COMMUNICATIOXS TO THE EDITOR
1086
dation rates are 1.00,0.548 and 0.211, respectively.) However, 1-methyl- 1-p-methoxyphen ylbutaiie oxidized a t the lowest rate (relative rate 0.199). This niay be explained by iiotirig that l-methyl-lp-methoxyphenylbutaiie-1-hydroperoxidemay decompose in the same manner as cumene hydroperoxide5 yielding p-methoxyphenol which is ai1 inhibitoi..6 (It may be noted that p-methoxytoluene and aidsole appear to be good retarders in the oxidation of cumeiie.3) ( 5 ) ISmk a n d Lange, Bel. , 7'7, 257 (1944). (6) G. E. Penketli, J . A p p l Chrm., 7, 512 (1957)
Vol. 74
If the slow oxidation of l-methyl-l-p-methoxyphenylbutane is due t o the inhibition by p-methoxypheiiol formed during the oxidation, then the former compound should have an oxidation inhibiting effect 011 other compounds. Earlier work' has show-n that bis-(l,3-diniethylbutyl) adipate autoxidizes rapidly. However, when mixed n-ith 25 mole yo of' 1-methyl-1-p-methoxyphenylbutaiie the oxidation rate is reduced by n factor of lo3. I t is apparel1t that 1-methyl- 1-pinet hosyphenylbutane has a strong inhibiting effect.
COMMUNICATION TO THE EDITOR THE FREE-R;1DICA4LCLE.IT'dGE OF PEROXYBEXZOIC ACID
Sir: We have fouiid evidence that peroxybeiizoic acid in benzene solutioiis can decompose into free radicals, a type of reaction which had not been known for peroxycarboxylic acids. Solutiolis of this peroxyacid in benzene mere studied using a small amount of a,a-diphenyl-B-picrylhydrazyl as a free-radical scavenger, as described by B a m a i d 1Iellish. The decrease in optical density at 520 mp was followed with a Beckman Model DL spectrophotometer until most of the hydrazyl had been consumed, thus giving measurements over a wide range of hydrazyl concentration. The data follow The rate law -dD/dt
= k
[PI
TABLE
1
RESULTS IN BEKZENE Temp., Run
OC.
Initial concn. Iiydrazyl,
23 26
30 :31 32
60 60 60 60 60 60 60 GO
4 . 0 x 10-5 4.0 1.0 4.0 1.0 4.0 4.0 8.0
0.959 x 1 0 - 2 1,918 0.959 1.341 0.575 1.918 1.918 1.$118 A4veragc
1.21 x 10-7 1.12 1.01 0.95 1.05 0 , $16 1.05 1.31 1.08
33 34 35 36 37
65 65 65 65 65
8.0 8.0 8.0 8.0 8.0
0.959 0,575 0.1916 1.3$2 0,381 hverago
2.07 2.1-1 2.18 L94 2.:35 2.14
27 28
m
rnole/l.
Concn. PBA, [PI niole/l.
k
x
sec. - 1
where 'r> is the optical density, and [PI is the concentration of peroxy acid. This rate la^ with a large excess of peroxybenzoic acid is best explained by a first order cleavage of peroxide molecules into This difference in behavior may be due t o the two free radicals ( I ) , presumably by rupture of the difference in structure of peroxybeiizoic acid in 0-0 bond. Result? of a number of experiments these two solvenk. In benzene, peroxybenzoic are listed in the table. The rate constant, It, acid exists in the internally hydrogel1 bonded form is for the first order, free-radical decomposition of (I)2;in 1-butanol the ring presumably is opened the peroxide. The activation energy for this proc- and the acid is hydrogen bonded to the solvent ess is approximately 30 kcal., about the same as (11). In t'he open form (11) the elect,roiis in the the actiration energy for the corresponding free- 0-0 bond are displaced t'on-ard the oxygen atradical cleavage of benzoyl peroxide. The specific 0 0 rate coilstants, however, are only one-tenth as / \ / \ large. C6H5-C 0 C6Hh-C OH------O-R In 1-butanol solutiolis the rate of decrease of I! I10 .--. H 0 ----- H H optical density is very much less than in benzene solutioii, and, depending on the history of the ()-It peroxyhenzoic acid, the reaction is first to second I I1 order in dipheiiylpicrylhydrazyl. A n y free-radical cleavage at a rate comparable to the rate in benzene tached to carbon. This polarization niay prevent would have given an over-all zero or fractional appreciable free-radical cleavage of the peroxide order reaction with hydrazyl. These results indi- bond. In the cyclic form (I) the elect'ron withcate that free-radical cleavage does not occur to drawing benzoyl group also acts through the hydrogen bond to draw electrons toward t'hc hyany significant extent in this solvent. ~
(1) C. I . H. B a a n a n d 3 (1991).
I-. Alellish, Trans. I'araday
Soc., 47, 1216
(2) D. Swern, L. P. I%-itnauer,C. R. E d d j a n d IV. E. Parker. J . .4m. Chem. Soc., 7'7, 5537 (1955).
drogen iii the 0-OH bond, in addition to drawing electrons toward the oxygen attached to carbon, thus decreasing the polarization of the 0-0 bond enough t o permit the observed slow free-radical cleavage of peroxybenzoic acid. B R 0 K - S USI\7ERBITY
PROTIDEWE 12. RHODE ISLAXI) R E ( T I T E I > JI.LY ~
1087
COMMUXICATIOSS TO THE EDITOR
August, 1960
T= 2126°K
STEPHES R. C o m s Y JOHS0 . EDWARDS 1, 1!l6O
~~
Gij Ijeliar-iiient of ( ~ h e n i i s t r y ,S o r t l i e a s t e r n t - n i v e r s i t y , Boston 1.i SIassarliiisrt ts.
-+ Y
OS THIS W E O F T,1STAILUJI KSTJDSES (:E rm ISHIGH TEMPERATURE THEK3IODYS;IJIIC STUDIES OF OXIDES
Sir: It has been shown by several investigators' -5 that the rolat'ility of oxides is greatly enhanced in the prescnce of tantalum. Chupka, et u Z . ! ~ have shown t'hat this is due to the formation of TaO kind Ta02. 1nasmuc.h as heats of 011 of these species have heeii reportedq6 it might appear that t,antalum cells would still he useful for oxide studies provided the pre-beiice of gaseous tantalum oxides in the effwing vapors is taken intmoaccount. The purpose of this note to point out that such cxorrections 11-ill not, necessarily lead to menniugful equilibrium c~mstantsfor reactions hecw~seof the maiiner in which TaO(g) :11id T a 0 2 ( g i are formed and escape the Iinudsen cell. 1:or some time we have l ~ studj-iiig ~ n the vaporization of rare earth oxides by the Knudsen effusiou techiiiquv. These oxides vaporize much more readily froin iautalum than from tungs'eii cruc*il~les.~ l'igure 1 shows the isot,hernial rate of weight loss of :I tantalum Knudsen rell' contaiiiing LazO, as a funrtion of the ratio of the tot,al weight of niater:al vaporized to the weight of the iiiitial It should be noted that the charge of I,a& steady r:rte of' vaporization persists beyond the poinf n-here rhr weight of material vaporized csveeds the weight of LasOa originally hi the crucible. This \viis assumed to result from the escape of TaO(5) and TaOz(g) through the orifice in equilihriurii n-ith the LaO(g) arising from La2Oa.j Subsequent niass spertrosmpic experiments, using a Beiidix time-of-flight iustrumriit, under idelltical coi1ditioi.s. ha\-e shon-11 that +his assumption \vas erroiieous. csperimciith, the on/!/ vapor ohser\-etl mi t'he Iiiiudsen orifice \vas TAO. Thc 1' La'+ varied with aniouiit vaporized iii :i n i a ~ i i i richitir:il ~ to that shou-ii in lig. 1. (1)
L.Brtjver and .i&.~ r ( ~ y.1. . . I f f & . Chern. So(,., 7 3 , ::(OS
fSY5l). and R. .J. Thorn, zbid., 7 8 , 4 l l i g (18,Xjj. Anz. C ~ m 7 , : i r S o r .BwZ1.,38,2.3; (1Y.59). iiiika. .J. Rerkonvitz and AI. G . Ingliiaiii, J . Glum.
.
.itlsli. €1. \V. Goldstein and 1). \Tllite. J . Am. Ccrurn.
.
I n g h r s i i i . IT. .t. Cliulika a n d .1. Berkoiritz. J. C'hcm. f1937). ( 7 ) The oiiipty tantalriin crui,iblo was tliorouglily degiisscd a t 2400' i n uucuo j u t t lirior t o tliis set of runs.
I.
(
I'iigs.. 2 7 , .i':Y
b
4[
oi,
I
I
I
I
2
4
6
8
I to
I 12
Weight vaporizc~d/initialwsight of L:dJ+ Fig. 1.
S o tant,alum oxides n-ere observed8uiitil beyond the flat, in the isotherm and then oiily when t.he teinperature was increased 100' or more. I+ is therefore concluded that most' of the oxygen from La20a is absorbed in the wall of the cell, through which It' diffuses with subsequent erolution of TaO(g) and Ta02(g)from the exterior surface of the c r ~ c i b l e . ~ 1T7e do not believe that any equilibrium involving ga,seous tant,aliim oxides is established 011 the cell. There is further evidence to support t'he conclusion reached here. Tant,alum Knudren cells containing TazOs lose weight at, a rate of 30 to 50 tinies that predicted from the reported thermodynamic properties of TaO(g'1 arid Ta02(gi6, become heavily etched on the orrt.side at the sample level. A "16" wucible wall has been eroded completely iu less t h m one hour at 2050°1roi\-art. G . De M a r i a , R . P. Burns a n d SI. c;. Inghraiii, J . C h r m . Phus., 34, 13Iili (ISliO). ( S I ) R. .J. .Ackerinann a n d R . J . T h o r n , .Arponne National Laliorat o r y Report :iKL-.i821, .Janiiarl-. 1858.
HA~T)IOISSIS OF FERROW 10s SOLI-TIOSS IS HE.11-Y IVLITEl
