ref: Org. React., vol 3., pp 240 - 265 (1946)
The direct replacement of a hydrogen atom by a thiocyano group through the use of thiocyanogen, (SCN)2, is commonly termed thiocyanation. This replacement reaction is limited practically to aromatic amines and phenols, although a few particularly reactive aromatic hydrocarbons can be thiocyanated.
The reagent is used in synthesis in essentially the same way as the halogens, with the exception that certain precautions must be observed owing to the instability of thiocyanogen. Thiocyanogen is a liquid which on cooling forms a colorless, crystalline solid melting between -3 and -2 C. At room temperature is polymerizes rapidly to a reddish orange, amorphous mass of indefinite composition known as pseudo- or para-thiocyanogen. Although relatively stable in inert, dry solvents, thiocyanogen may polymerize in solution, expecially under the catalytic influence of heat, light, moisture, or oxygen. Thiocyanogen is readily hydrolyzed to produce thiocyanic acid and hypothiocyanous acid.
(SCN)2 + H2O ---> HSCN + HOSCN
The latter is unstable and is converted into hydrocyanic acid and sulfuric acid, both of which occur as end products of the overall hydrolysis.
The extreme sensitivity of thiocyanogen toward hydrolysis and polymerization probably accounts for the long interval between its formulation by .... when thiocyanogen is employed in chemical reactions, it is prepared in solution and more commonly is produced in situ.
Thiocyanogen is often classified as a pseudohalogen because of its resemblance to halogens in its chemical behavior. It attacks even noble metals like gold and mercury; it reacts with nitric oxide, aqueous hydrogen sulfide, hydrazoic acid, ammonia, and hydrochloric acid. It is released from metal thiocyanates by the action of chlorine, bromine, and other oxidizing agents (imp: ahhhm, oxone perhaps?). Halogen-thiocyanogen combinations are formed with chlorine and with iodine. Thiocyanogen is similar to iodine in its chemical reactivity but is slightly less electronegative.
Thiocyanogen reacts with aromatic compounds that are highly susceptible to substitution with the introduction of a thiocyano group. Reactions reported thus far are mainly with phenols of the benzene and naphthalene series and with primary, secondary, or tertiary amines of the benzene, naphthalene, and anthracene series. Apparently the presence of other substituents, such as nitro, chloro, bromo, alkoxy, carboxyl, or carbethoxy groups, does not interfere with the reaction provided that an active position is still available; however, the presence of a sulfonic acid group may prevent the reaction, since it is reported that p-amino- and p-hydroxy-benzenesulfonic acids do not undergo thiocyanation.
The thiocyano group is introduced into aromatic amines with rapidity; it enters a free para position if available, otherwise an ortho position. For example, aniline is converted into 4-thiocyanoaniline (97% yield), o-toluidine into 4-thiocyano-o-toluidine (80% yield) and anthranilic acid into 5-thiocyanoanthranilic acid (80% yield). Acetylation of the amino group prevents thiocyanation.
The reaction of phenols with thiocyanogen has not been studied so extensively as that of amines. Phenol is converted to 4-thiocyanophenol in 69% yield, o-cresol into 4-thiocyano-o-cresol in 90% yield, thymol into 4-thiocyanothymol in 95% yield, and alpha-naphthol into 4-thiocyano-1-naphthol in 83% yield. The point of attack is again the para position if free; ortho substitution occurs when this position is blocked, as in the reaction of p-cresol and beta-naphthol (100% yield). The effect of a substituent other than an alkyl group in the position ortho to the hydroxyl group has been examines to only a limited exten; the yiled of the thiocyano product is lowered in the case of an alkoxyl (guaiacol, 21% yield), hydroxyl (pyrocatechol, 48% yield, and resorcinol, 60% yield).
Aromatic hydrocarbons of the benzene and naphthalene series do not undergo thiocyanation, but certain hydrocarbons with several condensed benzene rings do... Anthracene, benzpyrene, benzanthracene.
Miscellaneous Reactions
2RNH2 + (SCN)2 ---> RNHSCN + RNH3SCN
2R2NH + (SCN)2 ---> R2NSCN + R2NH2SCN
C2H5SH + (SCN)2 ---> C2H5SSCN + HSCN
The reagent is prepared by the action of an oxidizing agent upon thiocyanic acid or a metal thiocyanate. The oxidation of thiocyanic acid in an organic solvent is accomplished by means of such reagents as lead tetracetate, lead peroxide, or manganese dioxide, but the yield is so low that the preparation from metal thiocyanates is much to be preferred. Lead thiocyanate reacts rapidly and quantitatively with bromine to form thiocyanogen and lead bromide, which is removed readily by filtration.
Solvents that have been used with thiocyanogen include benzene, bromobenzene, carbon tetrachloride, chloroform, ether, ethylene bromide, carbon disulfide, pet. ether, methyl acetate, nitromethane, and anhydrous formic and acetic acids. At low temperatures such solvents as saturated solutions of alkali thiocyanates in methanol or acetone can be used. The yield in the thiocyanation of amines is 20-30% higher when the reaction is carried out in a neutral medium like methanol rather than in acetic acid. Ether is usually not satisfactory because the solvent is attacked and because some of the amine is precipitated as the thiocyanate. On the other hand, thiocyanation of phenols appears to give better yield in acetic acid solution than in neutral solvents.
Moisture must be excluded form thiocyanation solution in order to prevent hydrolysis. Another troublesome side reaction, particularly in concentrated solutions, is polymerization, which is induced by light, heat, and the presence of hydrolysis products. Polymerization is reported to be dependent upon the dielectic constant of the solvent. imp: Acetic acid is best by far
Lead thiocyanate, used advantageously in the formation of thiocyanogen, is prepared from lead nitrate and sodium thiocyanate. To an ice-cold solution of 45 g. lead nitrate in 100 cc. of water is added a cold solution of 25 g. sodium thiocyanate in 100 cc. of water. Lead thiocyanate precipitates as a fine, white powder. It is collected on a filter, washed free of nitrates with ice water, and then dried in vacuum over P2O5 in the dark. The product should remain perfectly white.
One part by weigh (in grams) of lead thiocyanate is suspended in 5 to 10 pars by volume (cc) of the desired solvent. The solution is cooled to 5-10 C, and a small portion of 10% bromine in the same solvent is added. The mixture is shaken vigorously until the color due to the bromine disappears. The process of addition and shaking is repeated until the calculated amount of bromine has been used. The suspended solids are allowed to settle, the thiocyanogen solution is decanted, and the residual solids are washed by decantation with small portions of the solvent.
Decoloration of the bromine solution by lead thiocyanate is usually immediate. As heat is evolved by the reaction, the flask must be cooled regularly during the preparation to maintain the low temperature necessary to stablize thiocyanogen. At the end of the reaction lead thiocyanate should remain in about 10% excess imp: if we choose to use thiocyanogen bromide, we will need to use equimolar amounts of Br2 and Pb(SCN)2. Solutions of pure thiocyanogen are water-clear and colorless.
Since the reaction between bromine and lead thiocyanate is quantitative, the amount of thiocyanogen present can be taken as equivalent to the amount of bromine added to the solution provided the reagent is used immediately.
Well, SWIM thinks this offers much useful information. The thiocyanogen chloride and the oxone/KSCN ideas in the first post on this thread are sounding better and better by the second. SWIM also has a paper detailing the preparation of thiocyanogen bromide (bromine is easier to handle than chlorine). This will come soon.