HC1 scavenger

Shepherd and Gilbert [812] have used SEC analyses with 50-, 100- and 500-A gel columns of TNPP and epoxidised oil (HC1 scavenger) extracted from PVC sheeting that was subjected to a controlled heat treatment. They were able to identify rapid breakdown of the hydrolytically unstable TNPP at 170 °C... 

Heat stabilisers for PVC act by HC1 scavenging and include organotins, mixed metal salt blends, and lead compounds. The latter account for nearly 64 % of volume (in 1994), followed by barium/cadmium and organotin compounds. Cadmium-based heat stabilisers are rapidly being replaced due to environmental concerns. Barium/zinc and calcium/zinc compounds show a high growth rate. It is expected that methyltin stabilisers will soon dominate the growing PVC pipe market. 

A series of 2-chloro-l,3,2-oxazaphospholidine derivatives 131a-f were prepared by reaction of six enantio-pure alcohols 129a-f with phosphorus trichloride carried out in the presence of an organic base as HC1 scavenger (Scheme 38) [69], The 31P and II-NMR spectra of crude 131a, d, e containing a small amount of the HC1 salt produced during the synthesis of 131, as well as the distilled samples, indicated that the formed chloro derivatives were ca. 1 1 mixtures of the cis and trans isomers. 

The order of addition should probably be reversed, i.e., the solution of 2-amino-3-naphthol in benzene should be reasonably dilute and be added slowly to warm thionyl chloride. The HC1 scavenger may either be dissolved directly in the benzene solution or added separately at an appropriate rate. 

Chlorination of steroids (4, 264-265 5, 352-353 6, 298-299). Breslow s method for chlorination of steroids at CV has been developed into a useful method for conversion of 17a-hydroxyprogesterone (1) into the triene (4), a useful intermediate to highly active corticoids.2 The hindered I7a-hydroxyl group is esterified with m-iodobenzoic anhydride and 4-dimethylaminopyridine. Irradiation of the ester (2) with C6H5IC12 in the presence of NaOAc (HC1 scavenger) affords 3 in quantitative... 

The historical direct reaction route, which utilized phosgenation of a solution of BPA in pyridine, proved inefficient commercially because of the need for massive pyridine recycle. Calcium hydroxide was used as an HC1 scavenger for a period of time. In the historical transesterification process, BPA and diphenyl carbonate are heated in the melt in the presence of a catalyst, driving off by-product phenol, which is recycled to diphenyl carbonate. Using a series of reactors providing higher heat and vacuum, the product polymer was eventually produced as a neat melt. 

The study of PVC stabilisation by stannic compounds in the presence of HC1 scavengers shows that the essential stabilisation process is nucleophilic substitution of the allylic chlorine by the thioglycolate or mercaptopropionate groups. It is claimed that their capability to form complexes with HC1 and transport this degradation catalyst to HC1 scavengers are essential for the marked synergism observed between these two types of stabilisers. 14 refs. 

Tabazadeh, A., and R.P. Turco, Stratospheric chlorine injection by volcanic eruptions HC1 scavenging and implications for ozone. Science 260, 1082, 1993. 

Use of TiCl and HC1 scavenger allows the Claisen rearrangement to be carried out under mild conditions. 

The practical importance of Reaction 4 stems from the fact that HC1 in rocket exhausts behaves as an electron scavenger, rapidly reducing electron concentrations. This fact, together with the above discussion, emphasizes the need for a more thorough investigation of the values of 4 and k 4. Detachment of electrons from Cl- through... 

Addition of aluminum alkyls enhances the reactivity of RuCl2(PPh3)3/ H2-catalyzed systems primarily by scavenging HC1 according to reaction (22), and this promotes hydride formation, cf. Eq. (4) (122, 123). 

For reasons which will become apparent in section 1.8.2, it is assumed that each electron-ion pair eventually produces one molecule of hydrogen. In an earlier section (p. 160) it was seen that in HC1 an additional 0.20+0.2 hydrogen atoms per ion pair should come from dissociative ionisation and charge transfer processes. In the absence of hydrogen atom scavengers they will undergo the reaction... 

The yield Gc, of scavengeable electrons was 3.8+0.2, in agreement with Gion)sation (= 4.00 + 0.07). t was found to be 1 x 10-9 sec in HC1 at 560 torr. This is several orders of magnitude smaller than the lifetimes if there were homogeneous ion combination at conventional dose rates, viz. 

The scavenging of (HC1)( ) by trace concentrations of chlorine explains the abrupt initial drop in the yield of hydrogen from HC1 at —77 °C (see Fig. 5(b)). The more gradual reduction of G(H2) over the [C12]/[HC1] range from 10-4 to 10 2 has been attributed to a competition between HC1 and chlorine for hydrogen atoms, viz. 

It should be unaffected by the presence of Cl2 and Br2, which could not compete effectively with these reactions at the halogen HC1 ratios shown in Fig. 5(a) and (b). Thus hot atom reactions can account for the residual (or unscavengeable) hydrogen, GH = 2.2, as well as contribute to the yield of scavengeable hydrogen atoms. 

Fig. 6. Effects of scavengers on the hydrogen yields from y-irradiated liquid HC1. +Br2, Cl2 O C2H4. Concentrations of Cl2 and C2H4 have been multiplied by h+ci2/ h+bt2 and kH+C2 J Ah+b,2 respectively (taken from the data of refs. 107,117). 

Ene and 2 + 2 cycloaddition reactions of acetylenic esters (7, 7 8 8, 13). In the early studies on the reaction of acetylenic esters with alkenes, Snider used A1C1( as the Lewis acid catalyst. The presently preferred catalyst is C2H5A1C12, which is a Lewis acid and also serves as a proton scavenger by reaction with HC1 to give ethane and AIC13. It is generally used in amounts close to 1 equivalent for a neutral Hlkene. 

Sulfonyl chlorides having an a-hydrogen are unstable under basic reaction conditions and can give variable results [96,97]. For base-labile sulfonyl chlorides, the use of O-silyl ketene acetals as scavengers for HC1 has been recommended [96]. Table 8.7 lists some illustrative procedures for the preparation of sulfonamides from primary amines on solid phase. Further examples have been reported [98-101]. 

Cyclization ofsilyl enol ethers of -acetylenic ketones or aldehydes. Reaction of these derivatives of ynones or -als with HgCl2 (1.1 equiv) and hexamethyldisi-lazane (0.2 equiv.) as an acid scavenger in CH2C12 at 30° followed by acidification with aqueous HC1 and Nal (2 equiv. each) results in cyclic 0,y-enones. Thermal cyclization of these substrates requires high temperatures, which lead to decomposition and rearrangements.1... 

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