1,3-Dehydrochlorination Subject

The direct reaction of other alkyl chlorides, such as butyl chloride, results in unacceptably low overall product yields along with the by-product butene resulting from dehydrochlorination. AH alkyl haHdes having a hydrogen atom in a P- position to the chlorine atom are subject to this complication. 

Taking this data into account, we subjected the chemically dehydrochlorinated polymer to thermal treatment firstly at 200°C for 2 h to enrich the product with carbon via thermal dehydrochlorination and then at 350°C for 30 min to allow the formation of carbon-like structures. 

Fig. 4.2 Thermogravimetric analysis of the chemically dehydrochlorinated olyvinyUdene chloride 1 - not subjected to thermal treatment 2 - heated at 200°C (for 2 h) 3 - heated at 200°C (2 h) and then at 350°C (0.5 h)...

Differential thermal analysis was used as a further measure of thermal stability. The polymer powder was pressed at 200°C in air to form a film. The film sample was heated at 10°C/minute from 25°-500°C in a nitrogen atmosphere. The the temperature at the onset of decomposition— i.e.y dehydrochlorination—as well as the peak endothermic temperature of the polymer which had already been subjected to a temperature of 200°C during film formation, were obtained from the DTA plot (Figures 3 and 4). 

The key step in the first reaction is, once again, a 1,4-dehydrohalogenation followed by a stereocontrolled prototropic shift and electrocyclic closure of the derived triene to a bicy-cloheptadiene (shown below) the dichlorocyclopropane then undergoes a typical sequence of 1,2-dehydrochlorination followed by prototropic shifts to lead to the [10]annulene. The chlorodiene 45 may be produced by a similar sequence of 1,4-dehalo-genation, prototropic shift and cyclization 00b. The mechanisms of these and related reactions have been the subject of labelling studies. 

The research team of J.E. Baldwin developed the first synthetic sequence for the preparation of N(5)-ergolines. The key step was a hetero-Diels-Alder reaction of a substituted phenyl butadiene to form the piperidine ring. The phenyl butadiene substrate was prepared via the Meerwein aryiation of 1,4-butadiene and a diazonium salt derived from 2,6-dinitrotoluene. The initially formed chlorinated product was subjected to dehydrochlorination using DBU as the base. 

Selective dehydrochlorination of chlorofluorocarbons (CFCs) is a very important environmental issue, and the need to replace these detrimental, ozone-depleting compounds by benign hydrochlorofluorocarbons (HCFCs) and/or hydrofluorocarbons (MFCs) has stimulated intensive work on the subject [172-182]. Palladium has been the most extensively investigated catalytic metal in this reaction, but the moderate selectivity for CH2F2 exhibited by Pd/Si02 (40%) can be significantly increased, up to 95%, with a 20-40 at%. Au addition [180], and, in Pd/C, from 70% to 90% with Au addition [181,182]. 

Several authors have proposed that the dehydrochlorination step of PVC involves autocatalysis by the released HC1.82-84 This phenomenon has been demonstrated by Patel et a/.,84 who studied the degradation of PVC samples previously treated with HC1 gas at different temperatures. Figure 4.13 compares the degree of dehydrochlorination at 150 °C for two PVC samples, one of them having been previously subjected to HC1 treatment. Dehydrochlorination of the... 

Color Development. The absorption spectrum in the visible region of PVC subjected to heat degradation shows several maxima that are related to the degree of conjugation of the polyenes formed by dehydrochlorination. Bengough and Varma (29) have studied the... 

Mechanism of Nonoxidative Thermal Dehydrochlorination. This subject is still very controversial, with various workers being in favor of radical, ionic, or molecular (concerted) paths. Recent evidence for a radical mechanism has been provided by studies of decomposition energetics (52), the degradation behavior of PVC-polystyrene (53) or PVC-polypropylene (54) mixtures, and the effects of radical traps (54). Evidence for an ionic mechanism comes from solvent effects (55) and studies of the solution decomposition behavior of a model allylic chloride (56). Theoretical considerations (57,58) also suggest that an ionic (El) path is not unreasonable. Other model compound decompositions have been interpreted in terms of a concerted process (59), but differences in solvent effects led the authors to conclude that PVC degrades via a different route (59). 

Previously, it was shown that, when chloroalkanes or chloroolefins are subjected to temperatures of 350—450 C, dehydrochlorination occurs. In the presence of oxygen and an oxidizing catalyst, however, most of the hydrogen chloride is converted to chlorine, which is found in the resultant chloroolefin. 

The dehydrochlorination of poly(vinyl chloride) has been the subject of much investigation, particularly with the view of developing greater stability in PVC... 

It followed that the same epimerization probably occurred during acid hydrolysis of the Ginseng saponins. The chloro compound (10) was obtained from the ether insoluble fraction of the crude hydrolysate of the saponin mixture with cone. HCl. The ether soluble fraction of the hydrolysate was subjected to dehydrochlorination to give an 20-epimer of (11) named 20(S)-protopanaxadiol (26) (Chart 2), the structure of which was elucidated by correlating it with (13) as follows. The dihydro derivative (27) of (26) was oxidized to give a diketone (28) which was identical with the diketone derived from (13) via (16) 16, 17, 19). Crystalline substances (1) and (12) of the 20(R)-series were also readily obtained from the less soluble fractions of the hydrolysates of the saponin mixture and from the hydrogenated saponin mixture, respectively, while the corresponding 20 ( S)... 

The dehydrochlorination of poly(vinyl chloride) has been the subject of much investigation, particularly with the view of developing greater stability in PVC polymers and copolymers. Like many polymeric reactions, dehydrochlorination is a complex process. The vinylene groups, created by the elimination of HCl from adjacent carbon atoms in the chain. 

Predict the major product formed when the following compound is subjected to dehydrochlorination with sodium ethoxide in ethanol. H3C ... 

Camphorketene 209, created via dehydrochlorination of 208, is subjected to dimerization to give 210 (Scheme 66). The structural elucidation including the stereochemistry of the dimer bearing pyronone moiety was determined by X-ray crystallography (2001JOC5832). 

PVC is intrinsically unstable because of molecular defects in some of the polymer chains (56,57) and when subjected to heat they initiate a self-accelerating dehydrochlorination reaction. Stabilizers neutralize the HCl produced and introduce nucleophilic substitution reactions that prevent further degradation (56,57) (see Degradation). 

This equation illustrates the complete usage of chlorine which is a result of the reaction. The advent of oxychlorination techniques is expected to cause a major shift to ethylene-based processes for the preparation of vinyl chloride. There is, however, also interest in balanced processes which use very cheap feedstocks consisting of a mixed dilute stream of acetylene and ethylene. In such processes, the mixture is firstly hydrochlorinated and the acetylene present reacts to give vinyl chloride which is removed by scrubbing. The exit gases are then subjected to vapour phase chlorination and dehydrochlorination, whereby the ethylene present yields vinyl chloride. 

As mentioned previously, the exposure of poly(vinyl chloride) to either ultraviolet light or heat leads to degradation, as a result of which discoloration and changes in mechanical properties can occur. The degradation of poly(vinyl chloride) has been the subject of extensive investigation and much information has accumulated. However, the reactions which take place are complex and are not completely understood at the present time. The principal processes which occur during degradation are dehydrochlorination and oxidation and these are now considered. 

E-Methyl cinnamate, E-methyl crotonate, and methyl 3-methylcrotonate are unusual in that they undergo multiple addition of dichlorocarbene when subjected to phase transfer conditions [53]. A detailed study of this reaction has resulted in evidence for an initial cyclopropanation followed by a series of dehydrochlorinations and trichloro-methyl anion additions. The resulting tetrachlorospiropentanes which result from this complex sequence are shown in equations 2.34—2.36. Note that in these cases, there is no electron releasing group on the same carbon as the Michael activating substituent. 

---