K. M. KEEFERAND L. J. ANDREW
434s
Val. T9
acetoacetate. Reaction took place with the evolution of amorphous powder. The powder was recrystallized froln heat and ended within several minutes. The reaction mischloroform or benzene solution, and again light yellow crysture was fractionated under reduced pressure. The volatals were obtained. The crystals (plate form) were stable tile liquid distillates were sliovr-n to be alcohols by measurein chloroform or benzene solutions, b u t they lost their ments of the refractive indices and boiling points. The crystalline form and turned to a light yellow aniorphous quantity of alcohol was 0.010 mole. .In equivalent amount substance, gradually in air and rapidly in a vacuum desicca(0.010 mole) of compounds I11 or IV remained as the residue. tor. The color gradually turned to brown over 200" and When the products were solid (see Table I), they were the compound decomposed t o a dark-red liquid a t 233". purified using diethyl ether or petroleum ether as solvent. ;1nal. Calcd. for TiCloH140s: Ti, 18.3; C, 45.9; I $ , The liquid products were distilled a t mm. with a pot-5.38. Found: Ti, 18.2; C, 45.6; H, 5 . 5 7 . still or "falling-film" type molecular still, maintaining the temperature under 70" to prevent heat decomposition. Hydrolyses of dipropoxy- and diethosy-titatiiuni-bisThese products are soluble in most organic solvents. (acetylacetonate) were carried out analogously with the ( b ) Reaction of Tetraalkoxytitaniums with Acetylacetone butoxy-compound, and the products obtained were conand Ethyl Acetoacetate in a Molar Ratio 1:2.-To 0.010 firmed t o be identical with the hydrolysis product of the dimole of each tetraalkoxytitaniuni was added 0.020 mole of butoxy-compound by chemical analyses. acetylacetone or ethyl acetoacetate. Reaction took place The aqueous solution of VI11 gave transparent gel-like with the evolution of heat and ended within several iiiinutes. substance after standing several days a t room temperature. From the reaction mixtures, 0.020 mole of alcohols Tvere reWhen heated over 4O0,the solution decomposed t o a n opalmoved under reduced pressure. The theoretical amount escent substance. (0.010 mole) of compounds V or VI remaiiied as the residue. Molecular Weight Determinations.-Molecular weights were measured cryoscopically in completely dried benzene. iyhen the products were solid (see Table 11), they were purified using diethyl ether or petroleum ether as solvents. T h e measurements were carried out in a shielded cryoscopic The liquid products were distilled with the molecular still. cell with a n electromagnetic stirring device after exclusion These compounds are soluble in most organic solvents. of air with a stream of dry nitrogen. Freezing points of Molecular Refraction.-MR values for ( PrO)?Ti(acac)2, benzene solutions of some compounds showed a tendency t o (PrO)zTi(etac)a, (BuO)iTi(acac)?and (BuO)?Ti(etac)L were decrease with time. 103.8, 116.6, 113.4 and 123.9, respectively. Since there are Measurements of Absorption Spectra in the Visible and no atomic refraction data for hexa-coordinated titanium in Ultraviolet Regions.-Ultraviolet absorption ~ i i e ~ ~ s u r e m e i i t ~ the literature, calculation of the molecular refractions was were made with a Becktnan D U spectrophlJti,rneter using impossible. Instead, the atomic refraction was calculated 1.0-cm. silica cells. The absorption spectra of I11 and I' from observed molecular refractions, and the values of 19 measured in appropriate alcohols showed similar curves 20.1, 20.2 and 20.2, respectively, were obtained for lie. having a maximum a t 325 nip (log E 3.3), The absorption spectrum of irradiated ( BuO)2Ti(acac)2in the visible region coordinated titanium froni these four X R values. Hydrolyses of the Compounds and Isolation of VII1.was measured in n-hexane and showed a ninsimuni :it 800 T h e Compounds 111, IV and VI were hydrolyzed readily by m p and a shoulder near 630 mp. The infrared absorption measurements were carricd out moisture. Gel-like substances were formed after standing several hours in moist air or instantly by addition of water. with a Perkin-Elmer recording spectropliotonietcr model In case of dialkoliytitaniuni-bis-(acety1acetoiiate)s (I-), ad11%. Liquid samples were measured in thin films and t h e solid sample W;LS measured using Sujol. dition of water gave light-yell~~w needles. To 1.62 g. of (BuO)2Ti(acdc)a was added 5 nil. of water. T h e crystals obtained were filtered and dried to give 0.74 g. of pale-yellow TOKTO,JAPAS
[COTTRIBUTION FROM
THE
DEPARTMEST O F CHEMISTRY, v X I V E R S I T P
OF
CALIFORNIA AT DAVIS]
The Kinetics of Aromatic Hydrocarbon Chlorination in Acetic Acid. The Use of Zinc Chloride as a Catalyst and of Iodobenzene Dichloride as a Halogen Source BY It. X I . KEEFERAND L. J. XNDREKS RECEIVEDFEBRUARY 4, 1957 I n acetic acid methylbenzenes undergo chlorination by a reaction which is first order in halogen. The reactioiis a r e I i o t inhibited by hydrogen chloride and are subject to relatively mild catalysis b y zinc chloride. Bromination and iodiuatioii reactions in this solvent are of higher order than first in halogen in the absence of catalyst. The reactions are accelerated by zinc chloride and in some cases are complicated by trihalide formation. These differences may be explained in part through consideration of the relative stabilities of trihalide ions in acetic acid and of the role of acetic acid in the substitution reaction. Polymethylbenzenes chlorinate too rapidly in acetic acid for kinetic measurement. T o investigate possible steric complications in the reactions of these hydrocarbons, iodobenzene dichloride has been tested as a potentiall?; less reactive chlorinating agent t h a n the free halogens, In this sense it has proved t o be unsuitable. It dissociates slowly and comes to equilibrium with its components in dilute solutions in acetic acid. Its rate of reaction with mesitylene :1!1d pentamethylbenzene in acetic acid is identical with its rate of dissociation.
The investigation of the relative susceptibilities of a series of polymethylbenzenes t o nuclear halogenation with bromine and with iodine nionochloride in acetic acid has been simplified by the use of zinc chloride as a catalyst.1s2 In the presence of this salt the orders of reactions with respect to these halogens reduce to unity and the kinetic complication of trihalide formation? is eliminated. I n addition, by varying the catalyst concentration, relative halogenation rates for hydrocarbons of (1) L
J Andrews and R
11 Keefer
11x1s J o u ~ s i r , ,78, 454')
1 1 9%)
( 2 ) R h4 Keefer a n d I
J A n d r e b s z b f d 78, iO28 11970)
widely different reactivities can be established without changing the halogenating 'igeiit. LVith these observations in mind the effects of zinc chloride on the kinetics of chlorination of various methylbenzenes in acetic acid have been studied, and the results are presented in this paper One of the original objectives of this investigatioii was to determine whether the methyl group steric effect, which so markedly influences the susceptibility of polymethylhenzenes to bromination or iodination,' is less apparent in chlorination reactions. However. the higher molecular weight hydrocnrlmls, even i n the absence of catalyst. re,ict
Aug. 20, 1957
AROhlATIC
HYDROCARBON CHLORINATION
too rapidly with chlorine for kinetic measurement. The possibility, therefore, has been considered t h a t iodobenzene dichloride might serve as a less reactive chlorinating agent for use in a comparative rate study of the more reactive hydrocarbons. Although in this sense iodobenzene dichloride has proved to be an unsuitable reagent in acetic acid, the results of kinetic investigations concerning its function in aromatic chlorination are sufficiently interesting t h a t they are included in this report. Experimental Materials.-The procedures used in preparing pure acetic acid, t h e aromatic hydrocarbons and dry zinc chloride have been described p r e v i o ~ s l y . ~ Eastnian ,~ Organic Chemicals cyclohexene and iodobenzene were used without further treatment except for distillation of the latter. Solutions of chlorine in acetic acid were prepared by passing cylinder gas (Ohio Chemical) directly into the solvent. Halogen analyses of the solutions were made iodometrically. In determining the extinction coefficients of chlorine solutions, the spectrum measurements were made first, and samples for iodometric analysis were then removed by pipet directly from the ground glass stoppered absorption cells to avoid volatility losses. Solutions of hydrogen chloride in acetic acid were prepared and analyzed as described elsewhere.2 Iodobenzene dichloride was made in small lots which were used immediately. The standard synthetic method5 was used with the substitution of carbon tetrachloride for chloroform as the solvent. Iodornetric analyses of weighed samples of the fresh product in acetic acid solution were generally within lY0 of the theoretical value. The Spectrum of Chlorine in Acetic Acid and in Solutions Containing Hydrogen Chloride and Zinc Chloride.-Contrary t o what was found for solutions of hydrogen bromide and bromine' and of hydrogen chloride and iodine monochloride2 in acetic acid, the ultraviolet spectra (Table I) of solutions of hydrogen chloride and chlorine in acetic acid do not offer positive evidence of the formation of trihalide ion (or HCI,). T h e spectrum of a chlorine solution containing zinc chloride, at a concentration typical of t h a t used in the rate runs, and hydrogen chloride is included in Table I. TABLE I THEEFFECTOF HYDROGEN CHLORIDEASD ZISC CHLORIDE ON THE SPECTRUM OF CHLORINE IN ACETICACID"
c i z x 103, HCI, 1l.I ZnCln, III
,v
A, mp
300 310 320 330 340 350 360 380 The B values are the chlorine.
Soh. I
Soh.I1
S o h . I11
9.75
9.75 0.112
...
9.75 0.112 0.200
t
€
.. .. f
40.4 42.2 44.0 61.6 62.6 64.8 74.6 75.6 77.7 76.0 76.6 77.9 66.5 66.5 68.7 50.5 51.0 53.2 34.5 35.3 37.2 12.9 13.9 15.3 molecular extinction coefficients of
It is apparent t h a t the salt also has no marked effect on the halogen spectrum. The Rates of Chlorinations of the Hydrocarbons.-.A. single batch of purified acetic acid was used in carrying out the kinetic experiments. Stock solutions of the hydrocarbons and of zinc chloride in acetic acid were prepared from (3) N. Ogimachi, L. J. Andrews and R. M. Keefer, THISJOURNAL, 77, 4202 (1955). (4) R . hl. Keefer, A . Ottenberg a n d L J. Andrews, ibid., 78, 235 i19Z6) ( 5 ) H. J Lucas and E. R . Kennedy. "Organic Syntheses," Coil. Vol 111, John \Vile? and Sons, Inc. K?w York, K-. Y , 1Slj5, 13 182
I N X C E T I C *ACID
4349
weighed samples of the reagents. Known volumes of these and of stock solutions of chlorine and hydrogen chloride were mixed at the temperature of the rate measurement. Samples of these mixtures, in silica absorption cells (10 cm., 2 cm. or 1 cm., depending on the initial concentration of chlorine), were placed in the housing of the Beckman spectrophotometer. Details concerning temperature control are given e l s e ~ h e r e . ~ T h e changes in chlorine concentration were followed, generally to more than 7556 of completion, by measuring optical densities of the rate samples (against :tu acetic acid blank) at a fixed wave length in the 340-370 mp region. The wave lengths were chosen t o provide initial readings, somewhere between 1.0-0.4. The data were, in certain cases, corrected for slight absorption by zinc chloride in the solutions. The optical densities of dilute solutions of chlorine in acetic acid were observed t o change negligibly during the time interval of even the longest rate runs. In other words errors resulting from the volatilization of chlorine were negligible. The Dissociation of Iodobenzene Dichloride in Acetic Acid.-Qualitative experiments showed t h a t the absorption in the near ultraviolet region of freshly prepared solutions of iodobenzene and chlorine in acetic acid were found to increase slowly with time. These changes were attributed to slow equilibration between iodobenzene dichloride and its components in acetic acid. I t was found that the spectrum of a n acetic acid solution which was 5.55 X lo-( JI in iodobenzene dichloride and 0,0989 JI in iodobenzene did not change with time. The increases in absorption of solutions of chlorine and iodobenzene occurred more rapidly as the iodobenzene concentrations of the media were raised. The optical density readings a t equilibrium for two solutions, initially 2.39 X 10-8 J I in chlorine and 0.164 and 0.0184 .If in iodobenzene, were (after correction for the absorption of iodobenzene) nearly the same. I t was assumed, therefore, t h a t in acetic acid solutions in which the free iodobenzene concentration is of the order of 0.1 -lf,the conversion of chlorine to dichloride is essentially complete. Extinction coefficients for iodobenzene dichloride over a short wave length range were calculated from equilibrium optical density readings of two of the solutions described above. These are recorded in Table 11. Details of fur-
TABLE I1 THE SPECrRUM
DICHLORIDEI S ACETIC ACID" x, 1n/l 350 360 370 380 390 E of s o h . ID 309 233 147 85.6 t o f s o h . 11" 312 224 144 86.0 45.6 Based on optical densities after attainment of equilibrium. The optical densities were corrected for iodobenzene absorption. hSoln. I was originally 5.55 X .If in S o h . I1 was originally CsHjICln and 0.0989 31 in CeHJ. -71 in Cls and 0.164 in CeHSI. 2.39 X O F IODOBESZESE
ther experiments to determine the forward and reverse rate constants for dissociation of iodobenzene dichloride are included with the results. The Kinetics of Reaction of Iodobenzene Dichloride with Mesitylene and Pentamethvlbenzene.-aill rate measurem e n t s were carried out spe&ophotometrically using 1-cm. absorption cells and an acetic acid blank. The changes in optical density with time recorded during a run were corrected for any small absorption which was observed a t infinite reaction time. Several hours prior t o starting rate runs, a stock acetic acid solution of chlorine containing more than a n equivalent amount of iodobenzene was prepared. The rate samples were prepared by mixing known volumes of this solution with known volumes o f solutions of the 1iydrocarl)on a n d , in some cases, of iodobenzene. h single batch o f purified acetic acid was used in studying both kinetics uf dissociation of iodobenzene dichloride and of chlorinations with iodobenzene dichloride. The Product of Reaction of Pentamethylbenzene with Iodobenzene Dichloride in Acetic Acid.-To a freshly prepared solution of 0.65 g. (0.0024 mole) of iodobenzene dichloride in 80 nil. o f acetic acid was added 0.300 g. (0.00202 i n o l d o f ~ ~ c ~ i t : t i i i c t l ~ \ - l l'rlliq ~ c ~s0111tio11 ~ ~ ~ i ~ ~\I-:I< . :1IIo\vvtl t o
'4350
Ieen shown to he second nriier over-all; see, for example, A . E. Bradfield and B. Jones J . ( ' h e m .So< , 3073 (1928); B. Jones. ibid..418, 670 (1942) i l l ) 'The rrlative value? f o u n d f o r k a t for ti,l!iene. p-xylene and n-xi-lene are in the ratio l / C i 7 , 10 'l'heuc valucs compare F.ivurahl>- with those (l,Ij.3/13,4) calculated from t h e times of 20'0 reacti
