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The Dimerised fatty acids and its derivatives category covers C16 - C18 unsaturated fatty acids derived monomers, dimers and trimers, as well as their hydrogenated products in different proportions and in accordance with their corresponding production and purification processes. They are all prepared by the dimerisation of C16 - C18 unsaturated fatty acids. As UVCB substances derived from natural sources, members of this category are chemically similar as they are all essentially a complex mixture of C16 - C18 unsaturated and saturated, branched and linear fatty acids, their monomers, dimers and trimers with varying structural geometric isomers. All substances in the category have an overlap in regard to their composition. With reference to information of existing categories, the category of Dimerised Fatty Acids and Its Derivatives is based on similar physicochemical and toxicological properties and 2 sub-categories are further defined on the basis of their environmental fate and toxicity. The first sub-category covers three monomeric (by-)products of the dimerization process (readily biodegradable substances). The second sub-category covers the predominately oligomers (dimeric and trimeric products) of dimerization based on their lack of ready biodegradability and other environmental fate properties.

Sub category 2: predominantly oligomers (dimers, trimers)

ID No.


Common Name

Chemical Name



Crude dimer

Fatty acids, C16-C18 and C18-unsaturated, dimerized




Fatty acids, C18-unsaturated, dimers,



Hydrogenated dimer

Fatty acids, C18-unsaturated, dimers, hydrogenated




Fatty acids, C18-unsaturated, trimers

Derived from the same starting substance, all substances in this category have a homologous composition of fatty acids with a C16 - C18 carbon chain in diverse forms, that is susceptible to oxidation through metabolic processes. In view of the results of various QSAR analyses, the toxic hazard of these substances mainly depends on the number of carbons, on the chain “structure”, such as branching, unsaturation, grade of cyclics and aggregation, as well as on their position in the whole molecular structure. In contrast, the number of the functional group “carboxylic acids” has no significant influence on the tox- and ecotoxicological profiles.

Sub category 2: predominantly Oligomers (dimers, trimers)

As aforementioned the similarity of the sub-category 2 members is justified, in accordance with the specifications listed in Regulation (EC) No. 1907/2006, Annex XI, 1.5, Grouping of substances and read across, on the basis of overlap in composition, representative molecular structures, physico-chemical properties, tox-, ecotoxicological profiles and is supported by various QSAR methods. There is no convincing evidence that any one of these chemicals might lie out of the overall profile of this sub-category. The key characters that the members share are:

  • Common origin of C16-18 unsaturated fatty acids
  • Similar/overlapping structural features (no hydrolysable groups, all members have a homologous composition of fatty acids with a C16 - C18 carbon chain in diverse forms, that are susceptible to oxidation by metabolic processes)
  • Similar metabolic pathways (same ADME pathways of fatty acids; absorbed fatty acids undergo rapid metabolism (via ß- or ω-oxidation) and excretion either in the expired CO2 or as a hydroxylated or conjugated metabolite in the urine in the case of cyclic fatty acids)
  • Similar physico-chemical properties (log Koc >5, log Kow is judged to be > 4, insolubility in water)
  • Common environmental fate & eco-toxicological profile of the two sub-categories (not readily biodegradable, no toxicological effects to aquatic organisms up to the water solubility limit)
  • Common levels and mode of human health related effects

Please refer to IUCLID Chapter 6.3 for more details on the category justification for terrestrial toxicity and to section 13 for the complete category justification.

Since the group concept is applied to the category members of Dimerised fatty acids and its derivatives (sub-category 2 predominantly oligomers) data was generated from a representative sub-category member to avoid unnecessary animal testing. The selection of Fatty acids, C18-unsaturated, dimers, (CAS No. 61788-89-4) as representative test substance for the sub-category “oligomers” was an iterative process of combination of different aspects:

CAS No.71808-39-4 crude dimer, Fatty Acids, C16-18 & C-18 unsaturated, dimerized also contains monomers, which are acknowledged as being readily biodegradable. This is also reflected in the screening test for biodegradation (OECD 301B test) where CAS No. 71808- 39-4 had a degradation value of 29.3 %. Thus, this substance is regarded as not suitable as a representative worst case for the sub-category dimerised fatty acids and its derivatives “predominantly oligomers” which comprises of substances that reveal a potential persistence due to their lack of readily and inherent biodegradability.

Trimers are expected to have a lower bioavailability when compared with dimers based on the molecular size (Lipinski rule of 5). They are also expected to have higher potential to adsorb to soil due to the increasing carbon chain length. Therefore, Fatty acids, C18-unsaturated, trimers (CAS No. 68937- 90-6) was evaluated not to be the most suitable representative worst case test substance for the sub-category predominantly oligomers”.

The only difference between the two remaining dimerised fatty acids, namely Fatty acids, C-18 unsaturated, dimers (CAS No. 61788-89-4) and Fatty acids, C-18 unsaturated, dimers, hydrogenated (CAS No. 68783-41-5) is the degree of saturation or hydrogenation, respectively. Both substances seem to be suitable representative test substances. Based on an evaluation conducted using the OECD Toolbox, the degree of saturation is estimated to not have any effect on the toxicity profile of C18 fatty acids. The profiles for C18 fatty acids containing 0, 1 or 2 double bonds are the same in terms of lack of structural alerts, toxic hazard classification by Cramer, aquatic toxicity mode of action by OASIS, aquatic toxicity classification by ECOSAR, etc. For the unsaturated fatty acids additional organic functional groups are indicated (allyl, alkene), but these are not coupled to any structural alerts for protein binding. Experimental data on the long-term toxicity of 16/18C fatty acids to daphnia seem to indicate that unsaturated fatty acids even have slightly higher toxic potential compared to the corresponding saturated fatty acids (MOE, 2008; MOE 2003, entered under IUCLID section 5.1). This observation could possibly be explained by the fact that additional steps are required for the β-oxidation process, when double bonds are present, and more energy is thus needed for the metabolism of unsaturated fatty acids (Berg, Tymoczko and Stryer, 2002). Based on this information, Fatty acids, C-18 unsaturated, dimers can be considered as a worst case read-across substance. Additionally, the production volume of the unsaturated form is much higher than the other sub-category 2 members. Therefore one can assume that the risk of any direct exposure to the soil would be highest here due to large production volumes.

Based on all aspects discussed above, it was decided to test Fatty acids, C-18 unsaturated, dimers (CAS No. 61788-89-4) as representative member of the sub-category 2 “predominantly oligomers”.

A chronic earthworm study was conducted according to OECD guideline 222 with the read-across substance Fatty acids, C-18 unsaturated, dimers (CAS No. 61788-89-4). As no effects occurred in the this study, the members of the subcategory 2 predominantly oligomers, are considered as not harmful to terrestrial organisms and no further testing is needed.

As the test substance is not classified as toxic or harmful, the substance is not considered to cause toxic effects in predators. Additionally, the uptake and bioaccumulation potential is low and thus the test substance is considered to cause no hazard to predators. Thus, a study with birds is not needed due to animal welfare reasons and no PNEC oral is derived from data on mammalian toxicity.


Berg, J.M., Tymoczko, J.L. and Stryer, L. (2002) Biochemistry, 5th edition, W.H. Freeman and Company

Ministry of the Environment, Government of Japan (2003). Daphnia magna, reproduction test by oleic acid. Food Research Laboratories. Report No. 14053. 2003-03-31.

Ministry of the Environment, Government of Japan (2008). Daphnia, reproduction toxicity test for palmitic acid. Mitsubishi Chemical Safety Institute Ltd.,Laboratory. Report No. A050381. 2008-02-14.