THE PHOTOLYSIS OF ACETONE IN THE PRESENCE OF HYDROGEN BROMIDE1

ABSTRACT

The presence of hydrogen bromide during the photolysis of acetone sharply inhibits the yields of carbon monoxide, ethane, and volatile methyl radical derivatives as measured by the function (CHtf2CzHs). The observed effects can be explained on the assumption that both acetyl and methyl radicals react rapidly with HBr.

INTRODUCTION

The photolysis of acetone has been extensively used as a source of methyl radicals. The subsequent reactions of these with other substances are accessible to study when mixtures of acetone and a substrate are photolyzed. These methods have recently been applied by Raal and Steacie (6) to halogen- substituted hydrocarbons. I t has, however, been discovered that the photo- lysis of acetone in the presence of certain chlorine-substituted hydrocarbons is accompanied by complicating side reactions leading to the formation of hydrogen chloride (2, 7). Cvetanovic and Steacie (3) have studied the photo- lysis of acetone in the presence of hydrogen chloride and have shown tha t methyl radicals react rapidly with hydrogen chloride which is largely regener- ated by the abstraction of hydrogen atoms from acetone by chlorine atoms.

The work of Anderson and Kistiakowsky (1) indicates that methyl radicals react rapidly with hydrogen bromide also. However, there are reasons for believing that the photolysis of acetone in the presence of hydrogen bromide may not proceed entirely analogously with the hydrogen chloride case. In the oxidation of hydrocarbons the presence of hydrogen bromide profoundly modifies the reaction, the effect being attributed to the abstraction of a hydrogen atom from hydrogen bromide by peroxy radicals. Hydrogen chloride does not exert the same effect.

EXPERIMENTAL

Apparatus The optical system was as previously described and included a G.E. 935

phototube so that the intensity of the incident light could be controlled (7). The light source was a Hanovia S-500, medium pressure lamp. The experi- ments were carried out using a Corning 0.53 filter (Pyrex) with a "cut-off" below 2800 A. The analysis system mas of the type usually used in this labora- tory and included a Le Roy-Ward still, McLeod gauge, Toepler pump, and a gas burette. Since hydrogen bromide reacts slowly with mercury, a reaction

'~VFanziscript received August 3, 1954. Contribution from the Division of Pure Cltemistry, National Research Council, Ottawa, Canada.

Isslied as N.R.C. No. 547.9. 20tt leave of absence 1858-5.9 from the Division of Triboplzysics, Contntonwealtlt Scienti$c and

Industrial Research Organization, Melbourne, Azistralia.

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384 CANADIAN JOURNAL O F CHEMISTRY. VOL. 33

system was adopted in which taps greased with silicone grease were employed and contamination by mercury vapor was reduced to a minimum.

The acetone was prepared as previously described (7). Hydrogen bromide was prepared by adding bromine to tetralin. The crude hydrogen bromide was passed through a red phosphorus column to eliminate excess free bromine, and through a bubbler filled with a concentrated aqueous solution of hydrogen bromide in order to remove bromides of phosphorus. The gas was then sub- jected to three bulb to bulb distillations from acetone dry ice to liquid air temperatures, first and last thirds being rejected. The fraction retained was kept frozen in liquid nitrogen and samples were talien by warming to dry ice temperature. The frozen solid was quite white.

Preliminary Experiments

In view of the "tail-off" towards longer wave length in the absorption spectrum of hydrogen bromide, some preliminary experiments wei-e carried out to determine the effect on hydrogen bromide of light of the wave lengths passed by the filter.

Hydrogen bromide, 635 mm., was admitted to the reaction vessel a t 150°C. Within the experimental error the intensity of the emergent bean1 as deter- mined by the phototube was unaffected by the admission of the reagent. In the pressure region of the photolysis experiments absorption could therefore be safely neglected.

Hydrogen bromide, 101 mm., was illuminated for four hours a t 150°C. The fraction uncondensable a t -19G°C. was found to be negligibly small. In general, preliminary experiments inclicated that decomposition of hyclrogen bromide could be neglected. This conclusion is borne out in the experiments by the virtual absence of hydrogen among the products.

RESULTS

Products of the Photolysis of Acetone in the Presence of Hydrogen Bromide

The major products (Table I) were methane and carbon monoxide. A trace of hydrogen was detected in the run with 50 mm. hydrogen bromide, but none a t all in the run with 20 mm. hydrogen bromide. In none of the remaining experiments (in which the analyses were carried out by conventional gas analysis methods) was there any indication of the presence of hydrogen, and

TABLE I

Concentration, micromoles/cc. Products, ~llicromoles

--- Run Temp., Time, Acetone Hydrogen Methane Carbon Hydrogen Ethane

"C. hr. bromide monoxide

112 201 4 1.888 0.748 10.75 4.85 None 0.04 detected

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RIDGE A N D STEACIE: PHOTOLYSIS 385

it may therefore be disregarded as a significant product. With the quantities of hydrogen bromide employed here, ethane has been reduced to almost negligible proportions.

Effect of Concentration of Hydrogen Bromide The effect of hydrogen bromide over a range of concentrations is shown in

Table 11. The experiments were carried out with 50 mm. acetone a t 150'. Throughout the series the light intensity was maintained constant.

TABLE I1 THE EFFECT OF COXCENTRATION OF HYDROGEN BROMIDE ON THE PHOTOLYSIS OF ACETONE

Time of irradiation 4 hr.; Temp. 150°C.

Concentration, micromoles/cc. Products, microlnoles

-- Run Acetone Hydrogen Methane Carbon Ethane C H ~ + ~ C ? H G

bromide monoside CO

Table I1 shows that the addition of hydrogen bromide to acetone has resulted in a striking reduction of the carbon monoxide, volatile methyl radical (CH4+2CzHG) and ethane yields, the ethane yield being in most cases too small for accurate measurement. For small concentrations of hyclrogen bromide the effect is strongly dependent upon hydrogen bromide concentra- tion but becomes independent of hydrogen bromide a t higher concentrations.

Another striking effect of hydrogen bromide is the inhibition of the carbon nlonoxide yield. The addition of only 20 mm. of hyclrogen bromide (run 109) reduced the carbon monoxide yield to 10% of its value in the absence of hydrogen bromide. This behavior is contrary to that of hydrogen chloride, which produces no effect upon the carbon monoxide yield (3).

Tlze Effect of Temperature o n the Inhibi t ion by Igydrogen Bromide This is shown in Table 111. At each temperature two photolyses were carried

out a t the same light intensity. One of these was a blank with acetone alone. Throughout the series the time of irradiation ancl concelltration of reagents

were maintained constant, except with runs 194 and 195 where longer exposures were used so as to increase the amount of products. An attempt to maintain the illunlination constant over the whole series had to be abandoned owing to difficulties arising from the fogging of the cell windows, which became inore pronounced as the temperature was raised. Experilnellt pairs with the same number outside the bracket were carriecl out a t the same light intensity.

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386 CANADIAN JOURNAL OF CI-IEMISTRV. VOL. 33

TABLE 111

THE EFFECT OF TEMPERATURE ON HBr INHIBITION OF ACETOXE PHOTOLYSIS Time 4 hr.

Concentration, micromoles/cc. Products, micromoles

-- --- - R u n Temp. Acetone Hydrogell CH4 CO CzHs CH4+2C?Ht

bromide CO

'kFi~ne s i x Tiozirs

Table IV shows for each temperature the carbon monoxide deficiency, -ACO, obtained by subtracting the carbon monoxide yield for the run with hydrogen bromide from the value for the run a t the same temperature and light intensity with acetone alone, and the volatile metlzyl! radical deficiency, -ACH3, obtained

TABLE IV CARBON MONOXIDE AND VOLATILE METHYL RADICAI, DEFICIENCIES RI?SULTING FROM THI.:

ADDITION O F HYDROGEIi BROMIDE T O ACETONE

CO yield Te~np. , - ACO, -ACH:{, for acetone, -ACO -ACH:j

"C. micromoles micromoles ~nicromoles CO -ACO

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RIDGE A N D STEACIE: PHOTOLYSIS 387

in the same way from the function (CH4+2C2H6). The ratio of the carbon monoxide deficiency to the yield of carbon monoxide for the acetone photolysis is nearly unity up to 120°C. and then falls off as the temperature increases, but even a t 300°C. it is still 0.14. Over the whole range the value of -ACH3/ ACO is of the order of magnitude of unity.

The absolute values of the carbon monoxide deficiency and the volatile methyl radical deficiency are significant only as showing a general tendency, since illumination was maintained constant only over the pairs 113, 114; 105, 109; and 119, 120. I t is seen that they rise to maximum a t 150" and then fall away again.

The Effect of Time of Irradiation in the Presence of a Small Amount of Hydrogen Bromide

Table V shows the effect of increasing times of irradiation on 100 mm. of acetone a t 150" in the presence of 0.3 mm. of hydrogen bromide. The results of an experiment with the same pressure of acetone in the absence of hydrogen

TABLE V

EFFECT OF TIME OF IRRADIATION ON THE PHOTOLYSIS OF ACETONE I N THE PRESENCE OF A SMALL AMOUNT OF HYDROGEN BROMIDE

Acetone 100 mm.; hydrogen bromide 0.30 mm.; Temp. 150°C. -

Concentration, micromoles/cc. Time

Run of run. Acetone Hydrogen hr.

bromide -

130 3.779 - 3 132 3.774 0.0113 3 133 3.774 0.0113 6 131 3.779 0.0113 9

Products, micromoles CHI +~C~HO - CO - RCHa

CO Time. RC:HB Methane Carbon Ethane m icromoles/

mono?tide hr. -- --

5 89 7.73 3.34 1.G3 2.58 1.7G 9.89 7.18 1 .03 1 .66 2.39 9.60

18.11 15.13 3.18 1.62 2.52 5.70 24.73 51.28 4.93 1.03 2.37 5 .02

bromide are included for comparison. Although the carbon monoxide yields show more scatter than usual, it appears that even this amount of hydrogen bromide has inhibited the carbon monoxide yield to some extent.

The effect of the addition of 0.3 mm. of hydrogen bromide has been to increase the methane yield a t the expense of the ethane yield, the ratio CH4+2CZH6/CO remaining constant. The ratio RCR4/RC2E6 (RCE4 = rate of methane formation etc.) shows a drop as the time of irradiation is increased, presumably indicating that hydrogen bromide is slowly spent as the photo- lysis proceeds. However, the magnitude of the effect produced is such that a large part of the HBr reacting must be regenerated.

DISCUSSION

The effect of hydrogen bromide on the photolysis of acetone is particularly striking. As little as 1.5 mm. in 50 mm. of acetone a t 150" practically eliminates the ethane yield (run 110, Table 11), reduces the carbon monoxide yield to almost half its value in the absence of hydrogen bromide, brings about a considerable increase in the yield of methane and an increase in the ratio

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388 CANADIAN JOURNAL O F CHEMISTRY. VOL. 33

(CH4+2CzHG)/CO to a value well over 2. The last effect is a plain indication that the reaction is not a simple case of photodecomposition of acetone to give methyl radicals and carbon monoxide, followed by the abstraction of hydrogen from the hydrogen halide by the methyl radicals, as appears to be the case in the photolysis of acetone - hydrogen chloride mixtures (3).

That hydrogen abstraction (2) is, however, involved CH3fHBr = CH4+Br [21

is fairly clear from the results of the photolyses for various lengths of time, with very small additions of hydrogen bromide (see Table V). In this case the carbon monoxide yield was only slightly affected and the ratio (CH4+2CzH6)/ CO remained the same as for acetone alone, the drop in the ethane yield on addition of hydrogen bromide being accounted for by a corresponding increase in the methane yield. From the same series of experiments i t is likewise clear that the reaction

BrfCH3. CO. CH3 = HBrfCHz. CO.CH3 141

occurs sufficiently rapidly compared with other reactions of bromine atoms for the hydrogen bromide to be partly regenerated, as has already been discussed. This result is of significance for the photolysis of acetone in the presence of bromine substituted hydrocarbons, as i t indicates that the formatioil of even small quantities of hydrogen bromide will result in serious complications as far as interpretation of the results and derivation of activation energies concerned.

Hydrogen bromide in greater concentration brings about a marked reduc- tion in the volatile methyl radical yield (CH4+2CzHG); for example, a t 150' the addition of 9.8 mm. hydrogen bromide to 50 mm. acetone reduces the volatile methyl radical yield to less than half its value in the absence of hydrogen bromide (run 107, Table 11). This indicates that methyl radicals are either undergoing reactions not leading to methane or ethane produc- tion, or that the photolysis of acetone is so interfered with by the presence of hydrogen bromide, that there is a reduction in the number of methyl radicals produced.

A striking feature of the effect of hydrogen bromide is the sharp suppression of the carbon monoxide yield. This is almost certainly due to a rapid reaction of acetyl radicals with HBr to form acetaldehyde.

Table IV shows that the ratio -ACH3/-ACO is approximately unity over the range of temperature studied. If the values most liliely to be in error are omitted, viz. for 300" where the degree of inhibition is small, and for 50" where the amount of products is small, the average value is 1.1. However, this ratio, as calculated froin the figures of Table 111, alone is of limited significance. I t is, however, consistent within the experimental error with a ~nechanism in which the loss of CH, and of CO is due to loss of acetyl radicals.

Attempts were made to detect acetaldehyde ainong the products of photo- lysis of acetone - hydrogen bromide mixtures, but these were not successful. I t is felt, however, that this is not a serious objection as there was a considerable amount of tarry products and acetaldehyde may well have been lost in these.

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RIDGE AND STEACIE: PHOTOLYSIS 389

The Reaction Mechanism The observed effects are therefore accounted for by the following reaction

scheme : CHS. CO. CH,+hv = CH3. CO+CH, [I41 CH3. CO = CH3+CO I l l CH3+HBr = CH4+Br [2] CHZ.CO+HBr = CH3.CHO+Br [31 Br+CH3.C0 .CH3 = HBr+CH2.C0 .CH3 [4] CH3+Br = CH3Br [5]

~ince:the light used in these experiments was largely 3130 A, we assume tha t the initial decomposition of acetone proceeds predominantly t o a methyl and an acetyl radical (5). The acetonyl radical will presumably disappear by association to form bromoacetone or methyl ethyl ketone.

From the above scheme

d[CH3. CO]/dt = 14 - kl[CH3. CO] - k3[CH3. CO] [HBr] = 0. I Also d[CO]/dt = k,[CHz. COI, . . [CH3. CO] = d[CO]/dt . /kl. I I

Substituting I1 in I and rearranging

14 = d[CO]/dt+k3/kl. [HBr] . d[CO]/dt.

Identifying 14 with Rco, the rate of carbon monoxide formation for acetone alone, and setting RE0 equal to d[CO]/dt, the rate of carbon monoxide forma- tion in the presence of hydrogen bromide, we obtain on rearranging:

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390 CAN.ADIAN JOURNAL O F CHEMISTRY. VOL. 33

Fig. 1 shows log R C O / R ~ O - l plotted against l /T . The points lie about a straight line with a slope corresponding to about -13 kcal./mole for the difference E 3 - E l . The activation energy of reaction (I) is not linown precisely but has been estimated a t from 10 to 18 kcal./mole (4). E3 is unliliely to be more than 5 kcal./mole. Thus, in view of the drastic approximations made in the above derivation the value of - 13 kcal./mole for E3 - El is a reasonable one, and to this extent supports the mechanism suggested.

If the inhibition of the acetone photolysis by hydrogen bromide is due entirely to the reaction of acetyl radicals the yield of (CH4+2CzH6) for the inhibited reaction cannot be less than the carbon monoxide yield of the corresponding uninhibited reaction. However, reference t o Table I1 shows that the volatile methyl radical yield of the inhibited reaction falls appreciably below the carbon monoxide yield for acetone alone. This may be attributed to reaction (5). As noted previously with the experiments with 0.3 mm. hydro- gen bromide, hydrogen bromide is gradually spent despite the regenerative reaction. With the higher concentrations of hydrogen bromide used in the . experiments over a range of temperatures, the production of bromine atoms must become appreciable.

ACKNOWLEDGMENT

We wish to thanli Miss F. Gauthier and Dr. F . P. Lossing of this laboratory for the mass spectrometric analyses, and Drs. R. J . Cvetanovic and R. M. Heggie for many interesting discussions.

REFEREKCES

1. ASDERSOK, H. C. and KISTIAI~OTVSI~Y, G. 13. J. Chern. Phys. 11: 6. 19-13. 2. Cv~rawovrc, R., RAAL, F. A,, and STEACIE, E. W. R. Can. J. Chem. 31: 171. 1953. 3. CVETAXOVIC, R. and STEACIE, E. \V. R. Can. J . Chem. 31: 158. 1953. 4. DAVIS, W. Chem. Revs. 40: 201. 1947. 5. NOYES, \V. A,, JR. and DORFMAX, L. hI. J. Chem. Phys. 16: 788. 1948. 6. RAAL, F. A. and STEACIE, E. W. R. J . Chem. Phys. 20: 578. 1952. 7. RIDGE, M. J. and STEACIE, E. W. R. Can. J. Chem. In press. 1955.

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