Hydrogen chloride elimination

Preparation from Amines. The most common method of preparing isocyanates, even on a commercial scale, involves the reaction of phosgene [75-44-5] and aromatic or aUphatic amine precursors. The initial reaction step, the formation of N-substituted carbamoyl chloride (1), is highly exothermic and is succeeded by hydrogen chloride elimination which takes place at elevated temperatures. 

Because of its importance in biological areas, special efforts have been made with the synthesis of the thiazolo[2,3-t]-[l,2,4]thiadiazole derivative 308 <2001BML1805>. The pathway started from the benzothiazole derivative 305 which was treated with chlorosulfonylacetyl chloride to form an intermediate 306, which underwent cyclization to a second intermediate 307 with hydrogen chloride elimination. The last step is the attack of the first intermediate 306 at the thiadiazine carbon atom to form the final product 308. 

Chloramine-T has also been employed, both as a halogenating reagent and base the reaction proceeds in good yield with aromatic as well as with aliphatic aldoximes (81). The role of chloramine-T probably involves an initial chlorination of the aldoxime to give the hydroximoyl chloride, followed by base-catalyzed hydrogen chloride elimination to afford the nitrile oxide (81). 

Paudler observed alcohol addition to the N( 1)—C(6) bond of 1,2,4-triazine 2-oxides unsubstituted at the 6-position (96) in the presence of hydrogen chloride. Elimination of water followed, leading to 6-alkoxy-1,2,4-triazines (97) (77JOC3489). 

The compounds resulting from the reaction of 748 were characterized by HRMS directly coupled to the reactor. The stable products 750 and 751 were analysed by GC and 111 NMR spectroscopy. The formation of the cyclodisilazane 750 is explained by dimerization of the unstable silanimine 749 only in the cold trap, as the reaction is carried out under high dilution conditions (equation 247). It was also shown that the hydrogen chloride elimination did not occur in the ion source of the mass spectrometer. 

The head-to-tail structure of poly (vinyl chloride) permits the continuous regeneration of an allylic chloride moiety as hydrogen chloride elimination proceeds along a chain. Thus, once initiated, loss of hydrogen chloride may proceed along a polymer chain without abatement. 

The first term represents the unimolecular decomposition of allylic chloride. The first and fourth terms are responsible for hydrogen chloride elimination and do not result in stabilization. The second and third terms are responsible for stabilization. This work has provided an estimate of the rate constants for dibutyltin dilaurate and dibutyltin bis (monobutyl maleate). 

Table II includes the reaction half-time (6410 minutes) calculated for the unimolecular hydrogen chloride elimination for 4-chloro-2-hexene...

The behavior of dibutyltin bis(dodecylmercaptide) on reaction with 4-chloro-2-pentene proved interesting (Figure 3). With only the two reactants in chlorobenzene, virtually no reaction took place up to 5 hours. However, the addition of dibutyltin dichloride resulted in a rapid reaction. Furthermore, the addition of a few milligrams of azobisisobutyro-nitrile eliminated any induction period. This latter consequence is not interpreted to result from a free radical stabilization mechanism, but it is presumed to be caused by free radical-catalyzed hydrogen chloride elimination, resulting (by neutralization with the stabilizer) in the formation... 

Addition of tellurium tetrachloride to 2,2-diphenylpent-4-en-1-oic acid followed by hydrogen chloride elimination produced bis[3,3-diphenyl-2-oxotetrahydrofuran-5-yl-methyl tellurium, dichloride2. 

In addition to comonomers, nylons are frequently used in blends. The pyrolysis of blends typically shows little interaction between the compounds generated from the individual blend components. However, a study on the co-pyrolysis of several polyamides in the presence of PVC showed interactions [40]. The study was done on nylon-12, nylon-6,6 and poly(1,4-phenylene terephthalamide) (Kevlar) in the presence of poly(vinyl chloride). Polyamide-PVC mixtures (typical mass ratio 1 1) were pyrolyzed at 700 and 900°C. It was found that the presence of PVC promoted the hydrol ic decomposition routes of amide groups and volatile nitrile formation from all examined polyamides due to the hydrogen chloride eliminated from PVC under pyrolysis. In the presence of PVC, an elevated yield of alkenenitriles was observed from nylon-12. For Kevlar in the presence of PVC, it was noticed the evolution of benzeneamine, benzoic acid, benzenenitrile and benzeneisocyanate. At 900°C in the presence of PVC, an enhanced evolution of HCN from nylon-12 and nylon-6,6 was noticed. 

One can open the epoxide, however, by treating it with sodium acetate in acetic acid at 85 °C for 48 hours and obtain the gylcol acetate, 12, in quantitative yield. Treatment of tridiphane with one equivalent of sodium methoxide in methanol at room temperature causes hydrogen chloride elimination and formation of the vinyl epoxide, 13, which has little herbicidal activity. In contrast to sodium methoxide, sodium methylthiolate readily opens the epoxide ring giving the / -thioalcohol, 14 in quantitative yield. 

Hydrogen chloride elimination from -chloroaldimines 6 and -chloroimines 8 lead from allylic intermediates to cyclopropyl products 7 and 9, respectively. However, they are regarded as 1,3- and 1,5-elimination of hydrogen chloride, respectively, and, therefore, are treated in Section 1.1.2. 

Attempts to synthesize perchloropropenylbenzenes, such as a,p-dichloro-propenylbenzene, with reagent BMC had been unsuccessful (Ballester and Riera, 1960). An alternative way was devised, consisting in the condensation of perchlorotoluene with trichloroethylene in methylene chloride by means of aluminium chloride. Such a process had to surmount significant steric hindrance, particularly the formation of a sterically strained intermediate adduct, and therefore the reaction had to be conducted at moderate temperature. It was nevertheless expected that steric strain would assist the subsequent hydrogen chloride elimination to form the ethylene bond (72). 

Because practically no PVC is processed and used without the addition of stabilizers, one should know the residual stability of a used PVC product. For this the best way may be the determination of the hydrogen chloride elimination at 180°C under air or nitrogen [50]. The conversion-time curves so obtained provide indication of the residual stability from the induction period and also enable calculation of the rate of HCl split-olf after consumption of the stabilizers. In some cases, however, it maybe sufficient to use a simple Congo Red test (e.g., according to DIN 53 418) instead of the apparatus for measuring the HCl elimination. 

The hydrogen chloride eliminated in the reaction is neutralised by salt formation with some of the amide solvent... 

Mole and Turner 205a) have studied the effects of substituents on the rates of hydrogen chloride elimination to produce 10-chlorophenoxarsines. 

Such transient species can be remarkably strained. For example, hydrogen chloride elimination from chloroquadricyclane with f-butyllithium or n-butyllithium/potassium tert-bntoxide leads to the formation of didehydroquadricyclane which can be trapped as a Diels-Alder adduct. The olefinic strain energy (i.e. the difference between the heat of hydrogenation of the strained alkene and that of an unstrained reference alkene) for this alkene was evaluated as 71kcal/mol (Figure 2.28). ... 

Despite much investigation, the mechanism of thermal hydrogen chloride elimination is still disputed. It was originally assumed that the dehydrochlorination started at the tertiary chlorine atoms. But, according to recent evidence, PVC does not have any tertiary chlorine atoms. However, there are about 0.5-1.5 double bonds per 1 000 carbon atoms. The dehydrochlorination should start at these double bonds and proceed further by an unzipping reaction ... 

Reaction of bis-(2-chloroethyl) H-phosphonate with triethylamine produces a cyclic ester of p-chloroethyl phosphorous acid [288], The rate of this reaction is very low. It has been proposed that in the presence of triethylamine, bis-(2-chloroethyl) H-phosphonate isomerizes into bis-(2-chloroethyl) phosphite. The latter forms the corresponding 2-chloroethyl-l,3,2-dioxaphospholane, accompanied by hydrogen chloride elimination. 

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