Sulfonic acid

Source: Wikipedia, the free encyclopedia.
General structure of a sulfonic acid with the functional group indicated in blue

In

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.[a] Salts or esters of sulfonic acids are called sulfonates
.

Preparation

Ball-and-stick model of methanesulfonic acid.

Aryl sulfonic acids are produced by the process of

alkylbenzenesulfonic acids
:

In this reaction, sulfur trioxide is an

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

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

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

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

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

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

alcohol use disorder
.

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

ion exchange resins
used in water softening.

Lignosulfonates

In the

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

idealized scheme for lignin depolymerization by the Sulfite process.

Reactions

Hydrolysis

Main article: Desulfonation reaction

Arylsulfonic acids are susceptible to hydrolysis, the reverse of the sulfonation reaction. Whereas benzenesulfonic acid hydrolyzes above 200 °C, most related 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.[10]

Esterification

Sulfonic acids can be converted to

alcoholysis
of the sulfonyl chlorides:

RSO2Cl + R′OH → RSO2OR′ + HCl

Halogenation

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, usually chloride or fluoride. Chlorides are produced by chlorination of sulfonic acids using thionyl chloride. Sulfonyl fluorides can be produced by treating sulfonic acids with sulfur tetrafluoride:[11]

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.

benzenesulfonate, which can be generated readily from benzene.[12]

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.

benzyne mechanism), benzenesulfonic acid undergoes the analogous conversion by an SNAr mechanism, as revealed by a 14C labeling, despite the lack of stabilizing substituents.[14]
Sulfonic acids with electron-withdrawing groups (e.g., with NO2 or CN substituents) undergo this transformation much more readily.

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−).

References

  1. ^
    ISBN 0-471-60180-2
  2. ^
    ISBN 978-3527306732
    .
  3. ISSN 0001-4842
    .
  4. OCLC 708034394
    .
  5. OCLC 867050415.{{cite book}}: CS1 maint: location missing publisher (link
    )
  6. ISSN 1463-9262
    .
  7. doi:10.1002/14356007.a25_747
    .
  8. ^
    ISBN 978-3527306732
    .
  9. PMID 17973436
    .
  10. ISBN 978-3527306732
    .
  11. ISBN 978-0-471-26418-7
    .
  12. doi:10.1002/14356007.a19_299.pub2
    .
  13. ISSN 0009-2665
    .
  14. doi:10.1246/bcsj.39.1212
    .
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