DIFFCSION KISETICS OF' XETHYL RADICAL COMBINATIOX IN THE TIEItMAL DECOJlI'OSITION OF ACETYL PEROXIDE AT HIGH PRESSUItES' BY THOMAS C. VOCT,JR., AXD WILLIAM 11. I ~ A J I I L L Department of Cheinistry and the Xadialion I d o r a t m y , Univervity of iyutse Dame, n'otre Dame, Iruiiartu Received July 13, 106%
The theriniil decomposition of acetyl peroside in 3-metliylpcntnne st 80" .as s function of iodine concentration has kwi meusured at 1, 2000, ;Kid 5000 stm. The efficiency of geminato combination of methyl rndicnls, bused upon 2C~Ib/C03rind corrected for methyl acetate formation, indicates that initial C H I . . CIiDseparation and inem free diffusive diqhcement decrease with increasing pressure.
.
Introduction Since the first observation and description by Roy, Williams, and IIamillZ of the effect of added scavenger upon the diffusion controlled recombination of geminate atom-free radical pairs following neutron capture, similar phenomena havc been observed for thermal d e oomposition,a~4 photolysis,5-8 and radiolysis.9 130th Xoycs10 and Xlonchickll have since considered the t hrory of solute intervention in diffusion cboiitrolled rewtions. We shall use our previous description, for the sake of continuity with earlier ~ o r k . ~For - ~ an identical r d i r a l pair, the prohability of geminal e rtcombination IV a t infirlitc time is givcn by
for reaction probability Pi, su1)ject to intervention hy scavcngcr a t mole fraction X with reaction probability I'z per encounter. The mran initial scparation of the free rndical pair is P and thc mean diffusive displacw ment i, both normalized to the radical diametcr. A study of the change in quantum yield a(I12) from hydrogen iodide dissolved in a hydrocarbon, over a wide range of temperature, explained in terms of the dependence of t and i upon the temperature-dependent free volume of tho solvent, assuming ZJ1 and 1'2 to he independent of tempernture.6 It is clearly drsirable to re-examinc the effect upon f and P of variation of ihc free volume isothermally. As a matter of convenience we chose to study thc effect of pressure upon the thermal decomposition of acetyl peroxide sinw it previously was shown that the rcaction is suitable for a study of diffusion k i n ~ t i c s . ~'l'hcre is good evidence for the following mechanism, with iodine as free radical scavenger
(CII,@O2)2+ (CII,CO:!
(cIr,co,+ CII~COJ-+ (CII3 (CIIX
+
(C'II~ CII,)
+
+ CH3)
+ CIIaCO2)
+ !?c'oz + cIT3)
(2)
(3)
-+
CZHe
(4
-+
xrr,
(
