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METHYL ETHYL KETONE IN THE PHOTOLYSIS OF ACETONE VAPORL By niass speclrorr~etric analyses, niethyl ethyl ketone was determined as a product of the photolysis of acetone between 100 and 284°C. According to the post~~lated mechanisni, ~iiethyl ethyl ketor~e together with methane and e t h a ~ ~ e accounted for approsirnately 05y0 of the methyl radicals procluced. INTRODUCTION It is no\v accepted that the mechanism for the photolj-sis oi acetone vapor between 100°C. and 250°C. is (6,7) Reaction [I] was shown to proceed with a quantum yielcl oi ~inity in this temperature range (4). 'The kinetics of reactions [2] ancl [3] have been in- vestigated (3,G,'i) in some detail. Methyl ethyl ketone was identifecl as a product but was not st~tdied quantitatively (1). Icetene was also found a t temperatures above 200°C., presumably resulting from the decomposition of acetonyl radicals (2) Considering reaction [I] as the only source of methyl radicals, it car] be seen from the results of Trotman-Dickenson and Steacie (7) that reactions [2] and [3] account for from 70 to 100yG of the methyl radicals produced, the percentage depending on temperature and concentration. Tl~erefore, the inclusion of reaction [4] should account for all the methyl radicals, and the material balance where R,,,, is the rate of production of ethane etc., should be equal to 2 ~~nder all conditions in this temperature range. If reaction [GI is significant, then the above ratio should increase with temperature above 200°C. The purpose of this work was to study the material balances in the acetone photolysis when methyl ethyl ketone is included. It serves as the basis of an investigation of the kinetics of addition of methyl radicals to unsaiuratecl hydrocarbons, the loss of methyl radicals I)? acldition being related to a clecrease in the material balance. dfa~t~~~crip1 recelved Oclober 15, 1953. Conlribulio?~ fro~r~ lke Division of Pure Clre~nislry, ~Vatioi~al Re.searc11 C o ~ r t ~ c i l of Cat~adn, Ollarua. Canada. Issued as N.R.C. No. 31L9. ? !\;nlio,~al Research Cozrncil of canad; Posldoctoralr Fr.llolel 1.9,51-<5S. Can. J. Chem. Downloaded from www.nrcresearchpress.com by UNIV CHICAGO on 11/11/14 For personal use only.

METHYL ETHYL KETONE IN THE PHOTOLYSIS OF ACETONE VAPOR

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Page 1: METHYL ETHYL KETONE IN THE PHOTOLYSIS OF ACETONE VAPOR

METHYL ETHYL KETONE IN THE PHOTOLYSIS OF ACETONE VAPORL

By niass speclrorr~etric analyses, niethyl ethyl ketone was determined as a product of the photolysis of acetone between 100 and 284°C. According to the post~~la ted mechanisni, ~iiethyl ethyl ketor~e together with methane and e t h a ~ ~ e accounted for approsirnately 05y0 of the methyl radicals procluced.

INTRODUCTION

I t is no\v accepted that the mechanism for the photolj-sis oi acetone vapor between 100°C. and 250°C. is (6,7)

Reaction [I] was shown to proceed with a quantum yielcl oi ~ in i ty in this temperature range (4). 'The kinetics of reactions [2] ancl [3] have been in- vestigated (3,G,'i) in some detail. Methyl ethyl ketone was identifecl as a product but was not st~tdied quantitatively (1). Icetene was also found a t temperatures above 200°C., presumably resulting from the decomposition of acetonyl radicals (2)

Considering reaction [I] as the only source of methyl radicals, it car] be seen from the results of Trotman-Dickenson and Steacie (7) that reactions [2] and [3] account for from 70 to 100yG of the methyl radicals produced, the percentage depending on temperature and concentration. Tl~erefore, the inclusion of reaction [4] should account for all the methyl radicals, and the material balance

where R,,,, is the rate of production of ethane etc., should be equal to 2 ~ ~ n d e r all conditions in this temperature range. If reaction [GI is significant, then the above ratio should increase with temperature above 200°C.

The purpose of this work was to study the material balances in the acetone photolysis when methyl ethyl ketone is included. I t serves as the basis of an investigation of the kinetics of addition of methyl radicals to unsaiuratecl hydrocarbons, the loss of methyl radicals I)? acldition being related to a clecrease in the material balance.

d f a ~ t ~ ~ ~ c r i p 1 recelved Oclober 15, 1953. Conlribulio?~ f r o ~ r ~ lke Division of Pure Clre~nislry, ~Vat io i~a l Re.searc11 Co~rt~cil of Cat~adn,

Ollarua. Canada. Issued as N.R.C. No. 31L9. ? !\;nlio,~al Research Cozrncil of canad; Posldoctoralr Fr.llolel 1.9,51-<5S.

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Page 2: METHYL ETHYL KETONE IN THE PHOTOLYSIS OF ACETONE VAPOR

so CANAUIA.\' JOURNAL OF CHEJIISTRI ' . 1.0L. :?

The reaction cell consisted of a cylindrical quartz vessel with plane polished ends and 195 cc. in volunle, 10 cln. long, and 5 cm. in diameter. I t was kept in an aluminurn block furnace which had a quartz window a t each end. Three thermocouples were fastened to different points on the cell. Two tubes ex- tendecl from the cell through the furnace, one serving as a colcl finger and the other connected to a stopcocli located one inch from the top of the furnace. The total volume of the cell and connecting tube was 205 cc.

The analytical system consisted of two traps, a modified LVard still (5), a sinall mercury diffusion pump, a coillbination gas burette,and Toepler pump, and a copper oxide tube heated to 240°C., all in series. The cell, analytical system, and reagent reservoirs were suitably connected to a two-stage mercury diffusion puinp.

A Hanovia S-100 lamp served as the light source and proved to be fairly constant over long periods of use. The light was collimated by a stop and a series of lenses, thereby filling over goy0 of the cell and, for greater efficiency, was reflected back by an aluminum mirror 011 the rear window of the furnace. The light was filtered with a Corning No. 98G3 filter. Different light intensities were obtained by adding neutral density iilters. In a few experiments the full arc was used.

The acetone was Merck Reagent Grade. I t was dried over "Drierite", degassed by bulb to bulb distillation, and separated from the system by a mercury cutoff.

For an experiment, acetone was introduced into the cell to the desired pressure. After condensing in the cold finger and punlping to remove any traces of noncondensible gases, the acetone was photolysed to about seven per cent decomposition.

The products of main interest were carbon monoxide, methane, ethane, and methyl ethyl ketone. Methane and carbon monoxide were separated a t -196°C. and the carbon monoxide was combusted and separated in the copper oxide tube. Ethane was separated a t -175OC. and various samples were analyzed with the mass spectrometer. The remainder, consisting mainly of acetone, was collected into 150 cc. sample bulbs and analyzed for methyl ethyl ketone with the mass spectrometer.

The analyses for methyl ethyl ketone were based on the height of peak 72. Two synthetic samples containing amounts of acetone used in most experi- ments and 2.5 and l.Oyo methyl ethyl ketone were subjected to the same procedure as after photolysis. Mass spectroscopic analyses of these gave 2.2 and 0.85y0 nmethyl ethyl ketone respectively. and therefore all the results for methyl ethyl ketone, obtained by this method, are reported 15% higher than those obtained in the analyses.

Since it is possible that some of the heavier methyl ethyl ketone was ab- sorbed by the stopcock grease in the apparatus and in the sample bulbs, i t was felt necessary to check on the validity of applying the 15y0 correction factor. The apparatus was altered so that the methyl ethyl ketone would not be in contact with stopcock grease except for a short time when introduced

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Page 3: METHYL ETHYL KETONE IN THE PHOTOLYSIS OF ACETONE VAPOR

MANDELCORN A N D STE.4CIE: A CE7 '0NE I'APOR 81

into the mass spectroineter. Mercury cutoffs were appropriately installed t-o isolate the cell (volume of connecting t~lbing = 37 cc.) analytical system and gas burette. The acetone and methyl ethyl ketone mixtures were measurecl in a known volunle which was connected with a Inercury cutoff also serving as a manometer, and then collected in sample bulbs equipped with brealiseals. iVIass spectroscopic analyses of two synthetic samples (prepared ~incler stop- cock grease free conditions) containing 0.37 and 0.71y0 inethyl ethyl lietone gave here 0.36 and 0.72% respectively. Obviously, with this pnocedure the mass spectroscopic results for methyl ethyl ketone may be ~isecl unambig~iously.

The results of experiinents done a t three different acetone concentrations and a t various temperatures are given in Table I. Inclucled also, a t 184OC. is

TABLE I PRODUCTS OF PHOTOLTSIS OF ACETONE

Mean 'acetone conc.-1.76 X 1 0 + M./cc.

Mean acetone conc.-3.56 X A{./cc.

*Stopcock-grease free system.

Mean acetone conc.-.88 X Jl./cc.

the effect of variation of light intensity, seen from the rates of carbon monoxide production. Quantum yields of methyl ethyl ketone production are calculated on the assumption of a quantum yield of unity for carbon monoxide production.

0.15 0.2 0.3

138 5160 184 1 7200 240 7200

1.76 1.62 1.49

2.66 2 10 0.90

1.91 1.82 1.86

3.29 3.39 3.18

0.47 1.29 2.94

0 . 5 0.7 1.2

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Page 4: METHYL ETHYL KETONE IN THE PHOTOLYSIS OF ACETONE VAPOR

I t can be seen trom the material balances obtai~~ecl in the stopcock grease free system that the application of 157c correction to the rest of the results for methyl ethyl ltetone is valid.

Mass spectrometric analyses occasionally sho\vecI traces of propane in the ethane fraction. However, after removing the ethane no significant lraction could be obtained a t --150°C., i.e., propane. In the experiment a t 28J°C, about 20Yo of the ethane fraction consisted of ethylene. KO other products coulcl be found b\ the above method of analysis.

The constancy of the material balance ( ~ R c ~ H ~ + RcH( + RcH: ,coc~H~) /Rco ,

over the temperature range studied i~~dica tes that reaction [GI is relativelj- insignificant a t these conditions. Therefore, it is apparent that reactions [2] , [3 ] , and [4] account for 95 f 3% of the methyl raclicals produced, reaction [I] being the main source. The deviation falls within the possible error of 157" in methyl ethyl ketone analysis. Conversel\., these results lend support to the postulated mechanism for acetone photolysis a t these temperatures.

I t is possible to obtain some irlforrnation on the kinetics of reactions [2], [4] , and [5]. I f it is assumed that the acetonyl radicals produced in reaction [3] react only to produce methyl ethyl ketone and biacetonyl, then R(CH3COCH2)2 \vould be equal to (RCH4 - R C H ~ C O C " H ~ ) / ~ . Hence (k:!fk54)/kd could be calculated as it is equal to ( R ~ ~ ~ ~ I ~ R ~ ( ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ) / R ~ ~ ~ ~ ~ ~ ~ ~ ~ . Such calculations were made and an average value of 0.7 f. 0.3 was found for (k2ik5i)/k4. These values. although considerably scattered, are fairly constant with temperature and are also in the range of -0.5, the magnitude of (Z2+ZSi ) /24 , Z being the col- lision frequency. I t seems unlikely that this is due to an accidental equival- ence of pS+ and P4 when both are small, and the results strongly support collision efficierlcies for reactions [4] and [5] of the order of magnitude of that of [2], i.e., unit!-.

The authors wish to tender their sincere tharllts to Dr. F. P. Lossing for his Ilelpful discussions on the methyl eth\-I ltetorle analysis, and to fi1 iss F. Gauthier and Miss J. Fuller for the mass spectrornetric analysis.

REFERENCES 1. ALLEN, A. 0. J. Am. Chem. Soc. 63: 708. 1941. 2. FERRIS, R. C. and HAYNES, W. S. J . Am. Chem. Sac. 72: 893. 1950. 3. GOMER, R. and ~(ISTIAKOWSKY, G. B. J. Chern. Phys. 19: 85. 1951. 1. HERR, D. S. and NOYES, \V. A., JR . J . AIII. Chem. Sac. 62: 2052. 1940. 5. LEROY, D. J. Can. J . Research, B, 28: 492. 1950. 6. NOYES, \rV. A, , JR . and DORFMAN, L. &I. J . Chem. Phys. 16: 788. 1948. 7. 'TROT~~.~N-DICKEXSOS, A. F. and STEACIE, E. \Ir. R. J . Chem. Phys. 18: 1097. 1950.

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