840
D. M. MARCHAXD T. HENS~IALL
iodine series and is 20-40 times larger for a given iodine derivative than for the bromine analog. Discussion The number of independent data determined in this study are insufficient to determine unambiguously the effect of halogenation on the individual rate constants kz-lc6. However, from the several trends indicated above some meaningful conclusions may be drawn. From the relation T = l/ka -Iks it can be seen *hat kg k6 increases with halogenation and is larger for the iodine derivatives than for the bromine derivatives. The increased spin-orbit coupling due to iodine would increase 4. The effect of such substitution on k6 cannot be determined from the lifetime data alone, but there is evidence that chlorine substitution on naphthalene increases both ks and ks by a factor of five.18 The integrated absorption, a measure of the natural lifetime of S’, T~ = l/kz, varies by less than 30% within the group of halogen derivatives in aqueous The values of T~ computed from @F = 7/70 vary by a factor of two between ff uorescein and e0sin.1~ In the following discussion we will assume that k2 is nearly the same for all of the dyes in EPA solution. The intersystem crossing ratio, x, sometimes is nearly equal to This is not necessarily true in the case of the fluorescein dyes where we cannot he certain that 4 Br > I. The kinetic results have shown that in aqueous solutions the decrease in 4% is due to the increase of k8. The parallel trends suggest that a similar situation prevails in EPA solutions. This conclusion is supported further by examination of the trends within the group of iodine derivatives. @F = k2/(k2 ka l e d ) and since @Fis about 0.1,1/@~ E (ka Ic4)/lcz. k&s increases progressively with the number of iodine atoms, while @F does not decrease in a similar progression. It would seem that if IC4 >> ks such a trend would exist. One final point must be emphasized. The decrease in @F is not accompanied by an increase in @p. The sum @pp @F decreases with substitution in the order H = C1 > Br > I, therefore at least one of the non-radiative processes, S’ -t S or T -+ S, becomes more important in this sequence.
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THE KINETICS OF CYCLIZATION OF SOME %,a’-DIPHENIC ACIDS IN SULFURIC ACID BY D. M. MARCH‘ AND T. HENSHALL Department of Chemistrg, Sir John Cass College, London, England Received Septembsr 66,l 9 S i
The kinetics of cyclization of some 2,2’-diphenic acids in concentrated sulfuric acid have been studied using a spectrophotometric method of analysis. Simple first-order kinetics were found in each case, and also a linear dependence of log k on the Hammett acidity function. The energy and entropy parameters were found to vary in a characteristic manner. A mechanism is proposed and is discussed.
The kinetics of cyclization of 2,2‘-diphenic acid to fluorenone-4-carboxylic acid2 have been studied in aqueous sulfuric acid, and in sulfuric-acetic acid mixtures over the ranges 77-100y0 and 60-100% w./w., respectively. The measurements also have been extended to certain 5,5’-disubstituted-2,2‘-diphenicacids con(1) Material taken from a thesis in partial fulfillment of the requirements for the Ph.D., London University, 1961. (2) C, Qraebe and C. Aubin, Ann., 247, 261 (1888).
taining substituents of well defined electronic character; these were studied in the aqueous medium only. In every case, a linear dependence was found of log k on the Hammett acidity function Ho but the slopes differed from unity. The activation energies and entropies have been found to depend markedly on the solvent composition; a probable interpretation is offered (Discussion).
May, 1962
CYCLIZATION OF 2,2’-DIl”EI\;IC
ACIDSI N
SULFURIC
ACID
841
Experimental Materials.-These various diphenic acids were prepared for study; 2,2’-diphenic acid; 5,5’-dimethyl-; 5,5’-diethyl-; 5,5’-di-t-butyl-; 5,5’-dichloro-; 5,5’-dibromo-; and 5,5’-dinitro-2,2 ’-diphenic acids. Of these, the 5,5’-diethyl-, and 5,5’-di-t-butyl acids have not been reported previously; the physical constants and analytical figures are, respectively: M.p. 247’. Anal. Calcd. for ClsH1SO4: C, 72.5; H, 6.09. Found: C, 72.5; H, 6.20. M.p. 306’. Anal. Calcd. for CZZHZ~O~: C, 74.6; H,7.40. Found: (>,74.7; H,7.52. Preparation of :Reaction Solvents. (i) 100% Sulfuric Acid.-A commercial acid containing a slight excess of sulfur trioxide was partially frozen by immersion in icedwater. Distilled water then was added dropwise from a microburet, with stirring, until the melting point rose to a maximum of 10.4’ and remained constant as the whole was frozen. The stock acid then was kept in a tightly stoppered bottle, whose neck was protected by a Polythene sleeve. The melting point was checked periodically. (ii) Mixtures of sulfuric acid with water, and with acetic acid were prepared by the dilution of the titock acid. A known weight of the acid was diluted by the gradual addition of distilled water or glacial acetic acid, cooled in a desiccator and reweighed. The Analytical Method.-The fluorenone-4-carboxylic acids possess charaic teristic absorption spectra in the ultraviolet region; maxima occur around 2850 and 3150 A., the former being the more intense. The diphenic acids themselves show an appreciable absorption in the ultraviolet but the intensities of alkaline solutions decrease with increasing wave length, becoming negligible above 3000 A. Consequently the cyclization reaction is most conveniently followed by absorption spectrophotometry; and absorption above :3000 A. shown by samples of reaction mixture after dilution by N/10 sodium hydroxide iil due almost entirely to the reaction product. Obedience to Beer’s law was tested for each acid studied. Optical density measurements were made in quartz cells on a Unicam photoelectric spectrophotometer, model SP 5003; and a typical plot is shown in Fig. 1 which relates to fluorenone-4-carboxylic acid itself. Reaction Vessels and Temperature Control.-The kinetic runs on 2,2’-diphenJc acid itself were carried out in 40-ml. tubes fitted with ground glass stoppers. However, due to the decreased solubilities of the disubstituted acids in sulfuric acid of concentration less than 95% w./w., it was necessary in these cases to use the apparatus shown in Fig. 2 in order to obtain reaction solutions sufficiently concentrated for rate studies. I n operation, the solvent is held in A by the entrapped air. The reactant is added, and when sufficient has dissolved, the reaction solution is transferred to B by removing the ground-glass stlopper and ap lying slight suction; undissolved material is retained by t i e sintered-glass plate. This procedure was found to be highly satisfactory; and by using 3-4 cm. quartz cells for the measurement of optical densit
