Nitrosyl thiocyanate

Table 3-1. Equilibrium constants Kxno (Scheme 3-28) and rate constants for diazotization of aniline (Ar2, Scheme 3-29) and of 1-naphthylamine (k2 and k-2/k Scheme 3-34) in water by nitrosyl chloride, nitrosyl bromide, nitrosyl thiocyanate, S-nitrosothiuronium ion [(NH2)2CSNO], and dinitrogen trioxide at 25 °C.

When thiocyanate ions are added to nitrous acid in water, a pink colouration develops which is believed to be due to the formation of nitrosyl thiocyanate (equation 34), which is too unstable to be isolated but which can be used as a nitrosating agent in aqueous solution. Because the equilibrium constant for ONSCN formation81 is quite large (30 dm6mol 2) at 25 °C, thiocyanate ion is an excellent catalyst for aqueous electrophilic nitrosation. The well established82 series is Cl- < Br < SCN < (NH2)2CS. Thiocyanate ion is also a sufficiently powerful nucleophile to react in acid solution with nitrosamines in a denitrosation process (equation 35), which can only be driven to the right if the nitrosyl thiocyanate is removed by, e.g., reaction with a nitrite trap such as hydrazoic acid. 

Nitrosyl chloride in carbon tetrachloride reacts with silver thiocyanate to form silver chloride and nitrosyl thiocyanate ... 

The presence of nitrites should be avoided for in acid solution they form nitrosyl thiocyanate NOSCN which yields a red colour, disappearing on heating, similar to that with iron (III). 

Nitrites yield a red colour due to nitrosyl thiocyanate and therefore interfere with the test. 

Catalysis of nitrosation and diazotisation by thiocyanate anion is well known (Ridd, 1961), and is more pronounced than either chloride ion or bromide ion catalysis. This arises from the much larger value for the equilibrium constant for the formation of nitrosyl thiocyanate than for the two... 

Since /f ox l nown, it is easy to calculate the second order rate constants for the reaction (25) of nitrosyl thiocyanate with any general substrate A. 

Earlier studies with halide ion catalysis have been extended to include the reactivity pattern of nitrosyl thiocyanate towards a number of substituted aniline derivatives (Dix and Williams, 1982). Figure 2 shows the values of log k plotted against the pAT -values. The trend for both NOCl and NOBr... 

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. 

II. Very pure thiocyanogen is obtained by dissociation of nitrosyl thiocyanate in vacuum. 

The transient red colour of the solutions is ascribed to the formation of the nitrosyl thiocyanate. The copper(ii)-catalysed reaction of hydroxylamine and peroxonitrite in alkali has been investigated. In the absence of oxygen, the reaction ... 

The presence of nitrites is to be avoided, since in acid solution they form nitrosyl thiocyanate NO CNS. This compound gives a red color (which will disappear on heating) very similar to that of the iron thiocyanate. ... 

Large amounts of halides, especially iodides, give only a slight greenish color under these conditions. However, 6 y thiocyanate may be detected in the presence of 1000 times the amount of iodide, bromide, chloride, thiosulfate and acetate. The latter two ions, on evaporation with cobalt salts, give a green color which, unlike that developed by thiocyanate, does not enter the acetone. Nitrites interfere because they react with thiocyanates to give red nitrosyl thiocyanate. 

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