Organic thiocyanates

Organic thiocyanates are organic compounds containing the functional group RSCN. the organic group is attached to sulfur: R−S−C≡N has a S–C single bond and a C≡N triple bond.[1]

Organic thiocyanates are valued building blocks. They allow to access efficiently various sulfur containing functional groups and scaffolds.[2]

Synthesis

Several synthesis routes exist,[3] the most common being the reaction between alkyl halides and alkali thiocyanate in aqueous media.[4] Illustrative is the preparation of isopropyl thiocyanate by treatment of isopropyl bromide with sodium thiocyanate in boiling ethanol.[5] The main complication with this route is the competing formation of alkyisothiocyanates. "SN1-type" substrates (e.g., benzyl halides) tend to give the isothiocyanate derivatives.

Some organic thiocyanates are generated by cyanation of some organosulfur compounds. Sulfenyl thiosulfates (RSSO3−) react with alkali metal cyanides to give thiocyanates with displacement of sulfite. This approach has been applied to allyl thiocyanate:[6]

CH2=CHCH2Cl + Na2S2O3 → CH2=CHCH2S2O3Na + NaCl
CH2=CHCH2S2O3Na + NaCN → CH2=CHCH2SCN + Na2SO3

Sulfenyl chlorides (RSCl) also convert to thiocyanates.

Aryl thiocyanates are traditionally produced by the Sandmeyer reaction, which involves combining copper(I) thiocyanate and diazonium salts:[3]

[ArN2]BF4 + CuSCN → ArSCN + CuBF4 + N2

Some arylthiocyanates can also often be obtained by thiocyanogenation, i.e. the reaction of thiocyanogen. This reaction is favored for electron-rich aromatic substrates.[1]

Structure

In methyl thiocyanate, N≡C and C−S distances are 116 and 176 pm. By contrast, N=C and C=S distances are 117 and 158 pm in isothiocyanate]s.[7]

Typical bond angles for C−S−C are 100°.[3] By contrast C−N=C in aryl isothiocyanates is 165°. Again, the thiocyanate isomers are quite different with C−S−C angle near 100°.

In both organic thiocyanate and isothiocyanate isomers the SCN angle approaches 180°.

Reactions

Organic thiocyanates are hydrolyzed to thiocarbamates in the Riemschneider thiocarbamate synthesis.

Some thiocyanates isomerize to the isothiocyandates. This reaction is especially rapid for the allyl isothiocyanate:[6]

CH2=CHCH2SCN → CH2=CHCH2NCS

See also
  • Isothiocyanate, isomers of organic thiocyanates with the formula R−N=C=S
  • Methyl thiocyanate, the simplest organic thiocyanate

References
  1. R. G. Guy (1977). "Syntheses and Preparative Applications of Thiocyanates". In Saul Patai (ed.). Cyanates and Their Thio Derivatives: Part 2, Volume 2. PATAI'S Chemistry of Functional Groups. pp. 619–818. doi:10.1002/9780470771532.ch2. ISBN 9780470771532.
  2. Castanheiro, Thomas; Suffert, Jean; Donnard, Morgan; Gulea, Mihaela (2016-02-01). "Recent Advances in the Chemistry of Organic Thiocyanates". Chem. Soc. Rev. 45 (3): 494–505. doi:10.1039/c5cs00532a. ISSN 1460-4744. PMID 26658383.
  3. Erian, Ayman W.; Sherif, Sherif M. (1999). "The chemistry of thiocyanic esters". Tetrahedron. 55 (26): 7957–8024. doi:10.1016/S0040-4020(99)00386-5.
  4. "Synthesis of thiocyanates".
  5. R. L. Shriner (1931). "Isopropyl Thiocyanate". Organic Syntheses. 11: 92. doi:10.15227/orgsyn.011.0092.
  6. Emergon, David W. (1971). "The Preparation and Isomerization of Allyl Thiocyanate. An Organic Chemistry Experiment". Journal of Chemical Education. 48 (1): 81. Bibcode:1971JChEd..48...81E. doi:10.1021/ed048p81.
  7. Majewska, Paulina; Rospenk, Maria; Czarnik-Matusewicz, Bogusława; Kochel, Andrzej; Sobczyk, Lucjan; Dąbrowski, Roman (2008). "Structure and polarized IR spectra of 4-isothiocyanatophenyl 4-heptylbenzoate (7TPB)". Chemical Physics. 354 (1–3): 186–195. Bibcode:2008CP....354..186M. doi:10.1016/j.chemphys.2008.10.024.
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