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HYDROCARBON FREE RADICALS IN PHOTOPROCESSES HUGH S. TAYLOR Department of Chemistry, Princeton University, Princeton, New Jersey Received May 25, 1958 The communication by Leighton (10) on the mechanism of aldehyde and ketone photolysis has discussed in detail one group of photoreactions in which the primary process leads, at least in part, to the production of hydrocarbon free radicals. In the present communication will be reviewed a group of other photoprocesses in which the absorption of light also gives rise to the production of hydrocarbon free radicals, from the secondary reactions of which further data on the properties of such radicals and their reactivities with various atomic and molecular species can be deduced. From such studies are slowly accumulating a series of data, first qualita- tive and later quantitative, with the aid of which a more certain interpreta- tion of secondary reaction paths can be deduced. The photoprocesses leading, in the primary process of absorption, to the production of free radicals include the photolysis of alkyl iodides, the photodecomposition of metal alkyls, the mercury-photosensitized hydro- genations of unsaturated hydrocarbons, and the mercury-photosensitized decompositions of saturated hydrocarbons. As yet, the majority of the studies are confined to the simpler homologs of the several series of com- pounds, and the radicals for which the studies are least equivocal are the methyl and ethyl radicals. THE PRIMARY PROCESSES With the lighter alkyl iodides, which show regions of continuous absorp- tion in the ultraviolet with well-developed band spectra of greater intensity than the continua, beginning near 2000 A. and extending into the Schu- mann region, it is quite generally postulated (2, 7, 8, 22, 24) that the primary process leads to dissociation into a free radical and an iodine atom. Thus, with methyl iodide we assume the formation of CH3 + I, and with ethyl iodide the formation of C2Hs + I. There are no data which conflict with this point of view. Differences in reaction product arising from such photolyses are, as we shall see, to be ascribed to secondary processes rather than to any other products of the primary absorption. ~ Contribution No. 6 to the Third Report of the Committee on Photochemistry, National Research Council. 65 /
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66 HUGH S. TAYLOR Of the metal alkyls the best investigated are those of lead and mercury. The spectrum of mercury dimethyl shows diffuse bands below 2150 A., with an overlapping continuum which extends to about 2600 A. (12~. Terenin and Prileshajeva (20) interpreted this absorption as leading to primary dissociation into free radicals and detected such by their action in removing metallic mirrors. Linnett and Thompson (12), after first assuming that the major primary process was decomposition to mercury and ethane, finally decided that the facts could be best interpreted by the two possible free-radical decompositions Hg(CH3~2 + he = Hg(CH3) + CH3 Hg(CH3~2 + he = Hg + 2CH3 Leighton and Mortensen (11) confirmed the observations of Terenin (19) and of Duncan and Murray (5), that the absorption spectra of lead tetramethyl, tetraethyl, and tetraphenyl were all continuous, the long wave length limits of absorption by the vapors being 2800 A. for the methyl and 3500 A. for the ethyl compound. Lead tetraphenyl in solution in trimethylpentane gave an absorption limit around 2800 A. These con- tinuous spectra point to dissociation in the primary absorption act, with radicals or saturated molecules as possible products in addition to the metal atoms or metal radical complexes. In the mercury-photosensitized hydrogenation of unsaturated hydro- carbons, when hydrogen is present in any marked amount, the primary process occurs between excited mercury and molecular hydrogen genera- ing atomic hydrogen. The free radical arises as a secondary reaction between the atomic hydrogen and the unsaturated hydrocarbon. The simplest radical so produced is the ethyl radical from ethylene. All the recent evidence indicates that this is a process of good efficiency and that the presence of ethylene serves to reduce the stationary state concentration of atomic hydrogen to small values. For the saturated hydrocarbons Bates (1) has shown that the quenching efficiency of methane for excited mercury is very small, but that with the higher homologs there is an increased quenching efficiency. The data of Morikawa, Benedict, and Taylor (13) suggest that quenching of methane even at room temperatures gives rise to CH3 + H either directly or by col- lision with metastable mercury atoms. Indirect evidence suggests that the dissociation process may require an activation energy of ~4.5 kg-car. The efficiency increases with temperature. A recent study of Steacie and Phillips (17) is concerned with the inter- action, in a circulatory process, of excited mercury and ethane. They reached the conclusion that the products of the primary interaction be- tween excited mercury and ethane are two methyl groups. The reaction C2H6 + Hg' = C2H5 + H + Hg
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. FREE RADICALS IN PHOTOPROCESSES 67 was ruled out because of the absence of molecular hydrogen in the reaction products. In the corresponding experiments with butane, Steacie and Phillips found abundant production of hydrogen. More recent experi- ments by these authors, not yet published, indicate that, in single-pass experiments in place of a circulatory process, the formation of hydrogen from ethane definitely occurs. This means that, with circulation, the hydrogen formed is converted into atomic hydrogen by photosensitization and is consumed in further reaction processes the nature of which will later be discussed. THE QUANTUM YIELD The alkyl iodides show remarkably low quantum yields, especially in the vapor state. Methyl iodide has a yield of 0.02 in terms of iodine atoms per absorbed quantum in the gaseous state (2), and 0.05 in hexane solution in the region of continuous absorption (25~. With ethyl iodide, the yield is of the order of 0.01 at 2600 A., increasing to 0.1 at 2026 A. In the con- tinuous region, liquid ethyl iodide and its solution in hexane show yields of about 0.6, whereas at 2026 A. the yield has decreased to 0.24 (24~. The low yield in the continuum points to dissociation followed by re- co~nbination. This view is supported by recent experiments of West (23), in which photolysis in the presence of silver foil as a trap for iodine atoms increased decomposition fortyfold, with a marked change in the composi- tion of reaction products (see later discussion). The solvent molecules, in the experiments in the continuum, also-should repress dissociation, according to the Franck-Rabinowitch principle, so that the large influence of solvent on quantum yield needs special consideration. In the short wave banded region, West and Ginsburg assume the production of opti- cally excited molecules and their interaction with normal iodide molecules. With lead tetramethyl vapor Leighton and Mortensen ;~11) found quan- tum yields at 25°C. somewhat more in excess of unity (1.01-1.13) than could be attributed to experimental error. The authors therefore sus- pected the existence of short chains. Linnett and Thompson (12), with mercury dimethyl, found a quantum yield of unity at room temperature, but at higher temperatures the yield increased gradually to 2.2 at 190°C. Cunningham (4) found a more than threefold increase in mercury dimethyl vapor decomposed between 50° and 300°C. These results point to the better propagation of chains with increase in temperature. In the mercury-photosensitized hydrogenation processes the quantita- tive extinction of mercury fluorescence by hydrogen is well known, and hence the quantum yield of the total process is dependent on the efficiency of the secondary processes. In the hydrogenation of ethylene the evidence points to a 100 per cent utilization of the primary products in these second- ary processes. In the mercury-sensitized decomposition of ethane, Steacie
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. . 68 HUGH S. TAYLOR and Phillips found a quantum yield of approximately 0.2 in terms of ethane disappearing. This they ascribed, at least in part, to the in- efficiency of the primary process of quenching by ethane. With butane a higher quantum yield, 0.55, was obtained. THE SECONDARY PROCESSES The primary processes in the photolysis of methyl iodide, lead tetra- methyl, mercury dimethyl and, according to Steacie and Phillips, in the photosensitized decomposition of ethane, all lead to the production of methyl radicals. There are, however, conspicuous differences in the products finally obtained. In the case of methyl iodide, methane comprises 80 per cent of the prod- uct at room temperatures with from 4 to 12 per cent each of ethylene and ethane, for reactions in quartz vessels, packed or unpacked. With silver foil present, the methane yield fell to 28 per cent, while the ethylene and ethane yields rose to 18 and 54 per cent, respectively, ethane becoming in this case the major product. As much as 36 per cent of the methyl iodide disappearing is recoverable as CH2I2. Ethane predominates almost to the exclusion of other products in the photolyses of lead tetramethyl and mercury dimethyl. Linnett and Thompson found from 7 to 10 per cent of methane and O to 5 per cent of ethylene with about 90 per cent of ethane. They do not record any variation in product with temperature change. Cunningham found ethane with negligible amounts of methane and eth- ylene from room temperatures to 160°C. Beyond this temperature the methane yield increased to about 20 per cent of the hydrocarbon product at 300°C. In the photolysis of acetone similar results obtain. At 70° and 160°C. the hydrocarbon is more than 90 per cent ethane; at 300°C. equal volumes of ethane and methane are formed. In the Steacie-Phillips experiments with excited mercury any methyl radicals are formed in the presence of excess ethane. Hence no conclusion as to the amount of ethane which is regenerated can be given. Of the other hydrocarbons, nearly 60 per cent is methane, 23 per cent propane, and nearly 20 per cent butane. The butane undoubtedly arises from recombination of two ethyl radicals, since this gas is the predominant product in the mercury-photo- sensitized hydrogenation of ethylene (9~. The low quantum yield and the abnormal products in the methyl iodide decomposition are accounted for by West with the following sequence of reactions: CH3I + he = CH3 + I CH3 + I = CH3I CH3 + CH3I = CH4 + CH2I (1) (2) (3)
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FREE RADICALS IN PHOTOPROCESSES CH2I + I = CH2I2 CH2I + CH3 = C2H~I 69 (4) (4a) CH2I + CH3I = CH2I2 + CH3 (5) CH2I2 + he = CH2 + Is CH2 + CH2 = C2H4 CH2 + CH3I = C2H5I CH3 + CH3 = C2H6 I+I+X= I2+X (6) (7) (8) (9) (10) It is evident that the recombination reaction (reaction 2) must be ex- ceedingly efficient to account for the low quantum yield. All other proc- esses are minor in comparison with processes 3, 4, and 5, the most impor- tant to account for the products found. West estimates that an activa- tion energy of 10 kg-car. for reaction 3 in competition with reaction 2 would account for the observed quantum yield. In agreement with this estimate are some measurements of Ginsburg (6), indicating an increase of quantum yield with temperature in the case of ethyl iodide photodecomposition. Reaction 2 must also be rapid in comparison with reaction 9 which forms ethane. A steric factor of the order of 10-4 or 10-5 has been ascribed to the association of two methyls by Bawn (3~. When, however, the iodine atoms are trapped by silver foil, reaction 9 appears to compete favorably with the methane-producing reaction (reaction 4~. A sequence of reactions to account for the photolyses of mercury and lead methyls is suggested by the following scheme of Thompson and Linnett: Hg(CH3~2 + hv = Hg(CH3) + CH3 Hg(CH3) + X = Hg + CH3 + X (2) Hg(CH3) + Hg(CH3~2 = Hg + Hg(CH3) + C2H6 CH3 + Hg(CH3~2 CH3 + Hg(CH3~2 CH3 + CH3 Hg(CH3) + C2H6 Hg + CH3 + C2H6 C2H6 Hg(CH3) + Hg(CH3) = 2Hg + C2H6 In these cases the recombination process (3) (4) (5) (6) (7) HgCH3 + CH3 = Hg(CH3~2 must be of lower probability than in the case of the iodide in view of the quantum yield of ~1. That it occurs to some extent is known from
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70 HUGH S. TAYLOR work of Paneth and others on the removal of mirrors by methyl radicals. Taking this into account, the quantum yield of unity at room tempera- tures signifies that some chain propagation is already occurring at these temperatures. Reactions 3, 4, and 5 are chain-propagating stages. They must possess an activation energy, otherwise a quantum yield of ~ 106 might be expected. Thompson and Linnett calculate that an activation energy of ~11 kg-car. for the chain-propagating reactions is in accord with the quantum yield of unity at ordinary temperatures and of 2.2 at 200°C. The presence of only traces of methane in the room temperature product indicates that, owing to the weakness of the Hg C bond, this is broken in preference to the C H bond in the mercury alkyls struck by methyl radicals. Since about 20 per cent of the hydrocarbon is found as methane at 300°C. this suggests that the breaking of the C H bond must have an activation energy some 2 kg-car. higher than that required for the breaking of the Hg C bond. Molecular hydrogen does not readily react with methyl radicals until temperatures of 160°C. and upwards are reached (13, 18, 21~. The activa- tion energy of the process has a value of 9 at 2 kg-car. The interaction of atomic hydrogen with methane is to be assigned a somewhat higher value (16, 21) of 13 ~ 2 kg-car. and is insignificant below 250°C. Interaction with ethane is much more easily obtained, some interaction occurring at room temperatures. Trenner, Morikawa, and Taylor (21) ascribed this to a reaction H + C2H6 = CH4 + CH3 The argument developed by them to exclude a reaction sequence H + C2H6 = C2H5 + H2 C2H5 + H = 2CH3 (1) (2) (3) is not entirely compelling. It is well known that F. O. Rice's free-radical mechanism (15) is incompatible with the ready occurrence of the first of these interactions. The absence of methane, in the photosensitized hydrogenation of eth- ylene, until all the ethylene is hydrogenated, even though ethyl radicals and ethane are present, may be due to the low stationary state concentra- tion of atomic hydrogen in presence of ethylene. Careful test by Jungers and Taylor (9) failed to reveal any significant amounts of methane with reaction in vessels kept carefully free from hydrocarbon deposits of higher molecular weight. Under these conditions also, the predominant product is butane, obviously by combination of ethyl radicals. The low relative production of ethane or propane is additional evidence of low hydrogen atom and methyl radical concentrations in such systems. .
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FREE RADICALS IN PHOTOPROCESSES 71 In the photosensitized decomposition of ethane, Steacie and Phillips now favor the C—H bond split by a reaction Hg' + C2H6 = C2H5 + H + Hg They favor the reaction C2H6 + H = CH3 + CH4 (1) (2) to account for their observed methane formation. For the reasons just discussed one is forced also to consider the possibility C2H5 + H = 2CH3 (3) as the source of methyl radicals, subsequently converted to methane and propane by the recombination processes, CH3 + H = CH4 CH3 + C2H5 = C3Hs (4) (5) The recombination of ethyl radicals would produce the observed butane as in the photosensitized hydrogenation of ethylene. Unless one accepts reaction 3 as the mode of production of methyl radicals one is forced to conclude, as Steacie has called to the attention of the writer, that reaction 2 must be at least four times faster than H + C2H6 = C2Hs + H2 (6) Otherwise hydrogen would not be consumed in the photosensitized inter- action of hydrogen-ethane mixtures. The difficulties of the F. O. Rice chain mechanisms (15) would once more be acute. Rice and Teller, from a theoretical analysis, also strongly favor the mechanism. It is quite evident that there is need for further study in this field. Contrasting with the relatively large butane formation in the photo- sensitization experiments are the products from the photolysis of ethyl iodide vapor. Here, as West has shown, the products are predominantly ethane and ethylene, with no butane and with minor amounts of hydrogen and methane. As in the case of the methyl radicals from methyl iodide it is the secondary processes which must account for the non-formation of butane. The sequence of reactions suggested by West is: C2H5I + he = C2H5 + I (1) C2H5 + I = C2H5I (2) C2H5 + C2H5I = C2H6 + C2H4I (3) C2H4I + I = C2H4I2 (4) C~H4 I = C2H4 + I (4a)
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72 HUGH S. TAYLOR C2H4I + C2H5I = C2H4I2 + C2H5 (I) C2H4I2 + he = C2H4 + I2 I+I= I2 (6) (7) Of these reactions 3, 4, 4a, and 6 are the essential secondary reactions which account for the decomposition. The reactions must occur more readily than the methyl reactions, since even in presence of silver foil no butane was observed. The increased yield in the photolyses of the alkyl iodides in the liquid or dissolved state over that in the gaseous state is ascribed by West, at any rate in part, to the action of the solvent molecules in providing third bodies for the yield-increasing radical associations. This influence must be exercised preferentially on the atom reaction I + I which certainly requires a third body, whereas the radical recombinations and CH3 + I and C2H5 + I probably occur, at least in part, as association reactions. The interactions of the radical-iodine atom systems must be more efficient than the radical recombinations as the quantum yield is so low. West has shown that an inert gas such as carbon dioxide at a pressure of 10 to 40 atm. exercises an influence similar to solvent molecules in raising the quantum yield, a pressure of 40 atm. exercising an eEect approximately 13.5 per cent of that obtaining in hexane solution. In addition to the influence of solvent as third body in recombination processes there is a possible influence due to secondary reaction with the radicals. Recent work of Norrish and Bamford (14) has shown that radicals from the photolysis of ketones may remove hydrogen atoms from saturated hydrocarbon solvent molecules, becoming saturated thereby and producing, ultimately, unsaturation in the solvent. This possible effect has not yet been studied in alkyl iodide systems. REFERENCES (1) BATES: J. Am. Chem. Soc. 52, 3825 (1930~; 64, 569 (1932~. (2) BATES AND SPENCE: J. Am. Chem. Soc. 63, 1689 (1931~. (3) BAWN: Trans. Faraday Soc. 31, 1536 (1935~. (4) CIJNNINGEAM: Ph.D. Thesis, Princeton University. (5) DUNCAN AND MURRAY: J. Chem. Phys. 2, 640 (1934~. (6) GINSBURG: Thesis, New York University, 1934. (7) GOODEVE AND PORRET: Trans. Faraday Soc. 33, 690 (1937~. (8) IREDALE: J. Phys. Chem. 33, 690 (1929~. (9) JUNGERS AND TAYLOR: J. Chem. Phys. 6, 325 (1938~. (10) LEIGETON: J. Phys. Chem. 42, 749 (1938~. (11) LEIGE[TON AND MORTENSEN: J. Am. Chem. Soc. 68, 448 (1936). (12) LINNETT AND THOMPSON: J. Chem. Soc. 1934, 790; Proc. Roy. Soc. (London A160, 603 (1935~; 166, 108 (1936~; Trans. Faraday Soc. 33, 501, 874 (1937~. (13) MORIKAWA, BENEDICT, AND TAYLOR: J. Chem. Phys. 6, 212 (1937~.
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4~: FREE RADICALS IN PHOTOPROCESSES 73 (14) NOURISH AND BAMFORD: Trans. Faraday Soc. 33, 1521 (1937~. (15) Rice, F. O., AND Rice, K. K.: The Aliphatic Free Radicals, p. 191. The Johns Hopkins Press, Baltimore (1935~. (16) STEACIE: Can. J. Research 16, 264 (1937~. (17) STEACIE AND PHILLIPS: J. Chem. Phys. 6, 179 (1938~. (18) TAYLOR AND ROSENBLDM: J. Chem. Phys. 6, 119 (1938~. (19) TERENIN: J. Chem. Phys. 2, 441 (1934~. (20) TERENIN AND PRILESEAJEVA: Trans. Faraday Soc. 31, 1483 (1935). (21) TRENNER, MORIKAWA, AND TAYLOR: J. Chem. Phys. 6, 203 (1937~. (22) WEST: J. Am. Chem. Soc. 67, 1931 (1935~. (23) WEST: J. Am. Chem. Soc. 60, 961 (1938~. (24) WEST AND GINSBURG: J. Am. Chem. Soc. 66, 2626 (1934~. WEST AND PACL: Trans. Faraday Soc. 28, 688 (1932~. Addendum The previous discussion might also have included the recent work from the New York University laboratories on the photolysis of azomethane as the source of free methyl radicals (Burton, Davis, and H. A. Taylor: J. Am. Chem. Soc. 59, 1038, 1989 (19379; Davis, Jahn, and Burton: J. Am. Chem. Soc. 60, 10 (1938~. The quantum yield of this photolysis has been care- fully studied by Forbes, Heidt, and Sickman (J. Am. Chem. Soc. 57, 1935 (1935~) with six monochromatic radiations at four pressures from 180 to 665 mm. The quantum yield approached unity as its upper limit for initial decomposition and a temperature increase from 20° to 226°C. had no effect on the quantum yield. The latter fell with increasing pressure. These results led the authors to the conclusion that the photolysis was not a chain reaction. Burton, Davis, and Taylor have made a careful analytical study of products of the photolysis in the temperature range—22.5° to 223°C. In every case there is an excess of nitrogen formed, about 55 per cent at room temperatures rising to a maximum of 69 per cent at about 220°C. Meth- ane in the hydrocarbon product is about 7 per cent by volume at room temperature and increases to 70 per cent by volume at 220°C. Ethane, which represents more than 90 per cent at room temperature, decreases to 15 per cent at 220°C. In the higher temperature range propane and possibly butane, in small amounts, are increasingly produced. Hydrogen and unsaturated hydrocarbons are not formed in measurable amounts. There is no doubt that the majority of these results are consistent with a primary act producing free methyl radicals with minor, if any, intramolec- ular rearrangement to form stable molecules. The change in the char- acter of the hydrocarbon products with temperature is consistent with the data on the reactions already discussed. In the case of the azomethane photolysis, however, there is quite evidently a more marked interaction between the free radicals and the azomethane, presumably to yield hydra-
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mar 74 HUGH S. TAYLOR zincs, than is the case in several of the other substances described. The overall reaction becomes correspondingly more complex, the authors being of the opinion that the ethane formed results, not from the recom- bination of the methyl radicals, but frown decomposition of the more complex radicals and molecules. In view of the preceding discussion this conclusion should be accepted with great reserve.
Representative terms from entire chapter: