Sulfonic acid
Overview
 
Sulfonic acid usually refers to a member of the class of organosulfur compounds
Organosulfur compounds
Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odours, but many of the sweetest compounds known are organosulfur derivatives. Nature abounds with organosulfur compounds—sulfur is essential for life. Two of the 20 common amino acids are...

 with the general formula RS(=O)2–OH, where R is an alkyl or aryl
Aryl
In the context of organic molecules, aryl refers to any functional group or substituent derived from an aromatic ring, be it phenyl, naphthyl, thienyl, indolyl, etc....

. The formal part of acid, HS(=O)2–OH, are formally derivatives of the "parent" inorganic compound
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

 with the formula HSO2(OH).
Sulfonic acid is produced by the process of sulfonation. Usually the sulfonating agent is sulfur trioxide
Sulfur trioxide
Sulfur trioxide is the chemical compound with the formula SO3. In the gaseous form, this species is a significant pollutant, being the primary agent in acid rain. It is prepared on massive scales as a precursor to sulfuric acid.-Structure and bonding:Gaseous SO3 is a trigonal planar molecule of...

.
Encyclopedia
Sulfonic acid usually refers to a member of the class of organosulfur compounds
Organosulfur compounds
Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odours, but many of the sweetest compounds known are organosulfur derivatives. Nature abounds with organosulfur compounds—sulfur is essential for life. Two of the 20 common amino acids are...

 with the general formula RS(=O)2–OH, where R is an alkyl or aryl
Aryl
In the context of organic molecules, aryl refers to any functional group or substituent derived from an aromatic ring, be it phenyl, naphthyl, thienyl, indolyl, etc....

. The formal part of acid, HS(=O)2–OH, are formally derivatives of the "parent" inorganic compound
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

 with the formula HSO2(OH).

Preparation

Sulfonic acid is produced by the process of sulfonation. Usually the sulfonating agent is sulfur trioxide
Sulfur trioxide
Sulfur trioxide is the chemical compound with the formula SO3. In the gaseous form, this species is a significant pollutant, being the primary agent in acid rain. It is prepared on massive scales as a precursor to sulfuric acid.-Structure and bonding:Gaseous SO3 is a trigonal planar molecule of...

. 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
Arene
Arene or Arênê or Arène may refer to:*an aromatic hydrocarbon*Arene , a genus of marine snails in the family Areneidae*Arene , the wife of Aphareus and mother of Idas and Lynceus in Greek mythology...

 undergoes electrophilic aromatic substitution
Electrophilic aromatic substitution
Electrophilic aromatic substitution EAS is an organic reaction in which an atom, usually hydrogen, appended to an aromatic system is replaced by an electrophile...

.

Thiols can be oxidized to sulfonic acids:
RSH + 3/2 O2 → RSO3H


Certain sulfonic acids, such as perfluorooctanesulfonic acid are prepared by electrophilic fluorination
Electrophilic fluorination
Electrophilic fluorination is the combination of a carbon-centered nucleophile with an electrophilic source of fluorine to afford organofluorine compounds...

 of preformed sulfonic acids. The net conversion can be represented simplistically:
C8H17SO3H + 17 F2 → C8F17SO3H + 17 HF

Properties

Sulfonic acids are much stronger acids than the corresponding carboxylic acids. p-Toluenesulfonic acid
P-Toluenesulfonic acid
p-Toluenesulfonic acid or tosylic acid is an organic compound with the formula CH3C6H4SO3H. It is a white solid that is soluble in water, alcohols, and other polar organic solvents. The 4-CH3C6H4SO2- group is known as the Tosyl group and is often abbreviated as Ts or Tos...

, with a pKa of -2.8, is about a million times stronger acid than benzoic acid
Benzoic acid
Benzoic acid , C7H6O2 , is a colorless crystalline solid and the simplest aromatic carboxylic acid. The name derived from gum benzoin, which was for a long time the only source for benzoic acid. Its salts are used as a food preservative and benzoic acid is an important precursor for the synthesis...

, with a pKa of 4.2. Similarly, methanesulfonic acid, pKa = -1.9, is also about one million times stronger acid than acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

. 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
Methanesulfonic acid
Methanesulfonic acid is a colorless liquid with the chemical formula CH3SO3H. It is the simplest of the alkylsulfonic acids. Salts and esters of methanesulfonic acid are known as mesylates. Methanesulfonic acid is used as an acid catalyst in organic reactions because it is non-volatile, strong 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
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

.

Applications

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

Detergents and surfactants

Detergent
Detergent
A detergent is a surfactant or a mixture of surfactants with "cleaning properties in dilute solutions." In common usage, "detergent" refers to alkylbenzenesulfonates, a family of compounds that are similar to soap but are less affected by hard water...

s and surfactant
Surfactant
Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid...

s are molecules that combine highly nonpolar and highly polar groups. Traditionally, soap
Soap
In chemistry, soap is a salt of a fatty acid.IUPAC. "" Compendium of Chemical Terminology, 2nd ed. . Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford . XML on-line corrected version: created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN...

s are the popular surfactants, being derived from fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...

s. 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
Lignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...

 are components of drilling fluids and additives in certain kinds of concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

.

Dyes

Many if not most of the anthroquinone dyes are produced or processed via sulfonation. Sulfonic acids tend to bind tightly to protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s and carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

s. Most "washable" dye
Dye
A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

s are sulfonic acids (or have the functional sulfonyl
Sulfonyl
A sulfonyl group can refer either to a functional group found primarily in sulfones or to a substituent obtained from a sulfonic acid by the removal of the hydroxyl group similarly to acyl groups...

 group in them) for this reason. p-Cresidinesulfonic acid
Cresidinesulfonic acid
p-Cresidinesulfonic acid is used to make specialty dyes for food products. It is a synthetic organic chemical that the United States imports from China, India, and Japan. Its CAS number is 6471-78-3....

 is used to make food dyes.

Acid catalysts

Being strong acids, sulfonic acids are also used as catalysts. The simplest examples are methanesulfonic acid
Methanesulfonic acid
Methanesulfonic acid is a colorless liquid with the chemical formula CH3SO3H. It is the simplest of the alkylsulfonic acids. Salts and esters of methanesulfonic acid are known as mesylates. Methanesulfonic acid is used as an acid catalyst in organic reactions because it is non-volatile, strong acid...

, CH3SO2OH and p-toluenesulfonic acid
P-Toluenesulfonic acid
p-Toluenesulfonic acid or tosylic acid is an organic compound with the formula CH3C6H4SO3H. It is a white solid that is soluble in water, alcohols, and other polar organic solvents. The 4-CH3C6H4SO2- group is known as the Tosyl group and is often abbreviated as Ts or Tos...

, which are regularly used in organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

 as acids that are lipophilic (soluble in organic solvents). Polymeric sulfonic acids are also useful. Dowex resin are sulfonic acid derivatives of polystyrene
Polystyrene
Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...

 and is used as catalysts and for ion exchange (water softening
Water softening
Water softening is the reduction of the concentration of calcium, magnesium, and certain other metal cations in hard water. These "hardness ions" can cause a variety of undesired effects including interfering with the action of soaps, the build up of limescale, which can foul plumbing, and...

).Nafion
Nafion
Nafion is a sulfonated tetrafluoroethylene based fluoropolymer-copolymer discovered in the late 1960s by Walther Grot of DuPont. It is the first of a class of synthetic polymers with ionic properties which are called ionomers...

, a fluorinated polymeric sulfonic acid is a component of proton exchange membranes in fuel cell
Fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...

s.

Drugs

Antibacterial drugs sulfa drug
Sulfonamide (medicine)
Sulfonamide or sulphonamide is the basis of several groups of drugs. The original antibacterial sulfonamides are synthetic antimicrobial agents that contain the sulfonamide group. Some sulfonamides are also devoid of antibacterial activity, e.g., the anticonvulsant sultiame...

s are produced from sulfonic acids.

Esterification

Sulfonic acids can be converted to ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

s. This class of organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

s has the general formula R-SO2-OR. Sulfonic esters such as methyl triflate
Methyl triflate
Methyl trifluoromethanesulfonate, is commonly called methyl triflate, and has the chemical formula CF3SO2-OCH3, and is used as a very powerful methylating reagent in chemistry....

 are considered good alkylating agents in organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...

. Such sulfonate esters are often prepared by alcoholysis of the sulfonyl chlorides:
RSO2Cl + R'OH → RSO2OR' + HCl

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
Thionyl chloride
Thionyl chloride is an inorganic compound with the formula SOCl2. It is a reactive chemical reagent used in chlorination reactions. It is a colorless, distillable liquid at room temperature and pressure that decomposes above 140 °C. Thionyl chloride is sometimes confused with sulfuryl...

 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. An early method for producing phenol
Phenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...

 involved the base hydrolysis of sodium benzenesulfonate, which can be generated readily from benzene.
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. The nonfluorinated sulfonic acids tend to have low toxicities.

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. 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
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