Mercurous ion

Reference Electrodes and Liquid Junctions. The electrical cincuit of the pH ceU is completed through a salt bridge that usually consists of a concentrated solution of potassium chloride [7447-40-7]. The solution makes contact at one end with the test solution and at the other with a reference electrode of constant potential. The Hquid junction is formed at the area of contact between the salt bridge and the test solution. The mercury—mercurous chloride electrode, the calomel electrode, provides a highly reproducible potential in the potassium chloride bridge solution and is the most widely used reference electrode. However, mercurous chloride is converted readily into mercuric ion and mercury when in contact with concentrated potassium chloride solutions above 80°C. This disproportionation reaction causes an unstable potential with calomel electrodes. Therefore, the silver—silver chloride electrode and the thallium amalgam—thallous chloride electrode often are preferred for measurements above 80°C. However, because silver chloride is relatively soluble in concentrated solutions of potassium chloride, the solution in the electrode chamber must be saturated with silver chloride. 

After inorganic mercuric salts are absorbed and dissociated into the body fluids and in the blood, they are distributed between the plasma and erythrocytes. Aryl mercuric compounds and alkoxy mercuric compounds are decomposed to mercuric ions, which behave similarly. 

Chloride. Chloride is common in freshwater because almost all chloride salts are very soluble in water. Its concentration is generally lO " to 10 M. Chloride can be titrated with mercuric nitrate. Diphenylcarbazone, which forms a purple complex with the excess mercuric ions at pH 2.3—2.8, is used as the indicator. The pH should be controlled to 0.1 pH unit. Bromide and iodide are the principal interferences, whereas chromate, ferric, and sulfite ions interfere at levels greater than 10 mg/L. Chloride can also be deterrnined by a colorimetric method based on the displacement of thiocyanate ion from mercuric thiocyanate by chloride ion. The Hberated SCN reacts with ferric ion to form the colored complex of ferric thiocyanate. The method is suitable for chloride concentrations from 10 to 10 M. 

Hydrogen SulBde. Sulfide ion from 10 to 1 Af can be measured potentiometricaHy with an ion-selective electrode. Mercuric ion interferes at concentrations >10 M. The concentration of hydrogen sulfide can be calculated knowing the sample pH and the piC for H2S. 

The reaction is fairly exothermic. Cooling is advisable. An increase in acid and mercuric ion concentrations results in a faster reaction starting at a lower temperature. 

The best characterized of these reactions involve the mercuric ion, Hg ", as the cation. The same process occurs for other transition-metal cations, especially Pd, but the products often go on to react fiirther. Synthetically important reactions involving Pd will be discussed in Section 8.2 of Part B. The mercuration products are stable, and this allows a relatively uncomplicated study of the addition reaction itself The usual nucleophile is the solvent, either water or an alcohol. The tenn oxymercuration is used to refer to reactions in... 

The reactivity of mercury salts is a fimction of both the solvent and the counterion in the mercury salt. Mercuric chloride, for example, is unreactive, and mercuric acetate is usually used. When higher reactivity is required, salts of electronegatively substituted carboxylic acids such as mercuric trifiuoroacetate can be used. Mercuric nitrate and mercuric perchlorate are also highly reactive. Soft anions reduce the reactivity of the Hg " son by coordination, which reduces the electrophilicity of the cation. The harder oxygen anions leave the mercuric ion in a more reactive state. Organomercury compounds have a number of valuable synthetic applications, and these will be discussed in Chapter 8 of Part B. 

Allenes react with other typical electrophiles such as the halogens and mercuric ion. In systems where bridged-ion intermediates would be expected, nucleophilic capture generally occurs at the allylic position. This pattern is revealed, for example, in the products of solvent capture in halogen additions and by the structures of mercuration products. ... 

Knabe has introduced mercuric acetate plus ethylenediaminetetraacetic acid (EDTA) as an oxidizing agent for tertiary amines (74). The solvent employed is 1 % aqueous acetic acid. In this system, the complexed mercuric ion is reduced to elemental mercury. Knabe s studies have centered on the... 

Reaction of acetone with D30+ yields hexadeuterioacetone. That is. all the hydrogens in acetone are exchanged for deuterium. Review the mechanism of mercuric ion-catalyzed alkyne hydration, and then propose a mechanism for this deuterium incorporation. 

Alkoxymercuration reaction (Section 18.2) A method for synthesizing ethers by mercuric-ion catalyzed addition of an alcohol to an alkene. 

These figures furnish a handy summary of solubility behavior. We see from Figure 10-5A that few chlorides have low solubilities. The few that do contain cations of metals clustered toward the right side of the periodic table (silver ion, Ag+, cuprous ion, Cu+, mercurous ion, HgJ2, and lead ion, Pb+2) but they do not fall in a single column. This irregularity is not un-... 

The acceleration by anions under both conditions was attributed to displacement of one of the water molecules presumed to be tetrahedrally coordinated with the mercuric ion, the subsequent reaction being then envisaged as displacement of the anion or water molecule by the aromatic the anions which cause reaction to take place more slowly were presumed to be more tightly bound to the mercuric ion than water. It has, however, been pointed out that less tightly bound anions would be unlikely to displace the more tightly bound water molecules in the first place438. 

Consider the oxidation of mercurous ion by thallium(3+) ion in aqueous solution.13 The reaction and rate law are... 

The oxidation of mercurous ions by thallium (3+) ion shows an inverse first-order dependence on [Hg2+], which means that one Hg2+ ion must be subtracted out in figuring the composition.3 Thus, we have... 

Reaction of the environment with the starting material The commonest example of this type of interaction is the protonation of the substrate by acids in the electrolysis medium, but pH effects will be dealt with in a later section. There are, however, other chemical interactions which can occur. For example, the mechanism and products of the oxidation of olefins are changed by the addition of mercuric ion to the electrolysis medium. In its absence, propylene is oxidized to the allyl cation (Clark et al., 1972),... 

The hydration of triple bonds is generally carried out with mercuric ion salts (often the sulfate or acetate) as catalysts. Mercuric oxide in the presence of an acid is also a common reagent. Since the addition follows Markovnikov s rule, only acetylene gives an aldehyde. All other triple-bond compounds give ketones (for a method of reversing the orientation for terminal alkynes, see 15-16). With allqmes of the form RC=CH methyl ketones are formed almost exclusively, but with RC=CR both possible products are usually obtained. The reaction can be conveniently carried out with a catalyst prepared by impregnating mercuric oxide onto Nafion-H (a superacidic perfluorinated resinsulfonic acid). ... 

Thus, both elemental mercury and the mineral form cinnabar (HgS) can release Hg++, the mercuric ion. Bacteria can then methylate it to form sequentially CH3 Hg+, the methyl mercuric cation, and dimethyl mercury. The latter, like elemental mercury, is volatile and tends to pass into the atmosphere when formed. The methylation of mercury can be accomplished in the environment by bacteria, notably in sediments. 

Arsenite and mercuric ions react with the —SH groups of lipoic acid and inhibit pyruvate dehydrogenase, as... 

The free-radical scheme, however, fails to account for the following (i) It cannot be easily generalised to cover the identical kinetics of the Mn(lII) sulphate oxidation if -CH(C02H) has an oxidation potential comparable with Mn(Ill)/ Mn(II) pyrophosphate then it cannot appreciably reoxidise Mn(ll) sulphate, (a) If -CH(C02H) reoxidises Mn(II) sulphate then it should be capable of re-oxidising both V(1V) sulphate (of the V(V)/V(IV) pair, potential 1.0 V) and Mn(II) sulphate in the V(V) oxidation of malonic acid that it does neither can be seen from the rate laws of these oxidations which show no Mn(II)-retardation vide infra). Hi) The not dissimilar kinetics of the Mn(III) sulphate oxidation of formic acid vide supra) and mercurous ion °. 

A. Synthetic Methods.—There have been no strikingly new approaches to the general problem of phosphorylation, but several ingenious methods of preparing suitable active esters under mild conditions have been reported. Typical of these is the reactive intermediate (1) formed from reaction of a mono- or di-ester of phosphoric acid with (2), itself produced by reaction of triphenylphosphine with bis(2-pyridyl) disulphide (preferably in the presence of mercuric ion as scavenger for the 2-mercaptopyridine liberated). 

Compound (1) phosphorylates phosphate monoesters and alcohols, although with the latter a considerable excess of alcohol is necessary to obtain satisfactory yields. In the absence of mercuric ions the milder phosphorylating species (3) can be isolated which converts monoalkyl phosphates to pyrophosphate diesters in good yield but does not react appreciably with alcohols unless catalytic amounts of boron trifluoride are added. Amine salts of (3) are converted to phosphoramidates on heating. In the presence of silver ions, O-esters of thiophosphoric acid behave as phosphorylating agents and a very mild and convenient procedure suitable for preparing labile unsymmetrical pyrophosphate diesters, such as the... 

The addition reactions discussed in Sections 4.1.1 and 4.1.2 are initiated by the interaction of a proton with the alkene. Electron density is drawn toward the proton and this causes nucleophilic attack on the double bond. The role of the electrophile can also be played by metal cations, and the mercuric ion is the electrophile in several synthetically valuable procedures.13 The most commonly used reagent is mercuric acetate, but the trifluoroacetate, trifluoromethanesulfonate, or nitrate salts are more reactive and preferable in some applications. A general mechanism depicts a mercurinium ion as an intermediate.14 Such species can be detected by physical measurements when alkenes react with mercuric ions in nonnucleophilic solvents.15 The cation may be predominantly bridged or open, depending on the structure of the particular alkene. The addition is completed by attack of a nucleophile at the more-substituted carbon. The nucleophilic capture is usually the rate- and product-controlling step.13,16... 

This result can be explained in terms of a steric preference for conformation A over B. The approach of the mercuric ion is directed by the hydroxy group. The selectivity increases with the size of the substituent R.25... 

Cyclization induced by mercuric ion is often used in multistep syntheses to form five- and six-membered hetereocyclic rings, as illustrated in Scheme 4.6. The reactions in Entries 1 to 3 involve acyclic reactants that cyclize to give exo-5 products. Entry 4 is an exo-6 cyclization. In Entries 1 and 2, the mercury is removed reductively, but in Entries 3 and 4 a hydroxy group is introduced in the presence of oxygen. Inclusion of triethylboron in the reduction has been found to improve yields (Entry l).113... 

The most synthetically valuable method for converting alkynes to ketones is by mercuric ion-catalyzed hydration. Terminal alkynes give methyl ketones, in accordance with the Markovnikov rule. Internal alkynes give mixtures of ketones unless some structural feature promotes regioselectivity. Reactions with Hg(OAc)2 in other nucleophilic solvents such as acetic acid or methanol proceed to (3-acetoxy- or (3-methoxyalkenylmercury intermediates,152 which can be reduced or solvolyzed to ketones. The regiochemistry is indicative of a mercurinium ion intermediate that is opened by nucleophilic attack at the more positive carbon, that is, the additions follow the Markovnikov rule. Scheme 4.8 gives some examples of alkyne hydration reactions. 

The reactants can be made from allylic alcohols by mercuric ion-catalyzed exchange with ethyl vinyl ether.220 The allyl vinyl ether need not be isolated and is often prepared under conditions that lead to its rearrangement. The simplest of all Claisen rearrangements, the conversion of allyl vinyl ether to 4-pentenal, typifies this process. 

In contrast to the equilibrium electrode potential, the mixed potential is given by a non-equilibrium state of two different electrode processes and is accompanied by a spontaneous change in the system. Besides an electrode reaction, the rate-controlling step of one of these processes can be a transport process. For example, in the dissolution of mercury in nitric acid, the cathodic process is the reduction of nitric acid to nitrous acid and the anodic process is the ionization of mercury. The anodic process is controlled by the transport of mercuric ions from the electrode this process is accelerated, for example, by stirring (see Fig. 5.54B), resulting in a shift of the mixed potential to a more negative value, E mix. 

After Halpern and Maher (92) demonstrated that methylpentacyano-cobaltate would react with mercuric ions to give methylmercury as the... 

Huang JH, Wen WH, Sun YY et al (2005) Two-stage sensing property via a conjugated donor-acceptor-donor constitution application to the visual detection of mercuric ion. J Org Chem 70 5827-5832... 

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