104 1
THE ltADIOLYSIS OF CHLORO-, BItOJfO-, AXD IODOBESZENES WITH COBALT-60 r-RAYS BY SANGUP CHOILVD JOHNE. WILLARD Department of Chemistry of the Univetsity of Wisconsin, Madison, Wisconsin Received November $0,1961
G-viilurs have becri dt$cwiiiiicd for the productu of radiolysis of liquid CsHsCI, CcH&Br,and CGli,Ia t rooin tc~iiipriiturc :ind 100°, and in the prmenrc of radical scavengers. Thc inorganic products arc HCI from C6HaCl, HI3r and 13rt from (J6H613r, arid primarily 1% froin CeHJ. G H 2 is ioninit,ially containing 4 X lop2Ai Brz. (7) ,J. 13. E v a n s , .J. E. Quinlari, and ,J, E. %'illanl, Ind. E n g . Chem., 50, 192 jlH.58).
,June, 1962
RAUIOLYSIS O F CHLORO-, BROMO-, AND
A larger scale plot indicates that there may be an induction period for the production of HBr such as was observed in the production of bromine. G(€IBr) values determined from the slopes after the absorption of 1 X lozoe.v. g.-1 are 0.43 for the pure bromobenzene and 0.65 for the bromine solution. At an absorption of about 10 X lozoe.v. g.-l these have decreased to 0.24 and 0.27, respectively. Irradiation of the pure bromobenzene eolution was continued to 38 X lozoe.v. g.-l; the G-value for the interval from 13 to 38 X lozoe.v. g.-I was 0.13. Irradiation of the Brz-C6HJ3r solution also was continued to 53 X lozoC.V. g.-l; the G-value for the interval from 12 to 53 X lozoe.v. g.-l was 0.17. In a system such as this where both Brz and HRr are produced there is a possibility that the two may compete for free radicals and that this competition may be affected by adding one or the other prior to irradiation. Such an effect is suggested by the upper curve of Fig. 2. It also is suggested by the influence of added bromine and of added HBr on the rate of production of bromine as shown in Fig. 1. All concentrations of added IlBr tested from 0.01 to 0.06 M increase the initial rate of bromine production by the same amount, L e . , to a G-value of about 2.8. As irradiation continues the increased rate falls off, becoming progressively more similar to the normal rate. The approach to the normal rate is slower, the higher the concentration of the additive. This behavior is of the type that mould be expectcd if radicals such as phenyl radicals produced in the system can rcact with HBr to producc benzene and bromine atoms or with Brz to produce bromobenzene and bromine atoms. The decrease in G(Brz) to the same initial value for all added concentrations of Brz shown in Fig. 1 also is to be expected if such a competition prevails. The initial G(Brz) with added bromine is about -0.4. The negative value suggests that there are radicals other than C6H5 in this system, such as C6&Br, the net effect when these react with Brz to form C6H&rz being to reduce the t o b l bromine in the system. As the dose is increased and HBr builds up in the system it competes more and more effectively with Brz and so the rate of bromine production approachcs the rate in the absence of additives, When bromobenzene with both HRr and Rrz (0.06 Af HBr, 0.04 M .f3rz)added prior to irradiation was irradiated, the curve for bromine production fell between the curvcs for 0.01 and 0.03 A4 added HBr shown in Fig. 1, cxcept, a t the stwt whwe thc rate was slower than uiiy of the curves without d d d bromine. As further evidence as to the role of radicals in the radiolysis we have determined the rate of entry of radiobrominc into organic combination when the radioactivity is initially present as Brz. The data for five experiments ranging from initial bromine concentrations of 0.4 to 0.02 1M are shown at3 the plot of log (1 - F) against radiation dose in Vig. 3. (2-valucs calculated from the initial slopes of these curvcs by the rclation
+
G = -2.3('L[I~r~][Ca%Br]J(2[I3r~] [CeH~Br]))(log ( I - P)/e.v. g.-L)(N/lOOOd)( 100)
are (i.8, '7.0, 6.1,6.0, and 5.4 for the experiments at 0.4,0.%,0.04,0.01, and 0.02 M , respectively. /
