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					2078                                        JOSHUA            RABANI
                                                 JORTNER JOSEPH
                                                      AKD                                                              Vol. 66

                      CHEMICAL DATA
                                                   JORTNER JOSEPH
                                            BYJOSHUA    AND     RABANI
                               Department of Phvsieal Chemistry, Hebrew University,Jemsulem, Israel
                                                           Received December Bi. 1961

   The decomposition of aqueous solutions of chloroacetic acid in acid and neutral solutions by H atoms was investigated.
The rate constants for hydrogen and chloride abstraction from the chloroacetic acid and from the chloroacetic anion by H
atoms were derived. It was found that H atoms as such react with chloroacetic acid mainly by hydrogen abstraction.
The comparison of these results with radiation chemical data yields evidence for the nature of the reducing radicals produced
in the radiolysis of water. The acid form of these reducing radicals involves the H atom as such,

                    Introduction                     for 15 min. through 25 cc. of the evacuated solution kept
   Recent experimental studies indicate that the at4". Determination of dose of H atoms was carried Out by
reducing radicals formed in the radiolysis of water reduction of 10+M ferricyanide solution.8C During the runs
can exist in two forms, exhibiting widely different the dose rate of H atoms was checked. The mean dose
reactivity with specific scavengers, e.g., H202,1,2 in this series of experiments was 1 X 10-6 the ferricyanide
                                                     (2.6 X 10-8 mole sec.-l). After
                                                                                                  mob la-' sec.-*
Nt0,3 organic alcohols, and ketones4 and chloro- dosimeter, it was found necessary tousing atomic hydrogen
acetic acid.6s6 It was shown2 that the form of H through pure water in order to obtain reproducible results.
atom equivalent produced by irradiation of neu-        Analysis.-Chloride was determined by a turbidometric
                                                                             solution consisted of
tral aqueous solutions differs from the species method. 0s. reagent volumes of the reagent0.01N AgNO,
                                                     in N "
produced by the oxidation of Hz by OH radicals, the2 analyzed solution were mixed at room solution with
and from those produced by radiolysis of acid 25'. The turbidity was measured a t 400 mp with a Beck-
aqueous solutions.l~7 The possible pairs of the man D U spectrophotometer. The optical density increases
reducing species considered were eaq and H, or with time,on C1- and a maximum acid concentration. min.
                                                                                          value after 20-30
alternately H atom and H2+ion. Radiation chemi- maximum reading was taken. The measured optical
cal data do not make it possible so far to discrimi- density is linear with C1- concentration up to 0.d. 0.8, for
nate unambiguously between these possibilities.1,2,6 a I-om. light path.
   Comparative studies of the reactivity of H          The method is adequate for microdetermination of in-
                                                                                                      5 X lo-+--
atoms generated as such and introduced into the organic chloride in the concentration region chloroacetic
                                                     10-8 M. The results were found to depend on
solution* and of the reducing species produced in concentration higher than 0.5 M , and were duly corrected.
the radiolysis of water may yield information con- The accuracy of the method is +3%. At chloroacetic
                                                                           above 3 M the chloride production
cerning the nature of the reducing radicals produced durin concentrations was very fast, and no experiments were
in irradiated solutions under various experimental carries the in this region.
conditions. Recent studiessf of the decomposi-         Materials and Solution.-A.R. grade chloroacetic acid
tion of aqueous solutions of HzOz by H atoms (Hopkin & Wilfiams),.sulEuric acid, and KOH were used.
showed that the acid form of the reducing radical The pH usuallyphosphate buffer HnSOd and KOM. Solutions
                                                     cases lod8M
                                                                      was adjusted by
                                                                                       was employed.
                                                                                                          I n some
involves a hydrogen atom as such. In order to were freshly prepared and kept in ice until used. The
provide further evidence for the nature of these initial chloride concentration of solutions u to 0.5 M
species, the decomposition of aqueous solutions of did not exceed 2.5 X 10-6 A chloride. SoPutions in the
chloroacetic acid by atomic hydrogen was in- concentration region 1-3 M did not contain more than 2 X
                                                          M chloride.
vestigated. Radiation-chemical studies of this
system were recently carried 0ut.5,~ In the pres-                              Results
ent work the radiation chemical data are com-           The decomposition of chloroacetic acid by atomic
pared with experimental results for the decom- hydrogen was followed by measuring the formation
position of acid and neutral chIoraacetic solutions of inorganic chloride. It was found convenient
by atomic hydrogen.                                  to define the reaction gield R for the product in
                    Experimental                     terms of
   Production of H-Atoms.-Atomic       hydrogen was gen-                                                [Cl-lV
erated by a method previously described,* by an electrode-                                 R(C1-)   =   - At
less discharge in Ha gas at 30 mm. pressure. The pumping
velocity was 150 1. min.-1. Atomic hydrogen was passed                     where A is the dose rate of H atoms expressed in
   (1) A. 0 Allen and H.Schwarz, Proc. Intern. Conf. Peaceful Uses
            .                                                              mole sec.-l, V the volume of the solution expressed
At. Energy, Geneva, 29, 30 (1958).
  (2) N. F. Barr and 8 . 0. Allen, J . Phys. Chem., 6 8 , 928 (1959).
                                                                           in I., and t the duration of the run. The values
   (3) F. S. Dainton and D. B. Peterson, Nature, 186,878 (1960).           of R(Cl-) were obt,ained as mean values of at
   (4) J. T.Allan and G. Soholes, ibid., 187,218 (1960).                   least three runs, and were reproducible within
   (5) E. Hayon and J. Weiss, Proc. Intern. Conf. Peaceful Uses At.           20y0 or better.
Energy, Geneva, 29, 80 (1958).
   (6) E. Hayon and A. 0. Allen, J . Phys. Chem., 66, 2181 (1961).            Reaction Yields in Acid Solutions.-The experi-
   (7) J. T.Sworski, J . Am. Chem. Soc., 7 6 , 4687 (1984).                mental results for R(C1-) obtained in acid solu-
  (8) (a) G. Czapski and G . Stein, J . P h w . Chem., 63, 850 (1959);     tions from pH 0.4to pH 2 are presented in Table
(b) G . Czapski, J. Jortner, and G. Stein, ibid., 68, 1769 (1959); ( 0 )   I. I n acid solutions R(Cl-) is dependent on chloro-
G.Czapski and G . Stein, ibid., 64,219 (1960); (d) G.Czapski, J. Jort-
ner, and G. Stein, ibid., 66, 956 (1961); (e) 66, 960 (1961); (f) 66,
                                                                           acetic acid Concentration leveling off at high
064 (1961).                                                                concentration. The limiting value observed is
SIDV.,                         DECOMPOSITION
                                         OF CIILOROACETIC BY ATOMIC
                                                      ACID        HYDROGES                                             2079
Ro(Cl-) = 0.33. At constant chloroacetic acid                           I                I           I         I
                                                                                                                   '     I
concentration, the chloride yield is independent
of pH in the region 0 . 4 2 . 0 .
   Reaction Mechanism in Acid Solutions.-The
limiting chloride reaction yield reaches the value
of 0.33 at high scavenger concentrations. Thus
the experimental results are interpreted by assum-

ing competition between two first-order scavenging
re:x tion?, which involve hydrogen and chloride
abstraction from chloroacetic acid.

                  TABLEI                                                    II

                        ACETIC ACID AP 4'
Dose of    R    atoms/l0-6 mole 1.-' set.-*; duration of run
                             900 sec.
                        [CHzCICOOH],                                    0          1         2          3         4
          PH              mole 1. - 1         R(CI -)                            [CHzClCOOH], mole 1. -1,
          0.4               0.5                0.18              Fig. 1.-Chloride reaction yields in acid solutions: 0 ,
           "4                .05                .032           pH 0.4; A, pH 1; 0 , pH 1.9. Calculated curves: - . . -,
           .4                ,005               .02
                                                               homogeneous kinetics; -diffusion treatment; - --,
                                                               forced convection treatment.
          1.0               3.5               ( .33)
          1.0               3.3               ( .33)           The parameter W is expressed using the same nota-
          1.0               2.5                 .33            tion as in previous workad
          1.0               1.o                 .21             w=      -               k3A
                                                                        6 { (k1 k z ) [CH?ClCOOH]1 "'D"'cp (Iv)
          1.o                .I                 .07            where A is the dose rate of H atoms, D their dif-
          1.o                .05                .02            fusion coefficient, and cp the mean surface area for
          1.2                .5                 .18            mass transport.
          1.9               3.5               ( .28)              The chloride yield can be expressed in terms of
          1.9               0.5                 .16            the limiting yield RO(Cl-)
                                                                          R(Cl-) = IZo(cl-)   (A/")                or)
          3.3                .05                .06            Setting the experimental value for the dose rate
          4.2                .87                .08            A = 2.5 X           mole set.-' and using the con-
          4.2                .44                .07            8tants8ds8e = 4 X
                                                                         D                cm.2/sec. and IC3 = 101'
          4.2                .01                .03            mole-l cm.3 see.-', we obtain
          4.4                .5                 .08
          4.4                .os                ,07
          7.4                .Ol                .04
                                                     Best agreement with experimental data was
H + CHzClCOOH            --j   + CHClCOOH
                               Hz                       (1)
                                                     obtained setting (kl                    +
                                                                               Ic~)q'/~ 1.3 X lo5 mole-1

                                                     1. sec.-l CM.'/~ Analysis of the results according
                                                     to the forced convection mechanismsc leads to
                       C1- + CIIzCOOH + H +    (2)
The present data were obtained a t a relatively high R(Cl-) = RO(CI-)
                                                                           (h kz)p/.t[CHzClCOOH] +   X
dose rate of H: atoms, thus second-order radical                                      k3A
recombination reactions have to be included. The                                1caA
concentration dependence of R(C1-) is attributed
to t.he recombination reaction
                                                         In 1 + -
                                                                        {                +
                                                                   (ICI k~Jcp/.t[CHzClC00Hl
                                                     where p is the velocity of the liquid. Reasonable
                      2H +Hz                   (3)   agreement was obtained setting (kl       k2)qp = 2.5      +
  Evaluation of the Experimental Results.-:n          X IO5 1. mole-1 sec.-l ~ m sec.-l. Finally, the
                                                                                        . ~
                                                   a homogeneous kinetic treatment was employed
previous worksd approximate kinetic expressions
were derived for the reactivity of H atoms intro-                          V T[CHzClCOOHJ 2 X
duced from the gaseous phase into the solution. R(Cl-) = RO(Cl-)                     A
By application of the diffusion modelsd the sum of
the reaction yields for HZand C1- production is                           2A
                                                                                          - I} (V1II)-   )I"
                                                                 V r [CHZC~COOH]~+

                                                     where T = (kl k-J2/2k3and V is the volume of the
                                                     solution, The experimental results were fitted by
wh.ere A is obtained from the cubic equation         setting T =       I.-' mole sec.-l. The comparison
                                                     of the experimental results and calculated data
               2A3 4- 3A2 4- A = W           (111)
                                             ,---,   is presented in Fig. 1.
2080                                          JORTKER JOSEPH
                                         JOSHUA    AND     RABANI                                                                     Vol. 66
   Derivation of "Absolute Rate Constants.79-                           Decomposition by H Atoms at Neutral pH.-Low
I n a previous analysisscit was shown that the homo-                 chloride yields were obtained a t the neutral pH
geneous kinetic treatment yields the lower limit for                 region 3-8. The experimental results are pre-
the rate constants. The scavenging rate con-                         sented in Table I. The analytical accuracy is some-
stmts were estimated from the result of the                          what less satisfactory in this region, and only semi-
diffusion model. The effective surface area for                      quantitative conclusions can be derived. R(Cl-)
introduction of H atoms was estimated as 10-20                       was found to be independent of pH over this whole
cm.2.Sdp8e Setting 9'18 = 10 cm.'/' we obtain kl               +     region. The reaction yield was found to be inde-
k2 = 1.3 X lo4 1. mole-' sec.-l. This estimation                     pendent of bhloroacetate concentration in the region
is based on the values of k3819 of cp.                               0.05-1 M . The limiting yield in this region was
   Alternatively, similar conclusions can be de-                     found to beRO(CI-) = 0.08 0.02.         *
rived by comparing the present results with the                        As the dissociation constant of monochloroacetic
kinetic data previously obtainedBe under similar                     acid is pK = 2.8, the change of the reactivity when
experimental conditions for the oxidation of Fe+2                    passing from acid to neutral solutions is attributed
ion by H atoms. We have calculated the ratio                         to the change of the reacting species from the un-
k1 4- k2/ka + F a t 2 using the three kinetic models.                dissociated acid CH2ClCOOH to the anion CH2-
The calculation of the rate constants ratio k1 IC2/
kB: t2 yielded 1.42 X
   +Fe                            using homogeneous
                                                          +          ClCOO-. The reaction mechanism in the neutral
                                                                     pH region is presented in the form
kinetics, 1.74 X           using the diffusion model,
and 1.67 X             applying the forced convection                CHzClCOO-         + H +H2 + CHClC00-                               (1')
   These rate constants ratios are independent of                    CHzClC00-         +H+
the vdue chosen for k3. The rate constants ratio                                          C1- + CHzCOO- + II+                (2')
for H atom scavenging in these two different                         Application of eq. XI and the experimental value
systems is not changed by the kinetic treatment                      of Ro(C1-) leads to k2'/JClf = 0.09.
employed. This conclusion yields further support                        The levelling-off of R(Cl-) a t lower concentra-
to the validity of the kinetic data obtained8 by                     tions a t the neutral pH region than in the acid re-
using this experimental method. Using the rate                       gion indicates that the sum of the scavenging rate
constant ~ H + F ~ +6.7 X lo5 mole 1.-l set.-' ob-
                      = ~
tainedlo from comparison of radiation chemical
                                                                     Constants kl'     +  kz' is higher for the anion than the
results with gaseous phase data,IO we obtain ICl               +     corresponding sum for the acid. Using the ap-
                                                                     proximate value R(C1-)/Ro(C1-) = 0.5 a t 0.02
kz = lo4 1. mole-' sec.-l. Thus the two inde-                        M , then by application of the diffusion model me
pendent methods lead to similar results.                             estimate (kl'         +J c ~ ' ) c p ~ /= 1.5 X 106 1. mole-'
   I n the present treatment only the recomlnination
reaction 3 was considered. The recombination of
                                                                                      hence klf k2' = 1.5 X 105 1. mole-'
                                                                     sec.-l ~ m . ~ / a ;
H atoms with the radicals CK2COOH and CHC1-
COOH was not included. The nature of this ap-                                            Discussion
proximation will be considered briefly. Assuming                        The rate constants for the decomposition of
equal reaction rates of H atoms with these radicals                  chloroacetic acid by atomic hydrogen are presented
and equal rates for the recombination of these                       in Table 11.
radicals, the kinetic scheme should include the                         These results indicate the enhanced reactivity
reactions                                                            of the anion compared to the acid in the dehydro-
                   H+B--,HB                     (4)
                                                                     genation and chloride abstraction reaction by H
                                                                     atoms. Recent data indicate that the relative
                     2B --+ B  z                (5)                  efficiency of the dehydrogenation of the anion
where B represents both CHClCOOH and CH2-                            compared with the acid may be general for the
COOH.                                                                case of aliphatic acids. The most extreme case
   At relatively high chloroacetic acid concentra-                   is that of formic acid where ~ H + H C O O - / ~ H + H C O O H
tions, the steady state concentration of H atoms is                  > 100.11 The increased reactivity of the formic
determined by the first-order scavenging reactions                   acid anion compared with the conjugated acid was
l'and 2, and thus the effect of reaction 4 is negli-                 demonstrated in some other radical reactions.'281P
giblei The greatest effect of the recombination                                                   TABLE
reactions 4 and 5 is manifested in the low concen-                   RATSCONSTANTS FOR SCAVENGING O F H ATOMS
                                                                                                            BY                       CHLORO-
tration region of the scavenger. It can be shown                                        ACETIC ACID
that the sum of the rate constants kl k~ may then
be altered by a factor of 2 only. This will not
                                                                        Reacting species       k l , 1. mole-1 seo.-l    kz, 1. mole-1 sea.-'
                                                                        CHiClCOOH                  9 x 108                   4 x 10s
affect the general conclusions of the present kinetic                   CHzClCOO -                 1 . 3 X 10'    x 104      1.3
   The rate constants ratio k2/kl was obtained from                    I n the case of the reactivity of acetic acid with
the limiting value RO(Cl-) = 0.33 a t pH < 2. Thus
      .  .     -                                                     H atoms it was found that k ~ p+ c o o - / k ~ +
                                                                                                      ~            CH~COOH
we calculated                                                        = 3.13 Until recently, the different reactivity of
                                                                     H atoms with acids and their conjugated anions
           _ - RO(Ci-) = 0.5 i 0.1
           kz -                                                      was not taken into account in radiation chemlcal
           k'    1 - RO(C1-)                                         studies of such systems.
  (9) H. L. Friedman and A. H. Zeltman, J. Chem. P h ~ a .28, 878,     (11) J. Rabani, ibid., 66, 361 (1962).
(1959).                                                                (12) E. J. Hart, J. Am. Chem. Soc., 83, 567      (1961).
  (IO) P. Riesz and E. J. Hart, J. Phys. ChelrL., 63, 859 (1959).      (13) J. Rabani and G. Stein, to be published.
                    OF     HYDROGEN
Nov., 1962 REACTIVITY ATOMIC     I N ALKALINE                      ACID 2081
                                            SOLUTIONS O F CHLOROACETIC

   The rate constants ratios kt/iil 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                                                       and
                                                                        from the hydr~gen-oxygen'~ 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.
The product of the reaction with H+ ion is another
                                                                        eaq + CH2ClCOOII +C1- + CH2COOH (6)
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)
   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 p H 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 a t pH > 3 and of Hz+                      identification of the acid form of the reducing radi-
reacting a t 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 0.55 ;ierived-by Hayon and Allen6for the repe-                      NOTE    ADDEDIN 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
                                                                        complete agreement with the results of the present work.
radiolysis of water involves an H atom as such.                            (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).

                                                JORTNER JOSEPH
                                          BYJOSHUA   AND     RABANI
                              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
                   Introduction                                         solutions a t 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).