SULFONIC ACID: STRUCTURE, NOMENCLATURE, PROPERTIES, USES - CHEMISTRY - 2025

Is called sulfonic acid to any compound belonging to a family of organic acids having one or more -SO ₃ H. Some authors indicate that the parent compound is H-S (= O) ₂ -OH or H-SO ₃ H, called by some “sulfonic acid”, however, its existence has not been proven, nor that of its tautomer HO – S (= O) –OH known as “sulfurous acid” (H ₂ SO ₃), nor in solution nor in solid state.

The latter is due to the fact that sulfur dioxide gas (IV) (SO ₂) in aqueous solution is converted to bisulfite ions (HSO ₃ ^-) and pyrosulfite (S ₂ O ₅ ^{2 -}) and not to H – S (= O) ₂ –OH and HO – S (= O) –OH, which has already been proven by many chemical researchers.

Sulfonyl group. Choij. Source: Wikipedia Commons

In the chemical formula R-SO ₃ H of sulfonic acids, the R group can represent hydrocarbon skeletons such as alkane, alkene, alkyne and / or arene. These can contain secondary functionalities such as ester, ether, ketone, amino, amido, hydroxyl, carboxyl, and phenol groups, among many others.

Sulfonic acids are strong acids, comparable to sulfuric acid (H ₂ SO ₄). However, the rest of its properties depend largely on the nature of the R group.

Some have a tendency to desulfonate at high temperatures. This means losing the -SO ₃ H group, especially when R is an arene, that is, with a benzene ring.

Sulfonic acids have many uses depending on their chemical formula. They are used to make colorants, inks, polymers, detergents, surfactants, and as catalysts, among many other applications.

Structure

Sulfonic acids have the following general structure:

General formula of sulfonic acids. Benjah-bmm27. Source: Wikipedia Commons

Nomenclature

- Sulfonyl group: –SO ₃ H

- Alkyl or arylsulfonic acid: R – SO ₃ H

Properties

They are solid or liquid depending on the R group. Gaseous sulfonic acids are not known.

Their physical and chemical properties depend on the nature of the R group. By way of comparison, some physical properties of various sulfonic acids are presented, where pf is the melting point and pe is the boiling point at a pressure of 1 mm Hg:

- Methanesulfonic acid: mp 20 ° C; bp 122 ° C

- Ethanesulfonic acid: mp -17 ° C; bp 123 ° C

- Propanesulfonic acid: mp -37 ° C; bp 159 ° C

- Butanesulfonic acid: mp -15 ° C; bp 149 ° C

- Pentanesulfonic acid: mp -16 ° C; bp 163 ° C

- Hexanesulfonic acid: mp 16 ° C; bp 174 ° C

- Benzenesulfonic acid: mp 44 ° C; bp 172 ° C (at 0.1 mm Hg)

- p-toluenesulfonic acid: mp 106 ° C; bp 182 ° C (at 0.1 mm Hg)

- 1-Naphthalenesulfonic acid: mp 78 ° C; pe decomposes

- 2-naphthalenesulfonic acid: mp 91 ° C; pe decomposes

- Trifluoromethanesulfonic acid: mp none; bp 162 ° C (at 760 mm Hg)

Most are very soluble in water.

Sulfonic acids are even more acidic than carboxylic acids. They have acidity similar to that of sulfuric acid. They dissolve in aqueous sodium hydroxide and aqueous bicarbonate.

They tend to undergo thermal decomposition, causing their desulfonation. However, several of the alkanesulfonic acids are thermally stable. In particular, triflic acid, or trifluoromethanesulfonic acid, is one of the strongest and most stable acids.

Obtaining

Arylsulfonic acids

Arylsulfonic acids are generally obtained through the sulfonation process that consists of adding an excess of fuming sulfuric acid (H ₂ SO ₄ + SO ₃) to the organic compound that we wish to sulfonate. For example in the case of benzene:

C ₆ H ₆ + H ₂ SO ₄ <---> C ₆ H ₅ SO ₃ H + H ₂ O

ArH + H ₂ SO ₄ <---> ArSO ₃ H + H ₂ O

Sulfonation is a reversible reaction, because if we add a dilute acid to benzenesulfonic acid and pass steam over the mixture, the SO ₃ elimination reaction is generated and the benzene is distilled by vapor volatilization, shifting the equilibrium towards the desulfonation.

The sulfonating agent can also be chlorosulfuric acid, sulfamic acid, or sulfite ions.

Alkylsulfonic acids

These are generally prepared by sulfo-oxidation, that is, treatment of alkyl compounds with sulfur dioxide SO ₂ and oxygen O ₂, or directly with sulfur trioxide SO ₃.

RH + 2SO ₂ + O ₂ + H ₂ O <----> R-SO ₃ H + H ₂ SO ₄

Applications

Sulfonic acids for surfactants and detergents

Alkylbenzenesulfonic acids are used in the manufacture of detergents, being one of the most widespread uses.

P-methylbenzenesulfonic acid. His-no-G. Source = Selfmade. * Date = 21 Aug 2006. * Made with ChemDraw. Source: Wikipedia Commons.

They are used for powder and liquid detergents, heavy or light duty detergents, hand soaps and shampoo.

The active components are generally linear alkylbenzene sulphonates and sulphonated fatty esters.

Common surfactants are alpha-olefin sulfonates, sulfobetaines, sulfosuccinates, and alkyl diphenyl ether disulfonates, all of which are derived from sulfonic acids.

These surfactants find use in household cleaning products, cosmetics, emulsion polymerization, and the manufacture of agricultural chemicals.

Lignosulfonates are sulfonated derivatives of lignin, a component of wood, and are used as dispersing agents, wetting agents, binders in road pavements, concrete admixtures, and additives in animal feed.

A derivative of 2-naphthalenesulfonic acid is used to increase the flowability and strength of the cement.

Sulfonic acid based dyes

These are generally characterized by containing azo groups (R – N = N – R).

Acid Red Dye. Smokefoot. Source: Wikipedia Commons.

Sulfonic acid-derived dyes are useful in the paper industry, inks such as inkjet printing, textiles such as cotton and wool, cosmetics, soaps, detergents, food, and leather.

A derivative of 2-naphthalenesulfonic acid is also used as an ingredient in dyes for polyester fibers and for leather tanning.

Sulfonic acid-based amides

Acrylamidosulfonic acids stand out in this application, which are used in polymers, in electrodeposition of metals, as surfactants and in transparent anti-fog coatings.

They are also used in enhanced oil recovery, as soil release agents, in hydraulic cement mixtures, and as aminoplast resin cure accelerators.

Some sulfonamides are also used in insecticide formulas.

Fluorinated and chlorofluorinated sulfonic acids

In this group, trifluoromethanesulfonic acid (CF ₃ SO ₃ H), also called triflic acid, stands out. This compound has application in organic synthesis and catalysis of polymerization reactions.

Triflic acid. Capaccio. Source: Wikipedia Commons.

For example, it is used in the polymerization of aromatic olefins and in the preparation of highly branched paraffinic hydrocarbons with a high octane number for use as fuel additives.

Another of its applications is in the synthesis of conductive polymers, through the doping of polyacetylene, producing for example p-type semiconductors.

Pentafluoroethanesulfonic acid (CF ₃ CF ₂ SO ₃ H), known as pentaphilic acid, is used as an organic catalyst.

Triflic acid is reacted with antimony pentafluoride (SbF ₅) generating the so-called "Magic Acid", a superacid catalyst that serves for alkylation and polymerization.

Nafion-H is an ion-exchange resin of a perfluorinated polymeric sulfonic acid. It has the characteristic of being inert towards strong acids, strong bases and reducing or oxidizing agents.

In biological applications

One of the few sulfonic acids found in nature is 2-aminoethanesulfonic acid, better known as taurine, an essential amino acid particularly for cats.

Synthetic and natural taurine are used as supplements in pet food and pharmaceuticals.

Taurine molecule. Harbinary. Source: Wikipedia Commons

Taurocholic acid is a component of bile and participates in the digestion of fats. 8-Anilino-1-naphthalenesulfonic acid, a synthetic compound, is used in the study of proteins.

Copolymers of urea and biphenyldisulfonic acid have been used as inhibitors of the herpes simplex virus.

Some sulfonic acid derivatives, such as sulfonated polystyrene and some azo compounds, have shown inhibitory properties of the human immunodeficiency virus or HIV, without being excessively toxic against host cells.

In the oil industry

Certain mixtures of petroleum derivatives, which include a varied combination of aromatics, paraffins and polynuclear aromatics, can be sulfonated and then properly neutralized generating so-called petroleum sulfonates.

These petroleum sulfonates have the property of lowering the surface tension of petroleum in water. In addition, its production is very low cost.

For this reason, petroleum sulfonates are injected into existing oil wells helping to recover crude oil that is trapped between rocks below the surface.

In the formulation of lubricants and anticorrosives

Magnesium sulfonates are widely used in lubricant formulations as detergent-dispersant additives and to prevent wear.

Barium sulfonates are used in antifriction bearing lubricants used in high speed applications. Sodium and calcium sulphonates are used in gear lubricants and greases to improve extreme pressure properties.

Magnesium salts of sulfonic acids are also useful in anti-corrosion applications such as coatings, greases, and resins.

References

1. Falk, Michael and Giguere, Paul A. (1958). On the nature of sulphurous acid. Canadian Journal of Chemistry, Vol. 36, 1958. Recovered from nrcresearchpress.com
2. Betts, RH and Voss, RH (1970). The kinetics of oxygen exchange between the sulfite ion and water. Canadian Journal of Chemistry, Vol. 48, 1970. Recovered from nrcresearchpress.com
3. Cotton, FA and Wilkinson, G. (1980). Advanced Inorganic Chemistry. Fourth Edition. Chapter 16. John Wiley & Sons.
4. Kirk-Othmer (1994). Encyclopedia of Chemical Technology. Volume 23. Fourth Edition. John Wiley & Sons.
5. Kosswig, Kurt. (2012). Surfactants. Ullmann's Encyclopedia of Industrial Chemistry Vol. 35. Wiley-VCH Verlag GmbH & Co.