Acetylene formation

A striking result of this reinvestigation (128, 129) is the observation that the ratio of the product ketone to the acetylene formed from a-bromo-p-aminostyrene is a function of the pH (Table Vll) but that the rate at which they are formed is not. As the pH increases from 3.9 to 13.1, the relative yield of acetylene increases from 16% to 85%. Therefore, the acetylene formation by elimination of a proton from the vinyl cation (path b in route D in Scheme XI) is more susceptible to an increase in base strength than is ketone formation via the enol (path a). This observation is a rare case of pH control over product composition in a 1-El reaction. 

Since photoexcitation greatly enhances the reactivity of acetylenes, formation of the enol intermediate becomes faster than its rearrangement to ketone. As a result, the intermediate acetophenone enol in the hydration of phenylacetylene could be directly observed 42... 

Storch (65) reviewed the probability of acetylene formation in thermal reaction, as shown in experiments such as the early ones of Bone and Coward (13). Voge and Good (69) and Laidler (40) reviewed more recent results. In general, reaction seems to follow a chain mechanism of the type originally suggested by Rice and Herzfeld ... 

Under ordinary conditions, this may not be a major reaction, but for the problem of acetylene formation it is notable that Cvetanovic and Callear (19), and Mitchell and Le Roy (49), in studies of the photosensitized decomposition of ethylene, found a uni-molecular mechanism. Ingold and Stubbs (33) also found that the decomposition of propylene starts with a molecular rearrangement and decomposition ... 

A final possibility for a type of reaction resulting in acetylene formation which must be considered is that of combination of radicals. This is not likely to be significant in most systems, but in the case of methane reactions it becomes of particular interest. The production of ethylene, and acetylene in the thermal decomposition of methane requires some sort of combination reaction (40, 65). Kassel suggested a mechanism involving methylene radicals ... 

Since the energy of activation of the acetylene formation reaction is greater than thaif of its decomposition reaction quickly raising the reaction medium to elevated temperatue results in the increased production of acetylene. The following heating methods are used... 

One further observation on the flash photolysis of methane is the fact that acetylene formation is dependent on intensity this suggests a radical-radical reaction such as... 

No evidence has been found for acetylene formation in a reaction equivalent to (5A) at low temperatures, although at higher temperatures (>1000 K), it is considered important [9] as is reaction (67) because of the high O atom concentrations that exist. 

Besides rare oxime coupling with an acetylene (formation of 3 and 4, Scheme 4) or with acetylacetonate in [VO(acac)2],36 other N—O bond rupture reactions are known (see Scheme 5) usually involving the formation of an N-metal bond ... 

This solvent is preferred to ether for the Simmons-Smith reaction of acetylenes formation of polymeric resins is less pronounced. ... 

Littlewood2S2, 2S3 has made several modifications to the Ar plasma jet so that the pyrolysis efficiency with respect to acetylene formation could be improved. To ensure that the coal was introduced into the hottest region of the plasma, it was fed axially into the plasma through holes drilled in a hollow cathode holder. The principal argon stream was introduced tangentially into the plasma jet and the pyrolysis... 

Preliminary engineering studies revealed that a substantial capital saving would be made if the reactor stream could emerge at a pressure in vicinity of about 0.5 MPa. However, at pressures above 0.15 MPa the model showed that a substantial loss in acetylene selectivity occurred, which is not unexpected in view of the extensive literature on the subject of acetylene formation. Other results to emerge were ... 

The possibility of acetylene formation (Section III.B) may be responsible for the unusually low yields (4-9% after the chromatographic separation) obtained in the synthesis of isoxazolo[5.4-[Pg.187]

Highly endothermic reactions such as acetylene formation by pyrolysis. 

Acetylene Formation. About 27% of the C2H2 is produced by ion-molecule reactions (Table III), and the ionic species involved are CH4+ and CH2+ (Table V). The predominant ion-molecule reaction of the parent ion is Reaction 4a. Recent studies (I, 14) show that the secondary ions—CH5+, C2H5+ and C2H4+—are unreactive toward CH4. However, CH5+ does dissociate in the sequence Reaction 4b + 4c. Thus, a possible sequence involving CH4+ is Reaction 4a followed (48) by... 

The C18-C28 segment 502 was synthesized starting from epoxide 497 as shown in Scheme 71. The thioketal 500 was synthesized by coupling of 498 and 499, which were derived from epoxide 497. The thioketal 500 was converted into hemiketal 501 via adjustment of the silyl protective group, deprotection of thioketal, oxidative cleavage of the double bond, and acetylene formation. Hydrostannylation/iodination of acetylene 501 gave vinyliodide, and protective group manipulation afforded 502. 

A number of people studying very high temperature acetylene formation-decomposition have invoked radical recombinations to explain the fact that acetylene compositions greater than equilibrium were achieved in their studies. Plooster and Reed (12) postulated that high temperature equilibrium favors two carbon species, C2H2 and C2H, and that during the quenching sequence two mechanisms contributed to the acetylene measured in the product... 

A 2 1 addition compound of uncertain structure is formed when diphenyl ketene and triethyl phosphite are allowed to react at room temperature (235). A structural formula for the adduct analogous to that proposed for the 2 1 adduct obtained from o-nitrobenzaldehyde and triethyl phosphite is a reasonable likelihood. When this reaction is carried out under pyrolytic conditions, there is obtained diphenyl acetylene and triethyl phosphate in yields of each as high as 65%. The limited experimental information does not justify any conclusion regarding the origin of diphenyl acetylene. Formation of the latter by rearrangement of the unusual carbene intermediate C=CPh2 is an interesting possibility. 

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