Olefins in Aqueous Acid

Olefins dissolved in aqueous acid are in rapid reversible equilibrium with a carbonium ion formed by addition of a proton.288 This rapidly and reversibly formed carbonium ion has to be a non-classical one in view of the behavior of the isomeric pentenes XLVI and XLVII.28 Both pentenes react with dilute nitric acid to give the same tertiary carbinol. If the reaction is interrupted when half of the olefin has been converted to carbinol, the remaining olefin has its original structure in both cases. The first product of protonation of the olefin is therefore of such a structure that loss of a proton gives only the original olefin. The reversibly formed carbonium ion can not therefore be the classical one. 

The reversibly formed re-complexes precursor to the hydration of olefins have the proton imbedded in the re-electron cloud of the double bond somewhere between the two carbon atoms. They are therefore 

This means that the ionization and rearrangement need not be concerted and that symmetrical protonated ethylene can not be a major intermediate in the reaction. A similar experiment with isobutylamine and nitrous acid in heavy water gave products that contained no carbon-deuterium bonds. Since it is known that the -complex formed from isobutylene and acid is in rapid equilibrium with protons from the solvent, none of this can be formed in the nitrous acid induced deamination. This in turn makes it probable that the transition state for the hydrogen migration is of the sigma rather than the -bonded type.261 

In contrast to the simple olefins, aryl-substituted olefins dissolve in sulfuric acid to give comparatively stable carbonium ions, as is shown by the -factors, the spectra, and the recovery of the olefin on dilution.262 In some cases it is neccessary to extrapolate the freezing point depression to zero time owing to a slow sulfonation. Because of the similarity in the spectra it is believed that these carbonium ions have the classical structures shown below.263 

It is difficult to see why the spectra of the ions from diphenylethylene and triphenylethylene should resemble that of the ion from anthracene if the ions have the non-classical protonated double bond structure. 

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