Mercuric bromide groups

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. 

A second method is based on the abnormal course of the Koenigs-Knorr reaction with fluoroalkyl alcohols. Indeed, when there are two or three methylenes between the Rf group and the hydroxyl, the reaction does not lead to substitution of the anomeric bromide but instead affords an orthoester. In the presence of mercuric bromide, this orthoester can undergo a rearrangement into an (9-fluoroalkyl glycoside (Figure 6.47). 

Hence the observation that, for example, mercuric acetate reacts with a given substrate in a given solvent faster than does mercuric bromide can be interpreted in at least two ways (i) the mechanism of reaction is SE2(open) and mercuric acetate is a more powerful electrophile than is mercuric bromide, and (ft) the mechanism of reaction is SE2(cyclic) and mercuric acetate is better able to act as a bridging group in a six-centred transition state than is mercuric bromide in a four-centred transition state. The possibility that the two salts might be reacting by different mechanisms must also be considered. 

Most electrophilic allylic rearrangements involve loss of hydrogen, but they have also been observed with metallic leaving groups. Sleezer, Winstein, and Young found that crotylmercuric bromide reacted with HCl 10 times faster than n-butyl-mercuric bromide and the product was >99% 1-butene. " These facts point to an SeI mechanism (lUPAC designation cyclo- l >ID k nk ... 

Moreover, cyclopropyl radicals, generated by the NaBH4 reduction of cyclopropyl-mercuric bromide in the presence of excess olefins possessing one or two electron-withdrawing groups, yielded the addition product in good yields (60%) (Table 8). 

Air, the cheapest oxidant, is used only rarely without irradiation and without catalysts. Examples of oxidations by air alone are the conversion of aldehydes into carboxylic acids (autoxidation) and the oxidation of acyl-oins to a-diketones. Usually, exposure to light, irradiation with ultraviolet light, or catalysts are needed. Under such circumstances, dehydrogenative coupling in benzylic positions takes place at very mild conditions [7]. In the presence of catalysts, terminal acetylenes are coupled to give diacetylenes [2], and anthracene is oxidized to anthraquinone [3]. Alcohols are converted into aldehydes or ketones with limited amounts of air [4, 5, 6, 7], Air oxidizes esters to keto esters [3], thiols to disulfides [9], and sulfoxides to sulfones [10. In the presence of mercuric bromide and under irradiation, methylene groups in allylic and benzylic positions are oxidized to carbonyls [11]. 

Mercuric bromide, HgBrj (mp 236 °C), oxidizes methylene groups that are adjacent to double bonds or aromatic rings to carbonyls [11]. 

Methylene groups adjacent to aromatic rings are oxidized to keto groups by oxygen with chromium sesquioxide as a catalyst [1128] or by mercuric bromide [11], ceric ammonium nitrate [380, 417, 422], selenium dioxide [509], sodium dichromate [622, 625], pyridinium chlorochromate [607], manganese dioxide [814], potassium permanganate [866, 877], and alkyl nitrites [452]. 

The corresponding iodide is prepared in a similar manner to the bromide. It may be crystallised from benzene or acetone and melts at about 80° C. It is more readily decomposed than the bromide, and hydrochloric or acetic acids break it down to allyl alcohol and mercuric salts. The presence of the two hydroxyl groups has been shown by the fact that the iodide yields a dibenzoyl derivative, M.pt. 100° C. When the iodide is treated with iodine at 40° C. a liquid iodohydrine is obtained. 

The chloride is very soluble in acetone, soluble in hot alcohol or ether, only slightly soluble in water. W hen treated with bromine in potassium bromide for tw o hours on a shaking machine, and then extracted with ether, o-bromonitrobeiizene, M.pt. 41° to 42° C., is obtained, which shows the mercury to be in the ortho position to the nitro group. No mercuric sulphide is obtained when the chloride is acted upon by ammonium sulphide, but when boiled with hydrochloric acid nitrobenzene results. During the direct mercuration of nitrobenzene, mercury o-o dinitrodi henyl, M.pt. 206° C., is also formed. ... 

---