2662
M. Simic. P. Neta, and E. Hayon
Reactions of Hydroxyl Radicals with Unsaturated Aliphatic Alcohols in Aqueous Solution. A Spectroscopic and Electron Spin Resonance Radiolysis Study M. Simic,'" P. N e t a , l b and E. H a y o n *
lC
Pioneering Research Laboratory, U.S. Army Natick Laboratories, Natick, Massachusefts 0 1 760, Radiation Biology Laboratory, Zooiogy Department, University of Texas, Austin, Texas 7871 2, and Radjation Research Laboratories, Mellon lnstitute ot Science, Carnegie-Mellon University. Pittsburgh, Pennsylvania 15213 (Received May 29, 1973) Publication costs assisted by Natick Laboratories
The reactions of OH and 0- radicals with allyl alcohol. 1,4-pentadien-3-01, and 2,4-hexadien-l-o1 in aqueous solutions were studied using the spectrophotometric pulse radiolysis and the in situ steady-state radiolysis-esr techniques. The reaction rate constants of the OH radicals with these compounds ranged from 6.0 to 10.0 X 109 M - I sec-1 and of 0- radicals from 2.4 to 4.3 X 109 M - 1 sec-1. The transient absorption, extinction coefficients, and decay kinetics of the intermediates produced from the reactions of OH, 0-, and H atoms with these alcohols were determined. The OH radicals were found to add predominately to the carbon-carbon double bonds, but a measurable proportion (10-20%) were found to abstract hydrogen from allylic positions (a-hydroxy positions). With conjugated double bonds, e g , 1.4-pentadien3-01, the extent of abstraction by OH radicals increased by a factor of 2. The 0- radicals were found to react predominately by hydrogen atom abstraction. The presence of double bonds conjugated to the a hydroxy radical site was found to considerably increase the acidity of these radicals. Thus, the pKa of the CHz=CHCHOH and CKz=CHC(OH)CH=CH2 radicals are 9.6 I 0.1 and 8.9 I 0.1. respectively. The esr experiments confirm the abstraction reactions of 0- radicals with these compounds. The importance of the abstraction reactions of OH radicals from substituents in an N position to double bonds are discussed. These reactions may be of importance in the fields of radiation polymerization and biological radiation damage.
Introduction Differences in the reaction rate constants and the site of reaction of hydroxyl radicals, OH, and their conjugate base 0- with compounds containing carbon-carbon double bonds have recently been demonstrated.2 The equilibrium constant OH
---L
0-
+
H"
(1)
is well established, pK, = 11.g3 and 11.8.4Due to the relatively high dissociation constant of the OH radical and the lower reactivity of 0- radicals,5.6 the reactions of 0 have to be studied a t pH 14 or higher. In this work, the reactions of OK and 0- radicals with aliphatic alcohols containing a single double bond, and conjugated and nonconjugated double bonds have been examined and compared. Conjugated double bonds are found in many biological compounds, e . g . , carotenoids, porphyrins, etc., and the systems studied here can serve as simple model compounds. The technique of pulse radiolysis and kinetic absorption spectrophotometry was used to study to reactions of hydroxyl radicals with allyl alcohol, 1,4-pentadien-3-01, and 2,4-hexadien-l-o1 in aqueous solutions. The radicals produced in steady-state irradiated solutions have also been identified by esr. Experimental Section
The pulse radiolysis experiments were performed using single pu!ses of 2.3 MeV electrons and -30 nsec duration (Febetron 705 machine), and the technique and conditions employed have been described.738 The esr experiments were performed using the steady-state in situ radiolysis set-up previously described.$ The Journal of Physical Chemistry. Voi. 77. No. 22, 1973
Allyl alcohol (Eastman Organic) and both dienes (Chemical Samples Co.) were used without further purification. The p H of the solutions were adjusted using perchloric acid, potassium hydroxide, and 1 m M phosphate and tetraborate buffers. Dosimetry was carried out using 0.05 M KCNS solutions (1 atm NzO) and the extinction coefficients calculated on the basis G ( 0 H ) = G(eaq-) = 2.8. The solutions were prepared immediately before use and protected from exposure to light from the monitoring lamp using a synchronized shutter.
-
Results The radiolysis of water generates eaq-. OH radical. and H atoms with C; values 2.8. 2.8, and 0.6, respectively. In the presence of KzO (1 atm) eaq- is virtually completely converted to 0-, which is in rapid equilibrium with OH
+
OH OH0- 4 H-O (2) with k z = 1.2 X 1010 M - 1 sec-1 and h-2 = 9.2 X 10' sec-1 (ref 6). Also in E20 solutions, the H atoms contribute -1270 of the total radicals formed and this can be accounted for. In alkaline solutions. H atoms react with OH- ions €1 OHe,
