Reactions of Thiocyanates

Using a linear free energy relationship for the ki pathway, the one-electron potential for the NCS/NCS couple has been estimated at 1.66 V. The corresponding value for (NCS)2 has also been evaluated at 1.29 V. The data are combined with those (vide infra) for iodide in an extensive examination of the self-exchange parameters for these reductants. The oxidation of NCS by [Fe(CN)6] has also been studied. 

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Guanidine salts can be prepared by reaction of thiocyanates and sulfamates (22). 

Thiocyanates are rather stable to air, oxidation, and dilute nitric acid. Of considerable practical importance are the reactions of thiocyanate with metal cations. Silver, mercury, lead, and cuprous thiocyanates precipitate. Many metals form complexes. The deep red complex of ferric iron with thiocyanate, [Fe(SCN)g] , is an effective iadicator for either ion. Various metal thiocyanate complexes with transition metals can be extracted iato organic solvents. 

A variety of thiazolines and thiazoles were synthesised by the condensation reaction of thiocyanates and thioureas with a-haloketimines, these imines are a new type of reactive bielectrophOic reagent <96JHC1179>. 

On the basis of these results it can be stated that the fast oxidation of thiocyanate by peroxy acids gives rise to the induced reduction of hydrogen peroxide. In order to elucidate the mechanism of this interesting reaction let us have a look at reactions of thiocyanate with peroxy compounds of different types. 

King and Tsang (22) have recently studied the relative rate of bimolecular substitution in dibenzyl ethylsulfonium salt 17 and the cyclic sulfonium fluoroborate salt 75. They found that the reaction of thiocyanate anion with salt 72 which can easily take the arrangement 70 is 8000 times faster than the corresponding reaction with the cyclic sulfonium salt 75, which is tied up in the arrangement 7U... 

Another example of soft nucleophile attack according to the H5AB theory was given by the reaction of thiocyanate salts with 1-chloroethyl carbonates affording the corresponding 1-thiocyano and/or 1-isothiocyano ethyl carbonates in good yields (Ref. 101). 

Increased selectivity in the determination of metals by thiocyanate is obtained by the choice of acidity, thiocyanate concentration, masking agent, and metal oxidation state. For example, the presence of a reducing agent is necessary for colour reactions with Mo, W, and Re. The reducing medium precludes the colour reaction of thiocyanate with iron. 

In particular, A-phenylisothiazolium salts 57r,s and 62i bearing a benzo crown ether substituent were synthesized in 53-60% yield by treatment of the (3-thiocya-natovinyl aldehydes 55a,g with the crown aniline (01PS29). Furthermore, the 2, 2,-(benzene-l,4-diyl)bis(4,5-dimethylisothiazol-2-ium) diperchlorate 66 was prepared in 88% yield as yellow crystals by the reaction of thiocyanate 55a with benzene-1, 4-diamine (02HCA183). 

Aryl-4,6-bis(methylsulfanyl)-l,3,5-oxadiazinium salts, e.g. 28, obtained by the reaction of thiocyanates and acyl chlorides (cf. Section B.1.3.) react with arylcarboximidamides in dioxane at room temperature to form diaryl-1,3,5-triazines in moderate yield (23 -29%).436... 

The reactions of thiocyanate with [Os(bipy)3] + and [Os(phen)3] + have been studied. The rate law derived is of the form... 

A modification of the above reaction, which proceeds via the same intermediate, is the reaction of thiocyanate salts with ethylene carbonates (67)". 

Table 3 - Influence of the conditions on the reaction of thiocyanate ions with butyl bromide...

Small amounts of water act in synergy with sonication and solid-support catalysis, for instance, in the reaction of thiocyanate ions. As in the previous case, presonication of the reagent and the support was found to improve the selectivity. These methods permit the substitution reactions to be run even in apolar solvents. Two practically simultaneous papers have described the preparation of aryl sulfones by alkylation of sodium arylsulfinates with reactive alkyl chlorides.i The reaction with benzyl chlorides is best performed on alumina, and allyl bromide reacts quantitatively in a DMF-water mixture in a few minutes. Activated primary halides undergo substitution by sodium azide in aqueous solution to give the potentially explosive organic azides. 1 The paper discusses the possible role of the relative densities of the starting material, the aqueous solution of the reagent, and the product in the success of the preparation. 

Reactions of Thiocyanates and Isothiocyanates.—Although these functional groups are structiurally dissimilar from many points of view, a thiocyanate behaving as a sulphenyl cyanide, while an isothiocyanate shows the reactions expected of it on the basis of its heterocumulene structure, there are several examples now of isomerization reactions — S—C=N —N=C=S, partly justifying a unified treatment of their reactions. 

Complex Formation Labile Cations. Solvent effects on reactivity in the formation of complexes of metal(n) cations with unidentate ligands have been reviewed, with special reference to magnesium(n) and to the solvents methanol, acetonitrile, DMF, and DMSO. There has been controversy over the mechanism of reaction of thiocyanate with nickel(n) in DMSO, with supporters of the usual Eigen-Wilkins la mechanism and of a D mechanism. The most recent investigators of this reaction report rate constants and activation parameters and favour the la mechanism. There has been further discussion of the mechanism of the reaction between nickel(n) and bipy in DMSO an earlier suggestion that the rate-determining step is ring closure is not supported by recent observations. Rate constants for the reaction of acetate, of other carboxylates, and of pada with nickel(ii) in several non-aqueous solvents have been determined. 

There is consensus on the fact that oxidation reaction of thiocyanate occurs within the (III)/(II) redox couple. According to this, the mechanism first involves the coordination of the M(II)Pc system to the linker distributed as a SAM over the gold electrode. Then it follows the initial oxidation of the metal, from M(II)Pc to M (III)Pc activating the catalyst to finally oxidize the analyte to its products via M(III) Pc with the subsequent regeneration of the M(II) species. 

Recent studies include a number of substitution reactions accompanied by or preceding reductions or ligand reactions. Stopped-flow spectrophotometry on the reaction of thiocyanate with tetrachloroaurateQlI) or tetrabromoaurate(III) revealed two kinetically distinct reaction stages. The first stages were rapid stepwise substitutions to [AuX(SCN)3] [equation (25), n = 0-3]. The solvent (H2O) path was negligible, and no... 

The kinetics have been reported for the formation of several edta-5-sulfosalicylate ternary complexes of lanthanide ions, and ligand substitution in trimalonatovanadium(III) with edta and nta . Reports have also appeared on the formation of ternary complexes involving Fe(III), pyridoxamine, and pyridoxal and the formation of bis- and tris(acetohy-droxamato)Fe(III) and activation volume evidence has been provided for a dissociative mechanism in the reaction of thiocyanate with diaqua[mcso-tetrakis(Af-methyl-4-pyridyl)porphinato] cobalt(III). 

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