REACTIOIqS OF & X O I L
CHLORINE
ATOMS
271 1
Reactions of Recoil Chlorine Atoms with Cis and Trans Olefins] by W. S , Smith, S. H. Daniel, and Yi-Noo Tang” Ihpurtm.ent of Chemistry, T e r a s A & &I Unauersity, College Station, T e z a s 77843’ (Heceived illarch 19, 1978)
Reactions of recoil 38Clatoms with the cis and trans isomers of both 1,2-dichloroethylene and 2-butene have been studied. The percentage absolute yield for the sum of the two isomeric products of the former system ranges from 7 to 31%, depending on scavenging efficiency, while that of the latter accounts for only about 0.5% of the total aSClformed. The unequsl trans/cis product branching ratio from the two geometric isomers OI dichloroethylene prompts one to t,lie conclusion that. besides the usual adrfition-elirninatioIi re:iction observed For C1 :itom reactions, there is a certain contribution from a direct substitution process. T h e presence of the latter reaction is strongly support,ed by the 5 : 1 and 1:5 isomer branching ratio of products observed in cis- and trans-Bbutene systems. The equilibrium product branching ratio from the addition--eliminatioii Ijrocess alone has been demonstrated to be a function of the escitation energy of the decomposing radirals.
The usr of the nuclrar recoil method for thc studies of various chtmical reactions of free atoms has been extremcly successful, cspc.cially in thc case of recoil t r i t i ~ n i . ~H - ~o w e w ~ there , are rclativrly few studies drvoted t o thc rcactions of C1 atoms formed by this mranS.5 --:I Rrwnt experiments have established that thc ?eartang rrcoil specirs n hen the nuclear mode of production is 17Cl(n,y)38CC1,whcre the initial recoil energy is aa low RS 527 eV, are ground-statr chlorinr atom.; with cxccss liirictic rnt>rgg.6 T h e rcactions of thrrmal Cl atonis with czs- and ti a?zs-dichloroc~th~ lcne (DCE) has bwn a topic of longtcrni interest. 1 2 - I G Thcx chlorine atom reacts with DCl? by addition to thci doublr bond to give an rxcited trichlorocthyl radical which decomposes to givr a mixtiinc of the geomctrie isomers of DCE; thc excitation m w g y comw from the. kinetic energy of the reacting C1 atom and from t h e addition reaction, cxothermic by about 20 kcal /mol Bv employing the reaction rate of Cl ith propatill as a standard, the trans/cis branching ratio i\aS tva1ualc.d by Knox and ltiddick to be 0.28/1 0 in favor of‘ thr cis iuomrr Jlhich has a heat of formation 4.70 eai/mol more stable than its trans counterpart. On the othcr hand, Wai and Rowland have employed 3qC1atoms, rmoilirig from nuclrar transformation, for thc study of thc above Their gas-phase systrm of CII&1 n i t h 3% ’DCE forms the CHC13*C1CHCI* radicals ttlinust entirely by low-encrgy 3yCl atom addition. Thc trans/cis branching ratio in this systcm is 0.30 rfr 0.03 I-Ioucver, nhcn the reaction was carricd out in an e ~ c e s of’ s €IC instead of CHsCI, the tranq/cis branching ratio Lvas obsrrved as 0.49 0.02 xhhcri starting from czs-DCE and as 0.55 f 0.02 when qtarting from !rans-I_>CF:. Thc pirpow oi the‘ prcscnt study is to reveal tho naturc. of t h r recoil chlorine atom reactions with DCE ah 11ell as othcr oldin“ Wc h a w cxtrnded the study t o thc 2-butcmcs i n ordcr t o establish thc occurcnce of
*
direct substitution, and we have measured the scavcrigcr dependencr both for thc absolute percentage yirlds and for the trans/& branching ratio i n order to demonstrate the dependence of esptirimrntal results on thc cncrgctics of the reacting C1 atoms.
Experimental Section Chemicals. All the following gases v, w e obrained from thti Natheson Company: C1’ grade Irans- and cis-butene with 99.0y0 minimum purity, instrument grade 1,3-butadiene with 99.5% purity, Cl’3C1 with 99.07, purity, and prcpurified gradc argon 1% hieh was 99.998% pure. 130th oxygen and helium ol’ 99% pu(1) Presented in part at the 158th National Meeting of the :American Chemical Society, New York, N. Y . , Sept, 1969. (2) R. Wolfgmg, PTOQT. React. Kind., 3, 97 (190.5); i l m z ~ .Ilea. P h y s . Chem., 16, 15 (1965). (3) F. S. Itowland, “Proceedings of the 1nterii.ztional School of Physics, ‘Enrico Fermi’ Course XLIV---Molecular Reani :md Reaction Kinetics,” C . Schlicr, Ed., Academic Prcss, Kew York, S . Y . , 1970. (4) 14’. Schniidt-Rleck and F. S. Itowiand, A n g ~ u j .(:hem,, Int. E d . Enyl., 3, 769 (1964). (5) ”Chemical Effects of Suclear Tr:irisfortnRtiolls,” Vol. 2, Interrintionirl Atomic Energy Agency, Vienna, 1965, pp 221 :tnd 333. (6) C. A I . \&‘uti and E’. S. liowlrand, J . I’h~s. C h n . , 71, 2752 (1967); J . Amer. Chem. Soc., 90, 3638 (1968); J . J’hys. (‘hem.., 72, 3049 (1968). (7) C. M. Wai, P h D . Thesis, University of California a t Irvine, 1967. (8) L. Spicer and I was utsed as an internal monitor in order t o make a tiriies on various columns and quantimeasurements for 3861-containing prod eri by couiitixrg the radioactivity of the in each cwnpound. nuclear reaction "Ar(n,y)""Ar gives the radioactive isotope 41Ar as a product. By using the natural isou7ich9'-typegas proportional counter radioachive sample was injected tope alundance and the thermal neutron capture cross section for a761and 40Ar and the decay constant for matographic system with €Ie as a "(21 and "Ar, the following equation waj derived by a thermal conductivity cell for Wai and Rob\ land and was employed by us for the calThis same gas sheam then f2ow~dthrough the "inner9' column of the sandwich culation of the absolute yields for 3* counter ~;liilethe 6'otite~'' umns were filled with proY = P.31(2/(1 - 2 } ~ ~ ~ ~ * ~ pane, a countiny gar. A napxint unit wail used for x is the mole fraction of Ar relative to Ar and C1 the ~ e g ~ s ~ ,and ~ a ~t ~ oe ~ ~c of ~counts. s ~ ~ ~ ~ where ~ The usc of a11 cxternai counter is necessary because of atoms in the system; AC138 and AAk,4r are respectively the decay-corrected cumulatjve ~ ~ ~ ~ o a cobserved t ~ v i t ~ the ~ ~ e n c artli ~ ionr of ~ ~r ~~~ c h l o ~ Q e t h y ~ and e n eother lor 3sCl products and 4PAr. ha8ogenetc.d con1g K'i 8.7 IY Their high-electron affinity causes tht:rn to fcre seriously with the counting Results ~counter. t ~ ~ ~ a ~ process of an ~ Reaclions of Recoil 38Cl A t o m with Dickloroelhylene. Separation of i!7etimrlric Isomers. The majority of The reactions of recoil 38Clatoms with cis- and transthe sa,rmple sepsratinu was performed with a 50-ft tridichloroethylene have been carried out in the presence 0-ioly1 phospha.tt. (' P) column at 50". There was of diff erent kinds and diff erent concentrations of ii very wide aepziratio1-n beCween the two isomers of scavengers. The trans/cis branching ratio and the DCE with the trans compound emerging 65 min before total percentage absolute yield for each system are as the cis. A 25-Ft T F P column was also used a t times. shown in Table I. The types of sc&vengingconditions Even in this i-w~:, the separation between the two include nonscavcnged and 0 2 - (pressure, 5 and 15 cna) isomers wm: compiete. and butadiene-02-scavenged samples (butadicne has There is no pure. c!s- or trans-l-chloropropene avail- able ~ o m ~ e ~ for c ~calibration ~ l ~ y of columns. How(17) J. K. Lee, E. K. C. Lee, B. Musgrave, Y.-N. Tang, J. W. Root, Wer, R mixtune of the two could be obtained. A Caliand F. s. Rowland, Anal. Chem., 34, 741 (1962). bration of the in-txliire CUI the 50-ft TTP column at 50" (18) R. Wolfgang and F, s, Rowland, ibid., 30, 903 (1958). 7
Yhe Journal of Phgsieal Chemistry, Vol. '76,N o . 19,1979
+
0.49 f 0.04 0.61 rir 0.03 0.62 i0.04 0.67 3: 0.05 0.72 f 0.03
DCE AT@) DGEI -p OZ(5:; 4- h 4 2 ) DGE + O,(S) -& He(60) $- AI*(^) DCB CS,(SS) "i Ar(2) DGE O&) 4-bubadiene(5)
-+ +
+
31 i2 26 i 22 22 I 2 18 4 3
7 4 2
30 3: 2 25 i 2 22 f 3 18 _i- 3 8 i2
0.61 I O . 0 2 0.71 j l o . 0 5 0.73 A: 0.03
0.75 ?= 0.05 0.95 f 0.04
Ar(2) a
~ ~ ~ ~ S - D C E - ~ ~ C ~ / C L ;ratio. S-DGE-~C~
The nu.merical values bebind the compounds stand for pressure in cm.
_ l _ l _p___ . " Table X I :
Branchirig Ba,tio and Percentage Absolute Yields from Recoil 38C1Reactions with cis- and trans-%Butene cis-2-Butene--Sample compositiona
+
%Butene(%) 4-C'F&l(35j Ar(2j utene(3.5) $. GF&l(35) 3- Oz(5) f Br (2.) Z-Butene(5) CF&I(20) Oz(5) 4IXe(50) 4- Ar(2) 2-Butene(35) + GF&1(30) iOz(5) buta.diene(4) 4As(2)
-+
+
+
Q
See footnote u of Table I.
b
Trans/& ratiob
% absolute yield
0.20 rir 0.02 0.22 i 0.02
-------trans-~-Butone--Transicis
%; absolute
ratiob
yield
0.61 i0.10 0.71 I 0.10
6.3 =k 0 . 4 5.5 1 0 . 3
0.30 rir 0.07 0.25 3: 0 . 1 0
C
