Nov., 1962 REACTIVITYOF ATOMICHYDROGEN IN ALKALINE SOLUTIONS OF CHLOROACETICACID 2081 The rate constants ratios k!& and kzJ/kll indi- scavenged by chloroacetic acid lead to dehydro- cate that H atoms as such rea,ct with chloroacetic genation. On the other hand, radiation chemical acid ma,inly by hydrogen abstraction. These studies6 indicate that at pH 5.5 the chloride yield results slioyld be compared with recent radiation is G(Cl-) = 2.8 at 0.01 M chloroacetic acid.f chemical studies of this system.V Radiation This value is near to the standard yield of the chemical investigations of aqueous chloroacetic acid reducing radicals in neutral solutions, obtained solutions were interpreted by Hayon and Weisss from the hydr~gen-oxygen'~and ethanol-oxygen15 by assunning that transient negative ions formed systems. Thus the reducing radical formed as as primary products in the radiolysis of water the main product by radiolysis in the neutral pH react with monochloroacetic acid with the forma- region exhibits reactivity different from the H tion of chloride ion. These species are the pre- atoms. This H atom precursor is presumably the cursors of H atoms which react by dehydrogena- solvated electron eaq. The reaction mechanism tion,r The quantitative study of this system by in irradiated aqueous solutions of chloroacetic acid Hayos and Allen6indicates that the reducing radical involves the reactions formed from the radiolysis of water yields C1- ion. H+ and chloroacetic acid compete for this radical. eaq + CH2ClCOOII +C1- + CH2COOH (6) The product of the reaction with H+ ion is another radical which reacts with the acid to form either ea, + CHzClCOO- +C1- + CHZCOO- (6') C1- or H2. These studies cannot determine un- ambiguously the nature of the two reducing radi- eaq + €I+ -+ H (7) ca18.6 The experimental results obtained in the present and H atoms reacting by reactions 1and 2. work in the acid region indicate no pH dependence These conclusions are consistent with the results of the chloride yield in the pH region 0.4-2. of a previous worksf where the reactivity of H This result rules out the formation of Hz+ under atoms in HsOz solutions was compared with the our experimental conditions, and the participation reactivity of the reducing species formed from the of this radical in dehydrogenation reactions. The radiolysis of water.lj2 The results of the present concentration dependence of R(Cl-) at pH 1.9 and the previous studysf of the reactivity of H atoms and 3.3 (Table I) cannot be adequately interpreted in aqueous solutions make possible an unambiguous in terms of H atoms reacting at pH > 3 and of Hz+ identification of the acid form of the reducing radi- reacting at pH < 2. It thus appears that the Hz+ cal produced in the radiolysis of water. These molecule ion is of no kinetic importance in this conclusions are consistent with recent tentative system. This conclusion is consistent with the identifications of the pair of the reducing radi~als.~~6 low rate of formation of this species in acid solu- The primary reducing species eaq has a sufficiently tion.8b The ra,te constants ratio lc2/k, = 0.5 long lifetime to react with active scavengers. The obtained in our experiments using H atoms as conversion of eaqto H atoms proceeds by reaction 7 such is in good agreement with the rate constants rather than by reaction with a water molecule. ratio ;ierived-by Hayon and Allen6 for the repe- 0.55 NOTEADDED IN PRooF.-Recently, kinetic salt effects activity of the acid form of H atoms (denoted as were utilized (G. Czapski and H. A. Schwarz, J.Phys. Chem., H' by A1len)V in irradiated solutions of chloro- 66,471 (1962)) as an evidence that the reducing radical pro- acetic acid. This agreement indicates that the duced in the radiolysis of neutral aqueous solutions may be acid form of the reducing radical formed in the identified as the solvated electron. This conclusion is in radiolysis of water involves an H atom as such. complete agreement with the results of the present work. (14) C. J. Hoohanadel, J. Phys. Chem., 66, 687 (1952). Obtained in the (15) G. G. Jayson, G. Scholes, and J. Weiss. J. Chem. Sac., 1358 pH region indicate that about 90% of the H atoms (1967). THE DECOMPOSITION OF CHLOROACETIC ACID IN AQUEOUS SOLUTIONS BY ATOMIC HYDROGEN. 11. REACTION MECHANISM IN ALKALINE SOLUTIONS BY JOSHUAJORTNER AND JOSEPHRABANI Department of Physical Chemistry, Hebrew University, Jerusalem, Israel Received Decsmbsr PI, 1061 The reactivity of atomic hydrogen in alkaline solutions of chloroacetic acid was investigated. The pH and concentra- tion dependence yield kinetic evidence for the formation of HzO- in alkaline solutions: H + OH--. HzO-with ICH+OH- = 6.X lo6 1. mole-' sec.-l. These results are correlated with radiation chemical data, and the nature of the HzO- radical is discussed. Introduction solutions at neutral pHV differ in their reactivity In the preceding work' it was shown that H atoms with chloroacetic acid. Aqueous solutions of generated in an discharge and the re- (2) E. Hayon and J. Weiss, PTOC.Intern. Canf. Peaceful Uses At. ducing radicals produced by radiolysis of aqueous Enerev, Geneva, as, (1958). (1) J. Jortner and J. Rabani, J. Phys. Chem., 66, 2078 (1~262). (3) E. Hayon and A. 0. Allen, J. Phys. Chem., 66,2181 (1961). 2082 JosHU-4 JORTSER-4KD JOSEPHRABANI Yol. 66 region 0.1-0.5 A4 the chloride content of the blank solutians did not exceed 2 X M. Chloroacetic acid, KOH, NaHCOa, NazCOa, and formic acid used were of Analar grade. Water was triply distilled. Solutions were pared in freshly boiled water. As only a negligible e#::; 1. of added Na2C03 on the experimental results was obsarved, - no further recautions were necessar I pH measurements 4 4 were Carrie$ out with a Metrohm pgmeter equipped with u, an “alkaline” electrode. 0.5 Results Chloride Yields at Alkaline pH.-The decompo- sition of chloroacetic acid by atomic hydrogen was investigated over the pH region 8-13 varying the 4567 8910 11 12 13 chloroacetic concentration in the region 2 X 10-3- PH. 0.5 M. From the amount of chloride produced Fig. 1.-The pH dependence of R(Cl-): [CI-IZC~COO-] and the dose of H atoms, the reaction yields‘ = 0.01 M; temperature 4*; dose rate of H atom 10-6 mole R(CI-) for C1- production were obtained. The 1.y1 set.-'; volume of solution 25 cc.; duration of run 15 experimental results are presented in Fig. 1 and 2. min.; open circles H adjusted with KOH; full circles, 0.25 M NanCOs addex; curve calculated from eq. VI. These results indicate that at constant chloro- acetic acid concentration, the reaction yield for C1- production increases with increasing pH in the pH region above 11. In this region the reaction yield for C1- production decreases with increasing t pH=?2,? chloroacetic acid concentration at constant pH. 2 IJH =122 We attempted to ascertain that the increase of the chloride yield at pH > 11 is not due to the hy- drolysis of the reaction products, which as indi- cated by previous re~ultsl~~.~are presumably chloro derivatives of carboxylic acids. A 0.01 M chloro- acetic acid solution was decomposed by H atoms at pH 7 and the pH of the solution was then adjusted to 12.6 by addition of KOH. No increase of the C1- concentration was observed after 1 hr. This 0 I conclusion also is consistent with the observed con- centration dependence at high pH. In the pre- 0 vious work1 it was shown that in the neutral pH re- B. I gion the chloride yield is independent of chloroacetic 0 0.25 0.50 acid concentration in the concentration region [CH&lCOOH], mole 1.-’. 0.05-0.5 M. Thus in this concentration region at Fig. 2.-The concentration dependence of R( C1-) at neutral pH, the yield of the chlorodicarboxylic alkaline pH: experimental condition is stated in Fig. 1; acids is constant. However, in the alkaline region curves calculated from eq. VI. the concentration dependence is entirely different. chloroacetic acid manifest different reactivity with These considerations rule out the above mentioned the two forms of H atom equivalent1-3 and are trivial mechanism of chloride production in alkaline convenient for the study of H atoms in solutions. PH. We have recently investigated the decomposition Effects of Some Added Solutes.-In order to of chloroacetic acid in alkaline solutions. A establish the reaction mechanism, the effect of preliminary report of this work was given.4 In carbonate and bicarbonate ions on the chloride alkaline solutions an enhancement of chloride yields was investigated. Experiments mere carried abstraction was observed. These results were out in the presence of KaHCOa and Na2CO3(Table interpreted in terms of a mechanism involving I). No enhanced decomposition in the presence an interaction of the H atom and the OH- ion. of carbonate and bicarbonate ions was observed. The species thus formed reacts with chloroacetic On the contrary, R(Cl-) in the presence of 0.25 acid by chloride abstraction, In the present work, M Na2C03 at pH 12.6 is about 15% lower than ob- me present our detailed results of the reactivity of tained at this pH in its absence. H atoms in alkaline solutions of chloroacetic acid. Recent radiation chemical studies6 indicate that H atom precursors (Le., solvated electrons) react Experimental with the HC03- ion with the formation of the COP- The experimental technique waB the same as described ion. It may be argued that the COZ- radical ion in the previous work.’ Freshly prepared solutions were is formed in alkaline solutions by decomposition always used.