Thiol/thiolate 2-

Thioethers (sulfides) can be prepared by treatment of alkyl halides with salts of thiols (thiolate ions). The R group may be alkyl or aryl and organolithium bases can be used to deprotonate the thiol. As in 10-37, RX cannot be a tertiary halide, and sulfuric and sulfonic esters can be used instead of halides. As in the Williamson... 

Thioethers (sulfides) can be prepared by treatment of alkyl halides with salts of thiols (thiolate ions).7S2 R may be alkyl or aryl. As in 0-35, RX cannot be a tertiary halide, and sulfuric and sulfonic esters can be used instead of halides. As in the Williamson reaction (0-12), yields are improved by phase-transfer catalysis.753 Instead of RS ions, thiols themselves can be used, if the reaction is run in benzene in the presence of DBU (p. 1023).754 Neopentyl bromide was converted to Me3CCH2SPh in good yield by treatment with PhS in liquid NH3 at -33°C under the influence of light.755 This probably takes place by an SrnI mechanism (see p. 648). Vinylic sulfides can be prepared by treating vinylic bromides with PhS in the presence of a nickel complex,756 and with R3SnPh in the presence of Pd(PPh3)4.757 R can be tertiary if an alcohol is the substrate, e.g,758... 

Hydrogen and Electron-Transfer Reactions by Thiols/Thiolate Ions 144... 

Table VII. Second-order Rate Constants (M V1) for Nucleophilic Substitution of Halogenated Substrates by Thiols, Thiolate Anions, Thioacids and Thioethers Ocscn). Compared to kfco.sN (T=25°C> Unless Otherwise Indicated)...

The rate of the reaction depends on the pH of the solution. If it is around or higher than the p.JCa of the thiol, thiolate anion will be formed and this opens the epoxide much faster than does the unionized thiol. The nucleophile is regenerated by the oxyanion produced in the rate-determining step. A more familiar example is the base-catalysed hydrolysis of esters we have mentioned several times in this chapter. The full pH-rate profile (Chapter 13) for the hydrolysis of a simple ester such as ethyl acetate shows just two straight lines meeting each other (and zero rate) at about neutrality. Ethyl acetate hydrolysis occurs by SAC or SBC only. 

Processes involving nucleophilic addition of thiol, thiolate, and dithiocarboxylate to sp and sp carbons... 

This approach to the thiophene ring seems most direct and involves (1) intramolecular nucleophilic addition of thiol, thiolate, and dithiocarboxylate sulfurs and, in a rare case, sulfide sulfur to sp and sp carbons (2) electrophilic attack of sulfenium and sulfonium ions and their equivalents on unsaturated carbon-carbon bonds (3) addition of thiyl radicals to unsaturated carbon-carbon bonds (4) addition of vinyl and aryl radicals to the sulfur atom of sulfides and (5) electrophilic attack of a carbocation on the sulfur atom of sulfides. 

Reaction of Thiols, Thiolates, or Monoorganylsulfanes with Dichlorosulfanes... 

The reaction is carried out in acetonitrile, in the presence of triethyl-amine in stoichiometric quantity with the thiol compound. The introduction of triethylamine allows the thiol-thiolate balance to be changed to give the... 

Sulphonic acids may normally be obtained in 65-75% yields upon oxidation of thiols, thiolates or disulphides with 30% hydrogen peroxide234-240. Higher yields are realized if a tertiary thiol is used. The same products may also be realized by reaction of either thiols or disulphides with aqueous dimethyl sulphoxide (equation 37), in the presence of a catalytic amount of bromine, iodine or a hydrogen halide241,242. In this latter oxidation, dimethyl sulphide is formed as a by-product, but is easily removed by aspiration. 

These reductively desorbed long-chain thiols (thiolates), which are insoluble in the electrolyte, remain near the surface, probably forming micelles, and do not diffuse in the bulk solution. They can be redeposited back on the surface of the electrode as a monolayer even if there was no thiol in the electrolyte prior to desorption . The amount of material readsorbed depends directly on the solubihty of the thiol. Shorter-chain (more soluble) thiols redeposit to give monolayers with smaller coverage. Redeposition also depends strongly on the pH of solution. Thiols become less soluble at lower pH, and therefore form monolayers with higher coverage under these conditions. 

As with SAC, the rate of the reaction depends on the pH of the soiution. If it is around or higher than the pK of the thiol, thiolate anion will be formed and this opens the epoxide much faster than does the unionized thiol. The nucleophile is then regenerated by the oxy-anion produced in the rate-determining step. 

Pseudo steady-state titration was found to be a valuable method for the determination of stability of colloidal nanocrystals upon change of the pH in the solutions [56]. CdSe nanocrystals coated with hydrophilic deprotonated thiol (thiolate) ligands were studied systematically. For comparison, CdTe and CdS nanocrystals coated with the same types of ligands were also examined. The results showed... 

Cysteine is one of the two sulfur-containing amino acids its side chain contains a thiol, which is one of the most chemically reactive groups found in proteins. The thiol form (-SH) is in equilibrium with the thiolate (-S ), the relative amounts of each depend on the solution pH. Cysteinyl residues in proteins are most commonly found in the reduced form (thiol/thiolate) or as disulfide bonds. 

By taking advantage of their inherent chemical reactivity, the presence and amovmt of umnodified cysteinyl residues (thiol/thiolate) in a protein can be determined. Briefly, a colorimeteric substrate is reacted with the protein the amovmt of signal is proportional to the amovmt of thiol present. The most commonly used chemical method for the determination of thiols is Ellman s assay. When higher sensitivity is required, fluorescence-based assays can be used. 

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