Dehydrochlorination process

The primary product which arises from the degradation of PVC, whether induced thermally, photochemically or by high energy radiation, is a distribution of conjugated polyene sequences of various lengths produced by a dehydrochlorination process which may be written -... 

These remarks represent only the barest outline of at least two aspects of PVC degradation which have been the focus of attention for several years and remain incompletely understood namely the mechanism involved and the related problem of the involvement of HC1. Several excellent reviews give more comprehensive summaries of the earlier work (10, 11, 12). More recent work has made it clear that under appropriate conditions the presence of HC1 can affect the initiation, propagation and termination steps as well as influencing the distribution of polyene sequence lengths. In addition it can undergo photochemical addition reactions with the polyenes, i.e. the reverse of the dehydrochlorination process, as well as forming colored polyene/HCl complexes. These various possibilities will be considered in turn. 

These results emphasise the important role played by HC1 not only as a catalyst for the dehydrochlorination process but in influencing the distribution of polyene sequences which result from the primary part of the degradation process and the photochemical cross-linking reactions of the polyenylic cations. 

This is the primary degradation process accompanying processing of the polymer. The early stage of the dehydrochlorination process is uncomplicated by interfering processes. The only product observed by evolved gas analysis is hydrogen chloride (scheme 1). The sample... 

Heating organomercurial 34 resulted in the a-ehmination of phenyl mercury bromide, generating dichlorocarbene, followed by addition of benzaldehyde to form the corresponding carbonyl ylide 35. This ylide can be intercepted with dimethyl-acetylene dicarboxylate (DMAD) to produce dihydrofuran (36), which formed furan 37 through a dehydrochlorination process in 46% yield. 

Virtually all previously published opinions on thermally degraded poly (vinyl chloride) regarded the dehydrochlorination process as being in the first instance, a monomolecular process resulting in conjugated polyene structures, and that all oxidation and cross-linking reactions... 

Hiberty40 used the single determinant ab initio molecular orbital theory to study the unimolecular HC1 elimination from ethyl chloride. The calculations of three potential energy surfaces corresponding to -elimination, planar //-elimination, and nonplanar anti-elimination were performed and the dehydrochlorination process was predicted to be syn, and to proceed via a planar four-membered transition state. According to these estima-... 

An interesting pyrolysis product of 3-chloro-2,2-bis (chloromethyl) propionic acid at 200-220 °C is 1,1-bis (chloromethyl)ethene153. This reaction may well be associated with the five-membered dehydrochlorination process of a-halopropionic acids146,147. In this respect, the HC1 elimination may be considered to proceed through a mechanism involving a six-membered transition state as described in equation 69. 

Although the residence time of plastic melt in the dehydrochlorination process is very long, the amount of HCl gas evolved is less than in the liquefaction processes of Niigata and Sapporo. This may indicate that PVC and PVDC materials are removed effectively... 

The discoloration is due to a dehydrochlorination process restilting in the formation of long conjugated polyene sequences in the polymer chain [Eq. (7-10)]. Polyenes can give rise to photo-cross-linking reactions. 

Minsker" observed that plasticizers induce degrading effects as a consequence of solva-tation of the PVC chains, which was more pronounced when increasing the compatibility of the plasticizer. According to Wypychl plasticizers of rapid gelation (i.e., those more compatible), yield more viscous melts, hindering the diffusion of the HCl and other products formed, which catalyze the dehydrochlorination process. 

In the absence of a solvent and excess of chloral, the reaction rate for this reaction may be expressed by a third-order equation-second order with respect to dimethyl H-phosphonate and first order with respect to the chloral [177]. In dioxane solution and excess dimethyl H-phosphonate, the dependence of the reaction rate on the chloral concentration is the same. In addition to the chloro-containing a-hydroxyalkyl phosphonate, which is the main product under the above conditions, a side product formed as a result of dehydrochlorination of the main product, followed by phosphonate-phosphate rearrangement, has been also isolated. The presence of a base such as triethylamine, alkali metal alkoxides and hydroxides, or sodium carbonate accelerates the dehydrochlorination process. An example of this side reaction is the transformation of dialkyl-2,2,2-trichloro-l-hydroxyethyl phosphonates into dialkyl-2,2-dichlQrovinyl phosphates in the presence of sodium hydroxide [181,182]. 

Heat stabilizers belong to one of the two major classes primary heat stabilizers and secondary heat stabilizers. Primary heat stabilizers function both by retarding the dehydrochlorination process and by reacting with the liberated HQ to delay progress degradation. Secondary heat stabilizers or costabilizers are used to scavenge Uberated HCl from the PVC resin or to react with the metallic chloride by products of the primary mixed metal stabilizers. 

Unzipping may then proceed from the new allyl group by either a radical process or unimolecular elimination. In addition, of course, the radicals produced by thermal decomposition of the hydroperoxide may directly initiate the radical chain dehydrochlorination process in a manner similar to that described previously for initiator fragments. 

Chlorine and sodium hydroxide are the main products of the industrial chlor-alkali electrolysis that is described as a process example in Section 6.19. Hydrochloric acid is produced by reaction from the elements H2 and CI2 or by the reaction of chloride salts such as, for example, NaCl or CaCl2, with sulfuric acid. Other important sources of HCl are industrial chlorination processes using CI2 as chlorination agent (e.g., chlorination of benzene to form chlorobenzene and HCl or the chlorination of methane to give chloromethane and HCl) or industrial dehydrochlorination processes (e.g., production of vinyl chloride and HCl from 1,2-dichloroethane). The main uses of hydrochloric acid are addition reactions to unsaturated compounds (by hydrochlorination or oxychlorination), formation of chlorine in the Deacon process, production of chloride salts from amines and other organic bases, dissolution of metals, regeneration of ion exchange resins, and the neutralization of alkaline products. 

It is observed that the initial reaction with amines is accompanied by formation of colour ranging from yellow to a dark red-brown. This is consistent with the formation of a sequence of conjugated double bonds along the chain somewhat reminiscent of the dehydrochlorination process that occurs during the degradation of polyfvinyl chloride). 

Instead of preventing the dehydrochlorination process, which generates colored conjugated sequences, it may be more appropriate to use additives that react with the chromophoric polyene formed and reduce the length of the conjugated sequences. A hypsochromic effect is observed in this case, which decreases the absorption in the visible range. 

Reddy, Sripathi Narayan Rao, Pamulaparthy Shanthan Anuradha, Gundamaraju Narsaiah, Banda Rambabu, Yadla Srinivas, Punnamraju Venkata Satya Surya Ravin-dranath, Kajjam. Dehydrochlorination process for the preparation of 1,1,1,2-tetrafluo-roethane from 1,1-dichlorotetrtafluoroethane. IN 2005DE00475 A 20070105 (2007). 

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