Thermal reactions of ethylene

Figure 1. Selectivities of methane and propylene in thermal reaction of ethylene...

Thermal Reaction of Ethylene. Dahlgren and Douglas (5) reported that the primary products of the reaction were propylene, butenes, butadiene, and ethane at temperatures ranging from 480° to 582°C and at pressures from 9 to 137 mm Hg. However, analyses of reaction products were incomplete, and the primary products were not distinguished from the secondary ones. 

Table I. Addition of Butadiene in the Thermal Reaction of Ethylene at 7 53°C...

Selectivity of formation of methylcyclopentene decreases rapidly with conversion of propylene. In the thermal reaction of ethylene this compound was not identified. Formation of five-membered ring compounds—i.e., methylcyclopentene, cyclopentene, and cyclopentadiene— may be attributed to allyl type radicals (3,14). 

These products are consumed consecutively, probably to form benzene and polycyclic compounds. Toluene may also react consecutively to benzene. The ratio of toluene or xylenes to benzene was about twice that obtained in the thermal reaction of ethylene, respectively, at temperatures from 703° to 854°C and at conversions up to 40 mole %. The ratio of styrene to benzene was about one-third as large as that obtained in the thermal reaction of ethylene. Addition of butadiene in the thermal reaction of propylene increased the selectivity of cyclic compound formation, although the increase was smaller than in the case of ethylene. These facts support the mechanism for the formation of monocyclic aromatic compounds proposed by Wheeler and Wood (24) this is discussed in detail later. 

Formation of Cyclic Compounds When we noted that the addition of small amounts of butadiene increased the yield of cyclics formed in the thermal reaction of ethylene and propylene, an effort was made to relate directly the formation of cyclics in thermal reaction of ethylene and propylene, respectively, to the Diels-Alder reaction between feed olefins and product butadiene. Reactions between product olefins and product butadiene were neglected owing to their small concentrations. Cyclics were deferred as the sum of C rings with and without alkyl or vinyl groups. 

Figure 17. Rate of formation of C6 cyclic compounds in thermal reaction of ethylene at 753°C...

A similar mechanism has been proposed above for the formation of five-membered ring compounds in the thermal reaction of propylene. In fact, for propylene the ratios of the yields of toluene plus xylenes to benzene were about twice as large as those in thermal reaction of ethylene. 

For the pyrolysis of paraffinic hydrocarbons at 700- 800 C, yields of olefins such as ethylene, propylene, butenes, butadiene and cycloolefins increase during the initial stage of the reaction, pass through their maxima, and later decrease yields of aromatics, hydrogen and methane however increase monotonically throughout the reaction course. Sakai et al. (1 ) reported previously the result of a kinetic study on thermal reactions of ethylene, propylene, butenes, butadiene and these respective olefins with butadiene at the conditions similar to those of paraffin pyrolysis, directing their attention on the rates of formation of cyclic compounds. Kinetic features of the thermal reactions of these olefins are sunnnarized in Table I combined with the results obtained in later investigations for thermal reactions of cycloolefins ( 2) and benzene O). 

Thermal reactions of ethylene (A>A) require higher temperatures (750- 800 C) than the other olefins. Initial reaction products are butadiene, 1-butene, propylene, ethane and acetylene. 

C forming allyl radicals as the initial step of the reaction. At the same time, it was reported previously ( ) that a thermal reaction of ethylene itself scarcely proceeded below 700 C. Experiments on pyrolysis of diallyl in excess ethylene were conducted at temperatures between 580 and 700 C, and at very low concentrations of diallyl, between 0.2 and 0.7 mol %, so as to cause exclusively the expected reaction of allyl radical with ethylene. 

Alternatively, thermal cracking of acetals or metal-catalyzed transvinylation can be employed. Vinyl acetate or MVE can be employed for transvinylation and several references illustrate the preparation especially of higher vinyl ethers by such laboratory techniques. Special catalysts and conditions are required for the synthesis of the phenol vinyl ethers to avoid resinous condensation products (6,7). Direct reaction of ethylene with alcohols has also been investigated (8). 

Thermal Cracking. Thermal chlorination of ethylene yields the two isomers of tetrachloroethane, 1,1,1,2 and 1,1,2,2. Introduction of these tetrachloroethane derivatives into a tubular-type furnace at temperatures of 425—455°C gives good yields of trichloroethylene (33). In the cracking of the tetrachloroethane stream, introduction of ferric chloride into the 460°C vapor-phase reaction zone improves the yield of trichloroethylene product. 

Thermal dimerization of ethylene to cyclobutane is forbidden by orbital symmetry (Sect 3.5 in Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume). The activation barrier is high E =44 kcal mof ) [9]. Cyclobutane cannot be prepared on a preparative scale by the dimerization of ethylenes despite a favorable reaction enthalpy (AH = -19 kcal mol" ). Thermal reactions between alkenes usually proceed via diradical intermediates [10-12]. The process of the diradical formation is the most favored by the HOMO-LUMO interaction (Scheme 25b in chapter Elements of a Chemical Orbital Theory ). The intervention of the diradical intermediates impfies loss of stereochemical integrity. This is a characteric feature of the thermal reactions between alkenes in the delocalization band of the mechanistic spectrum. 

The thermal desorption curve following the reaction of ethylene or propylene with 0 shows no substantial products in the gas up to 450 °C, and at that temperature CH is the primary product (19). These results suggest the following reactions, using ethylene as an example ... 

The PET polymer structure can also be generated from the reaction of ethylene glycol and dimethyl terephthalate, with methyl alcohol as the byproduct. A few producers still use this route. The aromatic rings coupled with short aliphatic chains are responsible for a relatively stiff polymer molecule, as compared with more aliphatic structures such as polyolefin or polyamide. The lack of segment mobility in the polymer chains results in relatively high thermal stability, as will be discussed later. 

It is obvious, however, that the situation rapidly becomes complex if not completely intractable. Consider, for example, the problem of choosing a reaction coordinate describing as simple a reaction as the thermal elimination of ethylene from ethyl acetate. 

The radical polymerization of ethylene, in practice, is initiated by free-radical initiators, although radiation-induced,155 210 photoinduced,211-213 and thermal213,214 initiations are also possible. The temperature of high-pressure polymerization should not exceed 350°C since above this temperature a rapid exothermic (AH = —30.4kcal/mol) thermal decomposition of ethylene can take place leading to a runaway reaction ... 

In some cases it is the diene component from the retro reaction which is the desired product and extrusion of a volatile alkene such as ethylene is then ideal. Pyrolysis of the 1,4-oxathiin systems 21 proceeds in this way to give the a-oxothiones 22 which for R = Pr1, exists mainly as the enethiol tautomer 2323. Thermal extrusion of ethylene from 24 provides convenient access to the interesting fulvene 25 in quantitative yield24, and the corresponding reaction of 26 at 650 °C and 10-4 torr gives the cyclopentadienoben-zopyrene 28 in 95% yield, presumably by way of the intermediate 2725. 

A detailed study of the oxidation of alkenes by O on MgO at 300 K indicated a stoichiometry of one alkene reacted for each O ion (114). With all three alkenes, the initial reaction appears to be the abstraction of a hydrogen atom by the O ion in line with the gas-phase data (100). The reaction of ethylene and propylene with O" gave no gaseous products at 25°C, but heating the sample above 450°C gave mainly methane. Reaction of 1-butene with O gives butadiene as the main product on thermal desorption, and the formation of alkoxide ions was proposed as the intermediate step. The reaction of ethylene is assumed to go through the intermediate H2C=C HO which reacts further with surface oxide ions to form carboxylate ions in Eq. (23),... 

Table V summarizes the key photochemistry of trinuclear metal-metal bonded complexes. The first noteworthy photochemical study of trinuclear complexes concerns Ru3(CO)j2. This species was found to undergo declusterification to mononuclear fragments when irradiated in the presence of entering ligands such as CO, PPh3,°T ethylene. The intriguing finding is that thermal reaction of Ru3(C0)12 with PPh3 results in the substitution product indicated in equation (18) whereas irradiation yields the mononuclear species given by equation (19) (57). These results...

The thermal reactions of dihydrobenzo[c]furan 258 were studied behind reflected shock waves in a single pulse shock tube over the temperature range 1050-1300 K to lead to products from a unimolecular cleavage of 258 <2001PCA3148>. Intriguingly, carbon monoxide and toluene were among the products of the highest concentration, while benzo[f]furan, benzene, ethylbenzene, styrene, ethylene, methane, and acetylene were the other products. Trace amounts of allene and propyne were also detected. 

The thermal alkylation of ethylene-isobutane mixtures at high pressures in the gas phase has been studied in the presence and absence of HCl, and it has been found that HCl can (a) dramatically increase the total yield of alkylate, (b) increase the fraction of the alkylate which is C6 rather than C8> and (c) both increase and decrease the ratio of 2-methyl pentane to 2J2-dimethylbutane in the C6 fraction of the alkylate, this latter depending on the amount of HCl used. All of these effects can be explained readily in terms of the generally accepted free radical mechanism of thermal alkylation, provided one assumes that HCl acts as a catalyst for those reaction steps that involve transfer of a hydrogen atom between a free radical and a hydrocarbon. 

While it has been less generally recognized, there are free radical reaction systems in which hydrogen atom transfer between radicals is not rate controlling but does control the selectivity with which the various possible reaction products are formed. This chapter is a study of the effect of HCl on such a reaction system, the thermal alkylation of ethylene. (The effect of HCl upon this reaction was first disclosed in one of the authors patents (4). Several years after this disclosure, Schmerling (6) published a paper which, though differing in many details, showed... 

The thermal decomposition of vinyl thiol, CH2=CHSH, appears to proceed by a molecular mechanism, similar to that of ethane thiol. This conclusion was derived indirectly from the thermal decomposition of ethylene sulfide at 1000 °C vide infra), in a fast flow system in which the major products are C2H4, C2H2 and H2S, the latter two compounds formed in equal quantities if the reaction products are quickly trapped out of the effluent stream, vinyl thiol can be detected. Preliminary results suggest the following steps... 

---