Addition of thiocyanogen to unsaturated compounds

Anhydrous acetic acid containing some acetic anhydride provides the most stable thiocyanogen solutions. Very pure lead thiocyanate is shaken in the dark with bromine in a mixture of 6 parts by volume of acetic acid (distilled from 1 % of chromic trioxide), 1 part by volume of acetic anhydride, and 3 parts by volume of pure anhydrous CC14, all contained in dry vessels that give up no alkali, i.e., that have been previously boiled out with hydrochloric acid. Filtration of the mixture through a dry filter, with exclusion of moisture, gives clear thiocyanogen solutions that are stable for several weeks. 

The (SCN)2 content of the solutions is determined by treatment of a portion with an excess of 10% aqueous potassium iodide and titration of the liberated iodine with thiosulfate  

Addition of (SCN)2 to unsaturated acids has a place in the analysis of fats and drying oils. Whereas iodine adds to all C=C bonds, (SCN)2 is only partially added. From the differences between iodine numbers and thiocyanate numbers (see page 114) conclusions can be drawn about, e.g., the composition of fatty acid mixtures containing oleic, linoleic, and linolenic acid. For details of the technique see Kaufmann.785 

Kaufmann, Neuere Methoden der praparativen organischen Chemie, Verlag Chemie, Berlin, 1944, Vol. 1, p. 241. 

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