Sulfonic acid

General structure of sulfonic acid. R can also be equal to H.

Sulfonic acid (or sulphonic acid) usually refers to a member of the class of organosulfur compounds with the general formula RS(=O)₂–OH, where R is an alkyl or aryl.^[1] The formal part of acid, HS(=O)₂–OH, are formally derivatives of the "parent" inorganic compound with the formula HSO₂(OH).

Preparation

Sulfonic acid is produced by the process of sulfonation. Usually the sulfonating agent is sulfur trioxide. A particularly large scale application of this method is the production of alkylbenzenesulfonic acids:

RC₆H₅ + SO₃ → RC₆H₄SO₃H

In this reaction, sulfur trioxide is an electrophile and the arene undergoes electrophilic aromatic substitution.^[1]

Thiols can be oxidized to sulfonic acids:

RSH + 3/2 O₂ → RSO₃H

Certain sulfonic acids, such as perfluorooctanesulfonic acid are prepared by electrophilic fluorination of preformed sulfonic acids. The net conversion can be represented simplistically:

C₈H₁₇SO₃H + 17 F₂ → C₈F₁₇SO₃H + 17 HF

Ball-and-stick model of methanesulfonic acid.

Properties

Sulfonic acids are much stronger acids than the corresponding carboxylic acids. p-Toluenesulfonic acid, with a pK_a of -2.8, is about a million times stronger acid than benzoic acid, with a pK_a of 4.2. Similarly, methanesulfonic acid, pK_a = -1.9, is also about one million times stronger acid than acetic acid. Because of their polarity, sulfonic acids tend to be crystalline solids. They are also usually colourless and nonoxidizing, which is convenient. Because of their high acidity, sulfonic acids are often soluble in water or exhibit detergent-like properties.

The structure of sulfonic acids is illustrated by the prototype, methanesulfonic acid. The sulfonic acid group, RSO₂(OH features a tetrahedral sulfur centre, meaning that sulfur is at the center of four atoms: three oxygens and onecarbon. The overall geometry of the sulfur centre is reminiscent of the shape of sulfuric acid.

• Representative sulfonic acids and derivatives.
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  Taurine, a bile acid, and one of the few naturally occurring sulfonic acids (shown in uncommon tautomer).

• 

  PFOS, a surfactant and a controversial pollutant.

• 

  Toluenesulfonic acid, a widely used reagent in organic synthesis.

• 

  Nafion, a polymeric sulfonic acid useful in fuel cells.

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  Sodium dodecylbenzenesulfonate, an alkylbenzenesulfonate surfactant used in laundry detergents.

Applications

Although both alkyl and aryl sulfonic acids are known, most of the applications are associated with the aromatic derivatives.

Detergents and surfactants

Detergents and surfactants are molecules that combine highly nonpolar and highly polar groups. Traditionally, soaps are the popular surfactants, being derived from fatty acids. 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 drilling fluids and additives in certain kinds of concrete.^[2]

Dyes

Many if not most of the anthroquinone dyes are produced or processed via sulfonation.^[3] Sulfonic acids tend to bind tightly to proteins and carbohydrates. Most "washable" dyes are sulfonic acids (or have the functional sulfonyl group in them) for this reason. p-Cresidinesulfonic acid is used to make food dyes.

Acid catalysts

Being strong acids, sulfonic acids are also used as catalysts. 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 fuel cells.^[4]

Drugs

Antibacterial drugs sulfa drugs are produced from sulfonic acids.

Reactions

Esterification

Sulfonic acids can be converted to esters. This class of organic compounds has the general formula R-SO₂-OR. Sulfonic esters such as methyl triflate are considered good alkylating agents in organic synthesis. Such sulfonate esters are often prepared by alcoholysis of the sulfonyl chlorides:

RSO₂Cl + R'OH → RSO₂OR' + HCl

Methyl triflate

Chlorination

Sulfonyl halide groups occur when a sulfonyl functional group is singly bonded to a halogen atom. They have the general formula R-SO₂-X where X is a halide, almost invariably chloride. They are produced by chlorination of sulfonic acids using thionyl chloride and related reagents.

Displacement

Although the C-SO₃H bond is strong, the (aryl)C-SO₃ bond can be cleaved by certain nucleophiles. 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.^[3] An early method for producing phenol involved the base hydrolysis of sodium benzenesulfonate, which can be generated readily from benzene.^[5]

C₆H₅SO₃Na + 2 NaOH → C₆H₅ONa + Na₂SO₃ + H₂O

Environmental concerns

Sulfonic acid derivatives are generally not derived from natural precursors and tend to biodegrade slowly. Some, such as perfluorooctane sulfonic acid, have been detected in the serum of humans. This should be not surprising as all organic fluorine compounds have extreme stability, and low to zero biodegradability. Many are intended to withstand harshest chemical conditions, for example teflon.^[6]

References

1. ^ ^{a b} March, J. “Advanced Organic Chemistry” 4th Ed. J. Wiley and Sons, 1992: New York. ISBN 0-471-60180-2.
2. ^ Kosswig, K. “Surfactants” in Ullmann’s Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. doi:10.1002/14356007.a25_747.
3. ^ ^{a b} Hans-Samuel Bien, Josef Stawitz, Klaus Wunderlich “Anthraquinone Dyes and Intermediates” Ullmann's Encyclopedia of Industrial Chemistry 2002 Wiley-VCH, Weinhem. doi:10.1002/14356007.a02_355
4. ^ Guido Busca "Acid Catalysts in Industrial Hydrocarbon Chemistry" Chem. Rev. 2007, 107, 5366-5410. doi:10.1021/cr068042e
5. ^ Manfred Weber, Markus Weber, Michael Kleine-Boymann "Phenol" in Ullmann's Encyclopedia of Industrial Chemistry 2004, Wiley-VCH. doi:10.1002/14356007.a19_299.pub2.
6. ^ Bin Wang, Fukuya Iino, Gang Yu, Jun Huang, Masatoshi Morita "The Pollution Status of Emerging Persistent Organic Pollutants in China" Environmental Engineering Science. 2010, volume 27, p. 215-225. doi:10.1089/ees.2009.0337

Other reading

• J.A. Titus, R. Haugland, S.O. Sharrow and D.M. Segal, Texas red, a hydrophilic, red-emitting fluorophore for use with fluorescein in dual parameter flow microfluorometric and fluorescence microscopic studies. J. Immunol. Methods 50 (1982), pp. 193–204. doi:10.1016/0022-1759(82)90225-3 PMID 6806389
• C. Lefevre, H.C. Kang, R.P. Haugland, N. Malekzadeh, S. Arttamangkul, and R. P. Haugland, Texas Red-X and Rhodamine Red-X, New Derivatives of Sulforhodamine 101 and Lissamine Rhodamine B with Improved Labeling and Fluorescence Properties, Bioconj Chem 1996, 7(4):482-9 doi:10.1021/bc960034p