~ 1 ~ ~ ‘ I ’ IOF C S TlIE PIiODUCTIO?; OF C2 DTJKING TIIE PYROJIYSIS OF l3TI-IY LNXK BY
lv. TSANG, 8. H. RATTElI AND F. WAELBROECK
Department of Chcmistru, Cornell Universzty, Ithuca, New York Received August 14, 1961
The rate of production of CZin the lowest vibrational level of the X,3nUstate has been measured via absorption spectrometry at A5165 (0,O Swan band) in back reflected shocks propagated in 8% CBH4-92% Ar. The temperature range covered was 2600-X600°K., with shock densities one-third to two atm., NTI’. The initial rate was found to be of the first order 111 the ethylenc concentration; the rate constant has an activation energy of 50 kcal. Those data can be explained by assuming that the ethylene MXS drcomposed t o acctylene and hydrogen by the incident shock, either in whole or in part, depending on the shock strength. The acetylene thus generated thrn was converted to a large variety of C/H fragments by deconiposition, abstractions, polymerization, rt c. One of thr products of this complex chain was C2. A steady-state concentration of CZwas reached, which was essentially a t equilibrium with various C/H species (excluding solid carbon), prior to the appearance of a continuum absorption. l‘hc lltttcr presumably was due to high molecular weight conjugiltrd ring or ckiain polymers.
Introduction The thermal decomposition of simple hydrocarbons (CH4, C21ls, C2H4, CZH2,etc.) has been studied extensively. As yet, there is no general agreement on specific mechanisms for the progressive degradation of any of these to the elements. The key to unraveling these complex kinetics is the identification of intermediates. Unfortunately, these are short lived and, hence, present in low concentrations. Nevertheless, there have been attempts to identify them directly. Greene and eo-workers1 analyzed residues from acetylene shocked to about 2000°1niposiurn (International) o n CONIbustion," Butteruorths, t o appear.
1'.4BLc
C2H4 +
x + C',lIn*
I
+
+ + + +
AH%. krnl. mole
:1Y 8 x --+ CZIIZ H L CzHz CiIlr --+ CnII2 IIz 3 7 C ~ I - I ~ C ~ I----f I ~ ~ ~ €czri 1 ~ 74 C4Hz CzI12 --+ C4Hd -t CzII 34 C,HZ C ~ H--+ L CzlIJ C'411 58 CrJIa CzI-Iz +Ca& C2H3 21 CzH, CzHz --f CzFI, CzH 10 C,IIa CZH +C& C1 72.5 CZH CZH --+ CZHZ 1'3 35 CdH CzH +C4HZ 6; 22 C4H CZH --+ C2Hz 42 C4II CnH +CINz C4 CIH CsH +CaHz - 102
+ + + + + + +
+ + + + + +5 +
+ + + + + CzII + C2Hz +CyH3 + C CzH + C?Hz +CAH + CHz C4H + C I H z +Call + Ca& C I H + CzHZ +C3H3 + C3
-
7'6 23 35 5
58
Conclusions In thc pyrolysis of ethylene, Cz is produced by a pseudo-first order process, with an activation energy of 50 kcal. A steady-state Cz (TI)concentration soon is reached and its magnitude is compatible with the assumption that, C2 is in equilibrium with a number of low molecular weight C/€I fragments. C2 formation appears to be a conscqueiice of abstraction rcmtions. The principal problem encountered in these studies was the unccrtainty in the estimation of the tempcraturc. Iu future work, it will prove advisable to use lower ethylene concentrations and to make measurements in the incident shock rcgion. Of course, a direct measurerncnt of the tcmperaturc behind the shock wave would be desirable. Further pyrolytic studies must be carried out before suEcient data will be available to permit specification of a mechanism. The use of isotopically labeled molecules has been mentioned. Of special intcrest is the possible role of the singlet levels in attaining the equilibrium condition for Cz. To determine whcthcr the singlet levels arc populated via collisions, we propose to rccord thc simultaneity of absorption by the Swan and the Deslandres systems. Acknowledgments.--This m r k was supportcd hy the Department of the Air Forcc, Wright Air Development Center, under Contract No. AF 33(616)6694, lor which grateful acknowlcdgmcnt is made. We also thank Dr. Russell E. Duff (Los Alamos Scientific T.:Lboratory) for providing us with n r i IBM-704 program for thc computation of cqiiilibrium shock conditions in a rcacting gas niixturc and Mr. Paul Marrorx for supervising our ('omputatioris at the Cornell Aeronautical Laboratory facility.
