Geometrical Isomerization of Penta- 1,3-Diene Catalyzed byCobaltBY P. B. WELLS AND G. R. WILSONDept. of Chemistry, The University, HullReceived 3 1 st January, 1966Cobalt powder and alumina-supported cobalt catalyze the reaction :cis-penta-1 , 3-diene +trans-penta-l,3-diene.Molecular hydrogen is not required as a reactant. Isomerization takes place at temperatures above75"C, no other isomers of pentadiene have been detected, and self-hydrogenation is negligible.The isomerization of isotopically labelled cis-penta-l,3-diene at 160°C is described in detail. Theresults are consistent with a mechanism in which the first step is the dissociation of the dioleh byloss of hydrogen from the methyl group, and the overall process is the 1 : 5-transfer of hydrogen.When unsaturated hydrocarbons react with hydrogen on transition metal catalystsa large number of adsorbed states may participate in the mechanism.1 Economyof hypotheses requires the restriction of the proposed adsorbed states to the minimumnumber necessary for an adequate interpretation of the results, but by so restricting thediscussion an accurate account of the mechanism may not be given.This problemhas arisen in the isomerization reactions (i.e., cis-trans isomerization and double-bondmigration) that aliphatic olefins undergo at group 8 metal surfaces.In the past, mechanism (1) has been generally accepted.2-7 Here, olefin adsorptionis assumed to be associative, and hydrogen atom addition to adsorbed olefin precedeshydrogen atom abstraction from adsorbed alkyl.Consequently, this is termed theaddition-abstraction mechanism.R (3-43 R CH, H\ / -H \ // I \CH3\ ' +H -- H - C - C - H C - C' ' H " 6 7\HI H /" T c \ H -H tads9rbed alkyl[R .olkyl](As written here, the olefin is adsorbed as a metal-olefin complex1 ; but, for thispurpose, the essential features of the mechanism are not altered if adsorbed olefin iswritten as a species adsorbed to two metal atoms by two a-bonds.) The addition-abstraction mechanism interpreted the observation that isomerization occurred onlyin the context of hydrogenation, i.e. the presence of molecular hydrogen was consideredto be necessary.3 Tracer studies showed that adsorbed butyl was an intermediatein the formation of butane, and consequently there was good reason to associate23238 PENTADIENE ISOMERIZATJONisomerization with the decomposition of the butyl group.Other workers haveproposed concerted " hydrogen switch " mechanisms in which dissociative adsorptionof olefin and hydrogen atom addition occurred simultaneously.~9 9In 1962, it was reported that the dissociation of adsorbed olefin to give adsorbedn-allylic intermediates occurred readily as one of a series of consecutive dissociativesteps in the exchange of substituted cyclopentanes with deuterium catalyzed by pallad-ium films.10 It was suggested that analogous processes might occur in other reactions,and butene isomerization was quoted as an example. Mechanism (2) shows how iso-merization may proceed by an " abstraction-addition " mechanism, the abstractionstep being the dissociative adsorption of olefin to give adsorbed hydrogen and anadsorbed .n-allylic species.19 11R/ +ti-HrcT. ,C' ' H [R =alkyl]Butene isomerization and hydrogenation using deuterium as a tracer has beenstudied with each of the six noble group 8 metals as catalyst 6912 ; butene isomerization,butene hydrogenation, and the manner in which deuterium appears in both buteneand butane, was studied simultaneously. The conclusion was : " there is rarely anydefinite evidence for n-allylic intermediates : this is not to say that they do not exist,but rather that the observations may be rationally interpreted without them.',lHowever, the addition-abstraction mechanism is sufficiently versatile to account forthe observations, and so the relevance of the abstraction-addition mechanism cannotbe ascertained from this type of experiment.Clearly, if the dissociative adsorptionof olefin occurs, and if its consequences are to be detected, a new approach to theproblem must be made.An important prediction of the abstraction-addition mechanism is that isomeriza-tion should occur in the absence of molecular hydrogen if the rate of isomerizationis fast compared to the rate of removal of adsorbed hydrogen atoms (e.g., by com-bination). In 1964, we reported that butene isomerization did indeed take place inthe absence of molecular hydrogen at the surfaces of cobalt wire and cobalt-alumina,and circumstantial evidence supported the abstraction-addition mechanism.13Unfortunately, this observation does not provide proof of the mechanism, becausedissociative adsorption of olefin may be taking place on part of the surface to give anunreactive residue, and the hydrogen so liberated may then initiate isomerizationon the remainder of the surface by the addition-abstraction mechanism.We decided that studies of the cobalt-catalyzed isomerization of some isotopicallylabelled hydrocarbons in the absence of molecular hydrogen might enable a choice tobe made between the possible mechanisms.The reaction chosen for study was thecis-trans isomerization of penta-l,3-diene, using CH3--CD=CD--CD=CHD as thelabelled compound. This paper records our first experimentsP. B. WELLS AND G. R. WILSON 239EXPERIMENTACATALYSTSCobalt powder was prepared by reduction of grey cobalt ox de (B.D.H.) at 415°C in astream of hydrogen for 24 h.The powder weighed 0.07 g, its surface area measured by theargon adsorption method was 4.8 m2 g-1, and it consisted of a mixture of the a- andp-phasesof cobalt.The support used in the preparation of cobalt-alumina catalyst was Peter Spence type Aalumina. According to the manufacturers, this nlaterial consists of very small crystallites,less than 50 A in size, the surface area is 275 m2g-1, and the only distinguishable form ofalumina present is boehmite. They give the formula A1203. H20. The general methodof preparation of cobalt-alumina has been described elsewhere.14 The two samples used inthis work are designated Co-A1 and Co-A2; they each weighed 0.51 g and contained10 % by weight of cobalt.The reduction of the oxide to the metal was carried out at 414°C ;partial dehydration of the alumina occurred during the reduction. Catalysts prepared inthis way contained adsorbed hydrogen that was not removable by pumping the catalyst.This hydrogen exclianged rapidly with molecular deuterium to give IFD and €32 in the gasphase at temperatures above 100°C, and 1021 hydrogen atoms per g catalyst were exchange-able at 130°C. We conclude, from the magnitude of this quantity,15 that this exchangeablehydrogen was associated with the support. This adsorbed hydrogen was completelyexchanged for deuterium before the catalysts were used for the isomerization reactionsdescribed below.APPARATUSThe catalyst rested on the bottom of a 100 ml Pyrex reaction vessel which was connectedto a conventional high-vacuum system.Pressures were measured using a mercury mano-meter. Hydrocarbons extracted from the vessel were analyzed by gas-liquid chromato-graphy : the 10 ft column contained a saturated solution of silver nitrate in benzyl cyanidesupported on crushed firebrick. Reaction mixtures were separated into pure cis- and trans-pentadiene fractions using preparative gas-liquid chromatography, the stationary phase beingas cited above. The pure fractions were condensed from the carrier gas stream into U-tubesimmersed in liquid air.Mass-, infra-red and proton magnetic resonance spectra of pure samples of cis- and trans-pentadiene were obtained using (i) a modified A.E.I.M.S.3 mass spectrometer (gaseoussamples) ; (ii) a Unicam SP.100 infra-red spectrometer (gaseous samples), and (iii) a Perkin-Elnier 4.0 megacycle n.m.r. spectrometer and a Varian 100 megacycle spectrometer (liquidsamples in carbon tetrachloride solution).INTERPRETATION OF SPECTRAThe composition of a mixture of deuterated pentadienes was calculated from the massspectrum on the basis of the usual assumptions (i) that C-H and C-D bonds had equalchances of breaking in the fragmentation process, and (ii) that all hydrogen and deuteriumatoms in the molecule were equivalent. These assumptions are not likely to introducesubstantial errors because the fragmentation of C5H8 at the standard electron beam energyof 12.0 eV was slight (CsHZ = 100, C5H; = 15, C S H ~ = 1, C S H ~ and lighter fragmentswere not observed).However, the inadequacy of the assumptions was evident in onerespect. For the analysis of samples of pentadiene containing about 4 deuterium atoms,errors in the fragmentation correction accumulated at mass 68, so that pentadiene-do appearedto be 2-5 % of the mixture, whereas, from the general form of the distributions, it was expectedto contribute less than 1 %. Consequently, we put the contribution from pentadiene-doequal to zero and normalized the remainder of the distribution to sum to 100. For thisreason, these product compositions do not contain values for pentadiene-do.The infra-red spectrum of " light " trans-pentadiene (fig. la) contains a group of fiveabsorption bands; a very weak band at 2740 cm-1, and four strong bands at 2880, 2930,3010 and 3075 cm-1.The first three bands have been assigned to C-€3 stretching vibration240 PENTADIENE ISOMERIZATIONof the methyl group, whereas the other two bands are attributed to stretching vibrations ofbonds in which hydrogen atoms are bonded to olefinic carbon atoms. Four absorptionbands for C-D stretching vibrations corresponding to the four strong bands quoted abovewere observed at 2080,2125,2240 and 2325 cm-1 (see fig. l c and Id).I " ' " ' ' '3200 2800 2400 2000frequency (cm-1)FIG. 1 .-Infra-red spectra of pentadiene : (a)" light " trans-pentadiene, pressure in gas cellP = 13 mm; (b) labelled cis-pentadiene, P =41 rnm ; (c) and (d) labelled trans-pentadiene pro-duced by the 60 and 30 % isoinerization of thecis-isomer respectively, P = 15 mm and 7 mm.!L ! 3 4 5 b 7 8 97FIG.2.-Proton magnetic resonance spectra ofpentadiene : (a) " light " trans-pentadiene ;(b) " light " cis-pentadiene ; (c) labelled cis-pentadiene ; ( d ) labelled trans-pentadieneproduced by the 60 % isomerization of thecis-isomer. Spectra (a) and (b) were obtainedusing a 40 megacycle spectrometer, and (c)and (d) using a 100 megacycle spectrometer.Proton magnetic resonance spectra of " light " trans- and cis-pentadiene are shown infig. 2a and 2b respectively; the assignment given in fig. 2a is equally relevant to the otherspectra in this figure. All spectra were integrated by the usual procedures.The mean distribution of hydrogen in a given sample of deuterated pentadiene wascalculated from a knowledge of (i) the mean hydrogen content of the sample, obtained bymass spectrometry, and (ii) the integrals obtained from the p.m.r.spectra. Infra-red spectraprovide qualitative confirmation of the conclusions so obtained.MATERIALSPenta-l,3-diene as supplied by Koch-Light Laboratories Limited contained approximately65 % of the trans-isomer, 25 % of the cis-isomer and 10 % of an unidentified isomer. Purefractions of cis- and trans-pentadiene were obtained by preparative gas-liquid chromato-graphyP. B. WELLS AND G. R. WILSON 241Deuterium (98.7 atom % D) was prepared by the electrolysis of deuterium oxide and waspurified by diffusion through a heated palladium thimble.Isotopically labelled cis-pentadiene was prepared as follows.A solution of penta-l,3-diyne in ether was prepared by the method of Armitage, Jones and Whiting,l6 and the di-acetylene was extracted by preparative scale gas-liquid chromatography (using a 2 ft columnof hexane-2,Sdione supported on firebrick). Gas-phase deuteration of the diacetylenewas carried out in a static system using palladium supported on a-alumina as catalyst attemperatures between 20 and 80°C. Reactions did not proceed in well-defined stages, andproducts representing all possible stages of reduction were obtained simultaneously. Purecis-pentadiene was extracted from the mixture by preparative scale gas-liquid chromatography.The isotopic composition of the product was as follows :dl d2 d3 d4 d5 d6 d7 dg D.N.0.2 6.9 27.8 46.8 16.5 1.2 0-4 0.2 3.79Its p.m.r.spectrum is shown in fig. 2c and its infra-red spectrum in fig. lb. The deuteriumnumber D.N. represents the mean number of deuterium atoms present in the molecule. Themean formula of the material is given in table 1, where analysis I has been obtained fromone integration using the 100 megacycle spectrometer, and analysis I1 has been obtainedfrom a large number of integrations using the 40 megacycle spectrometer. The analysesagree moderately well and the almost complete absence of deuterium from the methyl groupis confirmed by the absence of absorption bands in the infra-red spectrum at 2080 and2125 cm-1 (see fig.lb).TABLE 1 .-ISOTOPIC COMPOSITION OF THE LABELLED CIS-PENTADIENE (REACTANT)[X = H or D]analysis I analysis I1total H content of 4 x 3 2.74 2.91 f0.03H content of each =CX- 0.21 0- 13 f0.02total H content of =CX2 0-84 0.91 fO.01Thus, the material that we obtained was not the specific substance CH3--CD=CD--CD=CHD, but we considered that our product was labelled sufficiently well for a meaningfulstudy of its isomerization to be undertaken.RESULTSThe interconversion of cis- and trans-pentadiene occurred at a measurable ratein the absence of molecular hydrogen when either isomer was contacted with thecatalysts under the conditions described. No isomers of C5Hg other than cis- andtrans-penta- 1,3-diene were detectable in the product.The cobalt powder rapidly catalyzed the isomerization of pentadiene at 160°Cand equilibrium proportions of the two isomers were obtained from 25 mrn of trans-pentadiene after a contact time of 10 min.Measurable initial rates of 2.8 % min-1were obtained at 116°C. The activity at 160°C was independent of the time for whichthe catalyst was pumped before use, for periods up to 3 h. 60 % of the activity(measured at 120°C) remained after the catalyst had been heated in va'cuo for 3 h at400°C: the loss of activity was probably associated with a decrease in the surfacearea of the powder.The identical alumina-supported catalysts Co-A1 and Co-A2 were active for theisomerization above 75°C. For example, 22.7 mm trans-pentadiene was convertedto an equilibrium mixture of cis and trans isomers over Co-Al in 160 min at 136°Cin a reaction that exhibited an initial rate of 3.7 % min-1. Reactions were stronglypoisoned by traces of oxygen in the reactant.Activity declined slowly in a serie242 PENTADIENE ISOMERIZATIONof reactions. The initial rate was approximately zero order in pentadiene pressure(temperature 142"C, pressure range 10-85 mm), and the activation energy for theconversion of the trans to the cis-isomer was 15 f 3 kcal mole-1 over the range 77-135°C. The initial rate of isomerization of the cis-isomer was about three times asrapid as that of the trans-isomer.The composition of the equilibrium mixture of cis- and trans-pentadiene wasmeasured at six temperatures in the range 133-230°C ; the concentration of the trans-isomer was 76-2 f 0.5 % at 133°C and the value decreased as expected,l7 to 73.0 k0.5 %at 230°C.When pentadiene remained in contact with Co-A1 at 160°C for several hourstrace quantities of pentenes were formed : their isomeric composition was not obtainedaccurately but trans-pent-2-ene was the major product.Preferential formation oftrans-pent-2-ene also occurs in the hydrogenation of cis- or trans-pentadiene over thesame catalysts at the same temperature.18Table 2 shows the extent to which hydrogen atoms in the hydrocarbon exchangewith deuterium which is associated with the support. Only a trace of deuteriumentered the initial products, but the deuterium content of the pentadiene increasedconsiderably as the hydrocarbons remained in contact with the catalysts.Clearly,transfer of adsorbed deuterium from the support to the metal was taking place, butthe rate of exchange of hydrogen for deuterium in the hydrocarbon was much slowerthan the rate of isomerization.TABLE THE EXCHANGE OF HYDROGEN ATOMS OF " LIGHT " PENTADIENE WITH DEUTERIUMATOMS ASSOCIATED WITH THE CATALYST SUPPORT. THE ANALYSES SHOW THE COMPOSITIONOF CIS-PENTADENE PRODUCED BY THE ISOMERIZATION OF 100 mm Hg SAMPLES OF TRANS-PENTADIENEcomposition isotopic composition of cis-pentadiene (%)temp. time of mixture("C) (min) (%cisisomer) do di dz 4 d4 ds D.N.135 1 2 99.5 0.5 0.0 0-0 0.0 0.0 0.005135 18 6 98-3 1.7 0.0 0.0 0.0 0.0 0.0171 60 61 25 84.8 13.9 1.1 0.2 0.0 0.0 0.167160 287 25 32.9 40.6 20.3 5.5 0.7 0.0 1.005160 1840 25 29.9 36.1 23.5 8.5 1-5 0.5 1.172Two series of reactions were carried out in which labelled cis-pentadiene wasisomerized over Co-A1 or Co-A2 at 160°C.In the first series, small pressures( N 15 mm) of reactant were employed, and the distribution of deuterium in the trans-pentadiene produced was almost independent of conversion until the reaction was 47 %towards equilibrium. The following initial distribution was obtained by extrapolationto zero conversion :dl d2 d3 d4 d5 d6 d7 ds D.N.0.4 5.8 24.8 43.5 21.2 3.7 0-5 0.1 3-93The exchange reaction with deuterium from the support was more noticeable in thisreaction, and in the reaction to be described below, than was expected from the resultsshown in table 2.In the second series of experiments, larger pressures (- 11 5 mm)of labelled cis-pentadiene were employed, and the results are shown in fig. 3. Theextrapolated initial distribution is similar to that shown above. Comparison of thep.m.r. spectra of the reactant (fig. 2c) and the product extracted after 60 % isomeriza-tion (fig. 2 4 shows that there has been a considerable redistribution of hydrogenbonded to the terminal carbon atoms. The mean formula of the product is giveP. B. WELLS AND G. R. WILSON 243in table 3 (as in table 1, analysis I has been obtained using the 100 megacycle spectro-meter, and analysis 11 has employed the 40 megacycle spectrometer). By 60 %isomerization, molecules of pentadiene must have adsorbed, isomerized and desorbedmany times.The net effect is observed when tables 1 and 3 are compared. The-t2 08 0u*- 10Y 82 .-500 10 2 0 3 0 4 0 5 0 6 0i% isomerizationFIG, 3.-The dependence of the isotopic composition of trans-pentadiene upon conversion a t 160°CInitial pressure of reactant (labelled cis-pentadiene) = 115 mm. Catalyst Co-A2 was used for thereaction taken to 60 % conversion, whereas the other reactions were catalyzed by Co-A1.hydrogen content of the methyl group has decreased considerably, whilst that of theterminal methylene group has increased. The hydrogen content of the internalmethine groups has increased only slightly, assuming that each of the three such groupsof the product have the same isotopic composition. The infra-red spectra shown inTABLE 3 .-bOTOPIC COMPOSITION OF LABELLED TRANS-PENTADIENE (PRODUCT)[X = HorD]total H content of -CX3 1-55 1-51 f0.02total H content of =CX2 1-11 1.27 f0.02H content of each =CX- 0.26 0.22 f0.02analysis I analysis I1fig. l c and Id confirm these results.Trans-pentadiene extracted after 30 and 60 %isomerization contained comparable amounts of deuterium in the methyl group, andfurther, deuterium and hydrogen are both bonded to olefinic carbon.DISCUSSIONCobalt powder and cobalt wire catalyze the cis-trans isomerization of butene andpenta-l,3-diene7 these reactions occuring in the absence of molecular hydrogen. Thi244 PENTADIENE ISOMERIZATIONis primd facie evidence either that the abstraction-addition mechanism is operative, orthat adsorbed hydrogen produced by the dissociative adsorption of pentadiene (togive an unreactive hydrocarbon residue) initiates reaction by the addition-abstractionmechanism. Initiation of the addition-abstraction mechanism by adsorbed hydrogenin equilibrium with absorbed hydrogen can be discounted because (i) the solubility ofhydrogen in cobalt is very l0~,19 and (ii) the cobalt wire was an active catalyst althoughit had never been treated with hydrogen.13Hydrogenation studies have shown that diolefins adsorb very strongly at nietalsurfaces,20 and the zero order of the pentadiene isoinerization, measured by the initialrate method, probably indicates that the surface was fully covered by adsorbedhydrocarbon over the range of pressures studied.A change in the distribution of deuterium took place when labelled cis-pentadienewas converted to trans-pentadiene over cobalt-alumina.The increase in the fractionof pentadiene containing six or more deuterium atoms is evidence that deuterium wasentering the methyl group ; this was confirmed by the infra-red spectra of the trans-pentadiene after the reaction had proceeded 30 and 60 % towards isomeric equili-brium (see fig. l c and Id). At the latter conversion, cis-pentadiene of general formulaA (below) had been converted to trans-pentadiene of general formula B.%,i: OO.79 H0-21 ' 0 - 7 9/An acceptable mechanism for the isomerization of labelled cis-pentadiene mustaccommodate the following observations (i) that hydrogen of the methyl group isexchanged for deuterium, (ii) that deuterium of the methylene group is exchanged forhydrogen, and (iii) that exchange of deuterium for hydrogen at the internal methinegroups is slight, amounting to not more than 0.09 hydrogen atoms per position,assuming that the isotopic composition at each methine group is the same in both thereactant and the product.In addition, molecules of pentadiene exchanged on average0.7 atoms of hydrogen for deuterium which was formerly associated with the support.Two mechanisms are discussed.ABSTRACTION-ADDITION MECHANISMThis mechanism may be written asX X x X(1)adsorbed cis-pen t adienP. B. WELLS AND G. R. WILSON 245x X\ 4C - c - 2a)adsorbed penta-1 ,Cdiene(v)adsorbed trans-pentadienecis-Pentadiene is assumed to interact associatively with the surface in the first instance.Structure (I) shows one of two conformations in which the diolefin may adsorb.Evidence that diolefins adsorb by the formation of two metal-olefin bonds has beendiscussed.1920 The first step in the isomerization involves the abstraction of a hydro-gen atom from the methyl group of (I) to give structure (11).Migration of adsorbedhydrogen atoms is expected to be rapid under these conditions, so the addition stepto form (111) may in principle involve either a hydrogen atom or a deuterium atom;since the relative chances of acquisition of H or D in this step are not known theadsorbed “ hydrogen ” atom has been denoted by the symbol Z. Species (111) isadsorbed penta- 1,4-diene.Assuming that (111) does not undergo conformationalchange, the next step must be the abstraction of Z which will regenerate (11) or give (IV).Addition of an adsorbed hydrogen or deuterium atom, as Z, to (IV) regenerates (III)or gives (V) which is adsorbed trans-pentadiene. Thus, the cis-trans isomerizationof pentadiene by this mechanism is effectively a 1 : 5-transfer of hydrogen. If thistransfer occurs a sufficient number of times, the hydrogen and deuterium atoms of theterminal methyl and methylene groups should equilibrate, and the ratio of the integralsfor the protons in these positions, measured from the p.m.r. spectra, should fall from3-2 in the reactant to about 1-5 in the product. Values of 1.4 and 1.2 were observedfor the trans-pentadiene (table 3) which suggest that this isotopic equilibrium wasachieved.According to the abstraction-addition mechanism the adsorbed “ hydrogen ”atoms Z are entirely H in the initial stages of reaction but, when the “ hydrogen ”atoms bonded to the terminal carbon atoms attain isotopic equilibrium, the chanceof 2 being H will be lower and its chance of being D will be appreciable.Further,when exchange of “ hydrogen” atoms between the metal and the support occurs,as it did in the present work, the composition of Z will be enriched in the isotope thatis associated with the support. IT, as suggested above, the “ hydrogen ” atoms ofthe terminal methyl and methylene groups in the product have attained isotopi246 PENTADIENE ISOMERIZATIONequilibrium, then they can be written as -CZ3 and =CZz respectively. Thus, thecomposition of Z after 60 % isomerization is readily determinable from table 3 as52-55 % H (from analysis I) or 50-63 % H (from analysis U).A feature of our abstraction-addition mechanism is that the isomerization processdoes not require the fission of any of the three C-X bonds, and consequently thedeuterium located at these positions is not expected to exchange with the adsorbed“ hydrogen ”.Experimentally, only slight exchange occurred, showing that otherprocesses which may give exchange at these positions (see below) are of minor im-portance. Our observations are consistent with this mechanism.ADDITION-ABSTRACTION MECHANISMIsomerization by this mechanism can be written as(1) (Wadsorbed cis-pentadieneil -H It0adsorbed trans-pentadiene adsorbed penta-l,+dieneThe addition of a hydrogen atom Z to (I) can occur in two ways to give species (VI)and (VII) and subsequent abstraction of a hydrogen atom X from (VI) or H from themethyl group of (VII) gives respectively adsorbed trans-pentadiene (VIII) and ad-sorbed penta- 1,4-diene (III).The formation of penta- 1,4-diene and its subsequentisomerization back to 1,3-diene provides the route whereby the exchange of hydrogenin the methyl group occurs according to this mechanism. The important feature ofthis mechanism is that all molecules of trans-pentadiene necessarily contain theinitiating entity Z bonded to the methyl-substituted carbon atom (see species (VIII)above).Information about the isotopic composition of Z can be obtained from anexamination of the behaviour of the terminal methylene group during isomerization.The processes to consider are+ z -Y- 2 I-xc YCY, - -xc --CYzt - -xc f C Y t * Y iIf, in the addition of Z, the chance of acquiring H is greater than the chance ofacquiring D then the hydrogen content of the terminal methylene group will increaseP. B. WELLS AND G. R. WILSON 247Experimentally, such an increase was observed, and in consequence, trans-pentadieneproduced by this mechanism must contain 2 as a hydrogen-rich entity. However, thereis no evidence for this from the p.m.r. spectrum of the trans-pentadiene. Formula €3is calculated on the assumption that the isotopic composition of each of the threeinternal methine groups is the same, However, even if the total increase in the H-content of the methine groups is attributed to hydrogen (H) bonded to the methyl-Me\/substituted carbon atom, the composition of this group would be C= whichHO.3 8DO. 62is deuterium-rich, and not hydrogen-rich as required by the addition-abstractionmechanism. This is an important failure of this mechanism.We consider finally the origin of the initiating species Z, with special reference tothe reactions catalyzed by cobalt powder. As stated above, initiation of the addition-abstraction mechanism on the powder apparently requires the dissociation of somepentadiene to give unreactive residues and adsorbed hydrogen atoms. We havefound no evidence for such residues.No induction periods or progressive poisoningduring isomerization were evident. Slow deactivation of the catalysts did occur withuse, which might have been due to the progressive formation of residues ; alternatively,it may have been caused by adventitious oxygen in the reactant.To sum up, we consider that the abstraction-addition mechanism interprets ourobservations successfully, whereas the addition-abstraction mechanism does not.FURTHER DISCUSSION OF THE ABSTRACTION-ADDITION MECHANISMThree further matters merit brief mention. First, a simplification to the abstrac-tion-addition mechanism may be valid which eliminates the necessity of postulatingpenta-1,4-diene as an intermediate. Species (II) and (IV) may be considered torepresent canonical forms of a resonance structure in which electron delocalizationextends over all five carbon atoms.Cis-trans isomerization would then be mostlikely to proceed by 1 : 5-transfer of hydrogen in one step, because addition of hydrogento the centre carbon atom would involve loss of resonance energy. However, notest of this can be made from the present work.Secondly, adsorbed penta- 174-diene, if formed, does not readily undergo thefollowing conformational interconversion, because the subsequent isomerization of(IX) to 1,3-diene would provide a molecule with deuterium exchanged for hydrogenat the centre carbon atom.Lastly, the successful geometrical isomerization of pentadiene by the abstraction-addition mechanism depends upon the conformational characteristics of the adsorbeddiene.Any 173-diene may adsorb in two conformationally distinguishable ways248 PENTADIENE ISOMERIZATIONand for each of the penta-1,3-diene isomers only one conformational situation willlead to isomerization.(4 Me no isomerization k- \Me(b) Meisomerizat ion(trans to cis)Meisomerization 0-L Me (cis to trans)-"pno isomerization (d)L=) - I-)From studies of penta-193-diene hydrogenation using this cobalt catalyst we haveconcluded that penta-193-diene prefers to adsorb as the transoid conformation, ratherthan as the cisoid conformation (probable preference -4 : l).lS If this is also trueduring pentadiene isomerization then, from the conformational standpoint, cis-pentadiene will isomerize more efficiently by the abstraction-addition mechanism thantrans-pentadiene.The authors acknowledge their gratitude to the Science Research Council for theaward of a maintenance grant (to G. R. W.) and for a grant for the purchase of themass spectrometer; to Dr. J. Feeney and Miss A. Heinrich of Varian AssociatesLimited, Walton-on-Thames, who obtained the p.m.r. (100 megacycle) spectra, andto Dr. D. E. Webster of this department for assistance in the interpretation of p.m.r.spectra.1 Bond and Wells, Adv. Catalysis, 1964, 15, 91.ZTwigg, Proc. Roy. SOC. A , 1941, 178, 106.3 Taylor, CataZysis (ed. Emmett, Reinhold, New York, 1957) 1957, 5, 296-379.4 Wagner, Wilson, Otvos and Stephenson, J. Chem. Physics, 1952, 20, 338, 1331.5 Hamilton and Burwell, Proc. 2nd Int. Congr. CataZysis, (Technip, Paris, 1961), 1, 1002.6 Bond, Phillipson, Wells and Winterbottom, Trans. Faraday SOC., 1964, 60, 1847.7 Bond, Catalysis by Metals, (Academic Press, London, 1962), pp. 252 ff.8 Taylor and Dibeler, J. Physic. Chem., 1951, 55, 1036.9 Turkevich and Smith, J. Chem. Physics, 1948, 16, 466.10 Gault, Rooney and Kernball, J. Catalysis, 1962, 1, 255.11 Rooney and Webb, J. Catalysis, 1964, 3, 488.12 Bond, Webb, Wells and Winterbottom, Trans. Faraday SOC., in press.13 Phillipson and Wells, Proc. Chem. Soc., 1964, 222.14 Joice, Rooney, Wells and Wilson, this discussion.15 Hall, Leftin, Cheselske and O'Reilly, J. Catalysis, 1963, 2, 506.16 Armitage, Jones and Whiting, J. Chem. SOC., 1952, 1993.17 Egger and Benson, J. Amer. Chem. SOC., 1965, 87, 3311.18 Wells and Wilson, unpublished work.19 Sieverts and Hagan, 2. physik. Chem. A, 1934, 169, 237.20 Bond, Webb, Wells and Winterbottom, J. Chem. Soc., 1965, 3218