Hydrogen Atom Reactivity toward Aqueous tert-Butyl Alcohol - The


Jan 9, 2012 - The ubiquitous use of tert-butyl alcohol in pulse radiolysis for investigating H atom reactivity and the results of this work suggest th...
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Hydrogen Atom Reactivity toward Aqueous tert-Butyl Alcohol Sergei V. Lymar* and Harold A. Schwarz Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *

ABSTRACT: 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.



INTRODUCTION Tertiary butyl alcohol (2-methyl-2-propanol, tert-BuOH) holds a special place in radiation chemistry; it is widely used to convert a highly reactive major primary water radiolysis product, OH radical, into a much less reactive C-centered 2hydroxy-2-methylpropyl radical

from tert-BuOH

(CH3)3 COH + H → •CH2C(CH3)2 OH + H2

This reaction must be considered when accounting for the fate of H and for the yield and further reactivity of •CH2C(CH3)2OH. Numerous measurements of the specific rate for reaction 3 were reported over the years, and there is a general agreement that k3 is 3−4 orders of magnitude below the diffusioncontrolled limit. The earliest estimate2 based on the competition between tert-BuOH and the DCO2− ion gives k3 = 1.3 × 105 M−1 s−1 when we apply the tabulated1 rate constant for H + DCO2−. Subsequent radiolysis/electron paramagnetic resonance (EPR) measurements3,4 yielded two different and disagreeing results for k3: 8 × 104 and 1.7 × 105 M−1 s−1 at 15 and 8 °C, respectively. The latter value, as measured more directly, was tabulated and accepted for years as the most reliable one.1 More recently, measurements based on direct observations of UV absorption of the •CH2C(CH3)2OH radical again gave contentious results for k3: 2.3 × 105,5 1.2 × 106,6 and

(CH3)3 COH + OH → •CH2C(CH3)2 OH + H2O k1 = 6.0 × 108 M−1 s−1

(1) 1

The relatively low specific rate of this reaction necessitates the use of large tert-BuOH concentrations, often 1 M or greater, but the even lower rates of tert-BuOH with the other two water radiolysis products, the solvated electron and hydrogen atom, allow studies of the reactivity of these species toward various compounds with little or no interference from OH. Especially frequently, the scavenging of OH by tert-BuOH is used in acidic solutions, so that the solvated electron is rapidly1 protonated

eaq − + H+ → H

k 2 = 2.3 × 1010 M−1 s−1

(2)

making H the dominant reactive species. Many hydrogen atom reactions with various compounds were investigated under these conditions: that is, high tert-BuOH concentrations and pH below 3. As with OH, H abstracts an H atom © 2012 American Chemical Society

(3)

Received: December 4, 2011 Revised: January 7, 2012 Published: January 9, 2012 1383

dx.doi.org/10.1021/jp2116593 | J. Phys. Chem. A 2012, 116, 1383−1389

The Journal of Physical Chemistry A

Article

Table 1. GC/MS Analyses of tert-BuOH impurities (content in wt %) sample 1 2 3 4 1RC 2AS 3MS a

treatmenta none, S-A no. 471712, lot 61496HJ none, S-A no. 308250, lot 43096KK none, S-A no. 308250, lot 26296TMV none, S-A no. 19460, lot BCBB6281 4 times recrystallization of 1 passing of 2 through Ambersorb XE-347 passing of 3 through molecular sieve 13X

majorb 2-PrOH (0.4) 2-PrOH (0.3) 2-PrOH (0.3) 2-BuOH (0.2)c 2-PrOH (0.1) 2-PrOH (