In
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 +2cycloaddition
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 pKa = 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.
See also
* 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 ...
* Thiocarbamate
In organic chemistry, thiocarbamates (thiourethanes) are a family of organosulfur compounds. As the prefix ''thio-'' suggests, they are sulfur analogues of carbamates. There are two isomeric forms of thiocarbamates: ''O''-thiocarbamates, (ester ...
* Thiocarbonate Thiocarbonate describes a family of anions with the general chemical formula (''x'' = 0, 1, or 2):
*for ''x'' = 2 it is monothiocarbonate ion
*for ''x'' = 1 it is dithiocarbonate ion
*for ''x'' = 0 it is trithiocarbonate ion
Like the carbonate d ...
* Thiocarbonic acid
* Thiocarbonate Thiocarbonate describes a family of anions with the general chemical formula (''x'' = 0, 1, or 2):
*for ''x'' = 2 it is monothiocarbonate ion
*for ''x'' = 1 it is dithiocarbonate ion
*for ''x'' = 0 it is trithiocarbonate ion
Like the carbonate d ...
References
{{Functional Groups
Organic acids
Reagents for organic chemistry
Organosulfur compounds
Functional groups