COMMUNICATIONS TO THE EDITOR
209 1
in the isopropyl nitrite-nitric oxide system, might be questionable, since some acetone (and therefore hydrogen) might come from (c), especially at our highest temperature (200”). We have repeated our earlier work at 200” at the extremes of pressure used and estimated hydrogen by analyzing the gases volatile at - 212”, and acetaldehyde and acetone, with the results shown in Table I. Carbon monoxide formation, via radical attack on acetaldehyde, is negligible.
Comment on the Communication “Acetone Formation in the Pyrolysis of Acetaldehyde”
Sir: Phillips,’ who has correctly implied that the rate constant for the decomposition of the isopropoxy radical (i-Pro.) will be pressure dependent at 523’ and 195 Torr, questions the suggestion by Batt2 that acetone formation occurs in the pyrolysis of acetaldehyde by a displacement mechanism 6H3
+ CH3CH0
a
0
i-PrO.* b
CH3COCH3 d
+H
Table I
(1)
Initial pressures, Product yields, mm mma (C€Ia)z- CHa-
IPNo=NOo
100 20.5
CO
CHO
Hz
6.22 0.62 0.004 0.65 0.09 0.0002
[CHa-
[(CHa)zCOI [CHaCHOI[NOla
0.100(0.104b) 0.362(0.363b)
CHOl [Hzl
155 450
a I P N = isopropyl nitrite. Our previous results (ref 3). Pressures are in 154-mlreactor at 200’.
It is evident that acetone formation by (c) is insignificant in our system. Acetaldehyde-H2 ratios increase markedly with decreasing pressure showing that, provided all hydrogen is derived from decomposition of the i-Pro radical, k, falls off more rapidly than kb which is contrary to Batt’s suggestion. It is virtually certain that (c) and (d) will be pressure dependent under Laidler and Liu’s conditions. Similar observations have been made for the two possible decomposition paths of the sec-butoxyl radical; we have previously reported pressure dependence for C-C bond fission.8 Batt’s conclusions on the mechanism of acetone formation in the pyrolysis of acetaldehyde must therefore be questionable; the good agreement between calculated and experimental values for acetone production is probably fortuitous and does not prove that it is formed solely by a displacement process involving the isopropoxyl radical. A concerted displacement process and/or the mechanism suggested by Bensong 9
+ CH2CO + ‘CHZCOCH, *CHzCOCHa + CHICHO + CH3COCH3 + CH3CO *CH3
Pressure dependence means that k, and k, can be expressed in the form A ( E - E*/E)S-’, where A and E* are the Arrhenius parameters for the unimolecular decomposition of i-Pro., and E is the total energy content. S is taken to be two-thirds of the total internal degrees of freedom in i-Pr0..3 Using the thermodynamic data in ref 2, we find that k b is 101o.l sec-I, and k, is lo8.’ sec-’ if E, is 22 kcal/mol.2 At 195 Torr, spontaneous decomposition is much faster than quenching, unless deactivation occurs on every collision which is very unlikely. Hence the rate of formation of acetone is given by d(acetone)/dt = k,(i-Pro Once again, the data in ref 2 may be used to calculate (i-Pro.). This leads to a value for dmol/ml sec compared to the (acetone)/dt of 40 X mol/ml sec14still suggesting, in observed 57.5 X agreement with Benson13that most, if not all, of the acetone is produced by a concerted, displacement process. At this stage, however, acetone formation by the two bimolecular steps below cannot be excluded. , CH3 CH2CO IfCH2COCH3 a ) .
’
+
CH2COCH3
+ CHSCHO +CH3COCHs
(1) L. Phillips, J . Phys. Chem., 73, 2090 (1969).
(2) L. Batt, J . Chem. Phys., 47,3674 (1967). (3) S. W. Benson, “Thermochemical Kinetics,” John Wiley & Sons, Inc., New York, N.Y., 1968. (4) M .T. H. Liu and K. S. Laidler, Can.J . Chem., 46,479 (1968).
DEPARTMENT OF CHEMISTRY OF ABERDEEN UNIVERSITY OLDABERDEEN, SCOTLAND
L. BATT
RECEIVED APRIL1, 1969
cannot be excluded. (8) R. L. East and L. Phillips, J . Chem. Soc., A , 1939 (1967). (9) S. W. Benson, “Thermochemical Kinetics,” John Wiley & Sons, Inc., New York, K.Y., 1968, p 142.
EXPLOSIVES RESE.4RCH AND DEVELOPMENT ESTABLISHMEXT MINISTRYOF TECHNOLOGY WALTHAM ABBEY,ESSEX, USITEDKINGDOM RECEIVED JANUARY 27, 1969
Identification of the Competitive Elementary Reaction of Hydrogen Atoms with Chlorine Monoxide by Infrared Chemiluminescence
L. PHILLIPS
Sir: From a recent study’ of the reaction of hydrogen atoms with ClzO in a fast-flow system in which the (1) C. G. Freeman and L. L. Phillips, J . Phys. Chem., 72, 3031 (1968).
Volume ‘78,Number 6
June 1969
