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Xerox University Microfilms 300 North Zeeb Road Ann Arbor, Michigan 48106 1 1 - l l p o n 8 LD3907 .G7 Zemany, Paul Daniel, 1916- 1951 Free radicals in photolysis and pyro- .Z5 ly sis of acetaldehyde. 36p. diagrs. Thesis (Ph.D.) - N.Y.U., Graduate School, 1951• Bibliography: p .35-36. C81995 ( *M f IJst' Xerox University Microfilms, Ann Arbor, Michigan 48106 THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED. IiII'KAPT "T Kpvr ycrr, r"TT"> OMVLR;. !7Y h. '• FREE RADICALS IN PHOTOLYSIS AND PYROLYSIS OF ACETALDEHYDE Paul D. Zemany Research Laboratory of the General Electric Company, Schenectady, New York and Department of Chemistry, New York University, University Heights, New York, New York A dissertation in the Department of Chemistry submitted to the faculty of the Graduate School of Arts and Sciences of New York University in partial fulfillment of the requirements of the degree of Doctor of Philosophy. H S | TABLE OF CONTENTS Introduction...................................................................................... M aterials................. ,..................................................... Apparatus ........................................................................................ General Procedure ...................................................... Results and Discussion.................................................................. 1. Photolysis at 140°C................................................. 2. Photolysis at 290°C........................................... 3. Pyrolysis at 510°C................................................... 4. Pyrolysis of Pure CHs CHO................................. 5. Pyrolysis of CD 3C£0 and CH3CH0 at 465°C 6. Photolysis at 25°C.................................................. 7. Additional Experimental Findings..................... Conclusion....................................................................................... Summary ....................................................................................... Acknowledgment ........................................................... Bibliography. ............................................................................... 9 . o A ^ \ A FREE RADICALS IN PHOTOLYSIS AND PYROLYSIS OF ACETALDEHYDE Paul D. Zemany INTRODUCTION The presence of free radicals in the pyrolysis of acetaldehyde has been the subject of many investigations, but it has not yet been definitely established. The Rice-Herzfeld theory of free-radical chain reaction^) accounted both for observed order and for activation energy of the pyrolysis of acetaldehydeU^) by the mechanism CH3CHO,----------------- » c h 3+ HCO (1) R + C H C H O ------------------» RH+CH CO (2) 3 3 CH CO--------------------> CH + CO (3) 3 3 R + R-----------------------* chain end . (4) F.O. Rice and his co-workers had used the Paneth m irror technique^?) to show production of free radicals in the pyrolysis at 743°C. Pearson and Purcell(15) and Letort(9) also found such m irror evidence. Allen and Sickman tested the assumption that methyl radicals propagated the chains by a demonstration that such radicals resultant from pyrolysis of azomethane induced decomposition of acetaldehyde at 300°cU); i.e., 150° lower than the minimum temperature at which pyrolysis of ace­ taldehyde had hitherto been observed. An objection offered to the major significance of free radicals in pyrolysis of acetaldehyde was that actual observations of free-radical production related to temperatures two to three hundred degrees above those of the kinetic studies. Patat and Sachsse used the para-ortho hydrogen conversion^4) in an effort to establish the degree of contribution of free-radical chain mechanism to the over-all reaction at 550°C. They found clear evidence for free- radical production but concluded that it was only about 1/100 that re­ quired to account for the observed rate. In consequence of these various results, considerable doubt existed that methyl radicals were present at all at the lowest temperature at which the pyrolysis of acetaldehyde had been studied. -1- In a test of this point Burton, Ricci and D avis(^) extended the lower limit of observation of methyl radicals to 475°C by experiments with radiolead m irrors. Practically at the same time, Stavely and Hinshelwood'22) concluded from studies of the pyrolysis of acetaldehyde (in which nitic oxide showed no inhibition) that no significant portion of the pyrolysis went by a free-radical mechanism at the lower temperatures of study. They suggested that the so-called 3/2 order of the reaction generally assumed was in reality a first order changing to second order, and that the kinetics were adequately explained in terms of a single rearrangement process c h 3 c h o ------------------ » c h 4+ co, (1') involving about 18 square-terms. It was found subsequently that propylene did inhibit the decompo­ sition of acetaldehyde, and later Smith and Hinshelwood(21) reinvestigated the effect of nitric oxide and found that inhibition did occur and had been masked by its catalytic effect. Hinshelwood’s earlier view shortly received substantial support from some work of Morris(12), who studied the pyrolysis of mixtures of carefully purified CH3 CHO and CD3 CDO (stored over hydroquinone) and was unable to find any significant quantity of CH3 D or of CD3 H, both of which would have been expected if the mechanism was according to the Rice-Herzfeld scheme of reactions (1) to (4). M orris suggested as one conclusion from his result that Burton, Ricci and Davis had found radicals which resulted perhaps from a side reaction involving a trace of oxygen in their reactant (cf. Letort(lO)). since the lead-m irror technique, very difficult at best, is insensitive in the presence of oxygen, Taylor and Burton(25) suggested the possiblity of a free-radical chain involving an induced internal conversion in one elementary step R + CH3CH0-------------------> R+CH4+C0, which might apply also in the photolysis. The present investigation was undertaken to discover whether the mass spectrograph could be used to prove that free radicals are present at kinetically significant concentrations during the photolysis and pyrolysis of acetaldehyde. The experimental method consists in the continuous analysis of mixtures of acetaldehyde and acetaldehyde-d4 while they are undergoing pyrolysis or photolysis. The mass spec- -2- trometer can be used to measure concentrations of reactants and of all the products, but it is the presence of the various possible methanes (CH^, CD3H, GHgDg, GHSD, CD^) that are of principal interest. Obviously, the methanes containing hydrogen in both isotopic forms can only be formed from free radicals. The experimental results show un­ equivocally that free radicals are present at kinetically significant concentrations during the pyrolysis and photolysis of acetaldehyde, and they permit other conclusions to be drawn about the mechanisms of these reactions. MATERIALS For the first few runs acetaldehyde obtained from the City Chemical Company of New York was distilled under nitrogen and trans­ ferred to the vacuum system where it was distilled several times from frozen CC14 to C02-acetone, and from C02-acetone to frozen acetone using middle fractions. The aldehyde was not allowed to freeze since it tends to polymerize on freezing. For most of the runs, however, the acetaldehyde was prepared from paraldehyde^) by de-polymerizing with HgS04, and purifying as described above. The hydroquinone was Eastman Kodak Company, white label; used without further purification. Two different batches were used. The deuteroacetaldehyde was prepared by the method of Zanetti and Sickman(29) from deuteroacetylene^l'. Figure 1 shows schematically the apparatus used to prepare C2Dg. Fifty grams of CaC2, from a freshly opened container, was put into vessel B; the vessel was sealed off at the constriction. Twelve and one-half grams of DgO (obtained through the United States Atomic Energy Commission, from the Stuart Oxygen Company, San Francisco, California) was put into container C and degassed. A furnace (two inches i.d., eight inches long) slipped over tube B, and the temperature was maintained at 450°C by means of a Variac for three hours while pumping continued. The pressure, measured by a thermocouple gauge dropped to under a micron (the limit of sensitivity). This period of heating should have been sufficient to dehydrate any Ca(OH)* to the oxide. After cooling, the DgO was slowly run into the CaCg, and the CgDg was trapped in A by liquid air; methane, Da, and other more vol­ atile substances were pumped off. *In Ephriam’s Inorganic Chemistry (reference 4) the vapor pressure of HgO over Ca(0H)2 is given as 760 mm at 430°C. -3- APPARATUS FOR PREPARATION OF C2 Dg &+— 12/30 ? —X— TO VACUUM SYSTEM TO VACUUM — X- SYSTEM ii J D l y FIG. IA B —X 12/30 12/30 TOPLER PUMP 4 LITER STORAGE BULB MERCURY CUT-OFF — XtZZ APPARATUS FOR PREPARATION OF FIG. IB cd3 cdo from g2d2 By transferring the liquid air to B and putting a trap of frozen acetone (-94°C) around A, the C2D2 was transferred, leaving less volatile m aterials behind; these were then pumped out. It was next transferred to A, then to D by the same series of steps. The C 2D2 was then expanded into a four-liter bulb fitted with a mercury trap, ready for the preparation of CD 3CD0. A fraction of the CgD2 was checked for isotope purity by the mass spectrometer. The peak heights for the Cg group observed are given in Table I. TABLE I Peak Total P.H. Portions Due to 24 2350 C C - 2350 12 12 25 91 C C - 54; C C H - 37 12 IS 12 12 26 9050 C12C1 2 D - 9050 27 578 C12C13D - 200’ C12C12 H0 -3?8 28 54300 C C D - 54300 12 12 g If C12 c 1# HD and C C D have comparable ionization efficiencies, they are present in the ratio ol their peak heights, .378 0.7 per cent. 54.300 The ratio of H/D is half this or about 0.0035. By comparing peaks 25 and 26 the ratio of H/D indicated was .0041. However, the blank on the spectrometer in this region was fairly high at the time, so these estimates are probably somewhat high. The deuteroacetaldehyde was prepared in another part of the same vacuum system used to prepare C D (Fig. IB). Excess C2D2 was stored in A, trapped by the mercury trap B. Da O placed in C was degassed by repeated freezing and thawing while pumping. -5- Pg O was placed in E and G. The P2 Os in E was sublimed into D by headng with a torch. A slow stream of nitrogen, dried by passing through the P 20s in G helped in the transfer. One-half gram of IigS04, which had been dehydrated by heating, had previously been placed in D. After the P 20g had been transferred, the tube D was sealed off from E, and the system evacuated. F was a section of Bourdon tubing which was maintained in a distended condition by two pins resting on lugs attached to the top and bottom. This proved to be an excellent way to provide a flexible con­ nection. A stirring motor was fitted to shake the vessel D at about 120 rpm; the amplitude was about three inches. The D O was distilled slowly into the Pg O - HgS04 ; then the C 2D2 (whicn had just been made) was run into the mixture by means of the Tbpler pump while stirring. The reaction proceeded very skjwly « and was run over night. ^ The volatile materials were then distilled from the mixture, the CD3CD0 was separated and fractionated, all in the vacuum system out of contact with air. The CD3 CDO was stored in sample tubes with standard taper joints for attaching to the mass spectrometer. £ Calibrating gases, methane, ethane, CO, were taken directly from tanks, as required. Impurities could generally be readily observed, identified and measured in the mass spectrometer. APPARATUS The mass spectrometer used was a 60° deflection Nier type manufactured by General Electric. It is described in Manual GEI 18292, although some modifications were used in this investigation. A block diagram, Fig. 2, shows the essential parts. The high- voltage supply (catalog no. 8257689 G) was adopted from another device. It could supply one milliampere at any voltage to 3000, and it was reg­ ulated to ±0.01 per cent. The normal operating voltage was set at about 2000 volts. Two filament supplies were used. The first, a battery-operated supply so arranged that appearance potentials could be measured ac­ curately, was used in another problem and was replaced early in the -6-