Organosulfur compounds can be classified according to the sulfur-containing
functional groups, which are listed (approximately) in decreasing order of their occurrence. Image:R-allicin-2D-skeletal.svg| Image:Cysteine.svg| (
R)-
Cysteine, an
amino acid containing a thiol group Image:L-Methionin - L-Methionine.svg|
Methionine, an
amino acid containing a sulfide Image:Diphenyl disulfide.svg|
Diphenyl disulfide, a representative disulfide Image:Dibenzothiophen - Dibenzothiophene.svg| Image:Perfluorooctanesulfonic acid structure.svg|
Perfluorooctanesulfonic acid, a controversial surfactant Image:Lipoic_acid.svg|
Lipoic acid, an essential cofactor of four mitochondrial enzyme complexes. Image:Penicillin core.svg|
Penicillin core structure, where "R" is the variable group. image:Sulfanilamide-skeletal.svg|
Sulfanilamide, a
sulfonamide antibacterial, called a
sulfa drug. Image:Sulfur-mustard-2D-skeletal.svg|
Sulfur mustard, a type of sulfide used as a
chemical warfare agent. Image:MartinSulfurane.svg|
Martin's sulfurane with a see-saw structure, like that of SF4
Sulfides Sulfides, formerly known as thioethers, are characterized by C−S−C
bonds Relative to C−C bonds, C−S bonds are both longer, because sulfur atoms are larger than carbon atoms, and about 10% weaker. Representative
bond lengths in sulfur compounds are 183
pm for the S−C single bond in
methanethiol and 173 pm in
thiophene. The C−S
bond dissociation energy for thiomethane is 89 kcal/mol (370 kJ/mol) compared to methane's 100 kcal/mol (420 kJ/mol) and when hydrogen is replaced by a methyl group the energy decreases to 73 kcal/mol (305 kJ/mol). The single
carbon to oxygen bond is shorter than that of the C−C bond. The
bond dissociation energies for
dimethyl sulfide and
dimethyl ether are respectively 73 and 77 kcal/mol (305 and 322 kJ/mol). Sulfides are typically prepared by
alkylation of thiols. Alkylating agents include not only alkyl halides, but also epoxides, aziridines, and
Michael acceptors. They can also be prepared via the
Pummerer rearrangement. In the
Ferrario reaction,
phenyl ether is converted to
phenoxathiin by action of elemental sulfur and
aluminium chloride. :
Thioacetals and
thioketals feature C−S−C−S−C bond sequence. They represent a subclass of sulfides. The thioacetals are useful in "
umpolung" of carbonyl groups. Thioacetals and thioketals can also be used to protect a carbonyl group in organic syntheses. The above classes of sulfur compounds also exist in saturated and unsaturated
heterocyclic structures, often in combination with other
heteroatoms, as illustrated by
thiiranes,
thiirenes,
thietanes,
thietes,
dithietanes,
thiolanes,
thianes,
dithianes,
thiepanes,
thiepines,
thiazoles,
isothiazoles, and
thiophenes, among others. The latter three compounds represent a special class of sulfur-containing heterocycles that are
aromatic. The
resonance stabilization of
thiophene is 29 kcal/mol (121 kJ/mol) compared to 20 kcal/mol (84 kJ/mol) for the oxygen analogue
furan. The reason for this difference is the higher
electronegativity for oxygen drawing away electrons to itself at the expense of the aromatic ring current. Yet as an aromatic
substituent the thio group is less electron-releasing than the alkoxy group.
Dibenzothiophenes (see
diagram), tricyclic heterocycles consisting of two benzene rings fused to a central thiophene ring, occurs widely in heavier fractions of petroleum.
Thiols, disulfides, polysulfides Thiol groups contain the functionality R−SH. Thiols are structurally similar to the
alcohol group, but these functionalities are very different in their chemical properties. Thiols are more
nucleophilic, more acidic, and more readily oxidized. This acidity can differ by 5
pKa units. The difference in
electronegativity between sulfur (2.58) and hydrogen (2.20) is small and therefore
hydrogen bonding in thiols is not prominent. Aliphatic thiols form
monolayers on
gold, which are topical in
nanotechnology. Certain aromatic thiols can be accessed through a
Herz reaction. Removal of the hydrogen atom gives a
thiyl radical, an unstable reaction intermediate.
Disulfides R−S−S−R with a covalent sulfur to sulfur bond are important for
crosslinking: in
biochemistry for the folding and stability of some proteins and in
polymer chemistry for the crosslinking of rubber. Longer sulfur chains are also known, such as in the natural product
varacin which contains an unusual pentathiepin ring (5-sulfur chain cyclised onto a benzene ring).
Inorganic thioesters Esters of thiols with inorganic acids (e.g.,
Bunte salts, formally from a thiol and
sodium hydrogen sulfate) generally have properties deducible from those of thiols and the corresponding acid. Some are, however, of biological interest.
Thiophosphate esters see extensive use in pharmacology and agriculture, as the moiety tends to stall enzymes that hydrolyze phosphates.
S-Nitrosothiols, also known as thionitrites, attach a nitroso group to a thiol, e.g. R−S−N=O. They have received considerable attention in biochemistry because they serve as donors of the nitrosonium ion, NO+, and nitric oxide, NO, which may serve as signaling molecules in living systems, especially related to vasodilation.
Thioic acid derivatives Thiocarboxylic acids (RC(O)SH) and
dithiocarboxylic acids (RC(S)SH) are well known. They are structurally similar to carboxylic acids but more acidic.
Thioesters have general structure R−C(O)−S−R. They are related to regular esters (R−C(O)−O−R) but are
more susceptible to hydrolysis and related reactions. Thioesters formed from
coenzyme A are prominent in biochemistry, especially in
fatty acid synthesis.
Thioamides, with the formula R1C(=S)N(R2)R3 are more common than thioketones and thioaldehydes. They are typically prepared by the reaction of amides with
Lawesson's reagent.
Isothiocyanates, with formula R−N=C=S, are found naturally. Vegetable foods with characteristic flavors due to isothiocyanates include
wasabi,
horseradish,
mustard,
radish,
Brussels sprouts,
watercress,
nasturtiums, and
capers. Few
thioacyl chlorides are stable.
Thiocyanates, R−S−CN, are related to
sulfenyl halides and esters in terms of reactivity.
Other unsaturated C–S bonding Compounds with
double bonds between carbon and sulfur are relatively uncommon, but include the important compounds
carbon disulfide,
carbonyl sulfide, and
thiophosgene.
Thioketones (RC(=S)) are uncommon with alkyl substituents, but one example is
thiobenzophenone.
Thioaldehydes are rarer still, reflecting their lack of steric protection ("
thioformaldehyde" exists as a cyclic trimer). The
S-oxides of thiocarbonyl compounds are known as thiocarbonyl
S-oxides: (R2C=S=O, and thiocarbonyl
S,
S-dioxides or
sulfenes, R2C=SO2). The thione
S-oxides have also been known as
sulfines, and while
IUPAC considers this term obsolete, the name persists in the literature. These compounds are well known with extensive chemistry. Examples include
syn-propanethial-S-oxide and
sulfene. Triple bonds between sulfur and carbon in sulfaalkynes are rare and can be found in
carbon monosulfide (CS) and have been suggested for the compounds F3CCSF3 and F5SCSF3. The unbranched compound HCSOH is also represented as having a formal triple bond.
Sulfur halides A wide range of organosulfur compounds are known which contain one or more
halogen atom ("X" in the chemical formulas that follow) bonded to a single sulfur atom, e.g.:
sulfenyl halides, RSX;
sulfinyl halides, RS(O)X;
sulfonyl halides, RSO2X; alkyl and arylsulfur trichlorides, RSCl3 and trifluorides, RSF3; and alkyl and arylsulfur pentafluorides, RSF5. Less well known are dialkylsulfur tetrahalides, mainly represented by the tetrafluorides, e.g., R2SF4, although at least one difluoride dichloride (R2SF2Cl2) is known.
S-oxidized moieties A
sulfoxide, R−S(O)−R, is the
S-oxide of a sulfide ("sulfide oxide"); a
sulfone, R−S(O)2−R, is the
S,
S-dioxide of a sulfide; a
thiosulfinate, R−S(O)−S−R, is the
S-oxide of a disulfide; and a
thiosulfonate, R−S(O)2−S−R, is the
S,
S-dioxide of a disulfide. All of these compounds are well known with extensive chemistry, e.g.,
dimethyl sulfoxide,
dimethyl sulfone, and
allicin (see
drawing).
Sulfonic acids have functionality R−S(=O)2−OH. They are strong acids that are typically soluble in organic solvents. Sulfonic acids like
trifluoromethanesulfonic acid is a frequently used reagent in
organic chemistry.
Sulfinic acids have functionality R−S(O)−OH while
sulfenic acids have functionality R−S−OH. In the series sulfonic—sulfinic—sulfenic acids, both the acid strength and stability diminish in that order. Sulfonamides,
sulfinamides and
sulfenamides, with formulas R−SO2N2, R−S(O)N2, and R−SN2, respectively, each have a rich chemistry. For example,
sulfa drugs are sulfonamides derived from
aromatic sulfonation. Chiral sulfinamides are used in asymmetric synthesis, while sulfenamides are used extensively in the vulcanization process to assist cross-linking.
S-oxidation with nitrogen Sulfimides (also called a sulfilimines) are sulfur–nitrogen compounds of structure R2S+N−, the nitrogen analog of sulfoxides. They are of interest in part due to their pharmacological properties. When two different R groups are attached to sulfur, sulfimides are chiral. Sulfimides form stable α-carbanions.
Sulfinyl nitrenes, despite their formal resonance structure as R–S(=O)≡N, instead behave primarily as
nitrenic sulfinamides. Sulfoximides (also called sulfoximines) are tetracoordinate sulfur–nitrogen compounds, isoelectronic with sulfones, in which one oxygen atom of the sulfone is replaced by a substituted nitrogen atom, e.g., R2S(O)=N. When two different R groups are attached to sulfur, sulfoximides are chiral. Much of the interest in this class of compounds is derived from the discovery that methionine sulfoximide (methionine sulfoximine) is an inhibitor of
glutamine synthetase. In
Sulfonediimines (also called sulfodiimines, sulfodiimides or sulfonediimides), a nitrogen replaces both sulfone oxygen atoms, e.g., R2S(=N)2. They are of interest because of their biological activity and as building blocks for heterocycle synthesis.
Sulfinylamines are bicoordinate sulfur-oxygen-nitrogen compounds, roughly describable as RN=S+–O−.
Sulfonium salts and ylides A
sulfonium ion is a positively charged ion featuring three organic substituents attached to sulfur, with the formula [R3S]+. Together with their negatively charged counterpart, the anion, the compounds are called sulfonium salts. An oxosulfonium ion is a positively charged ion featuring three organic substituents and an oxygen attached to sulfur, with the formula [R3S=O]+. Together with their negatively charged counterpart, the anion, the compounds are called oxosulfonium salts. Related species include alkoxysulfonium and chlorosulfonium ions, [R2SOR]+ and [R2SCl]+, respectively. Deprotonation of sulfonium and oxosulfonium salts affords
ylides, of structure R2S+−C−−2 and R2S(O)+−C−−2. While
sulfonium ylides, for instance in the
Johnson–Corey–Chaykovsky reaction used to synthesize
epoxides, are sometimes drawn with a C=S double bond, e.g., R2S=C2, the ylidic carbon–sulfur bond is highly polarized and is better described as being ionic. Sulfonium ylides are key intermediates in the synthetically useful
Stevens rearrangement. Thiocarbonyl ylides (RC=S+−C−−R) can form by ring-opening of
thiiranes, photocyclization of aryl vinyl sulfides, as well as by other processes.
Sulfuranes and persulfuranes Sulfuranes are relatively specialized functional group that feature
tetravalent sulfur, with the formula SR4 : It is prepared from the corresponding sulfurane
1 with
xenon difluoride /
boron trifluoride in
acetonitrile to the sulfuranyl dication
2 followed by reaction with
methyllithium in
tetrahydrofuran to (a stable) persulfurane
3 as the
cis isomer.
X-ray diffraction shows C−S
bond lengths ranging between 189 and 193 pm (longer than the standard bond length) with the central sulfur atom in a distorted
octahedral molecular geometry. ==Organosulfur compounds in nature==