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Thioacetic acid is an organosulfur compound with the molecular formula CH3COSH. It is a yellow coloured liquid with a strong thiol-like odor. It is used in organic synthesis for the introduction of thiol groups in molecules[1]

Thioacetic acid
Skeletal formula of thioacetic acid
Ball-and-stick model of the thioacetic acid molecule
Names
IUPAC name
Ethanethioic S-acid
Other names
Thioacetic S-acid
Thiolacetic acid
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.007.331
KEGG
UNII
Properties
C2H4OS
Molar mass 76.11756
Density 1.08 g/mL
Melting point −58 °C (−72 °F; 215 K)
Boiling point 93 °C (199 °F; 366 K)
-38.4·10−6 cm3/mol
Hazards
Safety data sheet Fischer Scientific
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Contents

Synthesis and propertiesEdit

Thioacetic acid is prepared by the reaction of acetic anhydride with hydrogen sulfide:[2]

(CH3C(O))2O + H2S → CH3C(O)SH + CH3CO2H

It has also been produced by the action of phosphorus pentasulfide on glacial acetic acid, followed by distillation.[3]

CH3COOH + P2S5 → CH3COSH + P2OS4

Thioacetic acid is typically contaminated by acetic acid.

The compound exists exclusively as the thiol tautomer, consistent with the strength of the C=O double bond. Reflecting the influence of hydrogen-bonding, the boiling point (93 °C) and melting points are 20 and 75K lower than those for acetic acid.

ReactivityEdit

AcidityEdit

With a pKa near 3.4, thioacetic acid is about 15x more acidic than acetic acid.[4] The conjugate base is thioacetate:

CH3COSH → CH3COS + H+

In neutral water, thioacetic acid is fully ionized.

Reactivity of thioacetateEdit

Most of the reactivity of thioacetic acid arises from the conjugate base, thioacetate. Salts of this anion, e.g. potassium thioacetate, are used to generate thioacetate esters.[5] Thioacetate esters undergo hydrolysis to give thiols. A typical method for preparing a thiol from an alkyl halide using thioacetic acid proceeds in four discrete steps, some of which can be conducted sequentially in the same flask:

CH3C(O)SH + NaOH → CH3C(O)SNa + H2O
CH3C(O)SNa + RX → CH3C(O)SR + NaX (X = Cl, Br, I, etc)
CH3C(O)SR + 2 NaOH → CH3CO2Na + RSNa + H2O
RSNa + HCl → RSH + NaCl

In an application that illustrates the use of its radical behavior, thioacetic acid is used with AIBN in a free radical mediated nucleophilic addition to an exocyclic alkene forming a thioester:[6]

Reductive acetylation

Salts of thioacetic acid such as potassium thioacetate can be used to do one-step conversion of aryl nitrates to aryl acetamides. This is particularly useful in pharmaceutical preparations, e.g. paracetamol.[7]

ReferencesEdit

  1. ^ Jeannie R. Phillips "Thiolacetic Acid" Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley. doi:10.1002/047084289X.rt096.
  2. ^ Ellingboe, E. K. (1951). "Thiolacetic acid". Organic Syntheses. 31: 105. doi:10.15227/orgsyn.031.0105.
  3. ^ Schiff, Robert (1895-08-09). "Preparation of Thioacetic Acid and its Importance for Chemical-Legal Investigations". Chemical News and Journal of Industrial Science. 72: 64. Retrieved 2016-11-02.
  4. ^ Matthys J. Janssen "Carboxylic Acids and Esters" in PATAI'S Chemistry of Functional Groups: Carboxylic Acids and Esters, Saul Patai, Ed. pp. 705–764, 1969. doi:10.1002/9780470771099.ch15
  5. ^ Ervithayasuporn, V. (2011). "Synthesis and Characterization of Octakis(3-propyl ethanethioate)octasilsesquioxane". Organometallics. 30 (17): 4475–4478. doi:10.1021/om200477a.
  6. ^ Synthesis of methyl 6-deoxy-4-O-(sodium sulfonato)-α-L-talopyranoside, its C-4 epimer and both isosteric [4-C-(potassium sulfonatomethyl)] derivatives László Lázár, Magdolna Csávás, Anikó Borbás, Gyöngyi Gyémánt, and András Lipták ARKIVOC 2004 (vii) 196-207 Link
  7. ^ Bhattacharya, Apurba; et al. (2006). "One-step reductive amidation of nitro arenes: application in the synthesis of Acetaminophen" (PDF). Tetrahedron Letters. 47: 1861–1864. doi:10.1016/j.tetlet.2005.09.196. Retrieved 2016-11-02.