Free Radical Mechanisms in the Mercury Photosensitized Reaction of Hydrogen With Acetylene

Abstract

The Hg(JPI) photosensitized reaction of hydrogen with acetylene has been investigated with particular emphasis being placed on the identification of products and their dependence on hydrogen pressure. The results have been interpreted in tertns of a number of elementary reactions in which vinyl and ethyl radicals play an important part. INTRODUCTIONAlthough the reaction of H atoms with acetylene has been the subject of a number of investigations, no colnpletely adequate mechanism has been proposed to explain the results obtained under different conditions. With t h e high H atom concentrations used in the discharge tube method acetylene appeared to act simply as a catalyst for atom recombination (3, 27,8).However, Geib and Steacie (9) found that when D atoms were used the acetylene was rapidly exchanged. They suggested that the catalytic effect could be explained in terms of one of the follo~ving mechanisms:[4]The catalytic effect was confirmed by Tollefson and Le Roy (24) and by Dingle and Le Roy (6). These workers used the hot filament method for producing H atoms and total pressures of the order of 5 mm. The latter, using an improved technique, found the steric factor and activation energy of the rate controlling step, [I] or [3], to be 4X10a4 and 1.5 ltcal. per mole, respective1 y.Le Roy and Steacie (18) used the method of mercury pl~otosensitization t o produce H atoms in the presence of acetylene and found that appreciable quantities of ethane, ethylene, and butane were formed, as well as the polymer which had been observed by others.The wide difference in the nature of the results obtained by the discharge tube and hot filament methods on the one hand and photosellsitization on the other undoubtedly arises because of the very great differences in the relative concentrations of H atoms, H Z molecules, and perhaps certain radicals. Any reasonable mechanism for the fornlation of the gaseous products of the photosensitization experiments would seem to require the existence of vinyl radicals, regardless of whether the primary reaction is [I] or [3].In comparison with the saturated alkyl radicals, very little is known about the reactions of free vinyl radicals; the present reinvestigation of the mercury 'Manarscript received M a y 17, 1954.

The Reactions of Acetylene Photosensitized by Cd(3P1)

“…The products of the cadmium-photosensitization are different from those of the mercuryphotosensitization, [1][2][3][4][5][6][7][8] the direct photolysis,11) and the radiolysis.12)…”

The Mercury-Photosensitized Decomposition of Acetylene in the Presence of Nitric Oxide

“…The mercury-6 (3 )-photosensitized polymerization of acetylene has been the subject of a number of studies since the early work of Bates and Taylor.1 The main product is an involatile solid polymer which accounts for about 80% of the acetylene consumed. 2 The major volatile product is benzene although smaller amounts of hydrogen, butane, 1,3-butadiene, and ethylene are also formed. There has been little agreement on the mechanism of the reaction.…”

COMBINATION AND DISPROPORTIONATION OF ETHYL RADICALS: INFLUENCE OF THE REACTION H+C2H5 = C2H6

“…In their recent study of the mercury photosensitized hydrogenation of acetylene Cashion and Le Roy (4) found evidence for reactions [3] and [4], although the ratio (C~HG)/(C~HIO) varied from 0.77 to 1.25.…”

“…The followiiig local rate equations are obtained for the mechanism consisting of reactions [I], [2], [3], and [4]:…”