Leuckart thiophenol synthesis

What is Leuckart thiophenol synthesis?

The Leuckart thiophenol synthesis is a chemical reaction that was first discovered by Leuckart in 1890. This process involves the reaction between a diazonium salt and potassium ethylxanthate, which results in the formation of diazonium ethylxanthate. This compound decomposes upon heating to yield both diarylthiocarbonate and aryl xanthate. Thiophenol can then be obtained through alkaline hydrolysis. The Leuckart thiophenol synthesis is also referred to as the Leuckart synthesis.

Leuckart thiophenol synthesis - general reaction scheme - Leuckart synthesis
Leuckart thiophenol synthesis

During the reaction, aryl xanthate, diarylthiocarbonate, and ethyl ethylxanthate are all produced, with the yield of diarylthiocarbonate being slightly higher than that of ethyl ethylxanthate. It is important to note that aryl xanthate is not the intermediate in the formation of diarylthiocarbonate. The heterolytic mechanism is believed to be involved in the formation of diarylthiocarbonate since the yield of diarylthiocarbonate is lower in methanol than in water. Further evidence for this mechanism is the detection of phenol from aryl carbonium and the absence of biaryl in the reaction mixture.

The presence of a large group at the ortho-position of the diazonium salt can increase the yield of diarylthiocarbonate and the ratio of diarylthiocarbonate over aryl xanthate. However, there is no significant electronic effect from the substituent at the para-position, as similar yields of diarylthiocarbonate are obtained from diazonium salt with either a methoxy or a nitro group at the para-position. The orientation of substituents in both diarylthiocarbonate and aryl xanthates is also the same as in the initial diazonium salt.

Replacing the ethyl group with isopropyl or benzyl group in potassium ethylxanthate does not significantly affect the yield of diarylthiocarbonate. The best yield of diarylthiocarbonate is obtained when a 1:1 ratio of diazonium salt and potassium ethylxanthate is used, and increasing acidity decreases the yield of diarylthiocarbonate. The mesomeric form of the ethyl xanthate anion is believed to exist, which attacks the diazonium salt followed by the loss of ethanol, resulting in the intermediate ArSC(O)S−. This intermediate then attacks the second diazonium ion to form diarylthiocarbonate.

References

Leuckart, R. (1890), Eine neue Methode zur Darstellung aromatischer Mercaptane. [A new method for the preparation of aromatic mercaptans] J. Prakt. Chem., 41: 179-224. https://doi.org/10.1002/prac.18900410114

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