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Category name:
Category of Three Cresol Isomeres (o-, m- and p-cresol)

Justifications and discussions

Category definition:
The category members are isomers and therefore have very similar chemical structures and can be regarded as homologous structures. The only difference between the substances consists in the position of the methyl group. All isomers have the same empirical formula: C7H8O, and molecular weight: 108.14 g/mol. A methyl group is substituted either in meta-, ortho- or para-position to the hydroxy group of the phenol ring. The three isomers are slightly different in their reaction behaviour towards nucleophilic substitution or addition of the benzene ring. As there are no directly hydrolysable functions present, all are considered to be stable in water at ambient conditions and environmentally relevant pH values.
Category description:
All substances are structural isomers of methylated phenol. The structures consist of a benzene ring, a methyl group and a hydroxy group, substituted directly to the ring. They have a clear identity and are of high purity (> 98 %).
Category rationale:
The read-across approach should be used to support the assessment of m-cresol (3 methylphenol) and o-cresol (2-methylphenol) for data gap filling using ecotoxicity data of p cresol (4-methylphenol) and vice versa. All three substances are structural isomers of methylated phenol. The different substitution patterns lead to minor differences in chemical reactivity of the substances.
It is reasonable to assume and experimentally verified that the compounds have nearly identical behaviour in physico-chemical, environmental fate and (eco-)toxicological studies. A comprehensive data base and extensive evaluation thereof, shown in this document, support the read-across approach, underpinning the quantitatively similar effects of the substances. While all three isomers exhibit effects in the same range, no clear tendency can be derived. However, in some effects regarding the aquatic environment, p-cresol has been observed to be slightly more toxic than the other isomers. The read-across always considers the worst-case approach.