enzenesulfonic acid  is the simplest sulfonated aromatic compound and is simply a benzene ring to which is attached a sulfur(VI) as part of an SO3H group. As such, with three electronegative oxygen atoms, this group is strongly acidic. Needless to say, this would deplete the aromatic ring of its electron cloud making the molecule highly deactivated. It is nevertheless 3-directing to any species strong enough to be drawn to the remaining electron cloud around the aromatic nucleus.


The above resonance structures are given not to show that the ring is somehow furnishing the attached sulfonic acid group with electron density but merely to indicate the positions most likely to find relative electron density enrichment in what would otherwise be a generally deficient system vis-a-vis its electron cloud density. Comparatively speaking then, the 3-positions are slightly electron-density enriched and any suitable electrophile, seeking electron rich systems, will in all probability attack the benzenesulfonic acid ring in the 3- and 5- positions.




Both the laboratory as well as the industrial scale preparation of benzenesulfonic acids is carried out by direct sulfonation of the aromatic ring. This process can be done using concentrated sulfuric acid but this tends to be slow. Far better to use either fuming sulfuric acid, ‘oleum’ (SO3 dissolved in conc. sulfuric acid) or liquid SO3 as the reaction is then over in a matter of minutes.


Direct sulfonation of benzene



Two important points need to be mentioned here and these are firstly that the reactions are easily reversed, quite unlike a lot of other similar chemical (electrophilic substitution) reactions and secondly that further substitutions are directed to the 3 and 5 positions as is expected with this substituent.







 His functionality is the highest ranking functionality we come across and hence the compounds bearing this functionality are all derivatives of the arenesulfonic acid, benzenesulfonic acid in our case.







Physical Properties


Benzenesulfonic acids are white crystalline solids, very soluble in water giving strongly acidic solutions that will readily react with metals, carbonates, oxides, hydroxides and litmus in the expected fashion. (Often deliquescent as the solids they tend to be stored as the sodium salts)



Chemical Properties


A :          Reactions involving the SO3H group


a)                  Replacement of the SO3H with H – formation of parent arene    (PhSO3H   --> PhH )

Hydrolysis of the benzenesulfonic acids with dilute HCl at 150oC, under pressure, yields the parent arene (- not the phenol!)

The hydrolysis is the exact reverse reaction of the sulfonation process.


b)                  Replacement of SO3H with OH --formation of phenols    (PhSO3H   --> PhOH)

Fusion of the sulfonic acids with sodium hydroxide initially gives the phenate salt with the phenol being released by the subsequent addition of mineral acid.


c)                 Substitution of SO3H with CN -- formation of benzonitrile (PhSO3H --> PhCN)

An important reaction that easily gives us a small carbon chain extension (which may be very significant in some reaction sequences!) is that where sodium cyanide is heated with sodium benzenesulfonate, easily prepared from the sulfonic acid by neutralization. (The toxic gas HCN can easily be released if the sulfonic acid is treated directly with NaCN)



(This reaction is of some importance because the substitution of  Ar-Cl by CN is not possible, quite unlike the aliphatic halogenated compounds!)



d)                  Formation of Arenesulfonyl chloride, an acid chloride  (PhSO3H--> PhSO2Cl)

The OH group found on the SO3H group can be replaced by a Cl in the same way that it is for the carbon analogues. Treating the sulfonic acids with PCl5 affords the sulfonyl chlorides :

As acid chlorides, they give the same reactions seen with the carbonyl species except that they do not react with the same vigour, but will nevertheless give esters and amides, the sulfonate esters and sulfonamides respectively. (Reaction of the sulfonyl chloride with water is slow, unlike the carbon equivalent and it is for this reason that formation of esters can be brought about in aqueous conditions)




B :          Reactions on the ring

The ring is deactivated and the sulfonic acid group is 3-directing to electrophiles. With the exception of the Friedel-Crafts alkylations, which will not work on account of the ring being too deactivated, the electrophiles give 3-substituted sulfonic acids when harsher conditions than those needed for benzene are used.


    e.g.  Nitration 

Nitration of benzenesulfonic acid yields the 3-nitrobenzenesulfonic acid, as expected :






A very important class of compounds involving the arylsulfonic acids is the detergents. A typical detergent has the following structure :


A typical synthesis would be as follows :



With the calcium salts for these compounds being water soluble, there is no formation of the unsightly ‘scum’ --  not to mention the much desired, better washing abilities of these compounds as they are better at dispersing greases, oils and fatty materials even into ‘hard’ water. They function in exactly the same way as the ‘soaps’ otherwise. That is to say, their hydrophobic ‘tail’ end, with a long carbon chain which often includes an aromatic ring, dissolves in the oil and the water soluble, ionic sulfonate salt ‘head’ protrudes into the aqueous medium. A number of these ‘heads’ will eventually offer a good enough solubilising area to lift the oil off the fabric and disperse it into the watery world around. As an entity suspended in water, the globule can be removed permanently through rinsing.


Tests and Identifications


No specific tests are available for this class of compounds but they clearly must give acidic properties such as release of CO2 from carbonates as well as H2 with metals.

The give-away must be the presence of sulfur when doing elemental analysis (where the presence of S is shown by the sodium fusion [Lassaigne’s] test – it results in the formation of sulfides during the fusion reaction).

I`m done with the sulfonic acids, take me back please!