Sulfonic acid

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General structure of a sulfonic acid with the functional group indicated in blue FunktionelleGruppen Sulfonsaure.svg
General structure of a sulfonic acid with the functional group indicated in blue

In organic chemistry, sulfonic acid (or sulphonic acid) refers to a member of the class of organosulfur compounds with the general formula R−S(=O)2−OH, where R is an organic alkyl or aryl group and the S(=O)2(OH) group a sulfonyl hydroxide. [1] 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, HS(=O)2(OH), a tautomer of sulfurous acid, S(=O)(OH)2. [lower-alpha 1] Salts or esters of sulfonic acids are called sulfonates.

Contents

Preparation

Ball-and-stick model of methanesulfonic acid. Methanesulfonic-acid-3D-balls.png
Ball-and-stick model of methanesulfonic acid.

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. [1]

Alkylsulfonic acids can be prepared by many methods. In sulfoxidation, 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 surfactants. [2]

Direct reaction of alkanes with sulfur trioxide is not generally useful, except for the conversion methanesulfonic acid to methanedisulfonic acid.

Many alkane sulfonic acids can be obtained by the addition of bisulfite to terminal alkenes. Bisulfite can also be alkylated by alkyl halides: [2]

Sulfonic acids can be prepared by oxidation of thiols:

This pathway is the basis of the biosynthesis of taurine.

Hydrolysis routes

Many sulfonic acids are prepared by hydrolysis of sulfonyl halides 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, (C2H4(SO3)2), which in turn is obtained by the addition of sulfur trioxide to ethylene.

Properties

Sulfonic acids are strong acids. They are commonly cited as being around a million times stronger than the corresponding carboxylic acid. For example, p-Toluenesulfonic acid and methanesulfonic acid have pKa values of −2.8 and −1.9, respectively, while those of benzoic acid and acetic acid are 4.20 and 4.76, respectively. However, as a consequence of their strong acidity, their pKa values cannot be measured directly, and values commonly quoted should be regarded as indirect estimates with significant uncertainties. For instance, various sources have reported the pKa of methanesulfonic acid to be as high as −0.6 [3] or as low as −6.5. [4] Sulfonic acids are known to react with solid sodium chloride (salt) to form the sodium sulfonate and hydrogen chloride. [5] This property implies an acidity within two or three orders of magnitude of that of HCl(g), whose pKa was recently accurately determined (pKaaq = −5.9).[ citation needed ]

Because of their polarity, sulfonic acids tend to be crystalline solids or viscous, high-boiling liquids. They are also usually colourless and nonoxidizing, [6] which makes them suitable for use as acid catalysts in organic reactions. Their polarity, in conjunction with their high acidity, renders short-chain sulfonic acids water-soluble, while longer-chain ones exhibit detergent-like properties.

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.

Applications

Both alkyl and aryl sulfonic acids are known, most large-scale 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. [7]

Dyes

Many if not most of the anthraquinone dyes are produced or processed via sulfonation. [8] 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. [9]

Drugs

Acamprosate is a sulfonic acid-containing drug use to treat alcohol use disorder. Acamprosate structure.svg
Acamprosate is a sulfonic acid-containing drug use to treat alcohol use disorder.

Sulfa drugs, a class of antibacterials, are produced from sulfonic acids.

Sulfonates are the basis of most ion exchange resins used in water softening. CationExchCartoon.png
Sulfonates are the basis of most ion exchange resins used in water softening.

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 lignosulfonates, useful ionomers, which are soluble and can be separated from the cellulose fibers.

idealized scheme for lignin depolymerization by the Sulfite process. SulfiteProcessSimple.png
idealized scheme for lignin depolymerization by the Sulfite process.

Reactions

The reactivity of the sulfonic acid group is so extensive that it is difficult to summarize. [10]

Hydrolysis to phenols

When treated with strong base, benzenesulfonic acid derivatives convert to phenols. [11]

C6H5SO3H + 2 NaOH → C6H5OH + Na2SO3 + H2O

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:

R−C6H4SO3H + H2O → R−C6H5 + H2SO4

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. [12]

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

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: [13]

SF4 + RSO3H → SOF2 + RSO2F + HF

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. [8] An early method for producing phenol involved the base hydrolysis of sodium benzenesulfonate, which can be generated readily from benzene. [14]

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. [15] Unlike the mechanism for the fused alkali hydrolysis of chlorobenzene, which proceeds through elimination-addition (benzyne mechanism), benzenesulfonic acid undergoes the analogous conversion by an SNAr mechanism, as revealed by a 14C labeling, despite the lack of stabilizing substituents. [16] Sulfonic acids with electron-withdrawing groups (e.g., with NO2 or CN substituents) undergo this transformation much more readily.

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. [10]

Notes

  1. Neither the parent sulfonic acid nor the parent sulfurous acid have been isolated or even observed, although the monoanion of these hypothetical species exists in solution as an equilibrium mixture of tautomers: HS(=O)2(O) ⇌ S(=O)(OH)(O).

Related Research Articles

<span class="mw-page-title-main">Ether</span> Organic compounds made of alkyl/aryl groups bound to oxygen (R–O–R)

In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom bonded to two organyl groups. They have the general formula R−O−R′, where R and R′ represent the organyl groups. Ethers can again be classified into two varieties: if the organyl groups are the same on both sides of the oxygen atom, then it is a simple or symmetrical ether, whereas if they are different, the ethers are called mixed or unsymmetrical ethers. A typical example of the first group is the solvent and anaesthetic diethyl ether, commonly referred to simply as "ether". Ethers are common in organic chemistry and even more prevalent in biochemistry, as they are common linkages in carbohydrates and lignin.

Sulfur trioxide (alternative spelling sulphur trioxide, also known as nisso sulfan) is the chemical compound with the formula SO3. It has been described as "unquestionably the most [economically] important sulfur oxide". It is prepared on an industrial scale as a precursor to sulfuric acid.

<span class="mw-page-title-main">Nitration</span> Chemical reaction which adds a nitro (–NO₂) group onto a molecule

In organic chemistry, nitration is a general class of chemical processes for the introduction of a nitro group into an organic compound. The term also is applied incorrectly to the different process of forming nitrate esters between alcohols and nitric acid. The difference between the resulting molecular structures of nitro compounds and nitrates is that the nitrogen atom in nitro compounds is directly bonded to a non-oxygen atom, whereas in nitrate esters, the nitrogen is bonded to an oxygen atom that in turn usually is bonded to a carbon atom.

<span class="mw-page-title-main">Peroxy acid</span> Organic acid having a peroxide bond

A peroxy acid is an acid which contains an acidic –OOH group. The two main classes are those derived from conventional mineral acids, especially sulfuric acid, and the peroxy derivatives of organic carboxylic acids. They are generally strong oxidizers.

<span class="mw-page-title-main">Sulfone</span> Organosulfur compound of the form >S(=O)2

In organic chemistry, a sulfone is a organosulfur compound containing a sulfonyl functional group attached to two carbon atoms. The central hexavalent sulfur atom is double-bonded to each of two oxygen atoms and has a single bond to each of two carbon atoms, usually in two separate hydrocarbon substituents.

<span class="mw-page-title-main">Aromatic sulfonation</span> Chemical reaction which replaces a hydrogen on an arene with sulfonic acid, –NH–SO3H

In organic chemistry, aromatic sulfonation is an organic reaction in which a hydrogen atom on an arene is replaced by a sulfonic acid functional group in an electrophilic aromatic substitution. Aryl sulfonic acids are used as detergents, dye, and drugs.

<span class="mw-page-title-main">Isethionic acid</span> Chemical compound

Isethionic acid is an organosulfur compound containing an alkylsulfonic acid located beta to a hydroxy group. Its discovery is generally attributed to Heinrich Gustav Magnus, who prepared it by the action of solid sulfur trioxide on ethanol in 1833. It is a white water-soluble solid used in the manufacture of certain surfactants and in the industrial production of taurine. It is most commonly available in the form of its sodium salt.

Lignosulfonates (LS) are water-soluble anionic polyelectrolyte polymers: they are byproducts from the production of wood pulp using sulfite pulping. Most delignification in sulfite pulping involves acidic cleavage of ether bonds, which connect many of the constituents of lignin. Sulfonated lignin (SL) refers to other forms of lignin by-product, such as those derived from the much more popular Kraft process, that have been processed to add sulfonic acid groups. The two have similar uses and are commonly confused with each other, with SL being much cheaper. LS and SL both appear as free-flowing powders; the former is light brown while the latter is dark brown.

Polysulfones are a family of high performance thermoplastics. These polymers are known for their toughness and stability at high temperatures. Technically used polysulfones contain an aryl-SO2-aryl subunit. Due to the high cost of raw materials and processing, polysulfones are used in specialty applications and often are a superior replacement for polycarbonates.

<span class="mw-page-title-main">Triflic acid</span> Chemical compound

Triflic acid, the short name for trifluoromethanesulfonic acid, TFMS, TFSA, HOTf or TfOH, is a sulfonic acid with the chemical formula CF3SO3H. It is one of the strongest known acids. Triflic acid is mainly used in research as a catalyst for esterification. It is a hygroscopic, colorless, slightly viscous liquid and is soluble in polar solvents.

In inorganic chemistry, sulfonyl halide groups occur when a sulfonyl functional group is singly bonded to a halogen atom. They have the general formula RSO2X, where X is a halogen. The stability of sulfonyl halides decreases in the order fluorides > chlorides > bromides > iodides, all four types being well known. The sulfonyl chlorides and fluorides are of dominant importance in this series.

<span class="mw-page-title-main">Organosulfate</span> Organic compounds of the form R–O–SO₃ (charge –1)

In organosulfur chemistry, organosulfates are a class of organic compounds sharing a common functional group with the structure R−O−SO−3. The SO4 core is a sulfate group and the R group is any organic residue. All organosulfates are formally esters derived from alcohols and sulfuric acid although many are not prepared in this way. Many sulfate esters are used in detergents, and some are useful reagents. Alkyl sulfates consist of a hydrophobic hydrocarbon chain, a polar sulfate group and either a cation or amine to neutralize the sulfate group. Examples include: sodium lauryl sulfate and related potassium and ammonium salts.

Dodecylbenzene is an organic compound with the formula C
12
H
25
C
6
H
5
. Dodecylbenzene is a colorless liquid with a weak odor and floats on water.

<span class="mw-page-title-main">Methanesulfonic anhydride</span> Chemical compound

Methanesulfonic anhydride (Ms2O) is the acid anhydride of methanesulfonic acid. Like methanesulfonyl chloride (MsCl), it may be used to generate mesylates (methanesulfonyl esters).

<span class="mw-page-title-main">4-Toluenesulfonyl chloride</span> Chemical compound

4-Toluenesulfonyl chloride (p-toluenesulfonyl chloride, toluene-p-sulfonyl chloride) is an organic compound with the formula CH3C6H4SO2Cl. This white, malodorous solid is a reagent widely used in organic synthesis. Abbreviated TsCl or TosCl, it is a derivative of toluene and contains a sulfonyl chloride (−SO2Cl) functional group.

2,6-Dichlorophenol is a compound with formula C6H3Cl2OH. It is one of the six isomers of dichlorophenol. It is a colorless solid. Its pKa is 6.78, which is about 100x more acidic than 2-chlorophenol (8.52) and 1000x more acidic than phenol itself (9.95).

<span class="mw-page-title-main">Benzenesulfonic acid</span> Chemical compound

Benzenesulfonic acid (conjugate base benzenesulfonate) is an organosulfur compound with the formula C6H6O3S. It is the simplest aromatic sulfonic acid. It forms white deliquescent sheet crystals or a white waxy solid that is soluble in water and ethanol, slightly soluble in benzene and insoluble in nonpolar solvents like diethyl ether. It is often stored in the form of alkali metal salts. Its aqueous solution is strongly acidic.

Electrophilic aromatic substitution (SEAr) is an organic reaction in which an atom that is attached to an aromatic system is replaced by an electrophile. Some of the most important electrophilic aromatic substitutions are aromatic nitration, aromatic halogenation, aromatic sulfonation, alkylation Friedel–Crafts reaction and acylation Friedel–Crafts reaction.

<span class="mw-page-title-main">Alkylbenzene</span> Family of organic compounds

An alkylbenzene is a chemical compound that contains a monocyclic aromatic ring attaching to one or more saturated hydrocarbon chains. Alkylbenzenes are derivatives of benzene, in which one or more hydrogen atoms are replaced by alkyl groups. The simplest member, toluene, has the hydrogen atom of the benzene ring replaced by a methyl group. The chemical formula of alkylbenzenes is CnH2n-6.

<span class="mw-page-title-main">Desulfonation reaction</span>

In organic chemistry, the desulfonation reaction is the hydrolysis of sulfonic acids:

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