Thioacetic acid

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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 (Jmol)
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

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]

Synthesis and properties[edit]

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.

Reactivity[edit]

Acidity[edit]

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 thioacetate[edit]

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]

thioacetic acid application
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]

References[edit]

  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.