Thermal degradation isomerization

As far as the thermal degradation of alkenesulfonates is concerned, these authors have shown that the first step involves isomerization of the double bond followed by cleavage of the carbon-sulfur bond. Alkenesulfonate thermal degradation is catalyzed by acids. 

Scotter, M.J., Characterization of the coloured thermal degradation products of bixin from annatto and a revised mechanism for their formation, Food Chem., 53, 111, 1995. Zechmeister, L., Cis-trans isomerization and stereochemistry of carotenoids and diphenylpolyenes, Chem. Rev., 34, 267, 1944. 

Thermal degradation occurs when heat causes compounds to undergo structural changes, leading to the formation of simpler species. For example, many organophosphorus esters isomerize when heated... 

The isomerization of all-trans-[3-carotene under N2 atmosphere by photosensitization action of eight chlorophyll compounds naturally present in the extracts of green vegetables was investigated by illumination with fluorescent white light (3000 lux) at 12°C to minimize the thermal degradation (O Neil and Schwartz 1995). All chlorophylls showed similar isomeric distribution and efficiency, 9-d.s- 3-carotene is the main isomer formed. On the other hand, the illumination of all-fran.s-(3-car-otene without chlorophylls indicated that the main isomer formed was 13-d.s- 3-carotcnc, probably due to the population of a different triplet state manifold by direct photolysis. 

Figure 3. Structures of thermal degradation products of 4-hydroxy-(3-ionol (8) under SDE conditions (100°C pH 3.7). 5 = 2,2,6,7-tetra-methylbicyclo[4.3.0Jnona-4,7,9(l)-triene 10A/10B = E- and Z-megastigma-5,8-dien-4-ones 11A/11B = isomeric retro-o-ionones 12A-12D = isomeric megastigma-6,8-dien-4-ones N.I. not identified.

Figure 7. Structures of thermal degradation products of 3-oxo-a-ionol (19) under SDE conditions (100 C pH 3.7). 23A-23D = isomeric megastigma-4,6,8-trien-3-ones 24A/24B = isomeric megastigma-4,7,9-trien-3-ones. (Redrawn from ref. 16 and 17.)...

Since complexes 70 and 71 have been sho vn to be thermally stable in contrast to 69 [39], and hence their thermal degradation vas negligible. Although reaction of 70 with 1-octene performed at room temperature in ADMET conditions afforded mainly Ci4-olefin, significant amounts of Cy and C9-C13 olefins were observed in the reaction performed at 60 °C [40]. This result suggests that olefin isomerization is promoted to some extent at a higher temperature. 

The fate of diazirines on decomposition can be influenced by the addition of y -cyclodextrin (j6-CD). When 3-methyl-3-phenyl-3jf/-diazirine (3) was thermolyzed under argon, 1 -methyl-1,2-diphenylcyclopropane (4) was obtained as an isomeric mixture in close to 20% yield the main product (43% yield) was acetophenone azine (5). Cyclodextrin complexation prior to pyrolysis, however, increased the yield of 4 tremendously. The carbohydrate, therefore, facilitates both styrene (6) and cyclopropane formation.It is also interesting to note that the trans-41 cis-4 ratio increased concomitantly and that styrene appears as an isolable product after photochemical, but not after thermal degradation of the 3 /(-CD complex. 

Two additional phenomena contribute to the thermal degradation mechanism and can determine the formation of the relative maximum and minimum in the characteristic surface of the gaseous MWD as shown in Fig. 1. In the backbiting process, the end of polymer chain can also react to form gaseous products with a non-random distribution through a sequence of isomerization and /1-scission reactions. Finally, monomer units can also be released by an unzipping process through /1-scission of the terminal radicals. 

Fig. 24. Change of the wide-angle diffraction pattern of cw-polyacetylene during heating from room temperature with 11 °C/min. The disappearance of the (201) reflection at 150 °C indicates the cis/ trans isomerization, the broad peak above 290 °C indicates thermal degradation...

Co-milling polypropylene and zeolite Y was carried out by Audisio and co-workers. The product distributions as a function of carbon number at 673 K are reproduced in Figure 6 for NaY, HY and rare earth Y (REY, 10.7% of rare earth oxides) and compared with those observed for thermal degradation and in the presence of silica-alumina (also reproduced above in Figure 4). The product distributions for HY and REY were very similar, while a significantly lower fraction of C5-C11 products was formed when NaY was used. A detailed analysis of the products over HY and REY revealed that a significant amount of isomerization had occurred and olefins were formed in large yields. 

Thus, observed tacticity values using either tetramer pair, A and A, or B and B, are in fairly good agreement with those by H-NMR. This fact suggests that any appreciable thermal isomerization does not contribute to the thermal degradation of PMMA to yield the tetramers since the associated radical transfers to yield the tetramers occur only at methyl or methylene carbons which are not asymmetric in the polymer chain. Moreover, it has been demonstrated that the diad tacticity of MMA sequences can be also precisely determined by basically the same method even in the copolymers of MMA and various acrylates and their crosslinked polymers, which are difficult to characterize by NMR. 

The mass spectra indicate that the isomerization and condensation processes leading to these structures are quite extensive during the thermal degradation of PC. °... 

In addition, spectroscopic evidence shows that cis-trans isonaerizations as well as cyclizations occur in the process of thermal degradation of 1,4-polyisoprene. It is interesting that the cis-trans isomerizations were observed at temperatures as low as 200 C [460]. 

The first exotherm (205-250 °C) is caused by oxidation of ZDDP after its oxidative induction period, leading to the reaction of intramolecular isomerization. The second exotherm (250-280 °C) is thermal degradation of the ZDDP. A number of products of the thermal degradation at 280 °C can be seen in the gas chromatogram. 

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