In organic chemistry, thiocarboxylic acids are

organosulfur compound Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g., saccharin. Nature abounds with organosulfur compounds—sulfu ...

s related to

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...

s by replacement of one of the oxygen atoms with a

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...

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 hydr ...

s are possible: a thione form () and a thiol 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). A naturally occurring thiocarboxylic acid is pyridine-2,6-dicarbothioic acid, a siderophore.

Synthesis

Thiocarboxylic acids are typically prepared by salt metathesis from the acid chloride, 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 i ...

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 is ...

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 Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...

. Their conjugate bases, e.g. potassium thioacetate, 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 In chemistry, azide is a linear, polyatomic anion with the formula and structure . It is the conjugate base of hydrazoic acid . Organic azides are organic compounds with the formula , containing the azide functional group. The dominant applic ...

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 compounds, to give amides under mild conditions. 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. Unlike the Schmidt reaction or other nucleophilic-attack pathways, the reaction with an aryl or alkyl azide begins with a +2 cycloaddition; 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 with a

Grignard reagent A Grignard reagent or Grignard compound is a chemical compound with the general formula , where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride and phenylmagnesium bromide ...

: :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: :RCS2Na + R'Cl -> RCS2R' + NaCl Aryldithiocarboxylic acids, e.g.,

dithiobenzoic acid Dithiobenzoic acid is the organosulfur compound with the formula C6H5CS2H. It is a dithiocarboxylic acid, an analogue of benzoic acid, but more acidic and deeply colored. Synthesis and reactions It can be prepared by sulfiding benzal chlorid ...

, chlorinate to give the

thioacyl chloride In organic chemistry, thioacyl chloride is a functional group of the type RC(S)Cl, where R is an organic substituent. Thioacyl chlorides are analogous to acid chlorides, but much rarer and less robust. The best studied is thiobenzoyl chloride, a ...

References

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

Synthesis

Reactions

Dithiocarboxylic acids

See also

References