organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, ...

, thiocarboxylic acids are organosulfur compounds related to carboxylic acids by replacement of one of the

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

atoms with a sulfur atom. Two

tautomer Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert. The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hyd ...

s are possible: a thione form () and a

thiol In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl gro ...

form (). 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 Thioacetic acid is an organosulfur compound with the molecular formula . It is the sulfur analogue of acetic acid (), 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 ...

). A naturally occurring thiocarboxylic acid is pyridine-2,6-dicarbothioic acid, a

siderophore Siderophores (Greek: "iron carrier") are small, high-affinity iron- chelating compounds that are secreted by microorganisms such as bacteria and fungi. They help the organism accumulate iron. Although a widening range of siderophore functions is n ...

Synthesis

Thiocarboxylic acids are typically prepared by

salt metathesis A salt metathesis reaction, sometimes called a double displacement reaction, is a chemical process involving the exchange of bonds between two reacting chemical species which results in the creation of products with similar or identical bonding a ...

from the

acid chloride In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group . Their formula is usually written , where R is a side chain. They are reactive derivatives of carboxylic acids (). A specific example o ...

, as in the following conversion of

benzoyl chloride Benzoyl chloride, also known as benzenecarbonyl chloride, is an organochlorine compound with the formula . It is a colourless, fuming liquid with an irritating odour, and consists of a benzene ring () with an acyl chloride () substituent. It is ...

to thiobenzoic acid using

potassium hydrosulfide Potassium hydrosulfide is the inorganic compound with the formula KSH. This colourless salt consists of the cation and the bisulfide anion . It is the product of the half-neutralization of hydrogen sulfide with potassium hydroxide. The compound ...

according to the following idealized equation: :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 pK_a = 2.48 vs 4.20 for PhC(O)OH. For thioacetic acid the pK_a is near 3.4 vs 4.72 for acetic acid. Their conjugate bases, e.g.

potassium thioacetate Potassium thioacetate is an organosulfur compound and a salt with the formula . This white, water-soluble solid is used as a reagent for preparing thioacetate esters and other derivatives. Synthesis and reactions Potassium thioacetate, which is ...

, serve as reagents for installing thiol groups via the displacement of alkyl halides to give the

thioester In organic chemistry, thioesters are organosulfur compounds with the functional group . They are analogous to carboxylate esters () with the sulfur in the thioester playing the role of the linking oxygen in the carboxylate ester, as implied by t ...

, which in turn are susceptible to hydrolysis. Thiocarboxylic acids react with various nitrogen functional groups, such as organic azide,

nitro Nitro may refer to: Chemistry *Nitrogen, a chemical element and a gas except at very low temperatures, with which many compounds are formed: **Nitro compound, an organic compound containing one or more nitro functional groups, -NO2 **Nitroalkene, ...

, and

isocyanate In organic chemistry, isocyanate is the functional group with the formula . Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyan ...

compounds, to give amides under mild conditions. This method avoids needing a highly nucleophilic

aniline Aniline is an organic compound with the formula C6 H5 NH2. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine. It is an industrially significant commodity chemical, as well as a versatile starti ...

or other amine to initiate an amide-forming acyl substitution, but requires synthesis and handling of the unstable thiocarboxylic acid. Unlike the

Schmidt reaction In organic chemistry, the Schmidt reaction is an organic reaction in which an azide reacts with a carbonyl derivative, usually an aldehyde, ketone, or carboxylic acid, under acidic conditions to give an amine or amide, with expulsion of nitrogen ...

or other nucleophilic-attack pathways, the reaction with an aryl or alkyl azide begins with a +2

cycloaddition In organic chemistry, a cycloaddition is a chemical reaction in which "two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity". T ...

; the resulting heterocycle expels N₂ and the sulfur atom to give the monosubstituted amide.

Dithiocarboxylic acids

Dithiocarboxylic acids, with the formula , are less common than the monothio derivatives. They are about 3x more acidic than the monothiocarboxylic acids. Thus, for dithiobenzoic acid pK_a = 1.92. Such compounds are commonly prepared by the reaction of

carbon disulfide Carbon disulfide (also spelled as carbon disulphide) is a neurotoxic, colorless, volatile liquid with the formula and structure . The compound is used frequently as a building block in organic chemistry as well as an industrial and chemical n ...

with a Grignard reagent: :RMgX + CS2 -> RCS2MgX :RCS2MgX + HCl -> RCS2H + MgXCl This reaction is comparable to the formation of carboxylic acids using a Grignard reagent and

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

. Dithiocarboxylate salts readily S-alkylate to give dithiocarboxylate esters: :RCS2Na + R'Cl -> RCS2R' + NaCl Aryldithiocarboxylic acids, e.g., dithiobenzoic acid, chlorinate to give the thioacyl chlorides.

References

{{Functional Groups Organic acids Reagents for organic chemistry Organosulfur compounds Functional groups

Synthesis

Reactions

Dithiocarboxylic acids

See also

References