Oxidation of thiocyanate

Whereas sulfonyl halides have been known for a long time and, especially the chlorides, have become of great synthetic value, sulfonyl cyanides were unknown until 1968. They were first prepared by van Leusen and co-workers from the reaction of sulfonylmethylenephos-phoranes with nitrosyl chloride. The same group also investigated part of their chemistry. Since then, two more, completely different, methods of synthesis were published from sulfinates with cyanogen chloride,and by the oxidation of thiocyanates. ... 

The thermal oxidation of thiocyanate by Fe(III) in a perchlorate medium involves two thiocyanato complexes of Fe(III) . The stoichiometry is thought to be... 

INDUCED REACTIONS INVOLVING INTERMEDIATES PRODUCED BY PARTIAL OXIDATION OF THIOCYANATE... 

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. 

The salt, prepared in carbon tetrachloride solution, exploded feebly several times as the reaction proceeded. Oxidation of thiocyanate by chromium(VI) is postulated. 

The lactoperoxidase system produces various compounds for antimicrobial protection of milk, the principal one of which is hypothiocyanite (OCNS ) as a result of the lactoperoxidase-catalyzed oxidation of thiocyanate (SCN ) by H2O2. No overheating should take place during the pasteurization process, to avoid damaging lactoperoxidase . 

Adsorption of thiocyanate ions on Au electrodes from alkaline solutions has been studied using in situ IR spectroscopy and SERS [83]. Even at negative potentials, adsorption of thiocyanate was observed, which occurred via Au—S interaction. At higher concentration of OH ions, the bond strength between the adsorbed SCN ion and the surface was decreased. At very positive potentials, both the oxidation of thiocyanate to cyanate and the... 

Small amounts of hydrogen peroxide in raw milk can activate the lactoperoxidase-catalyzed oxidation of thiocyanate to produce a bacterial inhibitor (Hogg and Jago 1970). Inhibitory compounds resulting from oxygen metabolism can produce initially slow starter culture growth in industrial dairy fermentations if the milk has been excessively agitated. 

Hydroxyl radical oxidation of thiocyanate ion in acid and neutral solution leads to the formation of a transient species which absorbs strongly at a wavelength of 475nm. Figure 2 shows the growth in absorption of this species at intervals from 20ns to 800ns. 

Thomas EL (1985) Products of lactoperoxidase-catalyzed oxidation of thiocyanate and halides. In Pruitt KM, Tenovuo JO (eds) The lactoperoxidase system. Chemistry and biological significance. Marcel Deikker, New York, pp 31-53... 

Example 30 The oxidation of thiocyanate ions by permanganate in acid solution is applied in quantitative analysis. Express the reaction equation. 

The oxidation of thiocyanate is a rather complex process, which may be dealt with in two separate steps. First, sulphate and cyanide ions are formed ... 

In oxidation of thiocyanate ion by nitric and nitrous acids the formation of nitrosyl thiocyanate and some of its properties have been convincingly demonstrated However, its role in the oxidation reaction is not yet entirely elucidated. 

Oxidation of thiocyanate is complicated by the variety of products which may arise, and by rates of reaction which limit the range of analytical procedures which may be used. The results suggest that the first step is analogous to (4). Further reaction of the (unidentified) product leads to sulphate, cyanide, cyanate and sulphur dicyanide. Paths proposed for their formation include intermediate formation of thiocyanogen (reaction (5)), but there are other alternatives. Only in the higher pH region (6-7.6) were the later steps sufficiently fast to produce simple kinetics. The general rate expression is of the form... 

Oxidations of thiocyanate ion by nitrous and nitric acids and by peroxomono-sulphate and peroxodisulphate are considered under the oxidants (p. 293 and 350). Oxidation by hydrogen peroxide and by iodine will be considered here. 

Chung J and Wood JL (1971). Oxidation of thiocyanate to cyanide catalyzed by hemoglobin. J Biol Chem, 246, 555-560. 

Other reactions of simple anions that may occur in the absence of coordination can also be observed to occur for the complexed form, with the rate of reaction usually changed significantly as a result of complexation. This is anticipated, since a coordinated ion is bonded directly to a highly-charged metal ion, which must influence the electron distribution in the bound molecule and hence its reactivity. Two well-known examples where the product is ammonia occur through either reduction of nitrite with zinc/acid or oxidation of thiocyanate with peroxide. The former example is exemplified in (6.42) below. 

Scheme 6. CAN-mediated radical addition by oxidation of thiocyanate le...

An interesting application of the electrochemical oxidation of thiocyanate ion is the preparation of alkyl and aryl thiocyanates via anodically generated thiocyanogen. Alcohols have been converted to the corresponding thiocyanates by constant current electrolysis of NaSCN in CH2CI2 containing triphenylphosphite and 2,6-lutidinium perchlorate. The yields were fair to good for the primary and secondary alcohols, but no thiocyanate formation was observed with tertiary ones. Similarly, various aromatic amines and phenols were thiocyanated in a two-step procedure, namely electrochemical preparation of (SCN)2 and subsequent reaction with the substrates k... 

This peroxidase, present in milk, saliva, and tears, is involved in the bacterial defense through the oxidation of thiocyanate ions to the antibacterial species hypothiocyanate OSCN and other higher oxyacids. Thiocyanate, the physiological substrate of LPO, is present in secreted fluids in much higher concentrations than in blood plasma or in the extracellular space of tissues. LPO is a glycoprotein with a molecular weight of 78,000 Da and has a high content of carbohydrates (10%). Despite the absence of an X-ray structure for LPO, physico-chemical studies have shown that the bovine milk LPO-heme is a l,5-bis(hydroxymethyl) derivative of heme b, linked to the protein by esterification with Glu-275 and Asp-125 as in MPO. ... 

The oxidation of thiocyanate is also well-known to cause electrode passivation. Although it is usually attributed to accumulation of electrolysis products, it may be just deactivation of the surface by consumption of the oxide. The latter occurs, for example, with the oxidation of As(III) at a Pt (oxide-coated) electrode.We have found that with an electrode modified with the ruthenium-containing inorganic film, mvRu, the problem is alleviated. The general characteristics of flow injection determinations of thiocyanate with an amperometric detector based on the mvRu electrode are summarized in Table 1. The sample throughput was calculated as described above. 

The linear dynamic range which extends up to 4 pM thiocyanate is not surprising considering that the peak current for the oxidation of thiocyanate by linear scan voltammetry at the mvRu electrode is limited by diffusion of the analyte in the bulk solution. As shown in Table 2, the peak current function, ipaV /, is independent of scan rate in the range 10 -300 mV/s. 

I , oxidation with I2, oxidation with lOT j oxidation of thiocyanate-metal complexes, bromination and reduction with I , Sn2 substitution and oxidation with Br2, and complexation of cations with ethylenediaminetet-raacetic acid (EDTA) in the presence of Pb(I03)2. 

The amplification of SCN can be used as a basis for the amplification of metals by oxidation of thiocyanate metal complexes and several examples of this have been published, such as the ampHfication of Pb after formation of Pb(SCN)3[N(C4H9)4], in which lPb=114I, or after forming Pb3[Cr(SCN)6]2, in which lPb= 1521, and the amplification of bismuth via formation of Bi[Cr(SCN)6] or Bi(SCN)6[N(C4H9)4]3, in which cases an amplification factor of 228 is obtained. 

Addition of materials capable of reacting with electrons reduces the lifetime of enabling kinetics of its fast reactions to be determined. The powerful oxidant OH (the hydroxyl radical) has only a weak absorption in the ultraviolet, and its reactivity is best measured by a competition method based on its very fast oxidation of thiocyanate ion CNS to yield the intensely absorbing (CNS) ion (Xmax 472 nm). Addition of a second substrate X will provide competition for OH, and the intensity of the absorption of (CNS)2 will be systematically reduced as [X] is increased, enabling a rate constant to be derived. 

The publications of these chemists are so significant that it is worthwhile to reconstruct an example of their thinking, as described by Swift (10) "These workers were interested in the determination of thiocyanate. They either knew or learned that the anodic oxidation of thiocyanate to cyanide and sulfate with 100% current efficiency is difficult if not impossible of attainment. However, it was known that thiocyanate could be quantitatively, stoi-chiometrically, and rapidly oxidized by bromine in acid solutions with 100% current efficiency. Therefore they added a relatively high concentration of a soluble bromide to an acid solution containing the thiocyanate, anodically produced bromine, and allowed this bromine to diffuse into the solution and to oxidize the thiocyanate. By working with relatively large samples of thiocyanate, and by measuring the quantity of electricity involved by means of a chemical coulometer, they demonstrated that an accuracy within 1 ppt could be attained."... 

Shaima VK, Burnett CR, O Connor DB, Cahelli D (2002) Iron(VI) and iron(V) oxidation of thiocyanate. Environ Sci Tech 36 4182-4186... 

The sulphur-containing substrates thiocyanate and thiourea undergo oxidation by 2,9-dimethyl-l,10-phenanthroline (dmp) complexes of copper(n). As with beds, this ligand forms thermodynamically stable and brightly coloured bis-complexes with copper(i), which is otherwise unstable in aqueous perchlorate media. Under the conditions used, the oxidant is mainly in the form [Cu(dmp)2] . Oxidation of thiocyanate obeys a rate law... 

It has been found that between pH 1 and 4 the oxidation of thiocyanate elec-trocatalyzed by MPcs involves two one-electron steps, complemented by Tafel plots that have shown the participation of one electron transfer as the rate-determining step with an unsymmetrical energy barrier [129, 130]. According to this, it has been suggested that the first electron transfer corresponds to the rate-determining step. 

Experimental evidence has proven that thiocyanate binds to the MPc through the metal center [131], thus pointing out that this interaction is the main responsible of the electrocatalytic power observed for the oxidation of thiocyanate. Also, there is a 1 1 stoichiometry between thiocyanate and MPs [132], which reduces the number of possible binding conformations of thiocyanate with the metal center. In agreement with this, the bond formed between the catalyst and the target molecule involves an inner sphere process, in which the observed reactivity of the catalysts is much higher than that predicted by its redox potential [130]. It has been also stated that it is even more complicated if the mediator is confined on the electrode surface. 

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