Aryl Carbonium Ions in Sulfuric Acid¹

H0 And Related Indicator Acidity Function

“…The initial studies were with the indicator 4,4', 4,/-trinitrotriphenylcarbinol in 80-95 per cent aqueous sulfuric acid. That compounds of this type actually react with acids in the manner indicated has been established by cryoscopic studies in 100 per cent sulfuric acid (75). The usefulness of the new acidity function was early evident from the fact that the behavior of this indicator closely paralleled the rate of nitration of nitrobenzene and other compounds, whereas the H0 acidity function did not (66,99).…”

“…The indicator acidity function increases quite smoothly for increasing concentrations of sulfuric acid. Even so, it must be noted that the absolute values of Ho cannot be considered to be very firmly established beyond about 75 per cent sulfuric acid. The reason is that beyond this there are two places where the overlap in the ranges of the indicators is so poor as to make the pK value of the weaker base somewhat doubtful.…”

“…from those of H0 by the approximation j[ = H0 + log aH¡¡o i.e., by assuming that the last term in equation 16 might cancel out. Further indicator studies by Williams and Bevan (100) have shown, however, that this approximation is unsatisfactory at the lower acid concentrations, although it does appear to be useful for relative Jo values in solutions containing more than 75 per cent sulfuric acid. Gold (30,31) has recently proposed other more satisfactory approximations for the J0 function.…”

Reactivity of sterically hindered diarylbenzhydryl carbocations – competing trapping by nucleophiles and elimination

“…It is also of interest to investigate the significance of the "buttressing effect". Newman and Deno 9 have argued that resonance in mesityldimethyl methylcarbocation is further sterically hindered by the introduction of two methyl groups in the 3-and 5-positions to form the pentamethylphenyldimethyl methylcarbocation. A similar observation might have been expected when methyl groups are introduced into the 3,5-and the 3',5'-positions in the mesityl carbocations 11b, hence giving rise to greater stability of the bis-(pentamethylphenyl) cations 11c relative to the bis-mesityl cation 11b.…”

Reaction of 4-Dimethylaminoazobenzene and Related Compounds with 4,4'-Bis-(dimethylamino)-benzhydrol1

“…Although the ionization is promoted by electron-releasing substituents,21 electron-withdrawing groups do not appreciably suppress the ionization. 19 Reactivities of carbonium ions formed are, on the other hand, enhanced by the presence of electron-withdrawing groups.20 In view of the possibility that Michler's hydrol may also be ionized to a carbonium ion with ammonium groups, as is suggested by the van't Hoff's ¿-factor of 4,4'bis-(dimethylamino)-triphenylcarbinol,19 the higher reactivity of Michler's hydrol over benzhydrol is conceivably due to strong activation by ammonium groups. Indeed Michler's hydrol is so reactive that DAB and its derivatives are readily alkylated in spite of deactivation of the latter compounds owing to salt formation in coned, sulfuric acid.22…”