Dehydrochlorination catalysis

Observations concerning the kinetic effects of HC1 have now come full circle with a report of the last remaining permutation HC1 is said to inhibit dehydrochlorination under certain experimental conditions (55) However, the inhibition effect is small and may have resulted from HC1 addition to double bonds in the closed degradation vessel used (55). Under most conditions, HC1 exhibits strong dehydrochlorination catalysis (1,2,3,4), which modem workers usually prefer to interpret in terms of an ionic path (1,2,3,55). Attempts to quantify the catalytic effect of HC1 have been made recently (3,61), and dehydrochlorination proceeding from chloroallylic groups has been suggested to be the only catalyzed step (61). 

Jamieson and McNeill [142] studied the degradation of poIy(vinyI acetate) and poly(vinyI chloride) and compared it with the degradation of PVC/PVAc blend. For the unmixed situation, hydrogen chloride evolution from PVC started at a lower temperature and a faster rate than acetic acid from PVAc. For the blend, acetic acid production began concurrently with dehydrochlorination. But the dehydrochlorination rate maximum occurred earlier than in the previous case indicating that both polymers were destabilized. This is a direct proof of the intermolecular nature of the destabilizing effect of acetate groups on chlorine atoms in PVC. The effects observed by Jamieson and McNeill were explained in terms of acid catalysis. Hydrochloric acid produced in the PVC phase diffused into the PVAc phase to catalyze the loss of acetic acid and vice-versa. 

The catalytic dehydrochlorination of tetra-chloroethane has been studied by Shvets, Lebedev, and Aver yanov [Kinetics and Catalysis, 10 (28), 1969]. 

Oxidative catalysis over metal oxides yields mainly HC1 and C02. Catalysts such as V203 and Cr203 have been used with some success.49 50 In recent years, nanoscale MgO and CaO prepared by a modified aerogel/hypercritical drying procedure (abbreviated as AP-CaO) and AP-MgO, were found to be superior to conventionally prepared (henceforth denoted as CP) CP-CaO, CP-MgO, and commercial CaO/MgO catalysts for the dehydrochlorination of several toxic chlorinated substances.51 52 The interaction of 1-chlorobutane with nanocrystalline MgO at 200 to 350°C results in both stoichiometric and catalytic dehydrochlorination of 1-chlorobutane to isomers of butene and simultaneous topochemical conversion of MgO to MgCl2.53-55 The crystallite sizes in these nanoscale materials are of the order of nanometers ( 4 nm). These oxides are efficient due to the presence of high concentration of low coordinated sites, structural defects on their surface, and high-specific-surface area. 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

Other substituent groups are usually introduced via Hydrosilation reactions. In this important reaction, Si-H bonds add across C=C or C=C bonds in 1,2 fashion to give products. Many transition metals and their complexes will catalyze hydrosilation, but the usual catalyst is platinum see Hydrosilation Catalysis). Thus CF3CH2CH2Si(Me)Cl2 is made by hydrosilation of 3,3,3-trifluoropropene with methyldichlorosilane (equation 12). n-Alkylmethyldichlorosilanes can be made similarly, by hydrosilation of l-aUcenes. Vinylmethyldichlorosilane may be obtained in analogous fashion by hydrosilation of acetylene, as shown in equation (13). An alternate route to the same compound is hydrosilation of vinyl chloride, followed by dehydrochlorination (equations 14 and 15). 

Benzylindazole and l-benzyl-4-bromo-3,5-dimethylpyrazole were efficiently debenzylated with potassium tert-butoxide in DMSO and oxygen at room temperature <2002TL399>. Alkylation of pyrazoles 443 with 2,2-dichloro-ethyl ether under conditions of phase-transfer catalysis gave A -[2-(2-chloroethoxy)ethyl]pyrazoles 444, which underwent dehydrochlorination with potassium hydroxide in DMSO giving A -(2-vinyloxyethyl)pyrazoles 445 (Scheme 51) <2004RJC1264>. 

A general method for the preparation of 1,3-diynes is exemplified by the synthesis of compound 45 condensation of the acetylene 43 with cis-1,2-dichloroethene in the presence of Pd(PPh3)4.CuI gives 44, which is dehydrochlorinated by the action of tetra-butylammonium fluoride in THF . Conjugated ( )-enynes 48 are obtained from terminal acetylenes 46 (R = Bu, Ph or Me3Si) and the ( -bromo ester 47 under catalysis by (Ph3P)2PdCl2. Cul and triethylamine. The same catalyst system promotes the condensa-... 

Nitriles are produced from aldehydes in high yield by reaction of the latter with hydrazine hydrate under cyanide-ion catalysis. A method of limited utility involves the dehydrochlorination of a-nitrosophosphonium... 

Blends of polychloroprene with PMMA do not show the chlorine radical migration effects observed in PVC/PMMA blends, nor the stabilization of the dehydrochlorination reaction, both of which are consistent with the nonradical mechanism proposed for HCl production in the former polymer, but there is a major effect of HQ in producing PMMA stabilization. Blends of PVC with PMMA also show the effect of HCl, but this is masked by the earlier destabilization due to Q radicals. Acetic acid from PVA also leads to anhydride rings in PMMA with similar effects. Blends of PVC with PVAC show mutual catalysis of the decomposition of each polymer by the primary decomposition product of the other. This is seen both in TVA studies similar to those illustrated for the PVC/PMMA system and from direct measurements of acid production, as illustrated in Figure 18, which show clearly the destabilization of each polymer. 

Nome, R, Rubira, A.F., Franco, C., lonescu, L.G. Limitations of the pseudophase model of micellar catalysis the dehydrochlorination of l,l,l-trichloro-2,2-bis(p-chlorophenyl)ethane and some of its derivatives. J. Phys. Chem. 1982, 86(10), 1881-1885. 

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