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In
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, 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.Clay ...
, a sulfoxide, also called a sulphoxide, is an
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—sulf ...
containing a sulfinyl ()
functional group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the res ...
attached to two
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes ...
atoms. It is a polar functional group. Sulfoxides are oxidized
derivatives The derivative of a function is the rate of change of the function's output relative to its input value. Derivative may also refer to: In mathematics and economics *Brzozowski derivative in the theory of formal languages *Formal derivative, an ...
of sulfides. Examples of important sulfoxides are alliin, a precursor to the compound that gives freshly crushed garlic its aroma, and
dimethyl sulfoxide Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula ( CH3)2. This colorless liquid is the sulfoxide most widely used commercially. It is an important polar aprotic solvent that dissolves both polar and nonpolar compounds ...
(DMSO), a common
solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
.


Structure and bonding

Sulfoxides feature relatively short S–O distances. In DMSO, the S–O distance is 1.531 Å. The sulfur center is pyramidal; the sum of the angles at sulfur is about 306°.. Sulfoxides are generally represented with the structural formula R−S(=O)−R', where R and R' are organic groups. The bond between the
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 formul ...
and
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 we ...
atoms is intermediate of a dative bond and a polarized
double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...
. The double-bond resonance form implies 10 electrons around sulfur (10-S-3 in
N-X-L notation In chemistry, a hypervalent molecule (the phenomenon is sometimes colloquially known as expanded octet) is a molecule that contains one or more main group elements apparently bearing more than eight electrons in their valence shells. Phosphorus pe ...
). The double-bond character of the S−O bond may be accounted for by donation of electron density into C−S antibonding orbitals ("no-bond" resonance forms in valence-bond language). Nevertheless, due to its simplicity and lack of ambiguity, the IUPAC recommends use of the expanded octet double-bond structure to depict sulfoxides, rather than the dipolar structure or structures that invoke "no-bond" resonance contributors. The S–O interaction has an
electrostatic Electrostatics is a branch of physics that studies electric charges at rest (static electricity). Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for am ...
aspect, resulting in significant dipolar character, with negative charge centered on oxygen.


Chirality

A
lone pair In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom in a covalent bondIUPAC '' Gold Book'' definition''lone (electron) pair''/ref> and is sometimes called an unshared pair or non-bonding pair. L ...
of electrons resides on the sulfur atom, giving it tetrahedral electron-pair geometry and
trigonal pyramidal In chemistry, a trigonal pyramid is a molecular geometry with one atom at the apex and three atoms at the corners of a trigonal base, resembling a tetrahedron (not to be confused with the tetrahedral geometry). When all three atoms at the c ...
shape (steric number 4 with one lone pair; see
VSEPR theory Valence shell electron pair repulsion (VSEPR) theory ( , ), is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is also named the Gillespie-Nyholm t ...
). When the two organic residues are dissimilar, the sulfur is a chiral center, for example, in
methyl phenyl sulfoxide Methyl phenyl sulfoxide is the 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., saccha ...
. The energy barrier required to invert this
stereocenter In stereochemistry, a stereocenter of a molecule is an atom (center), axis or plane that is the focus of stereoisomerism; that is, when having at least three different groups bound to the stereocenter, interchanging any two different groups cr ...
is sufficiently high that sulfoxides are optically stable near room temperature. That is, the rate of
racemization In chemistry, racemization is a conversion, by heat or by chemical reaction, of an optically active compound into a racemic (optically inactive) form. This creates a 1:1 molar ratio of enantiomers and is referred too as a racemic mixture (i.e. con ...
is slow at room temperature. The enthalpy of activation for racemization is in the range 35 - 42 kcal/mol and the corresponding entropy of activation is -8 - +4 cal/mol-K. The barriers are lower for allylic and benzylic substituents.


Preparation

Sulfoxides are typically prepared by
oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
of
sulfide Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds l ...
s, sometimes referred to as
sulfoxidation in chemistry, sulfoxidation refers to two distinct reactions. In one meaning, sulfoxidation refers to the reaction of alkanes with a mixture of sulfur dioxide and oxygen. This reaction is employed industrially to produce alkyl sulfonic acids, ...
.
hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3% ...
is a typical oxidant, but periodate has also been used. In these oxidations, care is required to avoid over oxidation to form the sulfone. For example, dimethyl sulfide is oxidized to
dimethyl sulfoxide Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula ( CH3)2. This colorless liquid is the sulfoxide most widely used commercially. It is an important polar aprotic solvent that dissolves both polar and nonpolar compounds ...
and then further to dimethyl sulfone. Unsymmetrical sulfides are prochiral, thus their oxidation gives chiral sulfoxides. This process can be performed enantioselectively.


Aryl sulfoxides

In addition to the oxidation routes, di
aryl In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used as ...
sulfoxides can be prepared by two Friedel–Crafts arylations of
sulfur dioxide Sulfur dioxide ( IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic ...
using an acid catalyst: :2 ArH + SO2 → Ar2SO + H2O Both aryl sulfinyl chlorides and diaryl sulfoxides can be also prepared from arenes through reaction with
thionyl chloride Thionyl chloride is an inorganic compound with the chemical formula . It is a moderately volatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as a chlorinating reagent, with approximately per year bein ...
in the presence of Lewis acid catalysts such as BiCl3, Bi(OTf)3, LiClO4, or NaClO4.


Reactions


Deoxygenation and oxygenation

Sulfoxides undergo deoxygenation to give sulfides. Typically metal complexes are used to catalyze the reaction, using hydrosilanes as the stoichiometric reductant. The deoxygenation of dimethylsulfoxide is catalyzed by DMSO reductase, a molybdoenzyme: :OSMe2 + 2e- + 2 H+ → SMe2 + H2O


Acid-base reactions

The α-CH groups of alkyl sulfoxides are susceptible to deprotonation by strong bases, such as
sodium hydride Sodium hydride is the chemical compound with the empirical formula Na H. This alkali metal hydride is primarily used as a strong yet combustible base in organic synthesis. NaH is a saline (salt-like) hydride, composed of Na+ and H− ions, in ...
: :CH3S(O)CH3 + NaH → CH3S(O)CH2Na + H2 In the Pummerer rearrangement,
alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloal ...
sulfoxides react with acetic anhydride to give migration of the oxygen from sulfur to the adjacent carbon as an
acetate An acetate is a salt formed by the combination of acetic acid with a base (e.g. alkaline, earthy, metallic, nonmetallic or radical base). "Acetate" also describes the conjugate base or ion (specifically, the negatively charged ion called ...
ester. The first step of the reaction sequence involves the sulfoxide oxygen acting as a
nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they a ...
: :


Elimination reactions

Sulfoxide undergo thermal elimination via an Ei mechanism to yield vinyl
alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
s and sulfenic acids. :CH3S(O)CH2CH2R → CH3SOH + CH2=CHR The acids are powerful
antioxidant Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricants ...
s, but lack long-term stability. Some parent sulfoxides are therefore marketed as antioxidant
polymer stabilisers Polymer stabilizers (British: polymer stabilisers) are chemical additives which may be added to polymeric materials, such as plastics and rubbers, to inhibit or retard their degradation. Common polymer degradation processes include oxidation, UV-d ...
. Structures based on thiodipropionate esters are popular. The reverse reaction is possible.


Coordination chemistry

Sulfoxides, especially DMSO, form
coordination complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
es with transition metals. Depending on the hard-soft properties of the metal, the sulfoxide binds through either the sulfur or the oxygen atom. The latter is particularly common.


Applications and occurrence

left, 262 px, Esomeprazole, a blockbuster drug, is an enantiopure drug">blockbuster_drug.html" ;"title="Esomeprazole, a blockbuster drug">Esomeprazole, a blockbuster drug, is an enantiopure drug containing a sulfoxide functional group. The related drug omeprazole is the racemic version. DMSO is a widely used solvent. The sulfoxide functional group occurs in several drugs. Notable is [ someprazole, the optically pure form of the proton-pump inhibitor omeprazole. Another commercially important sulfoxides include armodafinil. Methionine sulfoxide forms from the amino acid methionine and its accumulation is associated with aging. The enzyme DMSO reductase catalyzes the interconversion of DMSO and dimethylsulfide. Naturally-occurring chiral sulfoxides include alliin and ajoene.


Further reading

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References

{{Authority control Functional groups