4568
J. Phys. Chem. 1988,92, 4568-4569 Acknowledgment. We thank Dr. B. D. Hockhart of the Queen’s University at Belfast and Dr. M. A. Salem of Tanta University of testing the ESR activity. We also thank Prof. Dr. N. Karl of the University of Stuttgart for the high-power nitrogen laser measurements.
This might indicate that the bathochromic shifts observed in the absorption spectra in dye 1 is due to simple protonation at the basic carbonyl centers. The blue species of dye 1 in concentrated H2SO4 give no electron spin resonance (ESR) signal, indicating the absence of radical species.
Registry No. 1, 83054-80-2.
(14) LukBE; Langhals, H. Chem. Ber. 1983, 116, 3524.
COMMENTS A fact that is so long that it appears to have been totally overlooked by S H R (and by their critics) is that many organic peroxides give significant yields of molecular hydrogen upon thermal decomposition in the absence of oxygen. The peroxides that thermalize to yield H2 have, without exception, a hydrogen atom attached to each peroxidic carbon atom; Le., they contain the HCOOCH moiety. In particular, H2has been shown to be a product from the thermal decomposition of di-sec-alkyl and of di-n-alkyl peroxides.21s22In many cases, the source of the H2 has been shown not to involve the intermediacy of H’ atoms. A concerted mechanism for H2 production which involves a cyclic six-membered ring transition state, originally proposed by Wieland and Wingler,’O has gained general acceptance.
Formation of Molecular Hydrogen in the Blmolecular Self-Reactlon of Hydroperoxyl Radicals In the Gas Phase’ Sir: Sahetchian, Heiss, and Rigny2 (SHR) have recently provided additional data which, so it is claimed, supports their earlier conclusion3 that molecular hydrogen is produced in ca. 8% of the bimolecular self-reactions of HOO’ in the gas phase at 150-200 OC and under atmospheric pressure; Le., k l / k 2= 0.086. In both HOO’
+
8%
/
HOO’
h
H2
+
H202
202
(1)
+
(2)
02
S H R generated HOO’ radicals by the thermal decomposition of di-n-alkyl peroxides, ROOR, in the presence of oxygen. The HOO’ radicals were presumed to be formed in the reaction RO’
+ O2
-
HOO’
+ product
A study of the thermal decomposition of di-sec-butyl peroxidelg has thrown some doubt on this concerted mechanism and even
(3)
where R = n-alk~l.~’It was found2 that the ratio [H2]/([H2] + [H202])was equal to 0.079 f 0.007 and was independent of the oxygen concentration. On this basis, it was correctly pointed out that atomic H’ could not be produced by the decomposition of ROOR or of RO’ (followed in either case by H’ + ROOR H2 product), nor could H2 be produced by the decomposition of RO‘. The earlier S H R study’ has been criticized on several grounds.e6 Thus, Glinski and Birks: using a different method for the gas-phase production of HOW, obtained k 1 / k 2< 0.0022 at 25 OC and 50 Torr of N1. These workers4perceptively suggested that it “would not be surprising if there were another source of hydrogen in this (the SHR) complicated, radical system.” Furthermore, Baldwin et alS5have shown that the formation of H2 by reaction 1 would represent a chain termination in the slow reaction of H2 with O2at 500 OC and that this would be totally inconsistent with the results of their very complete modeling of this oxidation process and of the H2-sensitized decomposition of
-
+
(8) Formation of H2 by the thermal decomposition of a-hydroxy peroxides is very well doc~mented.”~ It was first detected in 1898 by Blank and Finkenbeiner9 in the reaction of H202and H2C0 in basic solution, the generation of H2 being traced to the intermediacy of bis(hydroxymethy1) peroxide.1° The formation of H2 during the thermal decomposition a-hydroxyalkyl peroxides occurs in the liquid phase,”’Jc” the vapor phase,l2*I3and even in the solid state.1° (9) Blank, 0.; Finkenbeiner, H. Ber. Dfsch. Chem. Ges. 1898, 31, 2979-2981. (10) Wieland, H.; Wingler, A. Jusfus Liebigs Ann. Chem. 1923, 431, 301-322. (1 1) Rieche, A.; Hitz, F.Ber. Dfsch. Chem. Ges. B 1929,62,2458-2474. (12) Style, D. W. G.; Summers, D. Trans. Faraday Soc. 1946, 42, 388-395. (13) Jenkins, A. D.; Style, D. W. G. J. Chem. SOC.1953, 2337-2340. (14) Wurster, C. F., Jr.; Durham, L. J.; Mosher, H. S. J. Am. Chem. Soc. 1958,80, 327-331. (15) Durham, L. J.; Wurster. C. F., Jr.: Mosher. H. S. J. Am. Chem. Soc. 1 9 d , 80, 332-337. (16) Durham, L. J.; Mosher, H. S. J. Am. Chem. SOC. 1960, 82, 4537-4542. (17) Durham, L. J.; Mosher, H. S. J . Am. Chem. SOC.1962, 84, 281 1-2814. (1 8) (a) The formation of H2 by the thermal decomposition of primar and secondary dialkyl peroxides in the liquid phaseI9” and in the vapor pha.d321-N has also been reported. (b) The yield of H2 from di-sec-butyl peroxide is very much less in the vapor phase than in s01ution.l~ (19) Hiatt, R.; Szilagyi, S. Can. J . Chem. 1969, 48, 615-627. (20) (a) Hiatt, R.; LeBlanc, D. J.; Thankachan, C. Can. J. Chem. 1974, 52, 4090-4094. (b) This paper records the highest yield of H2. Thermal decomposition of Ph2CHOOCHPh2at 11C-130 OC in solution gave approximately 90% H2 Ph2C0 by a nonradical mechanism. Photolytic decomposition in toluene yielded no H2. (21) Arden, E. A.; Phillips, L. J . Chem. SOC. 1964, 5118-5125. (22) Livermore, R. A.; Phillips, L. J . Chem. SOC.B 1966, 640-643. (23) Walker, R. F.; Phillips, L. J. Chem. SOC.A 1968, 2103-2106. (24) Thynne, J. C. J.; Yee Quee, M. J. J . Phys. Chem. 1968, 72, 2824-2831.
H202.
(1) Issued as NRCC No. 29203. (2) Sahetchian, K. A.; Heiss, A.; Rigny, R. J . Phys. Chem. 1987, 91, 2382-2386. (3) Sahetchian, K. A.; Heiss, A,; Rigny, R. Can. J . Chem. 1982, 60, 2896-2902. (4) Glinski, R. J.; Birks, J. W. J . Phys. Chem. 1985,89, 3449-3453; Ibid. 1986, 90, 342. (5) Baldwin, R. R.; Dean, C. E.; Honeyman, M. R.; Walker, R. W. J . Chem. Soc., Faraday Trans. I 1984,80, 3187-3194. (6) Golden’s suggestion7 that H2 was formed from H’ arising from the decomposition of alkoxy1 radicals, RO’ H‘ + product, can be ruled out by the SHR results reported in ref 2. (7) Golden, D. M., private communication quoted in ref 2.
+
-
0022-3654188 , ,12092-4568$01.50/0 0 1988 American Chemical Societv I
-
4569
J. Phys. Chem. 1988,92, 4569-4569
raised the question as to whether or not H2 is actually produced by the thermal decomposition of peroxides containing the necessary HCOOCH moiety in the vapor phase.2s However, with relatively involatile compounds (SHR used di-n-heptyl peroxide2,’ and din-butyl peroxide2) it is difficult, if not impossible, to ensure that reaction occurs exclusively in the vapor phase:’ particularly when S H R “had to use high concentrations of peroxide to produce measurable amounts of H2 and H202at 150-160 oC.”2 It is also worth noting that when SHR used di-tert-butyl peroxide (a source of CH,’ and hence of CH30’) and O2 in their system under comparable conditions, the [H2]/([H2] + [H202]) ratio was ca. 1%, in contrast to the ca. 8% found with di-n-alkyl peroxides.2 For this system these workers therefore deemed it “necessary to introduce other reactions producing H202which do not form HPn2 A simpler explanation is possible, namely, that molecular hydrogen is most probably not formed to any significant extent in the gas-phase bimolecular self-reaction of HOQ radicals. Instead, in the di-n-alkyl peroxide/02 system%’H2is most probably formed directly from the peroxide (possibly on the surface of the reaction vessel or in some other condensed phase, vide supra), while in the di-tert-butyl peroxide/02 system, H2 may well be formed from the known reaction9 of HzO2with formaldehyde (the latter compound being an expected product of this reaction that was not detected). Of course, H2 may also be formed, in whole or in part, in the di-n-alkyl peroxide/02 system by reaction of H202with the aldehyde formed in the alkoxy1 radical/02 reaction: RCH20’ + 02 RCHO + H02’. In conclusion, the certainty that molecular hydrogen can be formed during the thermal decomposition of peroxides containing the HCOOCH moiety makes di-n-alkyl peroxides totally unsuitable for the use to which they were put by SHR. Indeed, the very need for this comment illustrates a fact well-known to chemists who study free-radical reactions in solution; namely, that far too many chemists working in the gas phase pay little or no attention to work done in the liquid phase. The reverse of this statement is not true.
-
(25) Mechanistic studies on the conceptually related, thermal decomposition of n-butyl peroxyacetate to butyraldehyde and acetic acid have also shown that a concerted six-center mechanism is very much less important in the gas phase than in solution.26 (26) Hiat, R. R.; Glover, L. C.; Mosher, H. S. J . Am. Chem. SOC.1975, 97, 1556-1562. (27) See footnote 6, ref 19.
Division of Chemistry National Research Council of Canada Ottawa, Ontario, Canada K I A OR6
K. U. Ingold
Received: July 23, 1987
Reply to the Comment “Formatlon of Molecular Hydrogen In the Blmolecular Self-Reaction of Hydroperoxyl Radicals In the Gas Phase” Sir: The interest taken by specialists of the liquid phase in the work of gas-phase kineticists and vice versa is very laudable, but the Ingold comment’ calls for some remarks. The main ones are 1. In connection with Baldwin et al.,2 SHR’ do not dispute the validity of the mechanism. The accuracy of some rate constants is the issue. 2. Ingold criticizes SHR, as well as other gas-phase kineticists. H e holds them guilty of taking no account of studies on the pyrolysis of organic peroxides (mainly in the liquid phase) which gives significant yields of molecular hydrogen, whereas he, himself, (1) Ingold, K. U. J. Phys. Chem., preceding paper in this issue. (2) Baldwin, R. R.; Dean, C. E.; Honeyman, M. R.; Walker, R. W. J. Chem. Soc., Faraday Trans. I 1984, 80, 3181. (3) Sahetchian, K. A,; Heiss, A,; Rigny, R. J . Phys. Chem. 1987, 91, 2382.
0022-365418812092-4569$01SO10
throws some doubt on his own a r g ~ m e n t . ~ 3. We agree with Ingold that the existence of heterogeneous reactions can never be entirely ruled o u t S However, he should not be unaware of the fact that the SHR work on this subject was begun by “a systematic study to reduce as much as possible the role of heterogeneous reactions in comparison with the homogeneous r e a c t i ~ n ” . ~If. ~the walls are correctly treated (boric acid coating and the slow reaction H2/02), H2and H202are formed simultaneously; if the treatment is not effective, neither H2 nor H202is observed.6 4. Other experimental conditions are not well understood by Ingold: the concentration of di-n-heptyl peroxide in O2or N2 at atmospheric pressure is about 10 ppm (ref 6) and that of di-n-butyl peroxide in mixtures of O2and N2,also at atmospheric pressure, varies from 30 to 120 ppm; these are relatively low concentrations. Experiments were also performed at lower temperatures (for example 90-130 “C) in the presence of O2with ROOR concentrations higher than 120 ppm: for these conditions, in the absence of peroxide decomposition, H2 was never observed. 5. No H2 is observed if the thermal decomposition of the peroxide is carried out in the absence of O2 between 150 and 200 OC. 6. During the decomposition of di-n-alkyl peroxides (di-n-butyl, di-n-pentyl, di-n-heptyl peroxides) in 02,only the H 0 2 radicals have been shown. 7. The concentration of H 0 2 radicals and its kinetics are in agreement with the decomposition of di-n-alkyl peroxides,’ so it has been possible to determine the rate constant of decomposition of these peroxides. 8. H2 and H202formed are in good agreement with the evolution of the concentration of H02 radicals. The activation energy for the formation of these radicals is -36 kcal/mol, when it would be markedly lower with a heterogeneous reaction. If, accordingly, the reaction of formation of H2 was heterogeneous, we should observe a strong decrease of the yield of H2 when the temperature is raised; that has never been observed. 9. With regard to the reaction suggested by Ingold H202 2RCHO H2 + 2RC02H (4)
+
-
it is very unlikely to occur in our system because it is a third-order one between species whose concentrations are about 1 ppm. Our results rule out this possibility, for the concentration of H2should then be proportional to [ROORI3, which was never noticed. 10. Concerning the decomposition of di-tert-butyl peroxide (100-200 ppm) in 02,it may be necessary to take in account the third-body effect, well established for self-reactions. In this system, used by SHR, H202and H 2 C 0 are present at rather high concentrations, and following the reasoning of Blank and Finkenbeiner,8this would have as a consequence the increase of formation of H2. In conclusion, as one can see from this comment and our answers, the analysis made by chemists working in the liquid phase is not in disagreement with the conclusions drawn by SHR. It seems interesting to us to carry on discussions as well as experiments, particularly to establish the reality of this reaction at ambient temperature. (4) Hiatt R.; Szilagyi, S. Can. J . Chem. 1970, 48, 615. (5) Proceedings of the Meeting on Influence of Heterogeneous Reactions on the Kinetics of Gaseous Systems, Paris, July 30-31, 1986, published in J . Chim. Phys. 1987, 84, 1-53. (6) Sahetchian, K. A.; Heiss, A.; Rigny, R. J . Can. Chim. 1982,60, 2896. (7) Sahetchian, K. A.; Rigny, R.; Blin, N.; Heiss, A. J . Chem. Soc., Faraday Tram. 2 1987.83, 2035. ( 8 ) Blank, 0.; Finkenbeiner, H. Ber. Dtsch. Chem. Ges. 1898, 3Z, 2979.
Laboratoire de Chimie GPnPrale CNRS UA 40870, 4Pme Ptage, Tour 55 UniversitP P. et M . Curie 4 Place Jussieu 75252 Paris Cedex 05, France
K. A. Sahetchian* A. Heiss R. Rigny
Received: September 9, 1987; In Final Form: February 24, 1988 0 1988 American Chemical Society
