Thiocarboxylic acid

Short description: Organic compounds with the tautomeric structures RC(S)OH or RC(O)SH

Thione form (carbothioic O-acid)

Thiol form (carbothioic S-acid)

In organic chemistry, thiocarboxylic acids are 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.

## Contents

* 1 Synthesis
* 2 Reactions
* 3 Dithiocarboxylic acids
* 4 See also
* 5 References

## Synthesis

Thiocarboxylic acids are typically prepared by salt metathesis from the 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

## 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.[4]

Their conjugate bases, e.g. potassium thioacetate, serve as reagents for installing thiol groups via the displacement of alkyl halides to give the thioester, which in turn are susceptible to hydrolysis.

Thiocarboxylic acids react with various nitrogen functional groups, such as organic azide, nitro, and isocyanate compounds, to give amides under mild conditions.[5][6] 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.[6] 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.[5]

## Dithiocarboxylic acids

Dithiocarboxylic acids, with the formula RCS2H, are less common than the monothio derivatives. They are about 3x more acidic than the monothiocarboxylic acids. Thus, for dithiobenzoic acid pKa = 1.92.[4] Such compounds are commonly prepared by the reaction of carbon disulfide with a Grignard reagent:[7]

RMgX + CS2 → RCS2MgX
RCS2MgX + HCl → RCS2H + MgXCl

This reaction is comparable to the formation of carboxylic acids using a Grignard reagent and carbon dioxide. Dithiocarboxylate salts readily S-alkylate to give dithiocarboxylate esters:[8]

RCS2Na + R'Cl → RCS2R' + NaCl

Aryldithiocarboxylic acids, e.g., dithiobenzoic acid, chlorinate to give the thioacyl chlorides.

## See also

* Thioester
* Thiocarbamate
* Thiocarbonate
* Thiocarbonic acid
* Thiocarbonate

## References

1. ↑ Cremlyn, R.J. (1996). An introduction to organosulfur chemistry. Chichester: Wiley. ISBN 0-471-95512-4.
2. ↑ Matthys J. Janssen (1969). "Thiolo, Thiono and Dithio Acids and Esters". in Saul Patai. Carboxylic Acids and Esters. PATAI'S Chemistry of Functional Groups. pp. 705–764. doi:10.1002/9780470771099.ch15. ISBN 9780470771099.
3. ↑ Noble, Jr., Paul; Tarbell, D. S. (1952). "Thiobenzoic Acid". Organic Syntheses 32: 101. doi:10.15227/orgsyn.032.0101.
4. ↑ 4.0 4.1 M. R. Crampton (1974). "Acidity and hydrogen-bonding". in Saul Patai. The Chemistry of the Thiol Group. Chichester: John Wiley & Sons Ltd. p. 402.
5. ↑ 5.0 5.1 Science of Synthesis: Houben–Weyl Methods of Molecular Transformations. 21: Three Carbon-Heteroatom Bonds: Amides and Derivatives; Peptides; Lactams. Georg Thieme Verlag. 2005. pp. 52–54. ISBN 9783131719515.
6. ↑ 6.0 6.1 Xie, Sheng; Zhang, Yang; Ramström, Olof; Yan, Mingdi (2016). "Base-catalyzed synthesis of aryl amides from aryl azides and aldehydes". Chem. Sci. 7 (1): 713–718. doi:10.1039/C5SC03510D. PMID 29896355.
7. ↑ Ramadas, S. R.; Srinivasan, P. S.; Ramachandran, J.; Sastry, V. V. S. K. (1983). "Methods of Synthesis of Dithiocarboxylic Acids and Esters". Synthesis 1983 (8): 605–622. doi:10.1055/s-1983-30446.
8. ↑ Frederick Kurzer, Alexander Lawson (1962). "Thiobenzoylthioglycolic Acid". Org. Synth. 42: 100. doi:10.15227/orgsyn.042.0100.

* v
* t
* e

Functional groups

| Only carbon,
hydrogen,
and oxygen|

| Hydrocarbons|

* Allene
* Alkene (Allyl group

|

Other|

* Acetoxy
* Acetyl
* Acryloyl
* Acyl
* Aldehyde
* Alkoxy (Methoxy)
* Benzoyl
* Carbonyl
* Carboxyl
* Carboxylic anhydride
* Dioxirane
* Epoxide
* Ester
* Ether
* Ethylenedioxy
* Hydroxy
* Ketone
* Methylenedioxy
* Peroxide (Organic)
* Ynone

Only one
element
apart from
C, H, O|

| Nitrogen|

* Amine
* Azo compound
* Cyanate
* Hydrazone
* Imide
* Imine
* Isocyanate
* Isonitrile
* Nitrene
* Nitrile
* Nitro compound
* Nitroso compound
* Organic amide
* Oxime

|

Phosphorus|

* Phosphonate
* Phosphonous

Sulfur|

* Disulfide
* Sulfone
* Sulfonic acid
* Sulfoxide
* Thial
* Thioester
* Thioether
* Thioketone
* Thiol

Selenium|

* Selenol
* Selenonic acid
* Seleninic acid
* Selenenic acid
* Selone

Tellurium|

* Tellurol
* Telluroketone

Other|

* Isothiocyanate
* Phosphoramide
* Sulfenyl chloride
* Sulfonamide
* Thiocyanate

See also chemical classification, chemical nomenclature ([[Chemistry:IUPAC nomenclature of inorganic, IUPAC nomenclature of organic chemistry|organic]])

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