Electrophilic Reactivity of the Triphenylmethyl Carbocation in Aqueous Solutions

Article abstract of DOI:10.1021/ja00282a029

The triphenylmethyl (trityl) carbocation has been generated as a transient intermediate by laser flash photolysis of 1:2 (v/v) acetonitrile:water solutions of trityl acetate and trityl 4-cyanophenyl ether.Identification of the transient as the free carbocation in the ground state was based on its characteristic absorption spectrum and upon conductivity changes.Rate constants have been measured for the reaction of the cation in this solvent with a series of ionic and neutral nucleophiles.The solvent rate constant at 20 deg C is 1.5 x 10⁵ s^-1.Azide ions reacts at 4.1 x 10⁹ M^-1s^-1; the directly measured azide:water ratio is compared to literature values determined by product analysis.Chloride ion reacts at 2 x 10⁶ M^-1 s^-1; with bromide the equilibrium addition can be observed with k(comb) = 5 x 10⁶ M^-1 s^-1 and k(ion) for Ph3CBr = 8 x 10⁵ s^-1.Rate constants do not adhere to the N₊ relationship.This predicts a slope of unity in a plot of log k(Ph3C⁺) vs.N₊, with the better nucleophiles reacting at the 10¹⁰ encounter-controlled limit.Azide is the only nucleophile of those studied to approach this.Sulfite and thiolate ions, which are better N₊ nucleophiles, react at 2-3 x 10⁸ M^-1s^-1, while amines react in the 10⁶-10⁷ M^-1s^-1 range.The plot vs.N₊ has a slope of 0.3-0.4.One explanation is that rate constants for the better nucleophiles do level, but this occurs considerably below the 10¹⁰ limit.Alternatively, the less than unit slope is real and this more reactive cation, in contrast to more stable analogues, is exhibiting selectivity.