Acetylenes, oxidation mechanism

In a polluted or urban atmosphere, O formation by the CH oxidation mechanism is overshadowed by the oxidation of other VOCs. Seed OH can be produced from reactions 4 and 5, but the photodisassociation of carbonyls and nitrous acid [7782-77-6] HNO2, (formed from the reaction of OH + NO and other reactions) are also important sources of OH ia polluted environments. An imperfect, but useful, measure of the rate of O formation by VOC oxidation is the rate of the initial OH-VOC reaction, shown ia Table 4 relative to the OH-CH rate for some commonly occurring VOCs. Also given are the median VOC concentrations. Shown for comparison are the relative reaction rates for two VOC species that are emitted by vegetation isoprene and a-piuene. In general, internally bonded olefins are the most reactive, followed ia decreasiag order by terminally bonded olefins, multi alkyl aromatics, monoalkyl aromatics, C and higher paraffins, C2—C paraffins, benzene, acetylene, and ethane. 

Although no reported work is available on vinyl acetylene oxidation, oxidation by O would probably lead primarily to the formation of CO, H2, and acetylene (via an intermediate methyl acetylene) [37], The oxidation of vinyl acetylene, or the cyclopentadienyl radical shown earlier, requires the formation of an adduct [as shown in reaction (3.142)]. When OH forms the adduct, the reaction is so exothermic that it drives the system back to the initial reacting species. Thus, O atoms become the primary oxidizing species in the reaction steps. This factor may explain why the fuel decay and intermediate species formed in rich and lean oxidation experiments follow the same trend, although rich experiments show much slower rates [65] because the concentrations of oxygen atoms are lower. Figure 3.13 is a summary of the reaction steps that form the general mechanism of benzene and the phenyl radical oxidation based on a modified version of a model proposed by Emdee et al. [61, 66], Other models of benzene oxidation [67, 68, which are based on Ref. [61], place emphasis on different reactions. 

Following the discussion from the preceding section, consideration will be given to the oxidation of ethene and propene (when a radical pool already exists) and, since acetylene is a product of this oxidation process, to acetylene as well. These small olefins and acetylene form in the oxidation of a paraffin or any large olefin. Thus, the detailed oxidation mechanisms for ethane, propane, and other paraffins necessarily include the oxidation steps for the olefins [28]. 

Although the oxidation mechanism of nitrite to nitrate in the heterotrophic nitrifiers has not been known at all on the enzyme level, the oxidation mechanism of ammonia to nitrite has been partially clarified. Ammonia is oxidized to nitrite through hydroxylamine also in the heterotrophic bacteria. The oxidation of ammonia to hydroxylamine is catalyzed by ammonia monooxygenase as in the enzyme of Nitrosomonas europaea. The enzyme purified from Paracoccus pantotropha GB17 (formerly Thiosphaera pantotropha GB17 or Paracoccus denitrificans GB17) catalyzes the oxidation of ammonia to hydroxylamine and contains copper, but its activity is not inhibited by acetylene (Moir et al., 1996), unlike the enzyme of Nitrosomonas europaea. 

Olson (10) where a series of ion/molecule reactions were combined with acetylene oxidation reactions and a computational model developed. This model gave ion profiles reasonably similar to those actually observed in sooting flames, and also predicted concentrations of ions sufficiently high that they might be considered as soot nucleation sites. Aspects of an ionic mechanism... 

As for the atmospheric dissipation of alkynes, the reaction with OH is the sole process. In addition, the initial reaction of OH with alkynes is similar to alkenes, and the reaction is in the high-pressure limit at 1 atm for alkynes with carbon number more than 3, although several atm is necessary to reach the high-pressime limit for acetylene (C2H2) (see Sect. 5.2.9) (see Table 7.1). The main alkyne in the polluted atmosphere is C2H2, and its oxidation mechanism by OH is shown in Reaction Scheme 7.8. 

It is well known that much of carbon chemistry is controlled by kinetic factors for example diamond would change spontaneously to graphite, and acetylene to benzene, at room temperature and atmospheric pressure if thermodynamics were controlling. In addition, nearly aU organic compounds are thermodynamically unstable to oxidation, and exist in the presence of air only because no suitable low energy oxidation mechanism is available. 

The electrochemical oxidation mechanism of a large group of unsaturated hydrocarbons (ethylene, acetylene, propylene, 1-butene, 2-butene, allene, butadiene, cyclohexadine, benzene) was investigated in detail by Bockris and co-workers [1, 15, 16,170, 191-196, 200]. The experimental results obtained in these papers can be formulated in the following way ... 

Flame or Partial Combustion Processes. In the combustion or flame processes, the necessary energy is imparted to the feedstock by the partial combustion of the hydrocarbon feed (one-stage process), or by the combustion of residual gas, or any other suitable fuel, and subsequent injection of the cracking stock into the hot combustion gases (two-stage process). A detailed discussion of the kinetics for the pyrolysis of methane for the production of acetylene by partial oxidation, and some conclusions as to reaction mechanism have been given (12). 

Grade C, Type II is typical of Hquid oxygen used as a rocket propellant oxidizer. Particulate content is limited because of the critical clearances found in mechanical parts of the rocket engine. In addition to water, acetylene and methane are limited because, on long standing, oxygen evaporation could cause concentration of these combustible contaminants to reach hazardous levels. 

Ca/NH3, ether or THE, 2 h NH4CI, H2O. 90% yield. Acetylenes are not reduced under these conditions. One problem with the use of calcium is that the oxide coating makes it difficult to initiate the reaction. This is partially overcome by adding sand to the reaction mixture to abrade the surface of the calcium mechanically. 

Bohlmann and Rahtz, in 1957, reported the preparation of 2,3,6-trisubstituted pyridines. Their method employed the Michael addition of acetylenic ketones 35 with enamines 36. The 5-aminoketones 37 are typically isolated and subsequently heated at temperatures greater than 120°C to facilitate the cyclodehydration to afford 38. Again one can see the parallels in this mechanism with that for the Hantzsch protocol. However, in this case the pyridine is formed directly removing the need for the oxidation step in the Hantzsch procedure. 

These conclusions were supported by the results obtained in a study of the reactions of various types of acetylenes with TTN (94). Hydration of the C=C bond was found to occur to a very minor extent, if at all, with almost all of the compounds studied, and the nature of the products formed was dependent on the structure of the acetylene and the solvent employed. Oxidation of diarylacetylenes with two equivalents of TTN in either aqueous acidic glyme or methanol as solvent resulted in smooth high yield conversion into the corresponding benzils (Scheme 23). The mechanism of this oxidation in aqueous medium most probably involves oxythallation of the acetylene, ketonization of the initially formed adduct (XXXV) to give the monoalkylthallium(III) derivative (XXXVI), and conversion of this intermediate into a benzoin (XXXVII) by a Type 1 process. Oxidation of (XXXVII) to the benzil (XXXVIII) by the second equivalent of reagent would then proceed in exactly the same manner as described for the oxidation of chalcones, deoxybenzoins, and benzoins to benzils by TTN. The mechanism of oxidation in methanol solution is somewhat more complex and has not yet been fully elucidated. 

The recently reported (757) conversion of 5-pyrazolones directly to a,j8-acetylenic esters by treatment with TTN in methanol appears to be an example of thallation of a heterocyclic enamine the suggested mechanism involves initial electrophilic thallation of the 3-pyrazolin-5-one tautomer of the 5-pyrazolone to give an intermediate organothallium compound which undergoes a subsequent oxidation by a second equivalent of TTN to give a diazacyclopentadienone. Solvolysis by methanol, with concomitant elimination of nitrogen and thallium(I), yields the a,)S-acetylenic ester in excellent (78-95%) yield (Scheme 35). Since 5-pyrazolones may be prepared in quantitative yield by the reaction of /3-keto esters with hydrazine (168), this conversion represents in a formal sense the dehydration of /3-keto esters. In fact, the direct conversion of /3-keto esters to a,jS-acetylenic esters without isolation of the intermediate 5-pyrazolones can be achieved by treatment in methanol solution first with hydrazine and then with TTN. 

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

The proposed mechanism (Scheme 7-20) includes (a) oxidative addition of Y-Ge (Y = S or Se) bonds to Pd(PPh3)n, (b) insertion of acetylene into the Pd-Y bond to give 92 or insertion of acetylene into the Pd-Ge bond to form 93, (c) formation of 91 by either a G-Ge or a G-Y bond-forming reductive ehmination with regeneration of Pd(PPh,),. 

Two possible routes are envisioned for X = B in Scheme 7-21. The authors favored a path involving the oxidative addition of the S-B bond to Pd(0), insertion of the alkyne into the Pd-S bond followed by C-B bond-forming reductive elimination. On the other hand, Morokuma et al. studied the mechanism of the addition of HSB(0CH2)2 (99) to acetylene (C2H2) using Pd(PH3)2 (100) as a catalyst to produce 101 using hybrid density functional calculations (Eq. 7.62) [5]. 

As corroborated by deuterium labeling studies, the catalytic mechanism likely involves oxidative dimerization of acetylene to form a rhodacyclopen-tadiene [113] followed by carbonyl insertion [114,115]. Protonolytic cleavage of the resulting oxarhodacycloheptadiene by the Bronsted acid co-catalyst gives rise to a vinyl rhodium carboxylate, which upon hydrogenolysis through a six-centered transition structure and subsequent C - H reductive elimina-... 

---