BARTON MILLIGAN AND RONALD L. BRADOW
2118
components of the higher boiling products a t higher column temDeratures were not successful. Ifydrolyzable fluoride was determined by refluxing equal weights of fluorocarbon and 1% sodium bicarbonate and titrating the aqueous layer for fluoride ion. The permanganate test for unsaturation was done with a freshly prepared 0.01 M KMnO4-acetone solution. Decreasing quantities of this reagent were mixed with 1 ml. of
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sample in acetone until a quantity was found which retained its characteristic permanganate color after 1min. of shaking. -
Acknowledgment.-The help of W. E. Rowe and R. J. Kunz is greatly appreciated in carrying out the experiments* Juri helpful with the electron irradiation.
was very
PHOTOCHEMICAL INTERCHANGE OF HALOGENS I N AROMATIC COMPOUNDS. 11. TEMPERATURE DEPENDENCE OF SOME SUBSTITUENT EFFECTS BY BARTON ~'IILLIGAS AND RONALD L. BRADOW Department of Chemistry, The Universtty of Mississippi, University, A!fisSissippi Received March l.d9196)
The changes with temperature of several substituent effects in photochemical aromatic halogen interchange have been determined. The enthalpy and entropy differences obtained from these data suggest the intervention of T complex formation prior t o the substitution step.
Introduction In a previous report' we described some general features of photochemical interchange of halogens in aromatic compounds, reactions represented by ArX
+ X' +ArX' + X
and reported some room temperature substituent effects in a number of these reactions. The mechanism of displacement was seen to be most probably a direct displacement on carbon by halogen atoms and one in which the bond from carbon to the halogen atom displaced is but little broken in the transition state. The substituent effects we report are generally small but readily measurable. These effects, determined by competitive experiments, fail to fit conventional linear free energy relationships and do not even form a consistent qualitative pattern. We suggested that the reason for these inconsistencies is the kinetic significance of n complexes as postulated by Miller and Walling2 for one case. I n this paper we wish to report data which lend great weight to this contention. Though often discussed, the formation of n complexes prior to the substitution step has never before received concrete experimental support for any aromatic displacement reaction. Experimental The materials and methods employed were those described previously.' Briefly, a solution of appropriate halogen in CCl4 was added to a solution of a substituted and an unsubstituted aryl halide in CCII. With two exceptions, aromatic concentrations were 0.10 M and halogen concentrations ranged from 0.01 to 0.1 M . When p-bromoanisole and mfluorobromobenzene were compared t o bromobenzene, 0.1: 0.5 and 0.5 : 0.1 molar ratios, respectively, were used. Aliquots of the solutions were placed in a thermostat bath re lated to 0.05' and irradiated by mercury arc lamps prov i g d with filters to remove ultraviolet radiation. The products were analyzed by gas chromatography. (1) B. Milligan, R. L. Bradow, J. E. Rose, H. E. Hubbert, and 8. Roe, J . A m . Chem. Soc., 84, 158 (1962). (2) B. Miller and C. Walling, ibid., 79, 4187 (1957).
Results The effects of several substituents on the rates of three halogen interchange reactions, each a t three or four temperatures, are given in Table I . When the reagent supplying the displacing halogen was IC1 or Brz, reaction was carried only to low conversion (5% or less), and the reactivity ratio given in Table I is the ratio of substituted to unsubstituted product. When chlorine was the reagent, conversion of the aromatics was approximately 20%, and the usual integrated expression was used.
W =
log ( A - x ) j A log ( B - x ) / B
The quantities in this equation were obtained by solution of simultaneous equations employing the ratio of products, from gas chromatography, and the total chlorine used, as determined by iodometric titration after reaction. Such a treatment is valid if chlorine is completely and quantitatively converted to br0mine.l We also have measured, a t different temperatures, the relative reactivities of iodo- and bromobenzenes toward chlorine from IC1 by means of the indirect method described ear1ier.l If iodo- and bromobenzenes mere compared directly in a competitive experiment, only one product, chlorobenzene, would be formed. However, in order t o minimize the possibility that the bromine displaced might react with iodobenzene to generate new bromobenzene, one would have to carry the reaction to very low conversion of IC1 and would thereby be faced with the difficulty of measuring the small difference between two large numbers. Therefore, we tagged first one and then the other of the compounds with a p-fluoro substituent and corrected the result for the substituent effect. The results obtained in this study as well as the previously reported value at 27.3' are given in Table 11. Plots of log TV us. 1/T are linear. The apparent enthalpy and entropy of activation differences
PHOTOCHEMICAL IKTERCHANGE OF HALOGENS IIC'AROMATIC COMPOUNDS
Nov., 1962
2119
TABLE I SUBSTITUENT EFFECTS Substnt.
Atom
Atom
leaving
entering
(X)
(X')
Br
c1 c1
ON PHOTOCHEMICAL
Br I I Br I
3-%
a
2-F Br 4-CHaO Br See text. 25.8'.
ratio ( W ) a (substd./unsubstd.)-------Temp., OC.
--Reactivity Reagt.
11.o
4-F
HALOGEN DISPLACEMENT
Clz IC1 BrZ Br c1 IC1 c1 Clz Br Brz c1 Clz c1 Clz 17.5'. 23.0". E 28.0'.
15.2 1.30
2.50 1.04 7.59
22.5 1.2gb 2.20 1.09 4.03 0.153*
1.05 0.204 0.238"
0.248 .781
27.3 1.23 2.08 1.11 3.10
0.120 .214 ,849 8.45
0.225
.80gd 34.9f
f
37.5
13.7e
14.0'.
TABLEI1 a plot of our data has negative slope, which might RELATIVEREACTIVITIES OF IODOBEKZENE AND BROMO- lead to difficulty in application of the isokinetic relationship discussed by Leffler.4 BENZENE TOWARD CHLORINE FROM IODINE CHLORIDE 27.3 22.8 11.o Discussion Temp., "C.
If photochemical halogen interchange were a simple direct displacement derived from the slopes and intercepts, respectively, of these plots are given in Table 111. The limits of error quoted reflect the variations between individual measurements. Immediately apparent from Table I11 is the fact that the enthalpies and entropies are always of opposite sign and more or TABLEI11 APPARENT DIFFERENTIAL ACTIVATIONPARAMETERS IS PHOTOCHEMICAL HALOGEN INTERCHANGR AAH*
Substnt. 4-F
Atom leaving Br Br I
I 3-F 2-F 4-CHaO Nonea a
Br I Br Br Br/I
Atom Unsubstd. entersubstd. ing Reagt. (koal./mole) C1 Clz 0.61 f 0.2 C1 IC1 1.8 f . 3 c1 IC1 9.8 f .4 Br Brz - 0.70 i. .05 C1 Clz 3.5 .1 Br Bm 0.98 =t .05 C1 Clz - 0.55 f . 0 5 1 1 0 . 7 i .4 C1 Clz C1 IC1 - 4.7 .1
+ + + + +
*
*
AAS*
-
Unsubstd. substd. (e.u./mole) 1.6 f 2 4.8 f 3 -28 f.5 2.0f. 1 -15 1 2 - 5.51.1 1.4f.O.5 -30 +3 +17 1 . 4
-
+ +
From Table 11.
less parallel one another in magnitude. A plot of A A H us. A A S gives a reasonable fit to a straight line passing through the origin. From the least squares treatment of all points A A H (cal.)
=
-330AAS
+ 158
The correlation coefficient, r, is 0.98. I n view of the great diversity of the reactions on which this plot is based, .the correlation is remarkably good and may be fortuitous. Petersen, Markgraf, and Ross3 have pointed out that random errors of measurement can lead to a linear plot of A H * us. AS* with slope T if the range of A H * is not large. Their treatment can be applied to our relative rate data with minor modifications. Accordingly, any correlation between A A H and A A S for about half of our data is open to suspicion, but the correlation of the larger values may have some significance. One should note that (3) R . C. Petersen, J. H. Markgraf, and S.D. Ross, J . Am. Chem. Soc.. 83, 5819 r1961).
x
X'
@ + ' X ' ICd-+ @ + x * '
the reactivity ratio obtained from a competitive experiment should be the ratio of the rate constants for the displacement step.
W
=
k/kO
Therefore In W
=
AHa" - A H *
RT
-
ASo" - AS*
R
The results presented in the previous section deal, saving two exceptions, with metn- and parasubstituted benzene derivatives. The exceptions are o-fluorobromobenzene and the comparison of iodo- and bromobenzenes. Very little or no differential steric interaction is expected in the latter case, since it could arise only from the larger bulk of the iodine atom over the bromine atom. In the former case the small size of fluorine should lead to a minimal steric effect. Since the absence of steric effects in meta- and para-substituted benzenes is widely accepted, one can fairly state that for a simple displacement mechanism the entropy term in the equation above should be zero or negligibly small in all cases studied. Therefore, plots of log W us. 1/T should be linear and pass through or near the origin. Furthermore, the slope of the line should have the same sign as log W. Examination of Table 111 reveals that these predictions fail. The photochemical reaction of bromobenzene with chlorine is a chain reaction2 (% = 50). 5 Since none of the bromoaromatics considered here is vastly different from bromobenzene in its reactivity toward chlorine, the assumption of chain kinetics in all of these cases is reasonable. On the other hand, the reactions of iodine chloride with bromides and iodides and of bromine with (4) J . E. Leffler,J . O i g . Chem., 20, 1202 (1955). ( 5 ) J. E. Rose, unpublished results. University of Mississppi.
2120
BARTON MILLIGAN AND RONALD L. BRADOW
Vol. 66
iodides are non-chain processes (a < 10-2).5,6 by the effect of the substituent on the enthalpy of One should note that examples of both chain and activation but also on the enthalpy of T complex non-chain reactions exhibit the inverse temperature formation. These effects need not (but may) be dependence. in the same direction. Another argument against such a simple mechThe inverse temperature effects we have observed anism lies in the slope of the log W us. 1/T plot for can be accounted for by a T complex mechanism the competition of p-bromoanisole and bromo- by postulating that the effect of the substituent benzene. According to the principles of chain re- on complex formation outweighs its a$ivating or action kinetics,’ all propagation steps must have deactivating influence on the substitution process. activation energies of no more than a few kcal. Furthermore, the abnormally large (nearly 11 kcal.) Therefore, for the reaction of bromobenzene to apparent enthalpy difference measured for the comhave nearly 11 kcal. more activation energy than parison of p-bromoanisole and bromobenzene can that of p-bromoanisole, as indicated by our data, be reconciled with chain kinetics by arguing that is inconsistent with a simple chain mechanism. the p-methoxy substituent not only lowers the An alternate explanation must be sought. enthalpy of activation for substitution but also A plausible explanation of our data is provided causes complex formation to become exothermic. by the intervention of n complexes prior to the Anisole is known to be a highly effective donor.g substitution step. Although a complete kinetic If the measured reactivity ratio contains equianalysis is complicated, one can show that under librium constants for n complex formation, the incertain limiting conditions, the measured reactivity tercept of the log TT vs. 1/T plot is composed of the ratio includes the ratio of the equilibrium constants substituent effect on both entropy of activation for n complex formation, as well as the ratio of rate and complex formation. In the cases of m- and constants for substitution. Whether the proper p-substituents, the former is expected to be small, equilibrium constant is that for the aromatic- but because of symmetry differences, the latter is halogen atom complex8 expected to have a substantial value. The flash photolysis studies of Strong, Rand, and X Brittscof aromatic iodine atom complexes reveal that they disappear by second-order kinetics with rate constants on the order of lo91. mole-1 sec.-l. It is worth noting that if similar considerations applied or the aromatic-hnlopen molecule complex in the cases we have studied, half-order kinetics X should be observed. However, we find that product ratios are proportional to the first power of reactant ratios in both chain and non-chain reU actions, effectively ruling out control of the steady depends on a number of factors which cannot be assessed a t present. In any event, the apparent state concentration of an aromatic-halogen atom differential enthalpy of activation obtained from the complex by bimolecular recombination reactions. Acknowledgment.-This work was supported in slope of a log W vs. 1/T plot is determined not only part by a grant from the Petroleum Research Fund (6) That this should be s o is reasonable because in these cases, propaadministered by the American Chemical Society, gation steps are endothermic by 8 kcal. or more. and grateful acknowledgment t o donors of the fund (7) C. Walling, “Tree Radicals in Solution,” John Wiley and Sons, Inc., New York, N. Y., 1957, p. 240. is hereby given. Earlier parts of the work were (8) The existence of such complexes has been amply demonstrated supported by a grant from Research Corporation. by a number of workers: (a) G. A. Russell, Tetmhedron, 8 , 101 (1960); (b) C. Walling and M. F. Mayahi, J . Am. Chem. Soc., 81,1485 (1959); and (0) R. L. Strong, S. J. Rand, and J. A. Britt, zbzd., 82, 5053 (1960)
(9) L. J. Andrews and R. &‘IKeefer, , ibzd., 75, 3776 (1953)f P. A. D. de Maine, J . Chem. Phys.. 26, 1189 (1957).
