Hydrogen Atom Reactivity toward Aqueous tert-Butyl Alcohol

Through a combination of pulse radiolysis, purification, and analysis techniques, the rate constant for the H + (CH3)3COH → H2 + •CH2C(CH3)2OH reaction in aqueous solution is definitively determined to be (1.0 ± 0.15) × 105 M–1 s–1, which is about half of the tabulated number and 10 times lower than the more recently suggested revision. Our value fits on the Polanyi-type, rate–enthalpy linear correlation ln(k/n) = (0.80 ± 0.05)ΔH + (3.2 ± 0.8) that is found for the analogous reactions of other aqueous aliphatic alcohols with n equivalent abstractable H atoms. The existence of such a correlation and its large slope are interpreted as an indication of the mechanistic similarity of the H atom abstraction from α- and β-carbon atoms in alcohols occurring through the late, product-like transition state. tert-Butyl alcohol is commonly contaminated by much more reactive secondary and primary alcohols (2-propanol, 2-butanol, ethanol, and methanol), whose content can be sufficient for nearly quantitative scavenging of the H atoms, skewing the H atom reactivity pattern, and explaining the disparity of the literature data on the H + (CH3)3COH rate constant. The ubiquitous use of tert-butyl alcohol in pulse radiolysis for investigating H atom reactivity and the results of this work suggest that many other previously reported rate constants for the H atom, particularly the smaller ones, may be in jeopardy.