In organic chemistry, sulfonic acid (or sulphonic acid) refers to a member of the class of organosulfur compounds with the general formula , where R is an organic alkyl or aryl group and the group a sulfonyl hydroxide.[ As a substituent, it is known as a sulfo group. A sulfonic acid can be thought of as sulfuric acid with one hydroxyl group replaced by an organic substituent. The parent compound (with the organic substituent replaced by hydrogen) is the parent sulfonic acid, , a tautomer of sulfurous acid, . Salts or of sulfonic acids are called .
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Preparation
Aryl sulfonic acids are produced by the process of sulfonation. Usually the sulfonating agent is sulfur trioxide. A large scale application of this method is the production of alkylbenzenesulfonic acids:
In this reaction, sulfur trioxide is an electrophile and the arene is the nucleophile. The reaction is an example of electrophilic aromatic substitution.
Alkylsulfonic acids can be prepared by sulfoxidation whereby alkanes are irradiated with a mixture of sulfur dioxide and oxygen. This reaction is employed industrially to produce alkyl sulfonic acids, which are used as .[
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From terminal alkenes, alkane sulfonic acids can be obtained by the addition of bisulfite.
Bisulfite can also be alkylation by :
Sulfonic acids can be prepared by oxidation of Thiol:
This pathway is the basis of the biosynthesis of taurine.
Hydrolysis routes
Many sulfonic acids are prepared by hydrolysis of and related precursors. Thus, perfluorooctanesulfonic acid is prepared by hydrolysis of the sulfonyl fluoride, which in turn is generated by the electrofluorination of octanesulfonic acid. Similarly the sulfonyl chloride derived from polyethylene is hydrolyzed to the sulfonic acid. These sulfonyl chlorides are produced by free-radical reactions of chlorine, sulfur dioxide, and the hydrocarbons using the Reed reaction.
Vinylsulfonic acid is derived by hydrolysis of carbyl sulfate, (), which in turn is obtained by the addition of sulfur trioxide to ethylene.
Properties
Sulfonic acids are strong acids. They are around a million times stronger than the corresponding acid. For example, p-Toluenesulfonic acid and methanesulfonic acid have p Ka values of −2.8 and −1.9, respectively, while those of benzoic acid and acetic acid are 4.20 and 4.76, respectively. The p Ka of methanesulfonic acid has been reported to be as high as −0.6
Because of their polarity, sulfonic acids tend to be crystalline solids or viscous, high-boiling liquids. They are also usually colourless and nonoxidizing,[
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The structure of sulfonic acids is illustrated by the prototype, methanesulfonic acid. The sulfonic acid group, RSO2OH features a tetrahedral sulfur centre, meaning that sulfur is at the center of four atoms: three oxygens and one carbon. The overall geometry of the sulfur centre is reminiscent of the shape of sulfuric acid.
File:Taurine.svg|Taurine, a bile acid, and one of the few naturally occurring sulfonic acids (shown in uncommon tautomer).
File:Perfluorooctanesulfonic acid structure.svg|PFOS, a surfactant and a controversial pollutant.
File:P-Toluenesulfonic acid structure.svg| p-Toluenesulfonic acid, a widely used reagent in organic synthesis.
File:Nafion3.svg|Nafion, a polymeric sulfonic acid useful in .
File:Sodium dodecylbenzenesulfonate skeletal.svg|Sodium dodecylbenzenesulfonate, an alkylbenzenesulfonate surfactant used in laundry detergents.
File:Coenzyme M (CoM).svg|Coenzyme-M, is a cofactor required for the biosynthesis of methane, found in natural gas.
Applications
Both alkyl and aryl sulfonic acids are known, most large-scale applications are associated with the aromatic derivatives.
Detergents and surfactants
and are molecules that combine highly nonpolar and highly polar groups. Traditionally, are the popular surfactants, being derived from . Since the mid-20th century, the usage of sulfonic acids has surpassed soap in advanced societies. For example, an estimated 2 billion kilograms of alkylbenzenesulfonates are produced annually for diverse purposes. Lignin sulfonates, produced by sulfonation of lignin are components of and additives in certain kinds of concrete.[Kosswig, K. "Surfactants" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. .]
Dyes
Many if not most of the anthraquinone dyes are produced or processed via sulfonation.
Acid catalysts
Being strong acids, sulfonic acids are also used as . The simplest examples are methanesulfonic acid, CH3SO2OH and p-toluenesulfonic acid, which are regularly used in organic chemistry as acids that are lipophilic (soluble in organic solvents). Polymeric sulfonic acids are also useful. Dowex resin are sulfonic acid derivatives of polystyrene and is used as catalysts and for ion exchange (water softening). Nafion, a fluorinated polymeric sulfonic acid is a component of proton exchange membranes in .
Drugs
Sulfa drugs, a class of antibacterials, are produced from sulfonic acids.
Lignosulfonates
In the sulfite process for paper-making, lignin is removed from the lignocellulose by treating wood chips with solutions of sulfite and bisulfite ions. These reagents cleave the bonds between the cellulose and lignin components and especially within the lignin itself. The lignin is converted to , useful , which are soluble and can be separated from the cellulose fibers.
Reactions
The reactivity of the sulfonic acid group is so extensive that it is difficult to summarize.[
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Hydrolysis to phenols
When treated with strong base, benzenesulfonic acid derivatives convert to phenols.
In this case the sulfonate behaves as a pseudohalide leaving group.
Hydrolytic desulfonation
Arylsulfonic acids are susceptible to hydrolysis, the reverse of the sulfonation reaction:
Whereas benzenesulfonic acid hydrolyzes above 200 °C, many derivatives are easier to hydrolyze. Thus, heating aryl sulfonic acids in aqueous acid produces the parent arene. This reaction is employed in several scenarios. In some cases the sulfonic acid serves as a water-solubilizing protecting group, as illustrated by the purification of para-xylene via its sulfonic acid derivative. In the synthesis of 2,6-dichlorophenol, phenol is converted to its 4-sulfonic acid derivative, which then selectively chlorinates at the positions flanking the phenol. Hydrolysis releases the sulfonic acid group.
Esterification
Sulfonic acids can be converted to . This class of has the general formula R−SO2−OR. Sulfonic esters such as methyl triflate are considered good in organic synthesis. Such sulfonate esters are often prepared by alcoholysis of the sulfonyl chlorides:
- RSO2Cl + R′OH → RSO2OR′ + HCl
Halogenation
Sulfonyl halide groups (R−SO2−X) are produced by chlorination of sulfonic acids using thionyl chloride. Sulfonyl fluorides can be produced by treating sulfonic acids with sulfur tetrafluoride:
Displacement by hydroxide
Although strong, the (aryl)C−SO3− bond can be broken by nucleophilic reagents. Of historic and continuing significance is the α-sulfonation of anthroquinone followed by displacement of the sulfonate group by other nucleophiles, which cannot be installed directly.[ An early method for producing phenol involved the base hydrolysis of sodium benzenesulfonate, which can be generated readily from benzene.][Manfred Weber, Markus Weber, Michael Kleine-Boymann "Phenol" in Ullmann's Encyclopedia of Industrial Chemistry 2004, Wiley-VCH. .]
- C6H5SO3Na + NaOH → C6H5OH + Na2SO3
The conditions for this reaction are harsh, however, requiring 'fused alkali' or molten sodium hydroxide at 350 °C for benzenesulfonic acid itself.
o-Lithiation
Arylsulfonic acids react with two equiv of butyl lithium to give the ortho-lithio derivatives, i.e., ortho-lithiation. These dilithio compounds are poised for reactions with many electrophiles.
Notes
