Dehydrochlorination Reactions

Millan (98) studied the effect of tacticity on the ionic dehydrochlorination and chlorination of PVC. For the dehydrochlorination reaction, both the reaction rate and the polyence sequence distribution depend markedly on the syndiotactic content. Chlorination appeared to be easier through heterotactic parts than through syndiotactic sequences as shown by C-NMR. 

A small portion of vinyl chloride is produced from ethane via the Transcat process. In this process a combination of chlorination, oxychlo-rination, and dehydrochlorination reactions occur in a molten salt reactor. The reaction occurs over a copper oxychloride catalyst at a wide temperature range of 310-640°C. During the reaction, the copper oxychloride is converted to copper(I) and copper(II) chlorides, which are air oxidized to regenerate the catalyst. Figure 6-1 is a flow diagram of the Transcat process for producing vinyl chloride from ethane. ... 

The di- and monoalkyltin compounds are considered to be effective as stabilizers because they (i) inhibit the onset of the dehydrochlorination reaction by exchanging their anionic groups, X, with the reactive, allylic chlorine atoms in the polymer (ii) react with, and thereby scavenge, the hydrogen chloride that is produced and that would otherwise induce further elimination (jii) produce the compound HX, which may also help to inhibit other undesirable side reactions and iiv) prevent breakdown of the polymer initiated by atmospheric oxidation, i.e., by acting as antioxidants. 

Figure 1. Relation among Log k of Dehydrochlorination Reaction, Median Lethal Dosage to Mosquito Larvae, and Substituent Constant for Trichloroethyl Compounds...

Chlorine-containing polymers such as poly(vinyl chloride) PVC undergo an autocatalytic dehydrochlorination reaction under the influence of elevated temperature and UV radiation. Since the HCl originating from the dehydro chlorination of the PVC chains is believed to sustain this autocatalytic process, stabilizers that irreversibly bond HCl can thus inhibit the degradation. Heavy metal compounds such as cadmium stearate or lead stearate are currently used for this purpose. However, alternatives are required due to environmental problems associated with the use of heavy metals. Indeed, the largest current application of LDH materials is in the polymer industry, mainly to stabilize PVC [3,229-232]. 

With the use of gene clusters of the natural products coronatine and kutznerides, the biosynthetic pathway of coronamic acid has also been elucidated by Walsh and coworkers. From the biosynthetic analyses, a nonheme Fe -dependent halogenase was identified as the chlorinating enzyme that converts L- //a-isoleucine to 7-chloroisoleucine. A second enzyme carries out a dehydrochlorination reaction to yield coronamic acid. The general biosynthetic pathway is shown below (Scheme 7). 

Guyot and coworkers have also noted that monomers stabilize poly(vinyl chloride) during mastication, probably because they act as free radical scavengers with the exception of basic monomers, such as vinyl pyridine, B. In this case dehydrochlorination reaction... 

Polyols. Polyols, such as pentaerythritol [115-77-5], dipentaerythritol [126-58-9], and sorbitol [50-70-4], most likely chelate the active metal centers to reduce their activity toward the undesired dehydrochlorination reaction. These additives are generally included in the stabilizer formulation, used in the range of 0.2 to 0.7 phr. 

The C-terminal tripeptide tail of oxytocin H-L-Pro-Leu-Gly-NH2 (melanocyte-stimulating hormone release inhibiting factor, MIF or melanostatin) is implicated as having a direct effect on the central nervous system. The ALeu analogue of melanostatin, [ALeu2-MIF] was prepared in view of the earlier work on the active conformation of melanostatin in which a (3-turn was implicated.11771 ALeu was prepared by the N-chlorination/dehydrochlorination reaction and then coupled with Boc-Pro-OH. 

From the temperature dependence of MC consumption shown in Fig. 8.17, an apparent activation energy for the reaction is estimated to be ca. 70 kJ mol-1. This kind of dehydrochlorination reaction from chloroethanes has previously been shown to occur on solid acids and bases.,8) Since the Ti02 surface has Lewis acid sites, the dehydrochlorination reaction may proceed on these acid sites. It is suggested that some of the HC1 produced remains on the Ti02 surface and may inhibit this reaction, but under photoillumination HC1 would be removed by a photocatalytic action of Ti02 as discussed below. 

In all cases, authors showed that such copolymers are stable only if intramolecular peroxides resulting from ozonization have been preliminary thermally decomposed before the above reactions. Their presence in the polymer backbones leads, in a very short time, to dehydrochlorination reactions and degradation of polymers. These works have been extended to other polymers, such as chlorinated polyethylene, vinyl chloride-vinyl acetate copolymers [127]. 

ECTFE has a thermal stability comparable to ETFE13 and can be stabilized by the addition of an ionomer, which considerably reduces dehydrofluorination and dehydrochlorination reactions and suppresses the discoloration of the polymer.14... 

In addition, the active species of plasma give intensive dehydrogenation and dehydrochlorination reactions of the phosphorus and halogen containing polymers with the generation of macroradicals (8). 

The principal reactions which have been studied in thermally degraded poly (vinyl chloride) are (1) the dehydrochlorination reaction ... 

The dehydrochlorination reaction causes a color change. Marvel and co-workers (17, 19) attributed the discoloration to a conjugated polyene structure. They suggested that random dehydrochlorination to form an olefinic bond would be followed by allylically activated removal of successive hydrogen chloride units to form the polyene structure. 

The formation of tri- and especially tetrasilanes which are already branched (tertiary Si-units) as the first reaction products (described elsewhere [4]) suggests the appearance of intermediate silylene species which could enter in insertion reactions of Si-Si as well as Si-Cl bonds. The tri- and tetrasilanes undergo thermal crosslinking reactions at reaction temperatures of 165-250 °C. In addition dehydrochlorination reactions initiated by acid H-abstraction of methyl groups cause the formation of carbosilane (methylene) units in the polymer framework. Table 1 shows the gross compositions of poly(methylchlorosilanes) which are determined by the reaction temperature. 

Plastic pellets conveyed from the pretreatment process are fed into molten plastic tanks by the feed conveyor. The pellets inclnding PVC are melted and agitated in the tank at 300°C for 2 h. At this stage, the dehydrochlorination reaction is sufficient to remove chlorine in the fluff. This is a characteristic of this process, since the modification catalyst of ZSM-5, sensitive to chlorine, is nsed in the cracking step. The generated gas is collected, and can be rensed to prodnce hydrochloric acid, bnt it is cnrrently nentralized with sodinm hydroxide and discarded in the sewage water. 

The latter reaction [Eq. (107)] would be similar to the dehydrochlorination reaction employed to generate derivatives of anthracene [Eq. (102)]. Reactions of 9-chloro-9,10-dihydrosilaanthracene with nitrogen bases gave polymeric products. 

Table 1 Relative Intensities in the Dehydrochlorination Reaction in a PVC Resin...

Both thermal and photochemical processes take the form of a dehydrochlorination reaction which leads to discolouration as well as extensive changes in the internal structure of the polymer which has an unfavourable effect on the desirable electrical and mechanical properties. It has become apparent that considerable similarity exists between the two degradation processes and that it is neither easy nor desirable to make a vigorous distinction between the two. Information gained from e eriments on thermal degradation are often directly relevant to the analogous photochemical process. 

The methylenecyclopropane products of the dehydrochlorination reaction can be further used as starting materials in the cyclopropanation reaction, thus giving rise to the series of methylenetriangulanes. The reaction is typically carried out at or near room temperature, as illustrated by the following general scheme, which is also used for double dehydrohalogen-ations in the preparation of bis(methylene)triangulanes (see Section 5.2.2.1.2.1.). 

To our knowledge, the analog of the double dehydrochlorination reaction for 1,1-dibromo-cyclopropanes has not been reported. While the reaction of 9,9-dibromobicyclo[6.1.0]non-4-ene with potassium terr-butoxide in dimethyl sulfoxide is known, only rearranged alkenes were obtained." On the other hand, many 1,1-dichloro- and 1,1-dibromocyclopropanes fused to unsaturated six-membered rings undergo double dehydrohalogenation to give cycloproparenes (see Section 5.2.2.I.2.5.). 

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