Mercurous Ions, Formation

These results have been interpreted in terms of trans addition of mercuric ion and nucleophile where the attack of the mercuric ion takes place from the more hindered side of the diene molecule. A transition state 197, involving an endo attack of mercuric ion with some stabilization by coordination to the 8,9-ethylenic bond to the mercury atom, has been proposed to support the suggested mechanism. Analogously, and in sharp contrast to the results obtained167 in the mercuration of norbomadiene which reacts with mercury salts via the usual scheme of exo-syn addition, the principal pathway in the mercuration of bicyclo[2.2.2]octa-2,5-diene is the formation of endo-syn products (equation 165). 

The BP utilises formation of a derivative in order to quantify penicillins in formulations. Some penicillins do not have distinctive chromophores a further problem with these molecules is that when they are in suspensions they are not readily extracted away from excipients since they are quite insoluble in organic solvents which are immiscible with water. Using the formation of a complex with the mercuric ion in the presence of imidazole as a catalyst, a derivative of the penicillin structure is produced, which has an absorption maximum between 325 and 345 nm. In the assay, comparison with pure standard for the particular penicillin is carried out rather than relying on a standard A(l%, 1 cm) value. This assay is used by the BP for... 

In contrast to this demonstration of bimolecularity, Hall and Lueck82 showed the possibility of acylium ion formation from benzoyl chloride by its reaction with mercuric perchlorate. In common with dimethylcarbamyl chloride, dimethylsulphamyl and tetramethyldiamidophosphochloridate, benzoyl chloride reacted readily to form the corresponding acylium ion /i-butyl chloroformate however was inert. Kivinen138 studied the effect of mercuric chloride on the ethanolysis of 4-methoxybenzoyl chloride, benzoyl chloride and 4-nitrobenzoyl chloride and obtained the following approximate relative rates for the effect of mercuric chloride (0.30 M) in ethanol, 4-MeO, 2.91 4-H, 1.00 4-NOz, 1.03, confirming the SN2 character of the 4-nitro-... 

The formation of the blue complex cobalt salt serves as a sensitive test for the detection of either cobaltous or mercuric ions with a sensitivity of ly Co or lOy of Hg in solution. Depending on the cation to be tested for, the test reagent consists of a concentrated solution of either mercuric chloride or cobaltous acetate containing alkali thiocyanate. The test may be carried out in one drop of liquid on a slide under a microscope (50-100X). Agitation of the drop with... 

In anodic dissolution of mercury in a solution of nitric acid, where both mercurous and mercuric salts are asumed to be completely dissociated, both the formed ions enter the solution in the ratio of their respective activities hKo+/ h1 ++ = 76. When alkali cyanide is used as electrolyte the bivalent ions formed on dissolution are predominantly consumed for the formation of the complex Hg(CN). As a result of the formation of this complex the concentration of free Hg++ jpns decreases considerably in accordance with the neghgible degree of dissociation of the above-mentioned complex, and consequently the dissolution potential of the system Hg/Hgt+ also decreases. For this reason, mercuric ions converted to mercuricyanide complex can be considered to be practically the sole product of the anodic process while the amount of univalent mercury ions is quite negligible. Contrary to this, on dissolving mercury in a solution of hydrochloric acid mercurous ions are predominantly formed due to the slight dissociation of mercurous chloride, the main product of the reaction. 

The relative yields of cyclized products increase greatly with the decreasing nucleophilicity (2a-c, 3 and 4 of Table 3) and with the increasing ionizing power of the solvent. Cyclobutanone derivatives (32) are usually obtained in much greater amounts than cyclopropyl ketones (33) but, when mercuric ions are added, formation of 33 is overwhelming. The effect of R in 31 on the product distribution is illustrated in cases 3b, 4a, 5a, 6a, and 7. 

Addition of mercuric ion to the complex (+)[Co(en)2(NCS)2]+, gives a (107) 1 1 adduct, which has the mercury atom on the two fold axis. It is the A transition which will be most altered by this adduct formation, and the component of the Cotton effect which shifted (87) was positive, so that (+)[Co(en) 2(NCS) z]+ has an A component with positive rotational strength and hence the D configuration. 

Should added unsaturation be desired, the condensation of a 2-cyclopentenone with lithio cyclopropylphenyl sulfide makes possible the formation of an ally lie alcohol which can be dehydrated quantitatively (Scheme 11). Thermal rearrangement, followed by mercuric ion-promoted hydrolysis of the vinylsulfide part structure completes the synthetic transformation. 

As fas as reaction conditions are concerned, two main approaches are usually taken. Either the nucleophilicity of the R5OH to be added is further enhanced by addition of base (normally R50 M +, or nitrogen bases of low nucleophilicity), i.e., base catalysis, or the electrophilicity of the accepting double bond is further increased by adding, e.g., mercuric salts (alkoxymercu-ration), or sources of halonium ions (formation of / -halohydrins). Clearly, the latter protocol, from now on abbreviated as "onium-methods , necessitates a subsequent step for the removal of the auxiliary electrophile, e.g., reductive demercuration of an intermediate /i-alkoxymercu-rial. Whereas base catalysis has successfully been employed with all varieties of acceptors, application of onium-methods thus far appears to be restricted to a,/ -unsaturated carbonyl compounds. Interestingly, conjugate addition of alcohols to a,/l-enones could also be effected photochemically in a couple of cases. 

As already oudined, inhibition is essentially complete when caused by reagents that react with sulfhydryl groups (for example, p-chloro-mercuribenzoate, p-mercuribenzoate, o-iodosobenzoate, L-ascorbic acid silver, cupric, and mercuric ions iodine, and ferricyanide) this inactivation can be reversed to some extent by hydrogen sulfide and by cysteine. Lineweaver—Burk graphs have shown that the action of L-ascorbic acid is noncompetitive, and L-ascorbic acid acting in the presence of cupric ions probably causes formation of an inactive cuprous-enzyme. The action of p-chloromercuribenzoate on barley beta-amylase has been shown to be a competitive inhibition. In contrast, the soya-bean p-chloromercuribenzoate inhibition is noncompetitive, and the extent of inhibition is inversely related to the concentration of acetate ion. The latter exhibits a protective effect, and there... 

Alkene oxymercuration is closely analogous to halohydrin formation. The reaction is initiated by electrophilic addition of (mercuric) ion to the alkene to give an intermediate mercurinium ion, whose structure resembles that of a bromonium ion (Figure 7.5). Nucleophilic attack of water, followed by loss of a proton, then yields a stable organomercury addition product. The final step, reaction of the organomercury compound with sodium boro-hydride, is not fully understood but appears to involve radicals. Note that... 

The rate of methylmercury formation by this process is largely determined by the concentration of methyl cobalamine compounds, inorganic mercuric ions, and the oxygen concentration of the water, with the rate... 

Metabolic, nutritional and toxicological aspects of 160-163 Se were studied. The transport of Se within the organism and the urine metabolites observed were different, depending on the form of Se administred. Rapid and efficient incorporation of Se into SeP in the liver and excretion into the plasma followed by the slow and steady incorporation of Se into GPx in the kidneys and excretion into the plasma described. The formation of Se-methyl-/V-acetylselenohexosa-mine in liver and its excretion via urine were reported. The mechanisms underlying the interaction between Se and mercuric ions in the bloodstream were explained by the formation of a ternary complex, [(HgSe) ], -selenoprotein P... 

In the mercuric ion-promoted reactions, decreasing electron density on the oxygen atom of the alcohol gave an increase in the proportion of a-glucosides and a decreased rate of formation of (3-glucosides. 

The electrode elements of an electrochemical sensor are often metallic, semiconductive, or inert. For metallic electrode elements, noble metals such as gold, platinum, and silver are often used in conventional electrochemical sensors. Mercury or amalgam as electrode materials for microfabricated electrochemical sensors are seldom used due to the difficulty involved. Because mercury has a relatively high vapor pressure, it does not lend itself well in any fabrication process using a vacuum or low-pressure environment. The formation of amalgam requires the use of mercuric ion containing reagent. This step can be elaborate and complicated. 

It must not be assumed that these two rules exhaust the possi bilities of bond formation. Thus mercuric ion combines with chloride ions to form mercuric chloride in which the mercury atom is bonded to the two chlorine atoms. In this process the mercury atom does not acquire an octet, and the chlorine atoms have the same octets in the chloride ions as in mercuric chloride. The bonds in this case may arise from a specific instability of the electron configuration of the mercuric ion. But the bonds which maintain themselves through the greatest vicissitudes arc those in which the atoms have achieved noble-gas electron configurations. 

The first step in the mercuric-ion-catalyzed hydration of an alkyne is formation of a cyclic mercurinium ion. (Two of the electrons in mercury s filled 5d atomic orbital are shown.) This should remind you of the cyclic bromonium and mercurinium ions formed as intermediates in electrophilic addition reactions of alkenes (Sections 4.7 and 4.8). In the second step of the reaction, water attacks the most substituted carbon of the cyclic intermediate (Section 4.8). Oxygen loses a proton to form a mercuric enol, which immediately rearranges to a mercuric ketone. Loss of the mercuric ion forms an enol, which rearranges to a ketone. Notice that the overall addition of water follows both the general rule for electrophilic addition reactions and Markovnikov s rule The electrophile (H in the case of Markovnikov s rule) adds to the sp carbon bonded to the greater number of hydrogens. 

Antidotes can change the chemical nature of a poison by rendering it less toxic or preventing its absorption. Formaldehyde poisoning can be treated with ammonia to promote formation of hexamethylenetetramine sodium formaldehyde sulfoxylate can convert mercuric ion to the less soluble metallic mercury and sodium bicarbonate converts ferrous iron to ferrous carbonate, which is poorly absorbed. Chemical inactivation techniques seldom are used today, however, because valuable time may be lost, whereas emetics, activated charcoal, and gastric lavage are rapid and effective. The treatment of choice for ingestion of either acids or alkalis is dilution with water or milk. Similarly, bums produced by acid or alkali on the skin should be treated with copious amounts of water. 

The formation of alkylmercurials from elementary mercury and from its inorganic substances in waters of oceans and lakes is the basic element in the whole chain of mercury conversion in the nature. In the upper sedimentation layers of the sea or lake bottom, methylation of elementary mercury or of mercuric ions occurs with the formation of methyl mercury. The methyl mercury is retained by small aquatic organisms which either serve as feed for fish or die, and during their decomposition dimethyl mercury vapours are released into the atmosphere. In the atmosphere, in acid medium (acid rain) dimethyl mercury can be demethylated to form methyl mercury, which can re-enter the nutritional chain in water. 

In addition to the standard Watson-Crick base pairing it has recently been discovered that thymine can selectively bind to mercuric ion (Hg ) via the formation of a thymine-Hg -thymine (T-Hg -T) complex (72). The presence of the Hg at the thymine-thymine mismatch leads to a significant increase ( 10°C) in the T . Mirkin and coworkers (40) incorporated this T-Hg "-T coordination chemistry in the DNA-AuNP system to design a novel colorimetric assay for detecting Hg " "(Fig. 12.15). In a typical assay, two batches of AuNPs were functionalized with two complementary sequences, respectively, and combined to form aggregates. 

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