Registration Dossier

Categories

Categories

Category name:
Aromatic Sulphonic acids

Justifications and discussions

Category definition:
Aromatic sulphonic acids
Category rationale:
The aromatic sulphonic acids category consists of the following five (5) substances:
• Benzene sulphonic acid (CAS No. 98-11-3)
• Hydroxybenzene sulphonic acid (CAS No. 1333-39-7)
• Toluene-4-sulphonic acid (CAS No. 104-15-4)
• Xylene sulphonic acid (CAS No. 25321-41-9)
• Cumene sulphonic acid (CAS No. 37953-05-2)

Sulphonic acids are a class of organic acids with the general formula R-S(=O)2-OH, where R is usually a hydrocarbon (aromatic) side chain. The sulphonic acids are strong acids and in watery environments they are almost completely ionized even at low pH.
The five aromatic sulphonic acids in this category are chemically related and quite similar. Benzenesulphonic acid is an organic compound with the formula C6H5SO3H. It is the simplest aromatic sulphonic acid. Benzene with a hydroxyl group is hydroxybenzene (or phenol). Benzene with a methyl group is toluene. Benzene with two methyl groups is xylene. And benzene with an isopropyl group is cumene.
Acidity of the sulphonic acid group is influenced by two factors:
1. The substitution group exerts an electron donating effect and so makes the negative charge on the resulting sulphonate ion, after deprotonation, more or less stable.
2. The resonance effect stabilizes the negative charge on the sulphonate ion by dividing the charge on the oxygen atoms.
As a result, the acidity of the sulphonic acid group is not expected to change significantly among the five aromatic sulphonic acids. The measured pKa values confirm this expectation: 2.68 for benzene sulphonic acid, 2.50 for hydroxybenzene sulphonic acid, 2.96 for toluene-4-sulphonic acid, 2.88 for xylene sulphonic acid and 3.17 for cumene sulphonic acid. Thus the reactivity of the sulphonic acids are very similar and they can each be used as a surrogate for the others.
A full set of 2010 guideline physical-chemical studies demonstrates the similar chemical and physical properties and behavior of the 5 sulphonic acids in the category.
The sulphonic acid moiety is the primary driver for mammalian toxicity and any difference between the benzene, xylene, cumene, and toluene moieties would be insignificant given the relatively high level of corrosivity of all five substances in the category.
As noted, the aromatic sulphonic acids are almost completely ionized in watery environments.
These acids are precursors to hydrotropes (or “sulphonates”) which include ammonium, calcium, potassium and sodium salts. Because of the chemical similarities and because much of the production of the aromatic sulphonic acids goes to manufacturing the salts, the extensive database for the hydrotropes is also used as a source of read-across for some of the endpoints in this aromatic sulphonic acid dossier.