The 4,4'-dimethoxytrityl carbenium ion by ionization of 4,4'-dimethoxytrityl alcohol in acetonitrile - aqueous perchloric and nitric acids containing electrolytes: kinetics, equilibria, and ion-pair formation

Abstract

We have studied the equilibration shown in eq. [3] of 4,4′-dimethoxytrityl alcohol in aqueous perchloric and nitric acids containing low proportions of acetonitrile using stopped-flow kinetics techniques. The rate constants for the overall progress to equilibrium, k obs , have been resolved into forward and reverse components using the equilibrium UV absorbance and a value for the molar absorptivity of the 4,4′-dimethoxytrityl carbenium ion determined in concentrated aqueous perchloric acid. The forward reaction (rate constant k f ) is first order in both the alcohol and the acid concentrations; the reverse reaction (rate constant k r ) is pseudo first order with respect to the carbocation. At constant hydronium ion concentration, the forward rate constant increases linearly with the concentration of electrolyte, whereas the reverse rate constant decreases. These effects depend upon the nature of the anion, but not the cation, and are not ionic strength effects. At constant anion concentrations, k f in both acids, and k r in perchloric acid, are independent of hydronium ion concentration; however, k r decreases with increasing hydronium ion concentration at constant nitrate concentration. At nonconstant ionic strength, changes in k f and k r observed in increasing concentrations of perchloric acid are attributable wholly to changes in perchlorate concentration. A mechanism is proposed which involves pre-equilibrium protonation of the alcohol, heterolysis of the protonated alcohol to give a 4,4′-dimethoxytrityl carbenium ion -water ion-molecule pair, then conversion of this into a dissociated carbenium ion in equilibrium with ion pairs. To account for the strong effects of perchlorate and nitrate upon the forward rate constants, it is proposed that these anions provide additional reaction channels from the ion-molecule pair. However, we find no evidence of acid catalysis in the reaction of the ion-molecule pair (in contrast to our finding for the reaction of the corresponding ion-molecule pair formed from dimethoxytritylamine in acidic media). Some of the elementary rate and equilibrium constants of the proposed mechanism have been evaluated.

Acid‐catalysed hydrolysis of methoxy‐substituted trityl trifluoroethyl ethers: a kinetic and computational investigation of leaving group effects

“…[7,8,19,20] These wholly unprecedented results were accommodated by a mechanism involving ion-molecule pairs, Scheme 3, that is by invoking the distinction between ionisation and dissociation of Tr 0 À ÀNH 2 R þ . [9,10,12] The final equilibrium here in which the (substituted) trityl carbenium ion undergoes reversible nucleophilic capture by water is, of course, exactly the same as in the acid-induced deprotection of trityl ethers, Scheme 1, and the protonation/dissociation of trityl alcohols themselves (Scheme 2).…”

“…This reaction is closely similar to the protonation/dissociation of the corresponding alcohols Tr 0 À ÀOH, Scheme 2. [9,10] In the ether cleavage, the final equilibrium proportions of the trityl alcohol and trityl carbenium ion are the same as from the trityl alcohol itself under the same conditions. Indeed, the reverse of the mechanism of Scheme 2 (or its concerted equivalent) is an expansion of the final equilibration shown in Scheme 1.…”

“…[9][10][11][12] This reversible reaction shown in Scheme 2 is the basis of the well-known H R acidity function scale where H R relates to K R þ , [13][14][15][16][17] and the experimentally observed pseudofirst-order rate constant for the overall equilibration, k obs , is for the sum of forward and reverse processes (regardless of the initial extent of reaction. [9,10] (www.interscience.wiley.com) HYDROLYSIS OF SUBSTITUTED TRITYL TFE ETHERS 3 (pK a ¼ 5.07; [53] k o ¼ 37 s À1 ); 4-MeOC 6 H 4 NH þ 3 (pK a ¼ 5.29; [53] k o ¼ 45 s À1 ); 4-FC 6 H 4 NH þ 3 (pK a ¼ 4.52; [53] k o ¼ 43 s À1 ). k o values from references [7,8,50] Scheme 4.…”

“…equivalent). [9][10][11][12] This reversible reaction shown in Scheme 2 is the basis of the well-known H R acidity function scale where H R relates to K R þ , [13][14][15][16][17] and the experimentally observed pseudofirst-order rate constant for the overall equilibration, k obs , is for the sum of forward and reverse processes (…”

“…regardless of the initial extent of reaction. [9,10] (www.interscience.wiley.com) HYDROLYSIS OF SUBSTITUTED TRITYL TFE ETHERS 3 (pK a ¼ 5.07; [53] k o ¼ 37 s À1 ); 4-MeOC 6 H 4 NH þ 3 (pK a ¼ 5.29; [53] k o ¼ 45 s À1 ); 4-FC 6 H 4 NH þ 3 (pK a ¼ 4.52; [53] k o ¼ 43 s À1 ). k o values from references [7,8,50] Scheme 4.…”