Reactions of Propyne and Propadiene on Magnesium Films Part 1 .-Self- hydrogenation BY YVONNE GAULT* Institut de Chimie, UniversitC Louis Pasteur, 67000 Strasbourg, France Received 27th June, 1977 The self-hydrogenation of propyne and propadiene to propene on evaporated magnesium films has been studied at 373 and 423 K. The species retained in the solid state in the course of the reaction were then desorbed by deuterium oxide and characterized as deuterated hydrocarbons. The structures of these hydrocarbons (mainly propyne and propene) and the variations in their deuterium distributions with temperature and contact time of the previous self-hydrogenation reaction are consistent with a mechanism involving two parallel processes : (a) dehydrogenation of propyne and propadiene into metalated propyne CH3-C=C-Mg- (deuterolysed to ['HJpropyne), stable at 373 K but rapidly further dehydrogenated at 423 K to the magnesium carbide Mg2C3 (deuterolysed to [2H41propyne) ; (b) two-step hydrogenation of the reagent to propene, via half-hydrogenated intermediates, stable at 373 K and deuterolysed to [2Hl]propene.Little is known about interactions of hydrocarbons with non-transition metals in heterogeneous conditions. However, reactions of ethylene, acetylene and propyne at the surface of liquid alkali metals ' 9 and self-hydrogenation of alkynes and dienes on magnesium and other metal films have been reported, suggesting that non- transition metals are able to react at least with some hydrocarbons. The present paper describes more detailed investigations of the mechanisms of self-hydrogenation of propyne and propadiene on magnesium films.EXPERIMENTAL MATERIAL A N D PRODUCTS Magnesium from Johnson-Matthey of 99.997 % metallic purity was used. Propyne and propadiene (Matheson) were purified by preparative g.1.c. Tetradeuteropropyne (isotopic purity 98.7 %) was provided by Mr. A. Janin (University of Caen). Mono- deuteropropyne CH3-bCD containing 1 0 % C3H4 was prepared by exchanging the acidic H of [2Ho]propyne with NaOD at room temperature ; no isomerization was detected under these conditions. Deuterium oxide and perdeuteropropene (isotopic purities 99.7 and 99 % respectively) were obtained from Merck, Sharpe and Dohme. Light propene was purified by g.1.c. APPARATUS A N D PROCEDURE A grease-free static apparatus was used, attaining vacua of N m-2 provided by a mercury diffusion pump.Magnesium films of 35 to 70 mg and %YO0 cmz geometrical area were evaporated at 273 K from Mg chips held in a tungsten spiral on the inner walls of a cylindrical reaction vessel ( z 300 cm3). The surface area of magnesium films, as determined by krypton physisorption measurements at 77 K following the procedure of Kaganer? was found to be roughly the same as the geometrical area ( ~ 5 . 1 0 ~ ~ atoms cm-2). 1 Torr (133.3 N m-2, lOI9 molecules) of the reacting hydrocarbon was usually introduced on the freshly evaporated film at the reaction temperature (373 or 423 K). Connection of the reaction vessel, through an adjustable leak, to a quadrupole mass spectrometer allowing fast 2678Y .GAULT 2679 repeated scanning, was used for continuous analysis of the gaseous reaction mixture (C3H4, C3H6 and Hz). In the deuterolysis experiments, the reaction was stopped at the desired stage by rapid cooling to room temperature and the gaseous products removed for further analysis. After evacuation of the reaction vessel during 30-60 min at room temperature, 0.2 cm3 of outgased deuterium oxide was added to the reacted film at 77 K and allowed to react for 120-150 min at 300 K. Deuterium formed from unreacted magnesium was then eliminated and the hydrocarbon mixture removed for gas-chromatographic analysis and separation, and mass-spectrometric analysis. ANALYSIS (1) For continuous analysis of the gaseous reaction mixture (quadrupole mass- spectrometer, ionization energy 70 eV), the parent peaks at m/e 40 and 42 were used.The ions at mass m/e = 40 were then corrected for the contribution of propene, and the number of ions at m/e = 42 multiplied by the appropriate sensitivity factor, determined before each experiment. (2) The gas-chromatographic analysis of the reaction products was effected on a dimethyl- sulpholane column. The deuterated hydrocarbons arising from deuterolysis were isolated on the same column. Propene and propane were collected together on DMS and further separated on a silica column. The chromatographic purity of the various molecules was checked before and after mass-spectrometric analysis. (3) The deuterated molecules were analysed with a Varian-Matt CH7 mass spectrometer operating under high resolution conditions (m/Am = 3500).Positive ion mass spectra (parent peaks) were obtained using an ionization energy of 70 eV. The deuterium distribu- tion of each deuteromolecule was obtained after corrections of the mass spectra for the natural abundance of I3C, and fragmentation corrections made on a statistical basis using the fragmentation patterns of the corresponding light and heavy molecules (determined before each analysis). Deutero-cis-butenes distributions were calculated according to the fragmentation patterns established by Touroude and Gault .5 RESULTS GASEOUS REACTION PRODUCTS Reactions of propyne or propadiene on magnesium films result in the formation of various gaseous products, which are : (i) at any temperature (273-423 K), the isomer of the reacting hydrocarbon and propene ; (ii) at 373-423 K, propane and c6 hydrocarbons.At 423 K, small amounts of hydrogen were also detected. (a) ISOMER OF THE REACTING HYDROCARBON Propyne-propadiene interconversion is fast in the range of temperature investigated (273-423 K). The equilibrium compositions are usually reached in <20 min at any temperature. They include 90 % propyne at 300K, 87 % at 373 K, 85 % at 423 K, according to the results of Cordes and Giinzler.6 (b) PROPENE The rate of propene formation at 273 or 300 K is too slow and irreproducible to allow accurate studies ; therefore the present study was carried out systematically at 373 and 423 K. Fig. 1 shows the variations with time of the gas phase concentrations of propyne and propene. Whereas propyne exhibits a sharp initial decrease, propene appears in the gas phase after an induction period of a few minutes. This figure does not reflect the stoichiometry of the over-all reaction, since the C6 hydrocarbons were not 1-852680 PROPYNE AND PROPADIENE REACTIONS ON Mg taken into account, but suggests that under the conditions used propyne is preferen- tially adsorbed.Increasing the pressure of propyne has no marked influence, while the rates of propyne disappearance and of propene evolution are film weight- and t emperat ure-dependent . 15 W z 2 2 $10 a w .3 I 5 - 4 - 3 - 80 60 40 20 20 40 60 80 - - time/min FIG. 1.-Production of propene from propyne. The composition of the gas phase is expressed as mol percent of propyne and propene. The initial pressure of propyne is 133.3 N m-z ( X l O I 9 molecules). A Propyne, A propene (reaction temperature 373 K, film weight 38 mg) ; 0 propyne, 0- propene (reaction temperature 423 K, film weight 38 mg) ; Cl propyne, propene (reaction temperature 423 K, film weight 66 mg).I 1 I 1 I I 20 40 60 80 100 timelmin F~~.&.-Variations with time of the ratio ANC~H~IANC~H~, where A N c ~ H ~ is the decrease in the number of molecules of gaseous C3H4, and a N ~ 2 ~ 1 - r ~ the related increase in the number of molecules of gaseous C3Hs. A Reaction temperature 373 K, film weight 38 mg ; 0 reaction temperature 423 K, film weight 35 mg ; 0 reaction temperature 423 K, film weight 59 mg ; 0 reaction tempera- ture 423 K, film weight 70 mg.Y , GAULT 2681 Similar curves were obtained for the reaction of propadiene.However, because of the very close fragmentation patterns of both C3H4 isomers, they do not give information about the propyne-propadiene composition as a function of time. Fig. 2 reports the variations with time of the ratio between the number of C3H4 molecules which have disappeared (ANc3H4) and the number of C3H6 molecules evolved in the gas phase (ANc3Ha). The shapes of the curves are temperature- dependent, whereas good reproducibility is observed at a given temperature whatever the weight of the film might be. After a sharp initial decrease, the curves reach more or less rapidly a stationary value, close to 3 at 423 K and 4 at 373 K; this means that after an induction period, 3 or 4 molecules of the reagent are consumed per molecule of propene evolved.(C) PROPANE Propane is not detected at low reaction temperatures, but accounts roughly for one-tenth of the propene at 373-423 K. It arises, presumably, from a weak self- hydrogenation of propene on magnesium, as shown by control experiments with propene. (d) c6 HYDROCARBONS These are not detected at low temperatures. They are branched hexens and hexadiens (mainly 2,3-dimethyl-l-butene and 2-methyl-l-pentene), amounting to 5-40 % of the propene, according to temperature and contact time. In addition, tiny amounts of other hydrocarbons (i.e. isobutene, isopentenes), assigned to the cracking of the C 6 hydrocarbons, were occasionally detected. (4) HYDROGEN Slight amounts of hydrogen were present in the reaction mixtures at 423 K.Since identical amounts of hydrogen are evolved from a fresh magnesium film heated at the same temperature, they are believed to arise from gases dissolved in the bulk metal and not completely evacuated during film evaporation. Indeed, only traces of D, and HD are obtained from tetradeuteropropyne at 423 K. Moreover, dissolved hydrogen does not participate in propene formation, since the reaction of a 20/1 mixture of deuterium and [2Ho]propyne gives pure [2Ho]propene. DEUTEROLYSIS EXPERIMENTS Addition of water or deuterium oxide on the film after reaction results in the desorption of a mixture of hydrocarbons. Chromatographic analysis of some hydro- or deutero-lysates are reported in table 1. It is seen that (i) very similar mixtures are formed from propyne and propadiene ; (ii) C3 hydrocarbons account for 75-90 % of the total products; (iii) depending on the temperature of the previous reaction, either propyne or propene is the mean product.Besides 2,3-dimethyl-l-butene (2,3DMlB), other c6 are present in trace amounts. Use of deuterium oxide results in formation of deuterated hydrocarbons. Deuterium distributions of propyne and propene are reported in tables 2 and 3. Depending on the conditions of the previous reaction, the distribution pattern of deuteropropynes has a maximum at C2Hl], a maximum at [,H4], or two maxima at [,HI] and [2H4], while the corresponding maxima in deuteropropenes are [2H,], [,€I6] or [2H1] and [,H6]. Propane, when present in sufficient amounts (previous reaction at 423 K) was collected and analysed: it was found in every case to be2682 PROPYNE AND PROPADIENE REACTIONS ON Mg mainly [2H,].No attempts were made to determine the deuterium contents of acetylene and propadiene on account of their low concentrations. 2,3 dimethyl-l- butene will be described in our next paper. TABLE 1 .-PRODUCT DISTRIBUTIONS IN HYDRO- AND DEUTERO-LYSATES OF REACTED MAGNESIUM FILMS (GAS-CHROMATOGRAPHIC DATA) expt. no. 439 497 460 396 506 reagent propyne propadiene propyne propadiene propyne previous 373K 373K 423 K 423 K 423 K reaction 1 90 min 90 min 90 min 30 min 180 rnin products % propane 3 3.7 14.6 17.1 17.3 propene 43.6 47.9 23.4 19.4 11.8 acetylene 0.9 0.8 1.7 1.9 4.8 propadiene 3.3 - 1.3 3.3 1.8 ProPYne 31.7 31.1 39.2 49.4 45.4 23DMlB 20.8 16.5 19.6 8.9 18.8 TABLE 2.-oBSERVED DISTRIBUTIONS OF DEUTEROPROPYNES IN DEUTEROLYSATES expt.no. 500 439 497. 460 396 429 506 reagent propync propyne propadiene propyne propadiene propadiene propyne reaction cond. 373 K 373K 373K 423 K 423.K 423 K 423 K 20min 90min 90min 20min 30mm 180 rnin 180 rnin % (a) (b) 31.7 31.1 39.2 49.4 (b) 45.4 [2H*I - 7.7 8.4 7.7 0.2 2.4 [2H11 58.9 54.6 43 45 38.5 12.8 15.3 IZH21 5.3 11.1 11.7 9.7 10.7 14.3 8.6 L2&1 5.8 11.8 4.5 4.1 22.9 29.8 13 w 4 1 30 14.9 32.3 33.4 27.6 40.7 63 a '' % " refers to the percent of total propyne among total deuterolysis products, as determined - by g.1.c. (cf. table 1) ; b not determined. TABLE 3 .-OBSERVED DISTRIBUTIONS OF DEUTEROPROPENES IN DEUTEROLYSATES expt. no. 500 503 439 497 460 506 reagent propyne propadiene propyne propadiene propyne propyne reaction cond.373 K 373.K 373 K 373 K 423 K 423 K 20 mm 20 mln 90min 90min 20min 180rmn % (4 (b) 33 43.6 47.9 23.4 11.8 12 11.7 19.3 - 12.9 - [2H11 58.2 51 47.4 51.4 48.2 13.4 [%I 7 17.3 10.8 22 6.3 15.2 [%I 9.3 5 9.5 5.7 4.4 4.6 rH41 4.2 1.1 2.1 4.9 1.8 3.6 12H51 2.3 1.2 5.7 3 2.7 13.4 [2H61 7 12.6 5.1 13.1 23.6 49.7 a " % " refers to the percentage of total propene among total deuterolysis products, as determined "Hol by g.1.c. (cf. table 1) ; b not determined. CONTROL EXPERIMENTS A number of control experiments were carried out in order to check the validity of the results reported in tables 1-3. (a) Hydrolysis of fresh Mg films yields traces of propyne and acetylene together with the corresponding olefins. These hydrocarbons undoubtedly arise from traces of Mg carbides contained in the metal.However, the amounts of hydrocarbonY . GAULT 2683 formed in this way are x500 to 1000 times smaller than the ones desorbed by hydrolysis of the reacted films, and therefore do not perturb the results. (b) In the absence of a Mg film, heavy propyne C3D4 exchanges one single D atom (presumably the acidic one) with H even at room temperature, most probably with traces of water remaining adsorbed on the glass, as is suggested by enhanced exchange in the presence of added water. Neither isomerization nor hydrogenation occurs in the absence of a Mg film in the range of temperature investigated (300-423 K). The following experiments [ (c)-(g)] were effected under the same experimental conditions (room temperature, 120-150 min) as in the deuterolysis of the reacted films : (c) In the presence of a Mg film previously isolated and heated for 1 h at 423 K, light propyne C3H4 exchanges one single H atom with deuterium oxide.Isomeriza- tion and deuteration into propene occur simultaneously. Propyne (68 % of the TABLE 4.-EXCHANGE OF LIGHT AND HEAVY PROPYNES AT 298 K ON MAGNESIUM FILM AFTER PREVIOUS REACTION OF LIGHT PROPYNE FOR 1 h AT 423 K initial composition final composition 46 14.5 - 23.4 - 18.2 [2H31 2.4 30.8 E"H41 51.6 13.1 ['Hal ['&I ['H21 TABLE 5.-EXCHANGE OF A MIXTURE OF LIGHT AND HEAVY PROPYNES AT 298 K WITH DEUTERIUM OXIDE ON MAGNESIUM FILM PREVIOUSLY HEATED FOR 1 h AT 423 K initial composition final composition 52.1 11.8 - 26.4 ['Hd ['I311 17.1 15.9 ['Hi] 45.6 28.5 reaction mixture) was recovered as 16 % [2H,] and 84 % [2Hl], while propene (27 % of the reaction mixture) was found to be mainly [2H2].The total absence of [2H3] propene suggests that deuteration of [2H,] propyne is faster than its exchange. (a) Under the same conditions, ['H0] propene remains unchanged and unex- changed. (e) On a Mg film previously reacted with propyne at 423 K (reacted Mg films are unsuitable for self-hydrogenation of fresh doses of reagent), a synthetic mixture of C3H4 and C3D4 interchanges all H and D (table 4), showing that the exchange is not limited to the H and D in acidic positions. (f) Under the same conditions, no exchange occurs between C3H6 and C3D6. (9) On a Mg film previously isolated and heated for 1 h at 423 K, a mixture of C3H4 and C3D4 was converted, in the presence of deuterium oxide, into a mixture of all deuteropropynes with 2 maxima at [l2H1] and [2H4] (table 5), as expected from superposition of (i) exchange of [2Ho] propyne with D20 to [2Hl], (ii) interchange of light species with [2H4].The resulting mixture reproduced approximately some of the propyne distributions reported in table 2. EXPERIMENTS USING LABELLED PROPYNES (a) Self-hydrogenation of monodeuteropropyne CH3-C=CD was investigated at 373 K (1 h) and at 423 K (3 h). The distributions of the alkynes before and after2684 PKOPYNE AND PROPADIENE REACTIONS ON Mg reaction and of the resulting propenes are given in table 6 . The residual gaseous reagents were recovered with some change in the H-D distributions, involving both a slight decrease in the total D/H ratio, and a slight scrambling of the H and D atoms.The resulting propenes were mainly [2H1], E2H2] and [2H3]. Species I2H0] was not detected, and species ['H4] and [2H5], which accounted together for < I % of the total propenes, are not reported in table 6. The observed distributions are compared with those calculated by assuming the random addition on C3H3D of D-H pools of 53-47 and 36-64 for the propenes formed at 373 and 423 K, respectively. TABLE 6.-sELF-HYDROGENATION OF CH3-CsCD (a) self-hydrogenation at 373 K (1 h) propyne propyne propene propene (miQa1) (final) (obs) (calc) 9 20.8 - - 91 73.5 19.1 22.1 - 4.7 55.6 49.8 - 1 25.3 28.1 - - 0.29 0.27 0.52 0.52 (6) self-hydrogenation at 423 K (3 h) propyne propyne propene propene (mtial) (final) (obs) (calc) C2H0] 10.5 31.1 - - ['Hi] 89.5 60.7 39.9 41.1 [2H21 - 6 48.5 46 [%I - 2.1 11.6 12.9 ['&I - I - - - - [%I L2H61 - - D/H 0.31 0.24 0.40 0.40 (b) Reaction of a 1 : 1 mixture of heavy propyne C3D4 and light 2-butyne C4H6 on a magnesium film for 45 min at 423 K was followed by isolation and mass- spectrometrical analysis of the residual 2-butyne and of the cis-2-butene (75 % of the total butenes) arising from the latter.Deuterium distributions of both C4 hydro- carbons are reported in table 7. More than 95 % of the 2-butyne was recovered as [2Ho], whereas cis-2-butene exhibited a much higher deuterium content, the main species being E2H0], C2HJ and ['H2] in increasing amounts. TABLE 7.-oBSERVED DISTRIBUTIONS OF RESIDUAL 2-BUTYNE AND Cis-2 BUTENE AFTER REACTION OF A MIXTURE OF f2&]PROPYNE AND ['H&2*BUTYNE ON A MAGNESIUM FILM (45 min AT 423 K) t2Hol I*Hd 12H21 PH31 PH4J PHs1 [*H6] PH71 I2HsI - - 2-butyne 95.5 3.3 0.9 0.3 cisZbutene 6.9 33.6 55.7 2 0.8 0.5 0.3 - - DISCUSSION As shown in fig.1, propene accounts for only a minor part of the reacted propyne or propadiene. Since the C6 hydrocarbons account at most for 40 % of the propeneY . GAULT 2685 (after long reactions at 423 K), an appreciable fraction of the reagent therefore remains on the magnesium fdm, probably as species including C-Mg bonds. As shown in table 1, these species are recovered after deuterolysis mainly as C3 hydrocarbons. Their structures and the variations in their deuterium distributions with temperature and contact time of the previous reaction (tables 2 and 3) support the general reaction scheme suggested below (scheme 1) in which part of the reagent is dehydrogenated and another part hydrogenated to propene.The dehydrogenation and hydrogenation processes will be discussed in turn. The additional hydrodimeriza- tion reaction, giving mainly branched hexenes, will be developed further in a future paper. + I + 1': H CH3 I C 111 CH2=CH-CH2 + CH2=C--CH3 c + H I I I I * * * I I * 1 1 Mg2C3 + 3H * (SCHEME 1) DEHYDROGENATION As shown in table 2, propyne in the deuterolysates consists mainly of species C2H1] and E2H4], in various ratios, depending on the conditions of the previous reaction. Since species [2H2] and [2H3] may result from exchange between lighter and heavier species, as shown by control experiments, they will not be further discussed.[2H4] propyne, which predominates after long contact times at 423 K, may be assigned with high probability to the magnesium carbide Mg,C3 , whose hydrolysis is known to supply mainly propyne with small amounts of propadiene.8-f0 The magnesium carbide Mg2C3 is the expected product of complete dehydrogenation of propyne or propadiene. Parallel formation of small amounts of the other magnesium carbide MgC2 is suggested by the presence of acetylene in the hydro- or deutero- 1 ysates. O Partial deuteration of [2H,]propyne into [2H6]propene and [2H,]propane in the course of deuterolysis is suggested by the observed correlation between the deuterium contents in the three C3 hydrocarbons : high contents in all three perdeuteromolecules were obtained after long reactions at 423 K, while the yields of [2H4]pr~pyne, f2H,]propene and total propane were low after the reactions at 373 K.2686 PROPYNE A N D PROPADIENE REACTIONS ON Mg Analogous formation of the alkali metal carbides Na,C, or K2Cz has been suggested by Pulham et al." to account for the stoichiometry of the reactions of acetylene and ethylene respectively on sodium and potassium surfaces, whereas reaction of propyne at 383 K on sodium (which does not form a carbide similar to Mg,C,) was assumed to yield metalated propyne CH,-C-CNa as sole solid product, as in the corresponding reaction in liquid ammonia.There is no report of isolation of magnesium salts of 1-alkynes. However [2H,]propyne, which predominates after all reactions at 373 K and after short reactions at 423 K, could be assigned to the deuterolysis product of dissociatively adsorbed or metalated propyne CH3-C-C-Mg--.Such a species would account for the very close compositions of the hydro- or deutero-lysates obtained from propyne and propadiene. One may assume indeed that dehydrogenation of propadiene on magnesium involves its conversion into metalated propyne, in the same way as allenic hydrocarbons transpose into the corresponding metalated 1 -alkynes in the course of their reactions with sodium or sodium amide,ll* l 2 or at the surface of non-transition metal oxides.' The formation of this dissociatively adsorbed propyne, stable at 373 K, could be, at 423 K, the first step in the rapid dehydrogenation process whose the final step is the carbide Mg,C,.However, this interpretation is weakened by the easy exchange with deuterium oxide of the acidic H atom of propyne : one could object that [2H,]propyne in the deuterolysates may arise from the exchange of [2H,]propyne. Self-hydrogenation of CH3-C=CD was investigated in order to obtain evidence for the occurrence of a metalation process. This molecule indeed is the best model to distinguish between acidic and methylic hydrogen atoms, and if, under some specific conditions, metalation does occur, not followed by carbide formation, one should observe then the exclusive participation of the acidic hydrogen atoms in the self-hydrogenation process. Although C3H3D was partly exchanged in the course of the reactions, as shown by the distributions of the recovered propynes, the distributions of the propenes obtained at 373 and 423 K are best interpreted by assuming the statistical additions on C3H3D of D-H pools corresponding to 53 and 36 % of D respectively (table 6).Were the hydrogenation process to involve exclusively the acidic D atom of C3H3D, only [2M3]propene should be observed. On the other hand, the participation, resulting from carbide formation, of all the H and D atoms would yield a randomized distribution of propene corresponding to the addition of a 25-75 D-H pool. The observed results correspond therefore to intermediate but unequal situations : at 373 K the acidic D atom is the major, but not the only, source of hydrogenating atoms ; at 423 K the hydrogen source is much more indiscriminate.These labelling experiments provide therefore a good support for a progressive dehydrogenation process as represented in scheme 1. One could discuss at this stage whether the formation of carbide and metalated species is a surface reaction or involves the bulk metal. Although the accurate material balance could not be established, one can estimate that at 423 K roughly 50 % of the reacting molecules are recovered as gaseous C3 and C6 hydrocarbons, and hence that an equivalent number (m 5 x 10' *) is retained as carbonaceous or hydrocarbonaceous residues. Since the number of superficial magnesium atoms, as determined by krypton physisorption measurements, is m these estimates suggest that = 50 magnesium layers are probably involved for metal- ation and carbide formation.Therefore the dehydrogenation reaction may be considered not as a surfaceY. GAULT 2687 reaction, but as a solid state reaction, and it is also tempting to consider the hydrogena- tion process as a true chemical reaction between magnesium hydride and propyne or propadiene. HYDROGENATION The hydrogen atoms released by the dehydrogenation process do not recombine and evolve as molecular hydrogen; therefore, they may be supposed to be temporarily retained as labile Mg-H species and to act for hydrogenation. The Mg-H bond of magnesium hydride has been shown to hydrogenate a variety of organic compounds, among which are unsaturated hydrocarbons,14* l5 so that the over- all self-hydrogenation process may be considered as the hydrogenation of a part of the reagent by magnesium hydride produced by the dehydrogenation of the other part.These H atoms can be transferred onto molecules different from the parent ones, as shown by the cross-reaction between [2H,]propyne and [2Ho]-2-butyne, in which the deuterium atoms arising from the first are transferred onto the second, which is directly converted into deutero-butenes without previous exchange (table 7). On magnesium, as on transition metals, the hydrogenation of alkynes and allenes to alkenes probably involves the stepwise addition of two H atoms with the inter- mediacy of half-hydrogenated states. * Two-step addition of H atoms onto propyne and propadiene to form propene is strongly suggested by the presence of [2H,]propene in the deuterolysates. This species, which cannot result from any exchange or deuteration, is best explained as resulting from the deuterolysis of half-hydrogenated states of propyne or propadiene, C3H5-Mg-, which may be considered also as insertion adducts of propyne or propadiene into Mg-H bonds.Whereas the half-hydrogenated states involved in catalytic hydrogenation on transition metals are highly unstable, a number of stable a-alkenyl metal complexes resulting from insertion of multiple C-C bonds into M-H bonds have been isolated. l Concerning the half-hydrogenated species (or insertion adducts) formed on magnesium films, their life-time appears to be long enough to allow their characteriza- tion by deuterolysis in the form of [2H,]propene.Moreover, results reported in table 3 suggest that at 373 K, in the course of the hydrogenation process, a large fraction of the reaction molecules remains frozen out on the magnesium film as half-hydrogenated species. That suggests that the addition of the second hydrogen atom required to form propene from the half- hydrogenated states occurs more slowly than the addition of the first one. The different rates of addition of the two hydrogen atoms could also explain the induction period required for the appearance of propene in the gas phase as shown in fig. 1 . GENERAL REACTION SCHEME The overall results related in this paper are consistent with the general scheme 1, where two additional assumptions are included : (a) existence of a direct hydrogena- tion pathway from propadiene to propene, (b) existence of two different half- hydrogenated species of propyne and propadiene, arising from the addition of one hydrogen atom either on the central or on the terminal carbon atom of the reacting hydrocarbons.Although no result in the present work argues for these assumptions, the first is suggested by the fact that self-hydrogenation of 2-butyne on magnesium films produces mainly cis-Zbutene, while 1-butyne gives 1-butene and 1,2-butadiene a mixture of2688 PROPYNE AND PROPADIENE REACTIONS ON Mg 1- and cis-2-butenes, showing that the structures of the olefins formed reflect the primitive structure of the reacting hydrocarbon rather than the equilibrium composi- tion of the C4H6 isomers.20 The second assumption, previously suggested by Yoshida and Hirota for propyne,17 is supported by the carbon skeletons of the main C6 hydrocarbons (2,3-dimethyl-l-butene and 2-methyl-l-pentene), for which the half-hydrogenated species of propyne and propadiene are the most consistent precursors.CONCLUSION The present work suggests that, under specific conditions, some reactions may occur on magnesium films which are not observed with magnesium when conventional procedures and solvents are used. Moreover, it shows that magnesium films, although not able to activate molecular hydrogen or deuterium, are able to use hydrogen or deuterium atoms produced in situ t o hydrogenate alkynes or dienes to alkenes. The observed reactions of propyne and propadiene on magnesium films may be paralleled with the following : (a) reduction of alkynes and allenes by alkali metals, which give the corresponding alkene and the metalated 1-alkyne ; (b) hydrogenation of unsaturated hydrocarbons on transition metal catalysts, which occur with the intermediacy of half-hydrogenated states ; (c) insertion of alkynes into metal- hydrogen bonds, in homogeneous catalysis, which give a-alkenyl adducts.We are indebted to Mr. A. Janin (University of Caen) for gift of a sample of tetradeuteropropyne, and to Mr. J. J. Ehrhardt (Centre de Cinetique Physique et Chimiqwe, C.N.R.S., Nancy), for help in krypton physisorption measurements. C. C. Addison, M. R. Hobdell and R. J. Pulham, J. Chem. SOC. A, 1971, 1704, 1708. G. Parry and R. J. Pulham, J.C.S. Dalton, 1975, 2576. Y . Gault, J.C.S. Chem. Comm., 1973, 478. M. G. Kaganer, Zhur. jiz. Khim., 1959, 33,2202. R. Touroude and IF. G. Gault, J. Catalysis, 1974, 32, 294. J. F. Cordes and H. Gunzler, Chem. Ber., 1959, 92, 1055. M. Hock, Bull. SOC. chim. France, 1963, 1422. R. C. Lord and P. Venkateswarlu, J. Phys. Chem., 1952, 20, 1237. J. F. Cordes and K. Wintersberger, 2. Naturforsch. 6, 1957, 12, 136. lo W. H. C. Rueggeberg, J. Amer. Chem. SOC., 1943, 65, 602. l1 (a) A. Faworskii, J. Prukt. Chem., 1888, 37,417 ; (6) A. Behal, Bull. SOC. Chim. France, 1888, l2 (a) M. Bourguel, Ann. Chim., 1925, HI, 205, 345 ; (6) M. Bouis, Ann. Chim., 1928, Tx, 459. l3 (a) J. Saussey, J . Lamotte, J. C. Lavalley and N. Sheppard, J. Chim. phys., 1975, 71, 818; l4 W. K. Henle and E. J. Smutny, U.S. Patent 3, 666, 416 (30/5/72); Chem. Abs., 1972, 77, 50, 629. (b) J. Saussey, J. C . Lavalley and N. Sheppard, J. Chim. phys., 1977, 74, 329. 64156~. L. H. Slaugh, J. Org. Clem., 1967,32, 108. l6 E. F. Meyer and R. L. Burwell, J. Amer. Chem. SOC., 1963,85,2881. l7 N. Yoshida and K. Hirota, Bull. Chem. SOC. Japan, 1975, 48, 184. (a) J. Grant, R. B. Moyes, R. G. Oliver and P. B. Wells, J. Catalysis, 1976, 42,213 ; (6) R. G. Oliver and P. B. Wells, J. Catalysis, 1977, 47, 364. l9 See for example : (a) K. Ziegler, Angew. Chem., 1956, 68, 721 ; (b) H. C. Brown, Organic Synthesis via Boranes (Wiley, N.Y., 1975), p. 10 ; (c) J. Trocha-Grimshaw and H. B. Henbest, Chem. Comm., 1968, 757; (d) J. A. Labinger and J. Schwartz, J. Amer. Chem. Soc., 1975, 97,1596 ; (e) D. W. Hart, T. F. Blackburn and J. Schwartz, J. Amer. Chem. Soc., 1975,97,79. 2o Y. Gault, unpublished results. (PAPER 7/1120)