Aromatic sulfonation is an organic reaction in which a hydrogen atom on an

arene Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The parent member of aromatic compounds is benzene. The word "aromatic" originates from the past groupin ...

is replaced by a sulfonic acid

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 re ...

in an

electrophilic aromatic substitution Electrophilic aromatic substitution is an organic reaction in which an atom that is attached to an aromatic system (usually hydrogen) is replaced by an electrophile. Some of the most important electrophilic aromatic substitutions are aromatic n ...

. Aryl sulfonic acids are used as detergents, dye, and drugs.

Stoichiometry and mechanism

Typical conditions involve heating the aromatic compound with sulfuric acid: :C₆H₆ + H₂SO₄ → C₆H₅SO₃H + H₂O Sulfur trioxide or its protonated derivative is the actual

electrophile In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carrie ...

in this electrophilic aromatic substitution. To drive the equilibrium, dehydrating agents such as

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 ...

can be added. :C₆H₆ + H₂SO₄ + SOCl₂ → C₆H₅SO₃H + SO₂ + 2 HCl Chlorosulfuric acid is also an effective agent: :C₆H₆ + HSO₃Cl → C₆H₅SO₃H + HCl In contrast to aromatic nitration and most other electrophilic aromatic substitutions this reaction is reversible. Sulfonation takes place in concentrated acidic conditions and desulfonation is the mode of action in a dilute hot aqueous acid. The reaction is very useful in protecting the aromatic system because of this reversibility. Due to their electron withdrawing effects, sulfonate protecting groups can be used to prevent electrophilic aromatic substitution. They can also be installed as directing groups to affect the position where a substitution may take place.T.W> Graham Solomons: ''Organic Chemistry'', 11th Edition, Wiley, Hoboken, NJ, 2013, p. 676, .

Specialized sulfonation methods

Many method have been developed for introducing sulfonate groups aside from direction sulfonation.

Piria reaction

A classic named reaction is the Piria reaction ( Raffaele Piria, 1851) in which nitrobenzene is reacted with a metal

bisulfite The bisulfite ion (IUPAC-recommended nomenclature: hydrogensulfite) is the ion . Salts containing the ion are also known as "sulfite lyes". Sodium bisulfite is used interchangeably with sodium metabisulfite (Na2S2O5). Sodium metabisulfite disso ...

forming an aminosulfonic acid as a result of combined nitro group reduction and sulfonation. Piria reaction

Tyrer sulfonation process

In the Tyrer sulfonation process (1917), at some time of technological importance, benzene vapor is led through a vessel containing 90% sulfuric acid the temperature of which is increased from 100 to 180°C. Water and benzene are continuously removed in a condenser and the benzene layer fed back to the vessel. In this way an 80% yield is obtained. Amino Sulfonic Acid Synthesis V

Applications

Aromatic sulfonic acids are intermediates in the preparation of dyes and many pharmaceuticals. Sulfonation of

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 ...

s lead to a large group of sulfa drugs. Allura Red AC Structural Formula V1

Sulfonation of polystyrene is used to make sodium polystyrene sulfonate, a common ion exchange resin for

water softening Water softening is the removal of calcium, magnesium, and certain other metal cations in hard water. The resulting soft water requires less soap for the same cleaning effort, as soap is not wasted bonding with calcium ions. Soft water also exten ...

Reactions of aryl sulfonic acids

As a

, aryl sulfonic acids undergo desulfonation when heated in water: :RC₆H₄SO₃H + H₂O → RC₆H₅ + H₂SO₄ When treated with strong base, benzenesulfonic acid derivatives convert to phenols. :C₆H₅SO₃H + 2 NaOH → C₆H₅OH + Na₂SO₄ + H₂O

References

{{DEFAULTSORT:Aromatic Sulfonation Substitution reactions