Pyrolysis, also carboxylates

The best yields are obtained when the ketene has an electronegative substituent, such as halogen. Simple ketenes are not very stable and must usually be generated in situ. The most common method for generating ketenes for synthesis is by dehydrohalo-genation of acyl chlorides. This is usually done with an amine such as triethylamine.167 Other activated carboxylic acid derivatives, such as acyloxypyridinium ions, have also been used as ketene precursors.168 Ketene itself and certain alkyl derivatives can be generated by pyrolysis of carboxylic anhydrides.169... 

Pyrolysis of Carboxylic Acid Esters. The second stage of our proposed ethylene/propylene synthesis (eq. 21) has been demonstrated also for typical intermediate ethyl/propyl esters. Pyrolysis of acyclic, aliphatic carboxylic acid esters to alkenes and the parent acid is well documented (72-74) and in our work C1-C3 alkyl propionates, prepared by the synthesis techniques of Table VI (expt. 1 and 11) and isolated by fractional distillation, were pyrolyzed to ethylene, propylene and propionic acid by the homogeneous gas phase method (see Table VIII). In the initial experimental series, 98+% ethyl propionate is passed over pyrex helices at 460 C (expt. 16). Ethyl propionate conversion averages ca. 30% per pass selectivity to propionic acid is 95-97% ethylene comprises 92% of the light gas fractions. 

The pyrolysis of perfluoro carboxylic salts can result both in mono and bimolecular products At 210-220 °C, silver salts give mostly the coupled products, at 160-165 °C in A -methylpyrrolidinone, the corresponding copper salts also give the simple decarboxylated compounds in nearly equal amounts The decomposition of the copper salts m the presence of lodobenzene at 105-125 °C results m a phenyl derivative, in addition to the olefin and coupled product [94] (equations 60-62)... 

Since 3-methylenecyclobutane-l,2-dicarboxylic anhydride is easily converted to 3-methyl-2-cydobutene-l,2-dicarboxylic acid, it is an intermediate to a variety of cyclobutenes. The dimethyl ester of 3-methylenecyclobutane-l,2-dicarboxylic acid is also a versatile compound on pyrolysis it gives the substituted allene, methyl butadienoate, and on treatment with amines it gives a cyclobutene, dimethyl 3-methyl-2-cyclobutene-l,2-di-carboxylate. ... 

Free-radical mechanisms are mostly found in pyrolyses of polyhalides and of primary monohalides,though they also have been postulated in pyrolysis of certain carboxylic esters/ Much less is known about these mechanisms and we shall not consider them further. Free-radical eliminations in solution are also known but are rare. ... 

Several studies have dealt with the problem of discriminating between mastic and dammar, and three marker compounds of mastic have been identified moronic, masticadienonic and acetyl masticadienolic acids [42], The chemical structure of (iso)masticadienonic acid and 3-0-acetyl-3-epi(iso)masticadienonic acid is characterized by a side chain, as for dammarane molecules, but with a carboxylic acid end group (Table 12.1). Under pyrolysis conditions this side chain is susceptible to cleavage as demonstrated by the presence, among the pyrolysis products of mastic, of 2-methyl-pent-2,4-dienoic acid, that perfectly matches with the chemical structure of the side chain end. In addition 3-(9-acetyl-3-epi-(iso)masticadienolic acid also loses the acetyl group and, in contrast to masticadienonic acid, is not detected at all. 

More recently on-line pyrolysis with HMDS has been performed successfully even if the pyrolysis interface was kept at 250°C. In fact, Domenech-Carbo and colleagues [57,58] have obtained very good results on a variety of art materials and on real paint samples as well. They have applied Py-GC/MS with on-line trimethylsilylation to the characterization of diterpenoid resins and, in contrast to previous literature data, the derivatization method enabled not only the identification of resinous carboxylic acids in the form of TMS esters, but also an efficient conversion of hydroxyl groups to TMS ethers. 

The usual sources used for the homolytic aromatic arylation have been utilized also in the heterocyclic series. They are essentially azo- and diazocompounds, aroyl peroxides, and sometimes pyrolysis and photolysis of a variety of aryl derivatives. Most of these radical sources have been described in the previous review concerning this subject, and in other reviews concerning the general aspects of homolytic aromatic arylation. A new source of aryl radicals is the silver-catalyzed decarboxylation of carboxylic acids by peroxydisulfate, which allows to work in aqueous solution of protonated heteroaromatic bases, as for the alkyl radicals. 

Annelation of a furan ring onto a thiophene is also possible by flash vacuum pyrolysis at 650 °C of the acrylate 368 and malonate 369, which gave 2-(methylthio)thieno[3,2-/ ]furan 370 and methyl-5-(methylthio)thieno[3,2- ]furan-2-carboxylate 371, respectively, in yields of 21% and 22% <1997J(P1)2483>. 

Table 2 also shows surface concentration of quinones and esters or lactones, often referred to as "neutral oxygen compounds" in the rubber literature actually these are basic oxygens, but not basic in an aqueous environment. The quinone content is estimated from the amount of carbon monoxide evolved in pyrolysis minus the weak acid content (presumed to be phenolic acids,not neutralized at pH 8.2), and the ester and lactone content is estimated from the carbon dioxide evolved in pyrolysis minus the strong acid content (presumed to be carboxylic acids neutralized at pH 8.2). 

Sauers (1959) has now oxidised camphor directly to the a-campholide by means of peracetic acid. It is also of interest to note that Otvos et al (i960) have shown, using labelled. - CH2C 02H (lkc), that in the pyrolysis of the calcium salt of homocamphoric acid to camphor, it is the labelled carboxyl group that is lost. 

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