Stable CarboniumIons.-Rates of reaction of the tris-(p-methoxypheny1)carbonium ion with water, hydroxide ion, and ammonia have been measured by a stopped-flow technique. The second-order rate constants are 0.28, 4-6 x lo3, and 2.1 x lo3 1. mole-1 set.-' respectively and a negative salt effect was observed. The spectroscopic properties of a large number of substituted and bridged trityl cations have been recorded2 and resolution of the first asymmetric carbonium ion (1) (by virtue of restricted rotation) has been achieved. The n.m.r. spectra of some diphenyl (2and 3-pyridinium)methyl dications (2) have shown that the heterocyclic ring protons are more deshielded than the phenyl proton^.^ Among new aromatic cations are the ditropylium dication (3) which exhibits a singlet n.m.r. spectrum at z 0.05 (cf: the tropylium ion at z 1-31) and is assumed to be twisted about the central bond.5 The tri-t-butylcyclopropenium ion appears to be less stable (pKR+ = 6.1) than the tri-isopropyl analogue, (pKR+ = 7.0) as measured by titration against sodium hydroxide in 50% aqueous acetonitrile.6 How much this is due to steric crowding and how much to a reduction in electron release by the t-butyl group is difficult to assess, but it may be relevant that the heptaphenyltropylium ion is less stable than '
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