should be the predominant reaction. For example, in the presence of 0.05 M perchloric acid, solvolyses of cis- and //- //7.s-5-methyl-2-cyclohexenyl p-nitrobenzoates are not only very much faster than in the absence of the acid, but polarimetric and titrimetric rates of solvolysis of optically-active esters were identical within experimental error. For these esters, the acid-catalyzed solvolysis was not accompanied by a detectable amount of isomerization. Braude reported, on the basis of indirect evidence, that isomerization accompanies acid-catalyzed hydrolysis of a-ethynyl-y-methylallyl acetate in aqueous dioxane. It was shown that, under some experimental conditions, the spectrophotometrically determined rate of appearance of the rearranged 1 -yne-3-ene chromophore exceeds the titrimetrically determined rate of hydrolysis,... 

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