Thiocarboxylic acid

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Thione form (carbothioic O-acid)
Thiol form (carbothioic S-acid)

In

organosulfur compounds related to carboxylic acids by replacement of one of the oxygen atoms with a sulfur atom. Two tautomers are possible: a thione form (RC(S)OH) and a thiol form (RC(O)SH).[1][2] These are sometimes also referred to as "carbothioic O-acid" and "carbothioic S-acid" respectively. Of these the thiol form is most common (e.g. thioacetic acid
).

A naturally occurring thiocarboxylic acid is

pyridine-2,6-dicarbothioic acid, a siderophore
.

Synthesis

Thiocarboxylic acids are typically prepared by

acid chloride, as in the following conversion of benzoyl chloride to thiobenzoic acid using potassium hydrosulfide according to the following idealized equation:[3]

C6H5C(O)Cl + KSH → C6H5C(O)SH + KCl

2,6-Pyridinedicarbothioic acid is synthesized by treating the diacid dichloride with a solution of H2S in pyridine:

NC5H3(COCl)2 + 2 H2S + 2 C5H5N → [C5H5NH+][HNC5H3(COS)−2] + [C5H5NH]Cl

This reaction produces the orange pyridinium salt of pyridinium-2,6-dicarbothioate. Treatment of this salt with sulfuric acid gives colorless the bis(thiocarboxylic acid, which can then be extracted with dichloromethane.[4]

Reactions

At neutral pH, thiocarboxylic acids are fully ionized. Thiocarboxylic acids are about 100 times more acidic than the analogous carboxylic acids. For PhC(O)SH pKa = 2.48 vs 4.20 for PhC(O)OH. For thioacetic acid the pKa is near 3.4 vs 4.72 for acetic acid.[5]

The conjugate base of thioacetic acid, thioacetate is reagents for installing thiol groups via the displacement of alkyl halides to give the thioester, which in turn are susceptible to hydrolysis:

R−X + CH3COS− → R−SC(O)CH3 + X−
R−SC(O)CH3 + H2O → R−SH + CH3CO2H

Thiocarboxylic acids react with various nitrogen functional groups, such as organic azide, nitro, and isocyanate compounds, to give amides under mild conditions.[6][7] This method avoids needing a highly nucleophilic aniline or other amine to initiate an amide-forming acyl substitution, but requires synthesis and handling of the unstable thiocarboxylic acid.[7] Unlike the Schmidt reaction or other nucleophilic-attack pathways, the reaction with an aryl or alkyl azide begins with a [3+2] cycloaddition; the resulting heterocycle expels N2 and the sulfur atom to give the monosubstituted amide.[6]

See also

References

  1. ISBN 0-471-95512-4
    .
  2. ISBN 978-0-470-77109-9
    .
  3. doi:10.15227/orgsyn.032.0101
    .
  4. doi:10.1080/03086648308080490
    .
  5. ^ M. R. Crampton (1974). "Acidity and hydrogen-bonding". In Saul Patai (ed.). The Chemistry of the Thiol Group. Chichester: John Wiley & Sons Ltd. p. 402.
  6. ^
    ISBN 978-3-13-171951-5
    .
  7. ^
    PMID 29896355
    .
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