N-tert-butylbenzothiazole-2-sulphenamide

Biodegradation in soil

Administrative data

Endpoint:

biodegradation in soil: simulation testing

Type of information:

experimental study planned (based on read-across)

Justification for type of information:

A simulation test is a standard requirement under REACH for substances in the tonnage band 100-1000 and >1000 to/a. TBBS is not readily biodegradable. The substance is a member of the group of Sulfenamides which also contains the substance CBS (CAS No. 95-33-0). TBBS and CBS both contain the common mercaptobenzothiazole moiety which is linked to an alkyl group (t-butyl group in case of TBBS, cyclohexyl group in case of CBS). Further, physico-chemical properties of the substances follow a trend considering hydrolysis behaviour and lipophilicity. TBBS hydrolyses rapidly with DT50 values of 1.8 h (MITI 1996 to 9.53 h (Monsanto 1984), CBS somewhat slower with a DT50 of 12.5h (both values at pH7 and 20°C). Both substances are lipophilic, but also a trend is observed: TBBS has a logKow of 3.36 and CBS a logKow of 5.0. In the context of EU REACH there does not seem to exist a strict definition on the adsorption potential. According to ECHA Guidance R.7c (2017, Table R.7.11-2, p. 148 & 158) substances with a logPOW or log KOC of > 5 fulfil the criteria for high adsorption. ECHA Guideline R.7b (2017, p 146) specifies the criteria for strongly adsorbing substances furthermore to a logPOW of > 5 and a logKOC of > 3, but also for high potential for adsorption to soil and sediment to a logKOC >4 (p. 226). Furthermore, ECHA Guidance R.7a (2017, p. 154) mentions a cut-off value of logPOW = 3, albeit with caution that this value might not be absolute. The adsorption behaviour to soil is typically expressed as logKoc values. The logKoc value for TBBS is calculated with several QSAR methods yielding the values 2.69 and 4.05. For CBS the logKoc values are in the range of 3.33 to. 4.12. According to the Framework Manual of the United States EPA in Section 5 (2012, p. 5-13), adsorption in the range of logKoc between 2.5 and 3.4 is defined as “moderate sorption”. As a conclusion, both substances,TBBS and CBS show a moderate to strong sorption behavior to soil.
The read-across from CBS to TBBS is considered a worst-case approach, justified owing to high structural similarity and a trend in physicochemical properties, here the adsorptive properties as reflected by logKOC and logPOW.

Structural similarity:
All four category members consist of the identical basic molecular structure with the common functional group of a benzothiazole-2-sulphenamide moiety and differ only in one structural feature, the substituent bound to the mercapto group: t-butyl group in case of TBBS, cyclohexyl group in case of CBS.
Both substituents are aliphatic hydrocarbon moieties with similar carbon numbers (4 in case of TBBS, 6 in case of CBS)

Adsorptive properties:
These only differ for TBBS and CBS based on the differing aliphatic hydrocarbon moiety. The larger the hydrocarbon moiety, the higher the sorptive properties as reflected by the higher logKOC and logPOW values for CBS versus TBBS.
Sorption itself comprises of different types of physical and chemical interactions (von Oepen et al. 1991). These interaction types can be grouped into three main categories: 1) physical and chemical adsorption, 2) covalent binding and 3) sequestration / entrapment. (ECETOC 2013). With respect to assessing non-extractable residues (NERs), various modes of binding have been postulated and recently also experimentally investigated (Kästner et al. 2018):
- Type I NER: non-covalent binding comprises of several interaction types (Kästner et al. 2018) with binding forces ranging from 0.5 (Van der Waals) to 350 (ionic) kJ/mol. CBS and TBBS are not surface active substances, therefore the weak van der Waals forces (0.5 - 5 kJ/mol) are the only non-covalent binding types deemed relevant for the hydrocarbon moiety while the benzothiazole-2-sulphenamide moiety is capable to participate in other binding types with considerably higher binding forces of up to 350 kJ/mol
- Type II NER binding is equivalent to covalent bonding which can be excluded for the hydrocarbon substituents as contained in TBBS and CBS.

In conclusion, the read-across from CBS to TBBS for simulation testing in soil is considered a worst-case approach as the difference in the physicochemical nature of the hydrocarbon substituent is responsible for the quantitative variation in the strength of adsorptive properties, with CBS being more adsorptive than TBBS while the nature of binding interactions to suspended matter (soil, sediment) must be considered identical for both. Accordingly, the Registrant has submitted a testing proposal for a soil simulation test for CBS. It is considered justified to use the result of this test also for TBBS.

Materials and methods

Test guideline

Qualifier:

according to guideline

Guideline:

OECD Guideline 307 (Aerobic and Anaerobic Transformation in Soil)

Deviations:

not applicable

GLP compliance:

yes (incl. QA statement)

Test type:

laboratory

Test material

Test material information

Constituent 1

Reference substance name:

N-cyclohexylbenzothiazole-2-sulfenamide

EC Number:

202-411-2

EC Name:

N-cyclohexylbenzothiazole-2-sulfenamide

Cas Number:

95-33-0

Molecular formula:

C13H16N2S2

IUPAC Name:

N-(1,3-benzothiazol-2-ylsulfanyl)cyclohexanamine

Test material form:

solid

Remarks:

colour: white

Radiolabelling:

yes