Thioacetic acid

Thioacetic acid is an organosulfur compound with the molecular formula CH₃C(O)SH. It is a thioic acid: the sulfur analogue of acetic acid (CH₃C(O)OH), as implied by the thio- prefix. It is a yellow liquid with a strong thiol-like odor. It is used in organic synthesis for the introduction of thiol groups (−SH) in molecules.^[4]

Thioacetic acid

Names
Preferred IUPAC name Ethanethioic S-acid[1]
Other names Thioacetic S-acid Thiolacetic acid
Identifiers
CAS Number	507-09-5 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:46800 Y
ChemSpider	10052 Y
ECHA InfoCard	100.007.331
KEGG	C01857 Y
PubChem CID	10484
UNII	PS92MLC0FQ Y
CompTox Dashboard (EPA)	DTXSID5060142
InChI InChI=1S/C2H4OS/c1-2(3)4/h1H3,(H,3,4) Y Key: DUYAAUVXQSMXQP-UHFFFAOYSA-N Y InChI=1/C2H4OS/c1-2(3)4/h1H3,(H,3,4) Key: DUYAAUVXQSMXQP-UHFFFAOYAO
SMILES O=C(S)C
Properties
Chemical formula	CH3C(O)SH
Molar mass	76.11756 g/mol
Appearance	Transparent, colorless to light yellow liquid[2][3]
Odor	Unpleasant, strong thiol-like
Density	1.08 g/mL
Melting point	−58 °C (−72 °F; 215 K)
Boiling point	93 °C (199 °F; 366 K)
Magnetic susceptibility (χ)	−38.4·10−6 cm3/mol
Refractive index (nD)	1.465
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	May cause severe skin and eye burns. Highly flammable liquid.
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H225, H301, H302, H314, H317
Precautionary statements	P210, P233, P240, P241, P242, P243, P260, P261, P264, P264+P265, P270, P272, P280, P301+P316, P301+P317, P301+P330+P331, P302+P352, P302+P361+P354, P303+P361+P353, P304+P340, P305+P354+P338, P316, P317, P321, P330, P333+P317, P362+P364, P363, P370+P378, P403+P235, P405, P501
Lethal dose or concentration (LD, LC):
LD50 (median dose)	75 mg/kg (mouse, intraperitoneal)
Safety data sheet (SDS)	Fischer Scientific
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Synthesis and properties

Thioacetic acid is prepared by the reaction of acetic anhydride with hydrogen sulfide:^[5]

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

It has also been produced by the action of phosphorus pentasulfide on glacial acetic acid, followed by distillation.^[6]

CH₃C(O)OH + P₂S₅ → CH₃C(O)SH + 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 75 K lower than those for acetic acid.

Reactivity

Acidity

With a pK_a near 3.4, thioacetic acid is about 15 times more acidic than acetic acid.^[7] The conjugate base is thioacetate:

CH₃C(O)SH → CH₃C(O)S⁻ + H⁺

In neutral water, thioacetic acid is fully ionized.

Reactivity of thioacetate

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.^[8] 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, where X = Cl, Br, I

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:^[9]

thioacetic acid application

Reductive acetylation

Potassium thioacetate can be used convert nitroarenes to aryl acetamides in one step. This is particularly useful in the preparation of pharmaceuticals, e.g., paracetamol from 4-nitrophenol or 4-nitroanisole.^[10]