GASPHASE PHOTOLYSIS OF ACETONE
2475
The Gas Phase Photolysis of Acetone at 3130 A in the Presence of Hydrogen Bromide. A Study of the Primary Photochemical Decomposition Processes of Acetone
by Carl W. Larson and H. Edward O’Neal’ San Die00 State College, San Diego, California
(Received December $7, 1966)
The effect of hydrogen bromide on the photochemical excitation of acetone a t 3150 A is described. Two primary decomposition processes are proposed : (1) spontaneous decomposition from upper vibrational energy levels of the acetone triplet state and (2) decomposition of thermally equilibrated triplet acetone molecules in the low-pressure (or falloff) region. Chemical quenching of the acetone triplet state by hydrogen bromide sometimes leading to the formation of isopropyl alcohol is also proposed. Quantitative estimates from quantum yield data are shown to give for the spontaneous decomposition (RRK) rate constant @(e) = veAst’R[(e - e O ) / e ] a - ’ ) , veAst/R = 10’5.’ sec-’, co = 17 kcal/mole, and s = 18; for the low-pressure thermal decomposition rate constant, 3 k d = 1012.2sX 10-9.95/e M-I sec-l; and for the triplet trapping rate constant, kT = X M-‘ sec-’. A mechanism for the formation of the unusual products observed (e.g., isopropyl alcohol, isopropyl acetate, and isopropyl bromide) is presented. The negligible yields of acetaldehyde and the regeneration of hydrogen bromide are also rationalized by semiquantitative calculations.
Introduction In an earlier study of the gas phase photolysis (3130 A) of acetone in the presence of hydrogen bromide, Steacie% reported that very small pressures of hydrogen bromide ( i e . , less than 0.1 mm at 25”) were sufficient to reduce ethane quantum yields to negligible proportions, trap almost all methyl radicals as methane, and reduce quantum yields of carbon monoxide to very low values. As in the acetone-hydrogen chloride photochemical system,2b hydrogen bromide (like HCI) was not used up in product formation but rather was regenerated in the course of the reaction. Although product analysis was confined to “noncondensable” gases (CHI, C2Hs, and CO), Steacie assumed trapping of the acetyl radicals by hydrogen bromide to produce acetaldehyde. The corresponding reaction of acetyl radicals with hydrogen iodide has since been well established.8 Initial results in these laboratories on the products of the acetonehydrogen bromide photochemical system confirmed Steacie’s findings with
regard to the very effective trapping of methyl radicals by hydrogen bromide and the low CO and C2He quantum yields. However, the “condensable” products of the photolysis contained almost no acetaldehyde. Surprisingly, the major condensable products were isopropyl alcohol, isopropyl acetate, and isopropyl bromide. In addition, the decomposition quantum yields of acetone were found to be strongly dependent on hydrogen bromide pressure and on temperature. These unusual results stimulated the further moderately extensive study of the acetone-hydrogen bromide photochemical system reported here. Although the nature of the secondary reactions was of considerable interest and has been fairly well illucidated, the most important results of this study (1) On leave of absence at Stanford Research Institute, Menlo Park, Calif. (2) (a) E. W.R. Steacie, Can. J . Chem., 33, 383 (1955); (b) R. J. Cvetanovi6 and E. W. R. Steacie, ibid., 31, 158 (1953). (3) E. O’Neal and S. W. Benson, J . Chem. Phys., 36, 2196 (1962).
Volume 70, Number 8 August 1986
CARLW. LARSON AND H. EDWARD O'NEAL
2476
concern the nature of the primary processes in the acetone photolysis. This is a subject which has received considerable a t t e n t i ~ n but , ~ nevertheless is one which has remained to a large degree both qualitatively and quantitatively unresolved.
Experimental Section Reagent grade acetone obtained from Matheson Coleman and Bell was distilled several times under vacuum and stored over calcium chloride. Gasliquid partition chromatography (glpc) indicated no low-boiling (Le., bp (105/@/10),the net effect of these additives must be to lower bromine atom concentrations appreciably. The much larger yields (essentially maximum values) of isopropyl alcohol products observed when toluene or propylene were present would thus be expected. Additional support for the mechanism of the termination processes proposed (i.e., reactions 2, 3, 5, 8, and 12) may be found in the methyl bromide yields. Again, by the method of relative rates one obtains
-
The Journal o j Physical Chemiatry
The average values found for the yield ratios Y(CH3Br)/Y(CH4) (which served as operational measures of the [(Br)/(HBr)],, ratio) are: l / 3 ~ , l/26, and 1/200 at 44, 96, and 126", respectively. Again, we see that reactions 5 and 6 should be competitive in the temperature range studied. Also, the suggested correlation (inverse relation) between the isopropyl alcohol product yields and the methyl bromide to methane yield ratios is generally consistent with the data. It should be noted that isopropyl alcohol was not produced in the acetone photolysis inhibited by toluene alone, e.@, run 92. Shima and Tsutsumi's results on the photolysis of liquid acetone in methylcyclohexane8strongly support the acetone triplet-hydrogen bromide trapping reaction proposed. These workers found that the major photochemical product in their system was isopropyl (8)
K. Shima and S. Tsutsumi, Kogyo Kaguku Za88hi, 64, 460
(1961).
GASPHASE PHOTOLYSIS OF ACETONE
2481
alcohol and concluded, in agreement with our own ideas, that the 2-hydroxyisopropyl radical was the important reaction intermediate. Chemical trapping of a photochemical excited state by HBr should not be unique to acetone but rather should represent a highly probable process readily observable in many other systems. The requirements for such a trapping reaction are that the excited states be chemically reactive and that they have lifetimes in excess of the effective collision frequency with HBr (-lo-' sec). The (n R * ) carbonyl absorption process produces a type of diradical since an unpairing of the pv nonbonding electrons occurs. Benson and De MoreB have proposed that the major energy requirement of a chemical reaction is the energy needed to produce unpairing of electrons in the reactant bond or bonds. By this definition, photochemically produced triplets (or singlets'O) should be very reactive species. Product identifications in a number of other carbonyl photochemical systems provide support for the excited state radical type reactivity proposed here. Whittemore and Szwarc have reported sizable yields of 3-hydroxybut-2-one accompanied by significant overall decomposition quantum yield reductions in the gas phase photolysis of hexduorobiacetyl-isopentane mixH atom abstraction from isopentane by tures." "triplet" biacetyl molecules offers a reasonable explanation for these observations. The unusual and wellknown Norrish Type I1 reactions may also be rationalized wia diradical intermediates. Thus, the mechanism would involve intramolecular H atom abstraction followed by decomposition. This is illustrated for the photochemical decomposition of methoxyacetone12 which yields formaldehyde and acetone as products.
0-H
I
CHaCECH2
-
CH3COCHB
The diradical produced after intramolecular H abstraction may either decompose (as in the Norrish Type I1 processes) or cyclize. As a reasonable confirmation of diradical intermediates in such systems, Orban, Shaflner, and J e g e P have observed cyclizations in a number of saturated ketones and aldehydes. The general mechanism proposed was
-f
( y) *
H
CHaCOCHzOCHa
- *i' hv
J
CHaC
\ / CH2
( f :.) H
CHaC:
\ / CH2
* 0-H CH,O
I
+
CH~C=CH~
\ *
H
/
0
>C
I1 (n >C \ /"\ I
hv c
7
-c **)
C
A
A
H H
/
j
-
\/\
0
I >c-cI 1 c-c A
A
A
By analogy with these and other examples14 of both intra- and intermolecular H atom abstraction reactions postulated for photochemically produced excited states, the acetone triplet trapping reaction proposed seems very reasonable. Other Products. The relatively high yields of isopropyl acetate and the identification of ketene are (9) W.B. De More and 8. W. Benson, Ann. Rev. Phys. Chem., 16, 397 (1965). (10) There is no reaaon why singlets should not be as reactive as triplets. The only reasonable justification for the aeaumption
normally made regarding the selective quenching of triplet states by such molecules as 01,NO, In, etc. would seem to be the rela tively short lifetimes ascribed to most singlet states. Thus, for a singlet having a lifetime of the order of 10-osec, 1 atm of quenching agent acting at unit collision efficiency would be required to observe chemical trapping. Since the acetone singlet (if it exists) is s u p posed to have a lifetime of the order of lo-* sec, all quenching by HBr in the present system is presumed to occur only with triplet state acetone molecules. (The existence of the singlet state in the acetone photolysis system will be discussed more fully in a communication to follow.) (11) I. M. Whittemore and M. Szwarc, J. Phys. Chem., 67, 2492 (1963). (12) R. Srinivasan, J. Am. Chem. SOC.,84, 2475 (1962). (13) I. Orban, K.Shaffner, and 0. Jeger, ibid.,85, 3033 (1963). (14) See the forthcoming article by S. W. Benson to be published in
honor of Semenov's 70th birthday.
Volume 70,Number 8 August 1966
CARLW. LARSON AND H. EDWARD O'NEAL
2482
evidence for reactions 3 and 9. We attribute the rather poor acetyl-methyl mass balances to a surface-sensitive ketene polymerization reaction. Slow polymer formation on the walls was observed.16 The very small acetaldehyde quantum yields, which originally seemed unreasonable in lieu of the exceptional efficiency of HBr as a radical trap, on closer inspection appear to represent near equilibrium yields. l6 Isopropyl acetate and isopropyl bromide were undoubtedly formed by the reaction of isopropyl alcohol with ketene and with hydrogen bromide, respectively (reactions 9 and 10). That these reactions occurred rapidly in our system even at room temperature was confirmed by simple mixing experiments. Both were probably heterogeneous. Primary Process. The Ratio of Rates of Decomposition and Trapping of the Acetone Triplet State. Quantitative estimates of some of the primary-process rate constants have been obtained from the decomposition quantum yield data. According to the mechanism, in the falloff region decomposition of triplets in thermal equilibrium with their environment competes with the hydrogen bromide triplet trapping reaction. This region is therefore capable of providing values for the rate constant ratio ('kd/kT). Assuming steady-state conditions, one obtains
(VIII) 'f$d represents the quantum yield of decomposition from thermalized triplets, and the rate constants are those defined in the mechanism. When the pressure of hydrogen bromide is such as to reduce the decomposition quantum yield for thermalized triplets to exactly one-half the value obtained in the pure acetone system: eq VI11 leads to the relation
the temperatures studied (Table V). An Arrhenius ] plot of the rate constant ratio [ k ~ / ' k d ( M ) obtained from (X) using these numbers with an average acetone pressure of JT = 70 mm is shown in Figure 5 . The line obtained is better than the data warrant15 since the values of (HBr)1la are quite sensitive to the smoothed curves drawn. However, any Arrhenius slope within the error limits noted will not give parameters significantly differentfrom those which were obtained. Thus, 3.2f0.65 x 10+Q.l*l.l/O (kT/3kd)'v 10The rate constant for triplet decomposition ('kd) can be obtained from the data of Groh, Luckey, and Noyes," who studied the 0 2 quenching of emission of the long-lived state of acetone at several temperatures. With reference to the proposed mechanism, the triplet lifetime in the presence of oxygen is given by ('7)
Smoothed curves through the data points of Figures 1-4 have been used to obtain the over-all decomposition quantum yields of pure acetone (#Ido), the limiting decomposition quantum yields in the presence of excess hydrogen bromide (#Id*), and the values of (HBr)lla a t The Journal of Physical Chemistry
'L(M) 4- k i c
JcP
+ kot(0z)
(XI)
(15) Since minute amounts of hydrogen bromide produced large changes in the decomposition quantum yields, the scatter of points in the falloff region was rather large (particularly at 44O). This is evident if the experimental data are applied directly (VIII). However, most of the scatter resulted from an experimental complication. During and after each run, the transmission characteristics of the reaction cell decreased. This was traced to a polymeric deposit which appreciably altered quantum yield results below 0.1 mm of hydrogen bromide. For example, with "polymer" the decomposition quantum yields of pure acetone were lowered between 0.2 and 0.5 unit. More surprising, the methane to ethane ratios were sometimes an order of magnitude higher than literature values. Successive runs on pure acetone gave quantum yields which gradually approached the expected value. Removal of the polymer by heating with Oz at 600° always gave the "normal" methane to ethane ratios and carbon monoxide quantum yields (e.g., compare runs 70 (with polymer) and run 102A (cleaned cell)). Small amounts of HBr wall absorption or reaction with the polymer on the walls was probably the source of these unusual effects. The smoothed curves used to obtain the (HBr)l pressures were drawn in such a way as to weight most heavily those runs made under comparable experimental conditions. (16) Using the mechanism and the suggested rate constants, one obtains (CHsCH0)oo = kc [(HBr)] - (CHaCO) 'v k-4 (Br) LIB
-
-
6d 0.3, [(A)/(HBr) 1 21 10, (A) 3.3 X 10-8 M , and 0 1.7 (conditions comparable to the average at T = 96O), one obtains (CHaCHO)oI 10-E.' M . This is equivalent to yields of about 4 X 10-8 mole, as observed. The following relations have been used in the above: (CHsCO),. %I.+d/ks(Br); ( B r h ? (.CHzCOCHa) x (HBr)/(A)(&); (*CHzCOCHs)m il! (1.7Ia/kd'/'. The high reactivity of the weak C-H bond in acetaldehyde toward H abstraction by bromine atoms is responsible for maintaining low acetaldehyde concentrations were only reduced by concentrations. Since (Br)&@ about a factor of 10 when toluene or propylene was added (compare rates of reactions 7 and ll), acetaldehyde was never an important product. However, with added toluene, acetaldehyde quantum yields were increased by about the expected amount (see Tables I1 and 111). (17) H . J. Groh, Jr., G. W. Luckey, and W. A. Noyes, Jr., J. Chem. Phys., 21, 115 (1953). (18) Noyes, et aZ,,were aware of a process which competed with phosphorescence in pure acetone and estimated its activation energy at about 10 kcal. However, they believed this process to be one of collisional deactivation.
If
where (HBr)l/a is the specific hydrogen bromide pressure in question and '#Ido is the total decomposition quantum yield of thermalized triplets in pure acetone. Equation IX rearranges to
=
GASPHASE PHOTOLYSIS OF ACETONE
2483
Table V: Data Used for Figures 5 and 6' T,
I,
"C
'$do
'$d*
mm
44 96 126 150
0.28 0.88 0.92
0.08 0.18 0.25 0.32
0.10 0.50 1.20 2.50
1.00
kio
cct,
'kd(M)
(ZM)
'
0.087
0.60 0 0 0
0.22 0.33 0.47
O r ) -1,d
T, OC
'#do
25 32 50 75
'
0.45 0.54 0.75 1.00
%(A),'
10-8
10-8
'$d*
900 -1
800 -1
0.05 0.06 0.09 0.13
5.0 6.5 10.0 21.0
2.1 3.3 7.2 21
'
Obtained by extrapolation of smoothed curves of present work (see footnote d ) . +O at 1.05 X 10+ M was estimated from the results of Heicklen and Noyes,' who studied the acetone photolysis a t low per cent conversions at -3 X lo-* M and at 40, 50, 60, and 70". +d* a t 1.05 X l o w 2M was estimated from our results and from those of S. Pavlov on the photolysis of acetone in the presence Taken from the results of Noyes, et aZ.,14who determined (%)-l a t 1.05 X 10-2 M . e ( M ) = of excess SiH4 a t 25O (to be published). +do = primary process decomposition quantum yields in pure acetone; +d* = quantum 1.05 x 10-2 M = acetone concentration. yield of the spontaneous decomposition (reaction 11); @do = @d f @(acetone) where @(acetone) = 1.65 (@(biacetyl)/@(ethane))'/8 and @d = *(CH4) 2@(C2He) @(MEK) - O ( C 0 ) ; 4 ' , = triplet decomposition quantum yield in hydrogen bromide-acetone system (reaction IV); *+d = (+d - +d*).
'
+
+
- &*) and the numbers of Table v, values of 'kd(M) at several temperatures were calculated. The Arrhenius plot of 'kd (Figure 6) gives 3kd = 1012.25 x 10-9.45/0 M-I sec-~
3.2
2.9
2.6
I .7
I .4
1.1
2.3
2.5 (I/T x
2-7
2.9
3.I
IO~)-OK-~
Figure 5. Arrhenius plot of the ratio of the triplet trapping ) the triplet decomposition rate constant rate constant ( k ~ to (?kd). Triplet decomposition is here represented as a second-order process although the full extent of the pressure dependence of this decomposition cannot be determined from this study.
-
where k p and ko, are the rate constants for phosphorescence and oxygen quenching, respectively. The intercepts of the Stern-Volmer plots of (XI) [Le., (")-' US. (O,)] represent measures of the triplet state lifetimes of pure acetone. Rearrangement of (XI) for zero 0 2 pressure and for k,
