289
RATECONSTANTS OF HYDRATED ELECTRON REACTIONS
Rate Constants of Hydrated Electron Reactions with Some Aromatic Acids,
Alkyl Halides, Heterocyclic Compounds, and Werner Complexes'
by A. Szutka,2J. K. Thomas, Shefield Gordon, and Edwin J. Hart Chemistry Dzvision, Argonne National Laboratory, Argonne, Illinois
(Receized August 13, 1964)
Hydrated electron rate constants for a number of aromatic acids, aliphatic halides, heterocyclic compounds, and Werner coniplexes of cobalt and chroniium have been measured using the pulse radiolysis technique. I n the series benzoic, phthalic, and triniesic acids, the second-order rate constants of the ions are 3.0 1.0 X lo9 IW1 set.-'; those of the undissociated acids are somewhat higher. Unconjugated acids such as phenylacetic and hydrocinnaniic have rate constants that are lower than the above group by a factor of 300. Furan, thiophene, pyrrole, and pyrrolidine are unreactive, whereas thiazole and succinimide are reactive. I n the alkyl iodides, methyl to butyl, the rate constants are 1.45 f 0.22 X 10'O 1l-I set.-'. The reactivity increases in the series: chloride, bromide, and iodide. The rate constants of the Werner complexes of chroniiuni and cobalt are all above 10'0 set.-' and the complexes containing the metal in the cationic part of the molecule react, on the average, three times faster than the anionic part.
*
Introduction The hydrated electron, e,,-, has been shown to react with bimolecular rate constants in the range of lo9 to 3 X 10'0 A P 1 set.-' with unhydrated aldehydes, ketones, conjugated double bond systems containing C=C, C=O, C=K groups, disulfides, peroxides, and certain heterocyclic derivative^.^ I n the present work, a more detailed study is reported on the reactivity of ea,- with some aromatic acids, alkyl halides, heterocyclic ring compounds, and Werner complexes . by following the decay of its optical absorption a t 5780 A.4
Experimental The irradiation, solution preparation, and dilution techniques are identical with those previously des ~ r i b e d . ~Briefly, ,~ the rate constants are calculated from the pseudo-first-order decay of the hydrated electron absorption #in electron-irradiated aqueous solutions. The 15-Nev. electron pulse introduces about M ea,l- in 0.2 or 0.4 psec. Solute concentrations range from 3 X low5M for the reactive compounds to 0.1 M for unreactive ones. Analytical grade alkyl halides were purified by washing with triply distilled water and drying over anhy-
drous sodium sulfate. Aromatic acids were purified by recrystallization and subsequent drying in an analytical oven. The solvents used in these purifications were triply distilled water for potassium hydrogen phthalate, triniesic acid, and hydrocinnaniic acid; a solution of methanol and water for benzoic acid ; methyl alcohol for cinnamic acid ; petroleum ether for phenylacetic acid; arid a solution of ethanol and benzene for phthalic anhydride. Heterocyclic coinpounds were distilled using a Podbielniak colunin and collecting the middle fraction of a distillate in the case of furan, pyrrole, pyrrolidine, t'hiophene, and thiazole. Succininiide was recrystallized from methyl alcohol and dried a t 100". For recrystallization of imidazole, the solvent was benzene and the drying temperature was 80". 2-Pyrrolidone was purified by repeated freezing of the coinpound and decanting of the liquid portion. (1) Based on work performed under the auspices of the L7. S. Atomic Energy Commission. (2) Research Associate from Vniversity of Detroit. Detroit, Mich. (3) E. J. H a r t , S.Gordon, and J. K. Thomas, .J. Phys. Chrm.. 6 8 , 1271 (1964). (4) E. J. H a r t and J. W. Boag, J . Am. Chem. Soc., 84, 4090 (1962). (5) S. Gordon, E. J. H a r t , 11. S. Matheson. J . Ilahani, and +J. K. Thomas, ibid., 85, 1375 (1963).
Volicme 89, S7~mber1
.Janirary t9A.i
290
A. SZUTKA, ,J. THOMAS, S. GORDON, AND E. HART
Table I: Werner Complexes No.
1 2 3 4 5 6 7 8 9 10 11
Name
:Mol. w t .
Potassium trisoxalatochromate( 111)3-hydrate Trisethylenediaminechromium(111) chloride hydrate Potassium bisoxalatodiaquochroniate( 111)2-hydrate Potassium bisoxalatodiaquochromate(II1) 3-hydrate Hydrogen ethylenediaminetetraacetatochromate(II1) Trisethylenediaminecobalt( 111) chloride trans-Dichlorobisethylenediaminecobalt(111)nitrate tzs-Dichlorobisethylenediaminechromium( 111) chloride hydrate" cis-Bisthiocyanatobisethylenediaminechromium(111)thiocyanate trans-Bisthiocyanatobisethylenediaminecobalt(111) chloride hydrateb cis-Bisthiocyanatobisethylenediaminecobalt(111)thiocyanate"
487 401 339 357 341 345 312 296 346 348 353
Formula
41 72 18 19 23 61 06 58 46 77 39
K3[Cr(C?04)3] ,3H20 [Cr(en)a]Cla.3.5H?O cis-K [ Cr(CLOI)2( H20 121 2H20 trans-K[Cr( C?O&(H20)2]4H20 H [Cr(EDTA)] [Co(en)31C13 truns-[Co(en)&l?]NO, cis-[Cr(en)&12]Cl.Ht0 C~S-[C~(~~)~(NCS)~]NCS trans-[Co(en)y(NCS)*]Cl.H~O ci~-[Co(en)~(NCS)?] NCS
Anal. Calcd.: C, 16.20; H, 6.11; N, 18.89. Found: C, 17.19; H, 6.33; N, 19.18. .4nal. Calcd.: C, 20.69; H, 5.20; N, 24.10. Found: C, 20.69; H, 5.43; N, 24.02. rlnal. Calcd.: C, 23.78; H, 4.56; N, 27.75. Found: C, 23.46; H, 4.50; N, 28.90.
Werner coniplexes were prepared by Dr. J. A. McLean and his group a t the University of Detroit. The list of Werner coniplexes with their names, molecular weights, and chemical formulas is shown in Table I.
Discussion of Results Our rate constants for reaction of eaq- with some aromatic acids appear in Table 11. I n the series benzoic acid, phthalic acid, and triinesic acid, the rate constants of the ions are 3.0 f 1.1 X IO9 M - I sec.-I. The rate constants of the undissociated acids are somewhat greater. Consequently, an increase in the xiuniber of carboxyl ions on the aromatic nucleus does riot increase the rate constant. Of major importance is the maintmance of conjugation to the ring as is shown by a comparison of benzoic acid (3.6 X lo9) and cinnamic acid (6.8 x lo9). Destruction of conjugation is illustrated by pheriylacetate (1.4 X IO7) and hydrocinnaniate (1.1 x IO7) ions. Of interest too is the fact that rinnamate ion has nearly the same rate constant as the fumarate ion, 7.5 X lo9. From this result we conclude that the aromatic nucleus functions by providing the conjugation required for high rate constant s. In thesr aromatic anions the initial point of eaqattack is in question. Diphenyl forms the dipheriylide ion in irradiated ethanol followed by rapid protonation.6 This reaction shows primary attachment to the ring, but in irradiated benzyl chloride, formation of the transient henzyl radical is evidence that initial attack may be on the side chairi.fi--8
CsH,CHzCl
+ enq- -+
C6H,CH,.
+ C1-
Since the isomeric phthalate ions form strongly absorbing transients directly from eaq-,9 it is likely in this case The J o u r n a l o j Physical Chemistry
Table 11: Rate Constants for Reaction of eas- with Aromatic Acids and Their Ions k. Compounds
Concn , MeOH, mll mM
Slope pH
X 10-5 Individual
8 X 109 5 4 X 109 1 x 109 9 x 109 4 x 109 o x 109 3 1 x 109 2 x 109 3 8 x 109 3 4 X 109 3 6 X lo9
Benzoic acid
1 0 0030 010 003 050 050 0 10 0 030
1 1 1 1 1 1 1 1
0 0 0 0 0 0 0 0
535 545 719 719 774 7i4 12 30 12 30
Phthalic acid
010 010 010 050 050 1 0 0 5 0 5
1 1 1 1 1 1 1 1
0 0 0 0 0
565 565 560 678 678 1257 1280 1280
2 2 2 2 2 7 4 4
025
1 0
025 0 125 0125 0 125 0 125
1 0 1 0 1 0 1 0 1 0
574 696 8 84 884 12 39 12 39
3 8 3 5 2 5 2 7 1 45 2 8 3 1 1 6
Trimesic acid
0 0 0
5 7 9 3 8 8 0 3
5 5 2 3 3 3
5 8 x 109 6 2 X 109 6 6 X lo9 6 2 X IO9 1 1 1 1 2
1 3 8 8
x x x
x
ox
109 109 1 2 109
109
109
1 9
x
109
x x
109 3 5 109 2 5
x x
109
100 105 3
109
2 3 2 3
4 2 4 2
x x x x
2 3 2 4
4 5 1 1
5 7 4 4
x x x x
107 107 5 1 107 107 1 4
x
107 4 9
x
107
ox
109 109 4 2
IO^
109
x
109
x
107
x
107
150 150 375 750
1 1 1 1
H ydrocinnamic acid
300 300
1 0 1 0
543 1214
6 3 1 5
4 9 1 1
1 0 1 0
722 1245
3 0 4 2
6 8 X lo9 6 8 X 109 9 7 x 105 Q 7 x 109
010 010
9 7 2 5
x
Phenylacetic acid
Cinnamic acid
0 543 0 543 0 1 2 3 8 0 1 2 2 8
250 6 6 130 4 5 6 5 7 0 16 5 4 45
J - 1
sec -1, av
I 1
x 107 x 107
(6) I. A. Taub, D. A. Harter, M. C. Sauer, and L. 51. Dorfman, J . Chem. Phys., 41, 979 (19641.. (7) M.S. Matheson and L. &I. Dorfman, ibid., 32, 1870 (1960). ( 8 ) M .Anbar and E. J. Hart. J . Am. Chem. Soc., 86, 5633 (19641.
RATECOXSTASTS OF HYDRATED ELECTRON REACTIONS
that initial reaction is with the electron-deficient aromatic ring. In a previous paper we reported that certain groups could increase the reactivity of benzene toward the hydrated electro~i.~The nitro group in nitrobenzene ( k = 3 X 10'0 ~11-lsec.-') and picric acid ( k = 3.5 X 1010 M-1 set.-'), the carbonyl group in phthalate ion ( k = 2 x 109 M-l sec.-l), and the ethylenic double bond in styrene ( I C = 1.3 X 1Olo M-' sec.-l) are such groups. Other groups either had no effect or decreased the reactivity, e.y., C1 in chlorobenzene, CHS in toluene, OH in phenol and hydroquinone, and KHz in aniline. The first series of "activating" groups are normally meta-directing groups apart from styrene, while the second series are normally ortho-para-directing groups. I n general, a meta-directing group tends to withdraw electrons from the aromatic nucleus while ortho-paradirecting groups tend to liberate electrons to the nucleus. The eaq- niay react with the positively charged aromatic: nucleus giving a negative ion which protonates and effectively gives a product identical with that obtained from H atom addition to the ring. It is noteworthy in Table I1 that the partially ionized benzoic and phthalic acids are more reactive than the fully ionized acids, presumably because the negative benzoate and phthalate ions tend to promote electron migration to the ring, hence decreasing the attraction of the eaq- for this reactive center. A siniilar explanation can be put forward to explain the reactivity of the ortho-para-directing C=C in styrene. Here the positive end of the system is the C=C group and the eaq- would tend to react here. A similar effect has been noted in the activation of the C=C bond by COOH, e.g., methacrylic acid, fumaric acid, and maleic acid, and by another C=C as in butadiene. This interpretation of the cap- reactions in terms of a charge separation of the niolecules may also explain the reactivity of the halogen compounds in Table IV. However, the differences in reactivity are not large enough to eliminate the effect of diffusion on these rate constants. Heterocyclic compounds, such as furan, thiophene, pyrrole, and pyrrolidine, are unreactive with hydrated electrons. However, certain modifications of the compounds increase their reactivity drastically. This is illustrated by the pyrrole derivatives in Table 111. Pyrrole has been found unreactive, while the rate constant for 2-pyrrolidone is 1.3 x lo7 and for succiriiinide, 7.2 X lo9 M-l set.-'. Consequently, introduction of a carbonyl group into a compound increases the rate constant. This is in accord with the previous finding that acetone has a rate constant of 6 X lo9 M-l sec.-'.
291
In spite of the inertness of pyrrole and of thiophene, the presence of nitrogen and sulfur a t o m in the same ring provides thiazole with a rate constant of 2.5 X lo9 AI-' set.-'. Also, the presence of two nitrogen atoms in the same ring slightly enhances the reactivity of imidazole to 3.7 X lo7M-'sec.-".
Table I11 : Rate Constants for Reaction of eaq- with Heterocyclic Compounds
Compound
Furan Thiophene Pyrrole Pyrrolidine 2-Pyrrolidone Succinimide Imidazole Thiazole
Concn., mM
MeOH, mM
13 2 100 100
88 59 0 0 loo 0 010 50 0 281 0 562
10 25 10 1 0
i o
100 50 25 50
k, M-' s e c . 3
Slope X 10-6
pH
7 94 6 73 10 29 1208 782 8 0 4 9 16 659 659
0 0 0 1 5 3 0 3 6
18 73 26 8 7 1 81 0 1
3 0 X lo6 6 5 X lo7 6 0 X lo6 42X106 13x107 72x109 3 7 X lo7 25X109 25X109
Table IV : Rate Constants for Reaction of eaQ-with Alkyl Halides' k, Compound
Concn., MeOH, mM mhf
Slope pH X 10-6 Individual
Methyliodide
0.040 0.080
6.25 6.85 3.3 12.50 6 . 8 5 5 . 3
Ethylbromide
0.131
Ethyliodide
0.062 0.124
6.25 6.75 4 . 1 12.50 6 . 0 4 7 . 3 4
Propylchloride
0.110
12.50 6.27
Propyl bromide
0.057 0.114
3.0
7.12
6.9
sec.-l, av.
M-1
1 . 9 X 1010 1 . 5 X 1010
1 . 7 X 10'0
1 2 X 1010
1 . 2 X 1010
1 . 5 X 10'0 1 . 4 X lO1o
0.33 3 8 X
1 . 6 X lOLo
lo9
6 9 X 108
6 . 2 5 6.15 2 . 0 12 50 6 . 1 5 4 . 4
8 . 2 X 109 8 . 8 X 109
8 5 X 109
0.051 0.102
6 . 2 5 6 21 12.50 6 21
3.5 4.9
1 . 6 X 10'0 1 . 1 X 10'0
1 . 3 X 1010
0.478 0.478
14 0
7.28 0.9 7.28 0.9
4 . 4 X 10s 4 . 7 X 108
4 . 5 X 10s
n-Butyl bromide
0.095
3.0
6.57
4 2
1 0 X 10'0
1 0 X
n-Eutyliodide
0.055 0 110
6.25 12 50
7 60 3 0 7.60 5.6
1 . 3 X 1010 1 2 X 1010
1 2 X 10'0
0.471
14.0
6 64
1 1
I j l X 108
5 . 1 X 108
Isobutylchloride 0 . 4 7 8
14.0
5.82
1 3
6 ,5 X 10s
5 . 1 X 108
Propyl iodide n-Butyl chloride
eec-Butyl chloride
14.0
1010
The rate constants for alkyl halides are higher than those of the other compounds studied. Icroiii Table IV, it is evident that the length of branching of the aliphatic chain has no effect on the rate constant. The rate constant for all of the iodides (methyl to butyl) is (9) S.Gordon, J. K. Thomas, and E. J. H a r t , J . Phys. Chem., 68, 1262 (1964).
Volume 69, iVumber I
January 1966
A. SZUTKA, J. THOMAS, S. GORDON, AND E. HART
292
~~
Table V : Rate Constants for Reaction of eaq- with Cobalt and Chromium Complexes Concn., m
Compound and metal ion
NO. 2
No. 9 NO. 6
No. 7
-+
[Crrrr(en),13+
+ [CrIIr(en)2(NCS)zl
+
+ [Corrr(en)313+ + [C~~~~(en)~Cl~]
No. 11 No. 10
+ cis-[Co~~~(en)~(NCS)~] +
+ trun~-[Cor~~(en)~(NCS)2] +
MeOH, mM
x
10-6
Individual
0.023 0.023 0.023
1.o 1.0 1.o
6.83 6.83 6.83
5.5 4.9 5.0
5 6 X 1O'O 5 0 x 10'0 5 1 x 10'0
0,020 0.020 0.010 0.010
1.0 1.o 1.0 1.0
5.55 5.55 5.55 5.55
6.4 6.1 3.3 2.9
7 1 x 10'0 7 7 x 10'0
0.020 0.020
1.0 1.0
5.65 5.65
3.7 3.7
4 2 4 2
0.023 0.023 0.012 0.012
1.0 1.o 1.o 1.0
6.55 6.55 6.55 6.55
7.5 6.3 4.0 3.5
7 6 8 7
0.020 0.010 0.010
1.0 1.0 1.o
5.55 5.55 5.55
6.6 4.0 3.7
7 6 x 10'0 9 1 x 1010 8 4 X 1O'O
0.020 0.020
1.0 1.0
6.00 6.00
6.5 4.8
8 0 X 1O'O 5 9 x 10'0
0.020 0,020 0.010 0.010
1.0 1.0 1.0 1.0
6.50 6.50 6.50 6.50
4.4 4.9 2.5 2.3
5 5 5 5
0.050 0.050 0.050 0.020
1.0 1.0 1.0 1.0
4.76 4.76 6.13 5.00
3.4 3.5 4.5 1.6
1 6 X 10'0 1 6 X 10'0 2 0 x 10'0 1 8 X 1O'O
0.097 0.097 0.048 0.048
1.0 1.o 1.0 1.0
6.40 6.40 6.40 6.40
4.9 5.5 2.5 3.1
12 13 12 15
0.097 0.097 0.048 0,048
1.0 1.o 1.0
5.3 5.8 3.7 3.8
1 1 1 1
3 4 7 8X
10'0
1.o
6.18 6.18 6.18 6.18
0.025 0.025 0.020 0.020
1.0 1.0 1.0 1.0
4.90 4.90 5.00 5.00
4.0 3.8 2.7 3.9
3 3 3 4
5 x 5 x 1x 5 x
10'0
1.45 f 0.22 X 1Olo M-' set.-'. The reactivity increases in the series: chloride, bromide, and iodide. The rate constants for the Werner complexes of cobalt and chromium are above 1O1O M-' set.-' and fall into two distinct groups. See Table V. Complexes containing metals in the cationic part of the molecule react, on the average, three times faster than molecules containing metal in the anionic part. If the metal ion is the reactive site of the molecule, the difference in the rate constants can be attributed to the electrostatic att,raction or repulsion of the hydrated electron toward The Journal of Ph.ysical Chemistry
Slope
PH
7 3
x
Average
5.3
x
10'0
7.1
x
10'0
10'0
6 6 X loLo
x x x
5 3 X 1X 1x
10'0 10'0 10'0 1OO ' 1O'O 10'0
0 x 10'0 6 X 1O'O 8 X 10'O 3 x 10'0
x x x x x x x
10'0 10'0 10'0
4.2 X 1O'O
7.3
x
10'0
Corrected for (NO3) 7.1
x
10'0
6 . 9 X 1O1O
5.4
x
10'0
1 . 8 X 10'O
1.3 X 1O1O
10'0
10'0 10'0
1 . 5 X 10O '
10'0 10'0 10'0 10'0
Corrected for acid content 2 . 6 X 1O'O
the cationic or anionic portion of the molecule. No distinction is observed between complexes containing chromium and cobalt metals in the series of compounds studied although Cr(CN)B3-reacts fourfold faster than CO(CN)2 -. lo Acknowledgment. The technical assistance of Rlr. Edward Hagen and Miss P. D. Walsh and the cooperation of Messrs. B. E. Clifft and E. R. Backstrom during Linac operations are appreciated. (10) M. Anbar and E. J. Hart, unpublished work.
