Mercuric bromide

Many mercury compounds are labile and easily decomposed by light, heat, and reducing agents. In the presence of organic compounds of weak reducing activity, such as amines (qv), aldehydes (qv), and ketones (qv), compounds of lower oxidation state and mercury metal are often formed. Only a few mercury compounds, eg, mercuric bromide/77< 5 7-/7, mercurous chloride, mercuric s A ide[1344-48-5] and mercurous iodide [15385-57-6] are volatile and capable of purification by sublimation. This innate lack of stabiUty in mercury compounds makes the recovery of mercury from various wastes that accumulate with the production of compounds of economic and commercial importance relatively easy (see Recycling). 

Mercurous Bromide. Mercurous hi.omide[15385-58-7] Hg2Br2, is a white tetragonal crystalline powder, very similar to the chloride, and prepared in much the same way, ie, by the direct oxidation of mercury by bromine or by precipitation from mercurous nitrate by sodium bromide. It is sensitive to light, less stable than the chloride, and is not of appreciable commercial importance. 

Mercuric Bromide. Mercuric hi.omide[7789-94-7] HgBr2 is a white crystalline powder, considerably less stable than the chloride, and also much less soluble in water (0.6% at 25°C). Therefore, it is prepared easily by precipitation, using mercuric nitrate and sodium bromide solution. Drying of the washed compound is carried out below 75°C. Mercuric bromide has a few medicinal uses. 

Unsaturation value can be determined by the reaction of the akyl or propenyl end group with mercuric acetate ia a methanolic solution to give acetoxymercuric methoxy compounds and acetic acid (ASTM D4671-87). The amount of acetic acid released ia this equimolar reaction is determined by titration with standard alcohoHc potassium hydroxide. Sodium bromide is normally added to convert the iasoluble mercuric oxide (a titration iaterference) to mercuric bromide. The value is usually expressed as meg KOH/g polyol which can be converted to OH No. units usiag multiplication by 56.1 or to percentage of vinyl usiag multiplication by 2.7. 

Tin tetrachloride has been used to prepare the stericaHy hindered triisopropylchlorosilane [13154-24-0] (119). Organobromosdanes are obtained under similar conditions through reaction with cupric and mercuric bromide. These reactions are most suitable for stepwise displacement of hydrogen to form mixed hydridochlorosilanes or in systems sensitive to halogen (120). Hydrides have also been displaced using organic bromides. Heating triethylsilane and... 

Mercuric bromide [7789-47-1 ] M 360.4, m 238.1°. Crystd from hot saturated ethanolic soln, dried and kept at 100° for several hours under vacuum, then sublimed. POISONOUS. 

It has been reported recently that 17/ -acetoxy-5a,14a-androstan epimerizes at C-14 when photolyzed in cyclohexane solution in the presence of mercuric bromide. When the reaction is carried out in perdeuterated cyclohexane, the product consists of 55 % d - and 12% d2-labeled species. This reaction may develop into an interesting deuteration technique if the incorporated deuterium can be securely assigned to the epimerized position. 

Swain and Eddy have queried the wide applicability of the S l and Sif2 mechanisms and favored a push-pull termolecular process for the reaction of pyridine with methyl bromide in benzene solution for example, they have suggested that the effects observed on the addition of methanol, phenol, p-nitrophenol, and mercuric bromide to the reaction mixture can be explained by an intermediate of type 168. ... 

Another hydro-de-diazoniation method in which the solvent is the reagent is that of dediazoniation in hexamethylphosphoric acid triamide (HMPT) it was discovered by Newman and Hung (1974). The mechanism is discussed in some detail in Section 8.9. We do not recommend it for synthetic purposes, however, as (solid) arenediazonium mercuric bromide complexes are used in all but one case. The authors mention only briefly that 4-toluenediazonium tetrafluorophosphate can also be used. 

Subsequent kinetic work has amply confirmed the mechanistic picture described above. For example, the reaction of diphenylmercury with Ph(COOEt).CH.HgBr gives an almost instantaneous reaction with precipitation of phenylmercuric bromide, whereas reaction of the soluble product with a second molar equivalent of mercuric bromide gave a very slow (ca. 2 weeks) precipitation of phenylmercuric bromide722, i.e. reaction involves (287) and (288)... 

The fast reaction clearly involves the transition state (XLVIII) as confirmed by the observation of Reutov etal.123 that all of the label in the mercuric bromide... 

The reaction starts spontaneously and is mildly exothermic. Moderating the temperature by use of a water bath diminishes the amount of bromine and product carried off by the carbon dioxide evolved. The reaction can be followed by use of a tetra-chloroethane bubbler, and at the end of the reaction the solvent in the bubbler can be used to wash the mercuric bromide. The checkers followed the reaction with a wet test meter presaturated with carbon dioxide 52-60% of the theoretical amount of carbon dioxide was evolved. 

The checkers used a sintered glass pressure filter (Coming Glass Works, Cat. No. 34020) rather than a suction filter in order -to minimize evaporation losses. An ordinary water aspirator can cause the mixture to boil at room temperature. The flask and filter can be cleaned readily with a little acetone, which dissolves mercuric bromide rapidly. 

In the Koenigs-Knorr method and in the Helferich or Zemplen modifications thereof, a glycosyl halide (bromide or chloride iodides can be produced in situ by the addition of tetraalkylammonium iodide) is allowed to react with a hydrox-ylic compound in the presence of a heavy-metal promoter such as silver oxide, carbonate, perchlorate, or mercuric bromide and/or oxide,19-21 or by silver triflu-oromethanesulfonate22 (AgOTf). Related to this is the use of glycosyl fluoride donors,23 which normally are prepared from thioglycosides.24... 

CIDNP has also been reported in reactions of organomercurials. Emission is observed from the couphng product of p-methylbenzyl-mercuric bromide and triphenylmethyl bromide (Beletskaya et al., 1971), while thermolysis of organomercury derivatives of tin such as t-C4H9HgSn(CH3)3 gave mixtures of isobutene and isobutane (by disproportionation of uncorrelated pairs of t-butyl radicals) showing A/E polarization (Mitchell, 1972). 

The reactions of mercuric iodide, mercuric bromide, and mercuric chloride with the excited species produced in the hexafluoroethane plasma were examined first, as the expected products were known to be stable and had been well characterized 13). Thus, these reactions constituted a "calibration of the system. Bis(trifluoromethyl)mercury was obtained from the reaction of all of the mercuric halides, but the highest yield (95%, based on the amount of metal halide consumed) was obtained with mercuric iodide. The mole ratios of bis(trifluoro-methyDmercury to (trifluoromethyl)mercuric halides formed by the respective halides is presented in Table I, along with the weight in grams of the trifluoromethyl mercurials recovered from a typical, five-hour run. 

Apart from the fluoride, mercuric halides react explosively with potassium like all analogues of the other metals already mentioned. With mercurous salts, the reaction seems less violent since with mercurous chloride, molten potassium causes the mixture to incandesce without ever combusting. It is likely that other metals react too an extreme violent reaction was mentioned between indium and mercuric bromide. 

In the same study12 trimethyl-dg-bromogermane was synthesized by treating trimethyl-d9-germane with mercuric bromide (equation 12). 

Although aliphatic alcohols are typically poor acceptors in the Mitsunobu-type glycosylation, Szarek and coworkers have highlighted one advance to this end [95]. For the triphenylphosphine and diethylazodicarboxylate promoted glycosylation of a monosaccharide acceptor, the addition of mercuric bromide is necessary to promote the reaction. For example, the (1,6)-disaccharide 44 was obtained in 80% yield using this modified Mitsunobu protocol. Unlike previous examples with phenol or N-acceptors, preactivation of the hemiacetal donor was performed for 10 min at room temperature prior to addition of the aliphatic alcohol nucleophile. 

An efficient mechanical stirrer is required. Near the end of the bromination the precipitate of mercuric bromide becomes quite heavy. 

Apart from the improvements to be geiined by attention to these factors, much better yields of disaccharides are obtained when acyclic alcoholic compounds eire used and in other modifications of usual procedures it has been recommended that mercuric oxide in the presence of small amounts of mercuric bromide acts efficiently as an acid acceptor and has the added advantage of low cost and photostability 28), German workers in particular have preferred to use trityl ethers rather than... 

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