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THEIODINE COMPLEXES OF FLUOROBENZEXES AND FLUOROTOLUENES
solutions. All the TiC14-aromatic solutions studieid have a red color with the exception of the orange-coloreid TiC14-CsH, solutions. Furthermore, the color deepens as the electron density of the aromatic ring is increased. We are currently investigating the thermodynamic properties of these liquid complexes.
Acknowledgment. The authors gratefully aclinowledge the support given this project by the Office of Army Research (Durham). We also thank Mr. Arnold Loveridge for his assistance with the freezing point measurements and Dr. R. T. Hawkins for help in the interpretation of the infrared spectra.
The Iodine Complexes of Fluorobenzeries and Fluorotoluenesl
by Milton Tamres Department of Chemistry, University o,f Michigan, Ann Arbor, Michigan
(Received A p r i l 18, 1964)
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The charge-transfer (c.t.) bands of a series of iodine complexes with mono-, di-, and trifluorobenzene (in CC1, and in n-heptane) and fluorotoluenes (in n-heptane) were studied. The c.t. bands appear in a region where the free donors also absorb. For those cases where the band maximum was not observed, it was estimated by extrapolation of the absorption curve. The effect of fluorine substitution is to decrease the equilibrium constant for iodine complexation, in correlation with Taft u* values. In most cases, there is an increase in the molar absorbancy index a t the band maximum. There is obsermd also an apparent blue shift of the c.t. band which is in a direction opposite to that expected from the ionization potentiah of the donor molecules. This may be due to the superposition of two c.t. bands which arise from the removal of the degeneracy of the highest occupied orbitals in benzene on substitution. A temperature dependence study of the fluorobenzene-iodine complex both in n-heptane and in CCI, gave a value of -1.4 kcal. mole-1 for the heat of reaction.
Introduction Previous studies on polymethylbenzene-iodine coniplexes213have shown that increased methyl substitution (a) increases the stability of the complex, (b) gives a red shift of the charge-transfer (c.t.) band, and (c) decrerises the molar absorbancy index of the complex (a,) a t the wave length of maximuni absorption (Amax). These observations, except for the last, are consistent with the inductive property of the methyl group and its effect of systematically lowering the ionization potential (I,) of the donor molecule on continued substtitution. The opposite trend in a, (reported also for olefin-iodine complexes4) has been explained as being due to the existence of several geometric arrangements
of the complex, each with its individual equilibrium constant ( K ) and a,, which could lead to a nonlinear log K vs. 1/T plot.6 Since multiple forins are more likely to be present in weak rather than strong complexes, because the latter would favor the existence of a single, stable structure, the fluorobenzenes would seem to be a favorable system to study this aspect. (1) Presented before the Division o f Physical Chemistry at the 146th National Meeting of the American Chemical Society, Denver, Colo., Jan., 1964. (2) (a) H. Benesi and J. H. Hildebrand, J . Am. Chem. Soc., 71, 2703 (1949); (b) L. J. Andrew and R. M. Keefer, ibid., 74, 4500 (1952). (3) M . Tamres, D. R. Virzi, and S.Searles, ibid., 75, 4358 (1953). (4) J. G. Traynham and J. R. Olechewski, ibid., 81, 571 (1959). (5) L. E. Orgel and R. S.Milliken, ibid., 79,4839 (1957).
V o l u m e 68, Number 9 September, 2964
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MILTONTAMRES
+
Fluorine is electron withdrawing and, by contrast to the methyl group, substitution in benzene and in toluene does not lead to a systematic change in the ionization potential of the donor molecule. It was thought of interest to study a series of iodine complexes with polyfluorobenzenes and fluorotoluenes to determine the effect of substitution on the above-mentioned properties.
All calculations were made using a program for the IBM 7090 computer, as mentioned in a previous paper.g No difference in results was observed in using the linear or the quadratic equation, as expected. l o
Results
Plots of the data for the various complexes using eq. 1 gave excellent straight lines. The slopes of these Experimental lines were well defined, thereby giving a quite good Apparatus and Procedure. All spectra were taken evaluation of the product (a, - aa)K,. Minor variaon a Cary recording spectrophotometer, Model 11. tions in the data have a small effect on the slope. On the other hand, even a small change in the slope has The temperature dependence study of the equilibrium constant was made with a Beckman quartz spectroa pronounced influence on the intercept (which is inversely proportional to a, - a,) because the line photometer, Model DU. The techniques for instrument calibration, temperature control, and making intersects the y-axis so close to zer0.l' Since a, - a, is evaluated prior to calculating K , up solutions have been described previously.fi The concentrations of reagents for work using the Cary from the intercept/slope ratio, any error in a, - a, instrument ranged from N A r = 0.02 to 0.94 (mole will produce a corresponding error in I
