Lifetimes of Oxocarbenium Ions in Aqueous Solution from Common Ion Inhibition of the Solvolysis of α-Azido Ethers by Added Azide Ion

Rate constants for hydration of the oxocarbenium ions derived from a series of carbonyl compounds have been determined from common ion inhibition of the solvolysis of the corresponding α-azido ethers by trapping of the oxocarbenium ion intermediate with added azide ion, assuming k_az= 5 X 10⁹M^-ls^-1: acetophenone, k_HOH= 5 X 10⁷; acetone, 1 X 10⁹; 4-methoxybutanone, 2 X 10⁹; methoxyacetone, 4 X 10⁹; benzaldehyde, 2 X 10⁹; propionaldehyde, 2 X 10¹⁰s^-1. Substitution on the reacting carbon atom affects log k_HOH0.4 as much as log K_eq.Resonance effects are much larger than polar effects of substituents on k_HOH, compared with K_eq; this represents imbalance in the expression of these effects in the transition state. Use of these substituent effects to estimate the lifetime of glycosyl and methoxymethyl oxocarbenium ions gives values of K_HoH~ 10¹²s^-1 for the glycosyl cation and 10¹²— 10¹⁵s^-1 for the methoxymethyl cation. It is concluded that the glycosyl cation has a short but significant lifetime in aqueous solution and that there is little or no barrier for hydration of the methoxymethyl cation. The effects of substituents on the rate constants for addition to oxocarbenium ions are smaller than those for addition to the corresponding carbonyl compounds. The decrease in reactivity toward water of protonated acetone compared with the corresponding oxocarbenium ion by ~ 10⁵ suggests that the proton occupies an intermediate position between the carbonyl group and water. The dependence of log k_ROH on pK_ROH for the addition of alcohols to oxocarbenium ions derived from derivatives of butanone and propionaldehyde in aqueous alcohol mixtures follows slopes of 0.5 and 0.1, respectively.