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Thiocyanate: formation, structure, types, uses

Thiocyanate is a curious species, because it is positioned on the border between organic and inorganic chemistry. It is perfectly part of both organic and inorganic compounds, only varying the way it binds or interacts. This pseudohalogenide has a strong biochemical link with cyanide ions and their metabolism.

Thiocyanate anion represented by a filled space model. Source: Benjah-bmm27 / Public domain.

In the image above you have a representation of the SCN  using a full-space model. The yellow sphere corresponds to the sulfur atom, while the black and blue ones are the carbon and nitrogen atoms, respectively. Thiocyanate has an oxygenated brother: cyanate, OCN  , in which the sulfur atom is replaced by an oxygen atom.

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Reaction between cyanide and sulfur

The SCN formula  allows you to see at a glance that its synthesis is based on the reaction of cyanide, CN  , with a species that donates sulfur atoms. Indeed, cyanide can either react with elemental sulfur, S 8 , or with thiosulfate anions, S 2 O 2- to produce thiocyanate:

8 CN   + S 8  → 8 SCN 

CN   + S 2 O 2-  → SCN   + S 2 O 2-

However, the second reaction is catalyzed by a system of enzymes composed of thiosulfate sulfurtransferases. Our body has these enzymes, and therefore, we are able to metabolize cyanides that come from cyanoglycosides (carbohydrates that have the CN group). In this way, the body gets rid of harmful CN  , which interfere with the processes of cellular respiration .

Thiocyanates are found dissolved in saliva and, to a lesser extent, in plasma. Its concentration levels reveal how exposed individuals are to cyanides, either by excessive intake of foods that contain it in its natural form (walnuts, almonds, legumes, flaxseeds, etc.), or by prolonged inhalation of smoke from the cigarettes and tobaccos.

Neutralization of thiocyanic acid

SCN  can be obtained by neutralizing its acid form: thiocyanic acid, HSCN or isothiocyanic acid, HNCS. Depending on the base used, a thiocyanate salt will also be obtained.

Structure

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Resonance structures of thiocyanate. Source: Ben Mills via Wikipedia.

The image above shows how the negative charge of the SCN  is distributed . Note that all the atoms have sp 2 hybridization , so they are located on the same line.

The pair of electrons can be located either on the nitrogen atom, or on the sulfur atom. This fact explains an important characteristic of thiocyanate: it is a bidentate ligand, that is, capable of binding in two different ways.

Isomerism

Bond isomerism is present in thiocyanate compounds. As can be seen in the image above, SCN  can be attached to a benzene ring or phenyl group either through its sulfur atom or the nitrogen atom. When it binds to S, it is called thiocyanate; while when it binds with N, it is called isothiocyanate.

Notice how the –SCN or –NCS looks like linear fragments. This linear geometry remains unchanged in both organic and inorganic thiocyanates.

The –NCS bond is stronger than the –SCN, because nitrogen, being smaller, better concentrates the negative charge of the pair of electrons with which it will form the covalent bond.

Interactions

SCN anions  cannot interact with each other because of electrostatic repulsions. Therefore, they need cations so that they can interact electrostatically, and thus “build” a crystal. Inorganic thiocyanates are essentially ionic compounds.

Meanwhile, for organic thiocyanates their interactions are based on Van der Waals forces; especially those of the dipole-dipole type. The SCN group, however attached, is polar and therefore contributes to an increase in the polarity of the compound. Obviously, dipole-dipole interactions are weaker than ionic attractions, present for example in KSCN (K + SCN  ).

Organic thiocyanates

Organic thiocyanates are represented by the formula RSCN. On the other hand, having bond isomerism, we also have isothiocyanates, RNCS.

Thus, it is enough to substitute R for alkyl or aromatic molecular fragments to obtain several compounds. For example, CH 3 CH 2 SCN is ethyl thiocyanate. In the previous section, R was replaced by a benzene ring, to obtain phenyl thiocyanate, C 6 H 5 SCN or φ-SCN.

Inorganic thiocyanates

Inorganic thiocyanates are considered salts of thiocyanic acid, HSCN, and can be represented as MSCN, where M is a metal cation or the ammonium cation. Thus, we have for example:

-NaSCN, sodium thiocyanate

-NH 4 SCN, ammonium thiocyanate

-Fe (SCN) 3, ferric thiocyanate

Many inorganic thiocyanates are colorless solid salts.

On the other hand, we also have thiocyanate complexes in solution. For example, an aqueous solution containing Fe 3+ ions will complex with SCN  ions to form [Fe (NCS) (H 2 O) 5 ] 2+ , which is blood red in color.

Similarly, SCN  is capable of complexing with other metal cations, such as Co 2+ , Cu 2+ and Ti 4+ , each giving rise to a colorful complex.

Applications

SCN anion  is used for photometric determinations of metals in aqueous solutions. This method is based precisely on the measurement of the absorbances of the colored complexes of thiocyanates with metals.

Outside of this specific use, the others are as varied as the thiocyanates that exist.

Organic thiocyanates are primarily used as building blocks for the synthesis of sulfur compounds used in medicine.

In contrast, inorganic thiocyanates with colorations are used for the textile industry or as additives for boat paints. Likewise, because they are good donors of SCN  ions , they are required for the production of insecticides and fungicides.

Of the thiocyanates, the most popular are NaSCN and KSCN, both in high demand in the drug, construction, electronics, and agrochemical industries.

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