Vol. 33, N o . 4, April 1968
HOMOLYTIC DECOMPOSITIONS OF HYDROPEROXIDES. I1 1421
Homolytic Decompositions of Hydroperoxides. 11." Radical-Induced Decompositions of t-Butyl Hydroperoxide R. H I A T TT. , ~ MILL, ~ K. C. IRWIX, AND J. K. CASTLEMAN Stanford Research Institute, Menlo Park, California Received June 88, 1967 Three factors account for deviations from the simplest pattern for the radical-induced decomposition of t-BuOzH (reactions 1-4 in part I ) : (1) variation in the ratio of terminating to nonterminating interactions of 2t-Bu02. with the viscosity of the medium; (2) competition between the propagation and cleavage reactions of t-BuO. radicals; and (3) competition between hydroperoxide and solvent for t-BuO radicals. Reactions in the gas phase illustrate all three factors. I n solution cleavage is important a t high temperatures and low concentrations of t-butyl hydroperoxide (as in 0.1 M t-BuOzH in benzene or CCl4 at 100") or in hydroxylic solvents ( AcOH or t-BuOH) a t moderate temperatures. Attack on alkane solvents increases with increasing temperature. Chain lengths of 1 to 50 are observed. Both cleavage and solvent participation result in shorter chains and complex kinetics.
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In benzene or chlorobenzene a t 25-50', t-Bu02H is cleanly decomposed by alkoxy radicals to 90% t-BuOH, 10% t-Bu202,and 100% 0 2 . The rate of decomposition is about ten times the rate of initiation and is independent of the concentration of t-Bu02H. After considering the free-radical-generating efficiencies of the initiators used and the stability of t-Bu02H in the absence of initiators, this paper deals with induced decompositions in several solvents at 45-100' and in the gas phase a t 100". We show that chain lengths and products vary and that kinetics may be complex, but that the results can be rationalized in terms of a few competing reactions. Experimental Section Materials.-t-BuOnH (90%) obtained from Lucidol Corp. was vacuum distilled to 99+% purity (by iodometric titration). sec-BuzOZ and ~-BuOZCGHII (C&I = cyclohexyl) were prepared by the methods of Mosher and coworkers .3 sec-BuZOz was shown to be pure by iodometric titration and by glpc. The contained about 57, of an unidentified contaminant. t-Bu2O2 obtained from the Matheson Co. was shown to be pure by glpc. Di-t-butyl peroxyoxalate (DBPO) was prepared by the method of Bartlett and coworker^.^ The solvents used were Matheson Chromatograde, refluxed and distilled from CaH2 before use. Isobutane, n-butane, cyclopentane, and cyclopentene were Phillips research grade, used without further purification. Analytical Procedures. Iodometric Titrations.-t-BuO2H, sec-Buz02, and DBPO were titrated iodometrically. In the basic technique the sample is added to a mixture of -20 ml of 90: 10 i-PrOH-HOAc; about 1 g of NaI is added; and the mixture is refluxed to hasten reaction and exclude air. For hydroperoxides, refluxing the mixture for 2-3 min gave quantitative liberation of 1 2 . DBPO liberated only 607, of the theoretical amount of 11, but did so consistently when the amount to be analyzed was kept below 0.05 mmol. Dialkyl peroxides gave no titer under these conditions. Di-sec-alkyl peroxides could be titrated quantitatively if a small amount of metal ion was added. Thus, sec-BuZ02 liberated 100 -f 27, of the theoretical iodine when the 20 ml of NaIHOAc-i-PrOH mixture contained 2 p1 of a Nuodex solution of iron octoate and reflux time was extended to a t least 5 min. Similarly, FeS04, FeC13, and Nuodex copper octoate were equally effective catalysts, while use of Nuodex solutions of cobalt or manganese octoate gave no Iz. t-Buz02 liberated no iodine even with higher concentrations of iron octoate and reflux times up to (1) (a) Part I: R. Hiatt. T. Mill, and F. R. Mayo. J . Ora. Chem., 85, 1416 (1968). Equations 1-16 appear in part I. (b) To whom all corre spondence should be addressed a t Brock University, St. Catherines, Ontario, Canada. (2) R. Hiatt, J. Clipsham, and T. Visser, Can. J . Chem., 41, 2754 (1964). (3) F. Welch, H. R. Williams, and H. S. Mosher, J . Amer. Chem. Soc., 77, 551 (1955). (4) P. D. Bartlett, E. Benzing, and R. E. Pincock, ibid., 81, 1762 (1960).
20 min. &Butyl cyclohexyl peroxide gave a maximum of 43% of the theoretical amount of iodine under these conditions. Gas Chromatography.-Most analyses of reacted solutions were made using the Wilkens Rlodel 90 or 350 gas chromatographs with thermal conductivity detectors and 10-ft columns of either 207, Carbowax 20M or 20% diisodecyl phthalate on Chromosorb P. t-Bu~Ozwas eluted without decomposition, but other peroxides and hydroperoxides were decomposed in the injector cavity or on the column. Solutions containing hydroperoxides were usually reduced to alcohols with triphenylphosphine5 (PhaP) before glpc analysis. sec-Bu202 and t-Bu02C6H11were analyzed on a Wilkens gas chromatograph with a flame ionization detector ("Hi-Fi") where they were eluted without decomposition from a 5-ft column of 2% diisodecyl phthalate on Chromosorb P. The low-loaded column permitted use of a low temperature (70"),thereby minimizing decomposition. In one instance a solution of t-BuOH, t-Buz02, and ~ - B u ~ ~ C in ~cyclohexane HI~ was analyzed by a combination of fractional distillation, quantitative ir analysis, and glpc. Procedures for Reactions.-Most reactions in solution were carried out in a series of eight or nine sealed Pyrex ampoules, initially degassed, or with an air atmosphere as indicated in text. Ampoules were immersed in a constant-temperature bath and withdrawn a t suitable intervals for iodometric and glpc analysis. Some ampoules were fitted with break-seals and opened on a vacuum line so that the amount of evolved gases could be estimated and samples analyzed by mass spectrometry. For solution reactions carried out under reflux, a cell consisting of a 50-ml bulb seaied to a condenser was immersed in a. bath 10-19" warmer than the reflux temperature of the solvent. The solution of hydroperoxide was brought to reflux and reaction was started by addition of the initiator. Gas phase experiments were carried out in 100-ml Pyrex bulbs with break-seals and capillary inlet tubes. Peroxide and hydroperoxide were added to the evacuated bulbs from a tared syringe. Hydrocarbons were added to some runs by vacuum distillation from a calibrated bulb. Formaldehyde was added in the form of paraformaldehyde. After being heated for appropriate time intervals, the bulbs were connected to the vacuum line and opened, and the contents were distilled into small liquid nitrogen-cooled traps which were then cut out of the line. Less volatile hydrocarbons were distilled with the hydroperoxide. Evolved gases were transferred to a gas buret with a Toepler pump, then analyzed using a Cu-CuO combustion furnace. Where large amounts of butanes were present, the bulb contents were fractionally distilled under high vacuum a t -80'. Blank experiments showed no significant loss of hydroperoxide under these conditions.
Thermal Decompositions of Initiators and of t-BuOzH
It was necessary to measure rates and efficiences of radical production by some of our initiators before chain lengths of induced decompositions could be evaluated. The possibility of some thermal decompo(5) L. Horner and W. Jurgleit, Ann. Chem.. 691, 139 (1955)
1422 HIATT,AfILL,
The Journal of Organic Chemistry
IRWIN, AND CASTLEMAN
TABLE I DECOMPOSITIONS O F INITIATORS
I%.Al'ES A N D PRODUCTS O F
Temp, O C
Solvent
[RzOzjo, M
.---Decomposition"--10bkkr, 8ec-1
---Yields on peroxides decomposed, %-----
%
100 100 100
CfiHs PhMe
0.0066 0.24;; 0.264
t-BuzOz 0.0875 0.102 0.0682
100 100 100
PhJIe PhXe Gas phase
0.10 0.027 0.0008
sec-BuzOz 0.27 83 0.27 75 0.15f 7-82f
CfiHfi
62 70 22
AcMe
t-BuOH
96 29
1.6 68
t-BuzOz
S-sb
-e
206 1ooc
23' 2.5.
Di-t-butyl Peroxyoxalate (DBPO) 25.0 t-RuOH 37.8 n-C,H12 81.5 c,rr,, a Apparent first-order rate constant toluene,b bibenzyl,c C I O H ?(CGH11)1.' ~,~
0.08 1.83 70 0,008 10.1 39 0 77 0,006 92 and extent of decomposition over which it was measured. f Average of seven runs. p See ref 7.
sition of t-BuO,H under the experimental conditions also had to be checked. Decompositions of Initiators in the Absence of Hydroperoxides. t-BuzOz.-At and above lOO", tBuzOzgave convenient rates of initiation. I n toluene a t loo", the first-order rate constant for decomposition of t-Buz02 was found6 to be 6.8 X10-7/sec, and the efficiency of radical production was 100% (measured by the yield of bibenzyl). This rate constant was used for calculation of chain lengths for induced decompositions in both gas phase and solution, although in some of the induced runs the disappearance of tBuzOr, as measured by glpc, was faster (see below). Typical results are shown in Table I. Di-t-Butyl Peroxyoxa1ate.-DBPO was used to initiate runs at 80" and below. Rate constants for the thermal decompositions of DBPO a t several temperatures in benzene and cumene have been given by Bartlett and c o ~ o r k e r s . ~We found the same rates in n-pentane and in t-BuOH. DBPO has been shown to give about 5% t-Bu202 by a cage reaction in solvents of moderate viscosity (pentane, benzene, HOAc) a t 3j-45O.7 In 1-BuOH and in Sujol, cage recombination accounted for 10 and 76%, respectively, of the radicals formed. For this work we assumed an initiator efficiency of 100% minus the per cent yield of t-Bu202 for the particular solvent. sec-Buz02.-Some runs at 100" were initiated by sec-BuzOz. Pryor, et d.,* have shown that this peroxide initiates the polymerization of styrene a t 80" about as rapidly as t-Bu20z does. We have measured rates of decomposition a t 100" in toluene and in the gas phase (Table I) using both glpc and a modified iodometric titration for peroxide analysis. Rates were first order in peroxide, and about four times as fast in solution as for the decomposition of t-Buz02, but the efficiency of radical production in solution, as measured by the yield of bibenzyl, was only 25%. Analysis of the other products of decomposition (Table I) gave an unexpected 2t5-30% hydrogen gas, which might be explained by a concerted decomposition of the i6) This value is close to the value (5.0 f 2.5) X lO-'/sec obtained by extrapolation from the 13C-15Oo gas phase data of L. Batt and S. W. Benson [ J . Chem. Phys.. 86,895 (1962)l. (7) R. Hiatt and T . G. Traylor, J . Amer. Chem. Soc.. 81, 3766 (1965). Huston, . T. R. Fiske, T . L. Pickering, and E. (8) W. A. Pryor, D. .\I Ciuffarin,ibid., 86, 4237 (1964).
108
29 62e Products of radical coupling: ,j.4
kind discovered by Mosher and Durhamg for primary hydroperoxy hemiacetals.
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