REACTION OF HALOALIPHATIC COMPOUNDS WITH HYDRATED ELECTROSS
27 1
The Reaction of Haloaliphatic Compounds with Hydrated Electrons’
by M. Anbar* and Edwin J. Hart Argonne .Tatinnu1 Laboratory, Argonne, Illinois
(Receized J u l y 27, 1964)
The reaction of hydrated electrons, e,,-, with haloaliphatic compounds has been investigated, using pulse radiolysis. The rate of reaction increases in the order F latheson C.P. reagents; and sodium iodoacetate was an Eastiiian White Label cheiiiical. All reagents were used without further purification.
An exploratory study of the reactivities of various organic coinpourids toward ea,- 3 , 4 followed the recent discovery of the hydrated electron, ea,-, which had been identified through its absorption spectruni.5 Aromatic compounds as well as haloaliphatic derivatives were found among the niost reactive organic species. The hydrated electron interacts with aromatic compounds by a mechanism analogous to nucleophilic substitution.6 This reaction results in the incorporation of the electron into the orbitals of the aromatic ring and in some cases to the formation of Results and Discussion aromatic carbanions of finite stability. Aromatic The biniolecular rate constants of a number of alkyl bronio and iodo derivatives are more reactive than halides and their derivatives are suniinarized in Table expected from their influence on the electron density I. Their reactivity toward eaq- increases in the series of the ring and this effect was attributed to the addiF < < C1 < Br < I and the relative rates of the four tional reactive center in these molecules. The de: 1 : 5 : 10. monohaloacetates, F : C1: Br : I, are 2 X chlorination of ohloroacetic acid was the first haloaliphatic reaction of ea,- to be d e n i ~ n s t r a t e d . ~ - ~ Iodo- and bronioacetic acids were also found to undergo (1) Based on work performed under the auspiies of the I-. S..Itomic d e h a l ~ g e n a t i o nand ~ the formation of halide ions as Energy Commission. (2) On sabbatical leave from the Weismaim Institute of Science. products of react ion may be considered as a rule for all Iiehovoth. Israel. ea,- reactions. In this study, we report the RX (3) E . J. Hart, S. Gordon, and J. K. Thomas, J . Phys. Chcm.. 68,1271 the rate of reaction of eaq- with different haloaliphatic (1964). compounds and suggest a niechanism for this process. (4) A. Szutka. J. K. Thomas, S.Gordon. and E. J. H a r t . t o be pub-
+
lished.
Experimental The rates of reaction of ea,- with haloaliphatic conipounds were determined by pulse r a d i ~ l y s i s . ~The experimental technique has been described previously, including in~trunientation~ and estimation of errors.fi All measurements mere carried out in the pH range
(5) J. IT.Boag and E. J. H a r t , .Tattire, 197, 45 (1963): J . 1’. Keene, i h i d . , 197, 47 (1963); E. J. Hart and J. W.Boag, J . -4nz. Chcm. Soc., 84, 4090 (1962). (6) 11.Aiihar and E. J. H a r t , ihid., 86, 5633 ( I W X ) , (7) E , Hayon and J . Weiss, Proc. 2nd I n f c r n . Conf.I’cacef~ri 17scs A t . Energy, Grncra, 1.968, 29, 80 (1959). (8) E. Hayon and d.0. .klIen, J . I’liys. Chim.. 65, 2181 (1961, (9) 11..Inbar. u1i1)uhlished results.
T‘oliime 69.
l’iimtiii
1
Janiiary 1,966
11. ANBAR
272
Table I : The Rat.e of Reaction of eaq- with Haloaliphatic Compounds Relative rate, Concn., Compound
CH3COO100 FCHzCOO40 FBCCOO100 ClCHzCHzCHzCHa 0 50 CH3CHClCHzCHI 0 50 ClCHzCHzOH 050 ClCHZCHZCOO0 50 CH3CHC1C000 50 ClCHzC000 50 ClCHzC6H5 020 C1~CCOO010 c13CCe.H~ 010 ClaCH 0 02 0 01 Cl3CCl BrCHZCH20H 0 50 BrCHZCHzCOO0 30 CH3CHBrCOO0 30 BrCHzCOO 0 167 ICH&HzCOO0 067 0 10 ICHzCOO-
kRx/ kCICH,CH,CH,CK,
k. M-' BeC.3
M X 102
2.0 2.0 2.6
+ 0.5" X + 0.5=X + 0.6" X
...
106
0.007
lo6
...
3 2 i 0 4 X 10" 5 1 f 0 8 X 10" 4 1 f 0 6 X 1 0 8 4 0 f 0 4 X 10' 1 4 f 0 2 X lo9 1 2 f 0 15 X 10*
1 00
5 8 8 3
5 5 3 0
i 0 5 x 1 0 9 f 1 0 X 1 0 g i o g x 1 0 9 f 0 5 x 10'Dd
3 0 f0 5 1 6 f0 2 7 f0 5 3 f0 6 2 f0 6 6 0
x
10'0d
2 X lo9 3 X loe 8 X log 7 X IOg 9 108
1 15 f 0 10
x 1010
1 60
1 28 1 25 4 38 3 75 172
266 260 94 94
5 0 (1 1 68e 3 4e 3 96 20 6 ( 1 0)r 1 75J
Value given for a Upper limit of rate constant (see ref. 6). comparison, c Value taken from ref. 5. d Value taken from ref. 4. e Relative rate toward BrCH2CH20H. f Relative rate toward ICHzCHzCOO-.
This relation suggests an analogy between the eaqR X and SN:! substitution reactions
+
eaq-
+ RX
-
T h e Joicrnal 01' Phglsical Chemistry
R
+ X-
eaq- -I- C6HjCHzCl
C6HjCHzC1-
followed by the decomposition of the latter in the ratedetermining step
+ C1-
( S N 1 mechanism)
one would expect a substantial increase in the rate for solvents of greater polarity.'O On the other hand, if C6HsCHzCl- is formed as an intermediate in the ratedetermining step, only a negligible solvent effect is expected. The formation of RX- as an intermediate is, in fact, an oxidation-reduction reaction by which RX is rex-. Considering it still as a nucleoduced to R philic interaction, it remains to be determined whether the reactive center is the carbon atom, as in classical nucleophilic substitution, or the halogen atom. The former mechanism would imply an enhancement of the
+
(10) A. Streitwieser, Chem. Rev.,56, 571 (1956). (11) (a) J , Hine, "Physical Organic Chemistry," AfcGraw-Hill Book CO., Inc., New York, x. Y., 1956. p. 158: (b) ihid.. pp. 131-133. (12) J. H. Baxendale, Radiation Res. Suppi., 4, 139 (1964). (13) G . E. B. Randles, Trans. Faraday Sor.. 5 2 ; 1573 (1956) (14) C. K. Ingold, "Structure and Mechanism in Organic Chemistry," Cornell University Press, Ithaca, N. Y.,1953. p. 345. (15) I. A. Taub. D. A. Harter. hl C. Sauer. arid L. hl. Dorfman. Chem. P h y s . , 41, 979 (1964). (16) S. Arai and L. hl. Dorfman, ibid., 41, 2190 (1964)
i.
(2)
E. J. HART
which formally resembles reaction 1, is expected to exhibit a solvent effect, provided reaction 2 is the ratedetermining step of the eaq- interaction. As the free energy of hydration of eaq- is -40 kcal.I2 and more ~ positive than the -70.7 kcal./niole for ~ h l o r i d e , 'an enhancement of the rate of reaction 2 is expected when the polarity of the solvent is increased.14 The rate of reaction of benzyl chloride with eaq- has been measured in water, methanol,15 and ethan01,'~and the rate constants are 5.5, 5.0, and 5.1 X lo9 M - I sec.-I, respectively. The solvation energy of the electron in water and ethanol is comparable, a result deduced from their similar absorption spectra. l 5 This lack of dependence of rate on dielectric constant of the solvent indicates that the activated complexes are equally solvated This is not surprising, assuming the formation of a large poorly solvated carbanion as the intermediate product of the eaq- reaction. I n the case of naphthalene. the formation of the naDhthalide ion has been denionstratedI6 and the rate of its formation was found equal in ethanol and in water.6t16 The formation of a chloride ion from benzyl chloride as the rate-determining step would imply a solvent effect owing to the difference in AH of solvation of C1- in the different solvents.I4 If a carbanion would be formed in a fast pre-equilibrium
C ~ & C H Z C ~+ - C6HSCH2.
since a relative reactivity of 5 x 1:50: 150 holds for a large number of systems.'" This analogy does not hold, however, when the effect of substituents on the halogen carrying carbon atom is examined. In Siv2 substitution, the secondary chloride reacts slower by a factor of 30-50 than the corresponding primary isomerl*jllain contrast with an enhancement by a factor of 1.6 in the eaq- reactions. The rate of nucleophilic substitutior of chlorine on C1CH2CH20H is 0.1-0.2 of that on rt-BuCllO as compared with an enhancement by a factor of 1.3 for the eaq- reaction. The addition Of a to the a-carbon has a retarding effect on the S N substitution,lo~"b ~ whereas it enhances the eaq- reaction by two orders of magnitude. Therefore. the siinilarjtv between the eao- reactions and bimolecular nucleophilic substitutions is superficial. The over-all reaction
AND
REACTION OF HALOALIPHATIC COMPOCNDS WITH HYDRATED ELECTRONS
273
Table I1 : A Correlation of Relative Rates of the e& Reaction with the u* Function
RX
+
R
*h
1.0
,:/ CH3CHCOO-
CH3CHzCHzCHz. CH~CHCHZCH~ .CH&H%OH .CHzCHzCOO CH~CHCOO.CHzCOO-
c
CaH,CCIz ClZCCOO HCClz ClsC. .CHzCHzOH .CHzCHzCOOCH3CHCO0.CHXOO -
0 0 0 0 0 0
00 20 f 0 1110 10 10 64 & 0 57 f 0 1 23 10 1 43 f 0 1 45 f 0 1 97 10 1 97 f 0 0 Ooe 0 23 $: 0 0 53 i 0 0 59 f 0
-0 13 -0 I 9 1 0 20" 0 38" 0 95* 1 05 0 215 1 53" 2 6jd 1 94 2 65 $0 20 0 38" 0 95* 1 05
12 10 10 12 10 10 12 12 22 22 126 13d 12d
calculated from 0 * ~ / 2 . 8 . ' ~* Calculat,ed from - u * c z ~ . Calculated from u*cci3 - U * C H ~ C I u * c ~ B s C H ~ . Calculated from u*ccil - U*CH%Cl U*CH~COOH. e Related to BrCH2CH20H. a U*RCB~
U*CH~COOA
+
+
rate with increasing polarization of the C+-X- bond. I n other words, The order of reactivity should be RF > RCl > RBr > R I , owing to the increase in electronegativity of the halogens.I7 Since the reverse order have been related to that of eaqn-BuCl yielding is found, an interaction of eaq- with the halogen atom c ~ ] . q-values, where q = log [ ~ R x ~ ' ~ ~ . B ~ Likewise, as the reactive center is likely. I t is also plausible rate constants of the RBr reactions were related to that the extra electron in R X - resides mainly on the that of OHCH2CHzBr. The 7-values were then halogen as the saturated carbon will show no tendency compared with r*-values taken or calculated from to accommodate an additional electron. Taft's data (Table 11). Plotting 8 against u* (Figure Because the dectron affinities of the halogens are l ) , a satisfactory correlation is obtained with the excomparable (3.63, 3.78, 3.54, and 3.24 v. for F, C1, ception of CdH6CHzClwhich reacts faster than expected. ' ~ ) ,rate of esq- reactions Br, and I , r e s p e c t i ~ e l y ' ~ ~the The higher reaction rate of berizyl chloride inay be exdoes not depend on this factor. A parameter which plained by an additional interaction of eaq- with the is more likely to affect the rate of reaction of eaq- with aromatic ring.6 The relation between 7- and u*a given halogen atom is the polarizability of the atom, values is satisfactory but it is far from perfect, which which substantially increases in the halogen series froni suggests that additional paranieters may contribute fluorine to iodine. Taking the monohaloacetates, the to the rate of eaq- interaction with the aliphatic halorates of the caq- reactions are 2 X lo6, 1.2 X lo9, gen. Tet the changes in electron density that are in6.2 X l o 9 , and 1.15 X 1O'O d/-l see. - I , compared with duced on the halogen atom by neighboring groups are the values of 0.38, 2.28, 3.34, and 5.11 X lopz4~ 1 1 1 . ~ probably the major factor in these eaq- reactions. for the atomic polarizabilities of F, C1, Br, and I, reIn a previous paper,6we pointed out that the reaction spectively. l9a In other words, the probability of an of eaq- with aromatic compounds is characterized by electron to be incorporated in a giver1 halogen atom a nucleophilic attack on an available or induced elecincreases with its size and number of atoniic orbitals. trophilic center on the organic niolecule. An anion As the polarity of the C-X bond decreases in the may then form as a primary product. The same niechseries F to I,I7 19b there is also a tendency toward anisni applies to the reactions of eaq- with haloaligreater electrophilic nature of the halogens. (17) H 0 Pritchard and €1 H Skinner, Chem Rea, 5 5 , 745 (1955) Followirig this line of reasoning, one expects an in(18) H 0 Pritchard, ibzd , 5 2 , 529 (1953) crease in electrophilic character of the halogen atom in(19) (a) L 31 Fergusori "The Modern Structural Theor3 of Organie duced by neigh boring electron-withdrawing groups. Chemistr\," I'rentice-Hall Inc Englemood Cliffs, X J , 1963, p 199: (b) ;bid.. P. 200. The electron withdrawal capacity of a given group may (20) K. W. Taft. "Steric Effects in Organic Chemistry," 31. s. be expressed in terllls of Taft's ,,*-f,lnction.20 BiNewman, Ed.. John Wiley and Sons, Inc.. Xew l-ork, K. T.,1956, molecular rate constants of t,he eaq- f RCl reaction Chapter 13.
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Volume 6.9, Sumher 1
January 196,5
274
phatic conipounds, but in this system the halogen atom becomes the reactive center toward eaq-. A similar behavior of haloaliphatic compounds is found in the oxidation of chromous ions which involves the halogen atom as the reactive center.21 The electrophilic nature of organically bound halogen atoms is a latent property which may be exhibited only in the presence of an extreiiiely reactive reducing agent. Thus in the case of haloaliphatic conipounds, eaqis not only the simplest and most reactive nucleophile but also tho most elementary reducing agent in aqueous solutions. As far as reduction reactions are concerned, eaq-, with its oxidation-reduction potential of
