Kinetics and mechanism of catalytic acetylene hydrochlorination with gaseous HCl on the surface of mechanically activated K<Subscript>2</Subscript>PdCl<Subscript>4</Subscript>

The catalyst for acetylene hydrochlorination with gaseous HCl at room temperature is prepared by mechanical pretreatment of K₂PdCl₄ in an acetylene atmosphere. The rate-determining step of the reaction is the chloropalladation of π-coordinated acetylene involving an HCl molecule. As a consequence, the replacement of HCl with DCl brings about a kinetic isotope effect of 2.8 ± 0.4, which differs substantially from that observed in the protodemetalation of the intermediate palladium(II) chlorovinyl derivative yielding vinyl chloride (6.8 ± 0.6).     

An ab initio quantum chemical study of reaction mechanisms in the C2H2/CH3OH/KOH/DMSO system

An ab initio quantum chemical study (MP2/6-311++G**//B3LYP/6-31+G*) of a number of possible interactions is performed for the gas phase system of acetylene—potassium hydroxide-dimethylsulfoxide(DMSO)—methanol and with regard to the solvent effect within the continuum model. Key structures in the vinylation reaction are shown to be methoxide ion complexes with the alkali metal hydroxide and acetylene molecules. The formation of these complexes results in the activation of the acetylene molecule and an increase in the nucleophilicity of the methoxide ion. In the C₂H₂/CH₃OH/KOH/DMSO reaction system, a proton exchange between the acetylene molecule and the anionic nucleophile ([OH]^- and [CH₃O]^-) is freely performed with the formation of systems with ethynideions, whereas the thermodynamically preferable formation of vinyl alcohol or methyl vinyl ether is determined by a barrier of 20 kcal/mol.     

Theoretical analysis of the conversion mechanism of acetylene to ethylidyne on Pt(111)

The conversion of acetylene to ethylidyne on Pt(111) has been comprehensively investigated using self-consistent periodic density functional theory. Geometries and energies for all of the intermediates involved as well as the conversion mechanism were analyzed. On Pt(111), the carbon atoms in the majority of stable C(2)H(x) (x = 1-4) intermediates prefer saturated sp(3) configurations with the missing H atoms substituted by the adjacent metal atoms. The most favorable conversion pathway for acetylene to ethylidyne is via a three-step reaction mechanism, acetylene → vinyl → vinylidene → ethylidyne. The first step, acetylene → vinyl, depends on the availability of surface H atoms: without preadsorbed H the reaction occurs via the initial disproportionation of acetylene, which resulted in adsorbed vinyl; with an abundance of preadsorbed H, acetylene could transform to vinyl via both the disproportionation and hydrogenation reactions. Conversions through initial dehydrogenation of acetylene and isomerizations of acetylene and vinyl are unfavorable due to high energy barriers along the relevant pathways. The conversion rate involving vinylidene as an intermediate is at least 100 times larger than that involving ethylidene.     

Quantum-chemical investigation of the mechanisms of nucleophilic addition reactions to acetylene

The interactions between an acetylene molecule and lithium hydrosulfide in the dissociated and undissociated states have been calculated in the framework of the semiempirical MNDO method. It has been established that acetylene can easily form molecular complexes with lithium hydrosulfide, which are stable toward all the possible types of nonradical dissociation. A comparison with the previously considered reaction between C₂H₂ and LiOH shows that in the case of LiSH, the activation barriers to the isomerization of the complex C₂H₂·LiSH to vinyl mercaptans are considerably lower.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1792–1796, August, 1989.     

Identifying crosslinks in polyethylene following gamma-irradiation in acetylene: A model compound study

Long-chain linear alkanes have been used as model compounds for polyethylene in an attempt to identify the chemical nature of crosslinks formed in polyethylene when it undergoes γ-irradiation in the presence of acetylene. IR and UV spectral analysis of alkanes and polyethylene following acetylene-sensitized irradiation shows the formation of vinyl, trans-vinylene, and diene groups. A correlation of the conditions of formation suggests that in polyethylene the vinyl groups are restricted to amorphous regions, diene groups are restricted to the crystalline regions, and trans-vinylene groups are formed in both regions. There is no information on the nature of crosslinks. ¹³C-NMR analysis of alkanes following irradiation of molten alkanes in the presence of ¹³C-enriched acetylene has shown that a range of saturated alphatic structures are formed by inclusion of acetylene molecules in the alkane structure. They include ethyl branches, γ-branches, CH(CH₃) , and  CH₂ CH₂ branches as the major species; the latter two are potential crosslink sites in the irradiation of polyethylene. In addition, the NMR analysis confirmed that the C atoms of the vinyl groups come from acetylene molecules and those of the trans-vinylene groups come from alkane molecules. Data on irradiation of the alkanes in the crystalline state showed that acetylene inclusion in the alkane structure is minimal under these conditions. The principal finding of this work is that acetylene can be incorporated as saturated aliphatic crosslinks in the amorphous regions of polyethylene during high-energy irradiation. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1549–1561, 1997     

Kinetics and thermochemistry of the reaction of acetylene and nitric oxide

The shock tube data of Ogura [5] on the pyrolysis of C₂H₂/NO mixtures (1100–1500 K) is shown to be consistent with a simple mechanism whereby radicals are both initiated and terminated by NO (Scheme I). The scheme accounts for the rate of formation of the main product, vinylacetylene (VA), the lesser products CO and HCN and a very minor product, propionitrile. It is also shown to be consistent with other studies below 900 K and observation at 300 K on the reactions of vinyl radicals with NO. The substantial inhibition of vinyl acetylene formation by 5% NO makes untenable any substantial role of vinylidene in the C₂H₂ pyrolysis above 1000°K. The reaction of NO with acetylene is an efficient source of HCN. It appears to be a general reaction of NO with substituted acetylenes and below 900 K a mechanism is presented to account for the production of acrylonitrile (AN) from the reaction of NO with VA. Thermochemical data are estimated on Δ_fH°₂₉₈ and S°₂₉₈ for some alkyl-NO, vinyl NO, and acetylene NO compounds and radicals and some new and some revised group values are estimated for estimating Δ_fH°₂₉₈ of derivatives of hydroxyl amines, imines, and isoxazolines. © 1994 John Wiley & Sons, Inc.     

Acetylene Pyrolysis in Tubular Reactor

Pyrolysis of acetylene was investigated in a tubular reactor of graphite with an internal lining of alumina. The temperature range was 850–1650 °C, and the pressure was about 0.133 bar (100 Torr). Pure acetylene and acetylene diluted with argon or hydrogen were used as feed. Carbon and hydrogen are the main products from acetylene pyrolysis particularly at higher conversion. At lower conversion of acetylene, other gas products were formed; the amount of these depended on temperature, dilution, and conversion. Benzene and vinyl acetylene are the main gas products from pyrolysis of pure acetylene below 1000 °C and at low conversion. Diacetylene increases with increasing temperature. Dilution with hydrogen changes the composition of the gas product, decreases the selectivity of vinyl acetylene and benzene, and increases the formation of methane and ethylene. Gas‐phase equilibrium may be approached between some components. The conversion of acetylene with argon dilution and low conversion was found to be of second order. Pyrolysis of pure acetylene at lower temperature and low conversion gave the rate constant k = 3.1 × 10⁹ · exp(?34.8/RT) L mol^?1 s^?1 with an activation energy of 34.8 kcal mol^?1. The initial reaction at 864 °C is a molecular formation of vinyl acetylene. The initial activation of acetylene in gas phase seems to be rate determining and of second order in acetylene. Decomposition of acetylene can take place both homogeneously and heterogeneously. Above a critical partial pressure of acetylene, the decomposition is apparently explosive with instant plugging of the reactor with carbon.     

The reaction of F atoms with C2H4. Vibrational Spectrum of the C2H4F Intermediate trapped in solid argon

When the products of the reaction between F atoms produced in a microwave discharge and C₂H₄ are frozen in a large excess of argon at 14 K, new infrared absorptions appear which can be assigned to the 2-fluoroethyl radical. Studies of the dependence of the product distribution on the F-atom concentration have confirmed that the stabilization of C₂H₄F₂ plays only a minor role under the sampling conditions typical of these experiments. Isotopic substitution experiments have demonstrated that the steric configuration about the CH bond is randomized as a result of the F-atom reaction. Upon irradiation of the sample with the full light of a medium-pressure mercury arc, absorptions of vinyl fluoride and acetylene and of the acetylene—HF complex grow in intensity, while those of FCD₂CH₂ and of FCH₂CD₂ diminish in intensity and those of FCH₂CH₂ a nd of FCH₂CD₂ are unchanged. The F-atom reactions and photolysis processes which occur in these experiments are discussed, and a tunnelling mechanism is proposed to explain the isotopic selectivity in the 2-fluoroethyl photodecomposition. The vibrational spectrum of FCH₂CH₂ is compared with that derived in a recent ab initio calculation.     

Cyclotrimerization of acetylene by the tris(acetylacetonato)titanium(III)–diethylaluminum chloride system

Reaction of acetylene with tris(acetylacetonato)titanium(III) and diethylaluminum chloride system leads to formation of benzene, a trace of ethylbenzene, and a small amount of polyacetylene. The isotopic composition of products obtained from cyclotrimerization of acetylene-d₂ and an equimolar mixture of acetylene and acetylene-d₂ is investigated to elucidate the mechanism of the cyclotrimerization. The results suggest a mechanism in which an acetylene inserts into the metal—ethyl bond formed by reaction of Ti(acac)₃ and Al(C₂H₅)₂Cl, followed by insertion of two acetylene molecules and elimination of a hydrogen atom from the first inserted acetylene to yield an ethylbenzene and a metal hydride intermediate. The metal hydride intermediate catalyzes acetylene cyclotrimerization to give benzene. During the reaction, the hydrogen atom in the metal hydride intermediate does not exchange with the hydrogen atom in the inserted acetylene molecules.     

On the mechanism of the OH initiated oxidation of acetylene in the presence of O2 and NO x

The mechanism of the oxidation of acetylene, in the presence of O₂ and NO _x , has been studied. Different levels of theory have been tested for the first step of the mechanism: the acetylene + OH radical reaction. Based on these results the meta-hybrid functional MPWB1K has been chosen for modeling all the other steps involved in the oxidation of acetylene. Different reaction paths have been considered and the one leading to glyoxal formation and OH regeneration is predicted to be the main channel, independently of the presence of NO _x . Two different mechanisms were modeled to account for formic acid formation, both of them involving cyclic intermediates. According to the computed activation free energies, the three-membered intermediate seems to be more likely to occur than the four-membered one. However, reaction barriers are very high and only a very small proportion of formic acid is expected to be formed through such intermediates. In the presence of NO _x , considered in this work for the first time, the main product of the tropospheric oxidation of acetylene is also expected to be glyoxal.     

Molecular orbital study of the selective hydrogenation of acetylene in aqueous metalcatalyzed tetrahydroborate systems

A computer program for geometry optimization based on a multivariate regression method is discussed. The configuration of the adsorbed species of acetylene on copper, silver and gold catalysts were obtained at the usual CNDO level. The adsorbed species of acetylene on copper and gold catalysts are M(ν² - C₂H₂) complexes, whereas that on silver is of vinyl form. The electronic-charge distribution, energy partitions and total Mulliken overlap population suggested that acetylene is effectively activated in these systems.     

Mechanism of the catalytic action of the mechanoactivated salt K2PTCL4 in the gas-phase hydrochlorination of acetylene

A heterogeneous catalyst for the hydrochlorination of acetylene by gaseous HCl is formed as a result of mechanical treatment of the solid salt K₂PtCl₄ in an atmosphere of acetylene, ethylene, or propylene by the formation of π complexes of platinum(II) as active centers in the surface layer under these conditions. The controlling stage of the catalytic reaction is chloroplatination of the π-coordinated acetylene by the HCl molecule. The reaction takes place as a concerted process, in which an intermediate β-chlorovinyl derivative of platinum(II), a complex of platinum with a coordination vacancy[PtCl ₃ ^* ]⁻, and a new molecule of HCl are formed simultaneously with cleavage of the H—Cl and Pt—Cl bonds in the metal complex adjacent to the π-acetylene complex. The catalytic cycle closes with rapid dissociation of the organoplatinum intermediate by the action of HCl, giving the final product and the initial complex [PtCl₄]²⁻. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 5, pp. 306–311, September–October, 2006.     

Nucleophilic addition to acetylenes in superbasic catalytic systems. 12. Vinylation of lupinine

The addition of a natural alkaloid lupinine to acetylene in the presence of superbasic catalytic systems (KOH—DMSO, KOBu^t—DMSO, KOH—dioxane) under elevated or atmospheric pressure affords O-vinyllupinine (in up to 88% yield), a promising optically active monomer and intermediate for the preparation of new quinolizidine alkaloids. The same vinyl ether was obtained (in 60% yield) by the reaction of lupinine with vinyl acetate in the presence of Hg(OAc)₂.     

Activity and selectivity of Ni nanoclusters in the selective hydrogenation of acetylene: A computational investigation

In this work, the Ni_n (n = 2–10) nanoclusters were investigated to design new catalysts for the selective hydrogenation of acetylene. Our results show that among the Ni_n nanoclusters, the Ni₆ nanocluster can be used as a catalyst in the reactions of hydrogenation. In the presence of the Ni₆ nanocluster, the E_a of the forward step in the reaction of conversion of vinyl to ethylene was 21.21 kJ/mol lower than that of the reverse step in the reaction of conversion of acetylene to vinyl. Also, the E_a of the forward step in the reaction of conversion of ethyl to ethane was 96.59 kJ/mol higher than that of the reverse step in the reaction of conversion of ethylene to ethyl. According to the obtained results, the Ni₆ nanocluster can selectively act in the hydrogenation of a mixture of acetylene and ethylene.     

Change of the reaction pattern by methodological variations in a multicomponent assembly promoted by Ni complexes

The pi-allylnickel complex formed by the addition of trimethylsilyl chloride (TMSCl) to a mixture of [Ni-(cod)2] (cod = 1,5-cyclooctadiene) and a vinyl ketone (Mackenzie complex) carbometalates an acetylene in a completely regioselective manner resulting in the formation of the corresponding vinyl nickel species. This intermediate is capable of controlled quenching in a variety of ways to give different types of compounds: under a CO atmosphere, an acylnickel species is formed that ensues from the carbometalation of the enol ether double bond to form cyclo-pentenone derivatives. Alternatively, if acetylene is present in excess and CO is absent, another acetylene moiety will replace the CO and cyclohexadienes will result instead. Finally, if only an excess of the vinyl ketone is used, the product from a slow double addition of the vinyl ketone across the triple bond is formed. The regioselectivities obtained by the present method are different from those obtained by the involvement of nickel acyclopentadienes as intermediates when the order of addition is reversed.     

Synthesis of 3-benzoyl-3,4-dihydro-2H- and 3,4-dihydro-2,2-disubstituted-benzopyrans via alkynyl grignard reagents

The versatility of the new route to a substituted chromane via a lithiated allene recently described by us [1] is reported. The relatively more stable alkynols 2–4 were readily identified and thus provide evidence for the formation of vinyl acetylene carbinol as an intermediate in the new route. Accordingly, phenylacetylene magnesium bromide [2] reacted with suitable aldehyde or ketone to give the alkynols 2–4 which condensed further with the same or different aldehyde or ketone to give 3-benzoyl heteroring-substituted chromanes 5–17.     

Correlating electronic properties of bimetallic surfaces with reaction pathways of C2 hydrocarbons

The rate and selectivity of chemical reactions on transition-metal surfaces can be controlled by using different bimetallic combinations. The interaction of bimetallic components leads to a change in the electronic properties of the surface, which in turn produces a change in chemical reactivity. In the current paper, we illustrate the correlation of the electronic properties of bimetallic surfaces with the reaction pathways of C2 hydrocarbons. Density functional theory (DFT) was used to study the binding of hydrogen, ethylene, acetylene, ethyl, and vinyl on monometallic and bimetallic transition-metal surfaces. The binding energies of these species were found to correlate with the d-band centers of these surfaces. The binding energies for hydrogen atoms on bimetallic surfaces were lower than for those on the corresponding parent metal surfaces. This trend was consistent for ethylene and acetylene binding. Comparative studies between acetylene and ethylene revealed that acetylene was more strongly bonded to the monometallic and the bimetallic surfaces than was ethylene. Bond order conservation (BOC) theory was used to calculate the activation barriers for ethyl dehydrogenation to ethylene and vinyl dehydrogenation to acetylene. The activation barriers for these reactions were correlated with the surface d-band center of the substrates.     

Effect of Calcination Temperature on La-Modified Al2O3 Catalysts for Vapor Phase Hydrofluorination of Acetylene to Vinyl Fluoride

A La-modified Al₂O₃ catalyst was prepared with deposition-precipitation method. The effect of calcination temperature on the reactivity for vapor phase hydrofluorination of acetylene to vinyl fluoride. The catalysts calcined at different temperatures were characterized using NH₃-TPD, pyridine-FTIR, X-ray diffraction, and Raman techniques. It was found that the calcination process could not only change the structure of these catalysts but also modify the amount of surface acidity on the catalysts. The catalyst calcined at 400 oC exhibited the highest conversion of acetylene (94.6%) and highest selectivity to vinyl fluoride (83.4%) and lower coke deposition selectivity (0.72%). The highest activity was related to the largest amount of surface acidity on the catalyst, and the coke deposition was also related to the total amount of surface acidic sites.     

A paradigm for the mechanisms and products of spontaneous polymerizations

In spontaneous vinyl and ring‐opening copolymerizations, polar and resonance effects on the intermediates from bond‐forming initiation offer a continuous spectrum of reactivities and polymer structures. In bond‐forming initiation, an electron‐rich donor monomer forms a bond to an acceptor monomer. The donor monomer may be a vinyl monomer with O, N, or aryl substituent or it may be an aza‐ or oxacycle. The acceptor monomer may be a vinyl monomer carrying CN, COOR, or SO₂R substituent or it may be a cyclic anhydride or maleimide. Beyond this, the donor may have a π‐like strained single bond, whereas the acceptor may be an electrophilic quinodimethane. Lewis acids may be used to enhance the electrophilicity of acceptor monomers. Reaction rates and polymer composition are determined by systematically varying the stability of the first intermediate, designated P (for polymethylene). The nature of the intermediate will vary from a highly reactive trans biradical, which initiates chain alternating copolymerization, to a cis/gauche zwitterion, which can initiate chain ionic homopolymerization, to an extremely stabilized zwitterion, which cannot add monomer, but builds up in concentration and terminates by combination, forming alternating copolymer. This model embraces the existing literature for a wide variety of monomers and possesses predictive power. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2009     

The Role of Halogens in the Plasma Polymerization of Hydrocarbons

Vinyl chloride, vinyl fluoride, and tetrafluoroethylene were polymerized in a radio frequency electric glow discharge. It was found that when compared with the unhalogenated simple hydrocarbons, the rates of polymer deposition are in the order vinyl chloride, acetylene, tetrafluoroethylene, vinyl fluoride, ethylene. This observation can be rationalized by considering the ease with which free radical and unsaturated species can be formed in the plasma. IR spectra show that the structures of plasma-polymerized vinyl chloride and vinyl fluoride are in many respects similar to the plasma-polymerized hydrocarbon. The spectrum of plasma-polymerized tetrafluoroethylene, however, does not resemble that of conventional polytetrafluoroethylene. Addition of dichlorodifluoromethane to the monomer stream dramatically increased the polymer deposition rate; the effect is more subdued for chloromethane and is negligible for tetrafluoromethane. Elemental analysis indicates that little of the added halogens is present in the resultant polymers. Thus the halogenated compounds appear to act as a gas phase catalyst for the plasma polymerization of hydrocarbons.