N,N'-bis(1,4-dimethylpentyl)-p-phenylenediamine

Endpoint summary

Additional information

Hydrolysis

A high number of hydrolysis tests is available for the different PPDs, but up to now no test according to OECD 111

has been performed for 77PD.

.

However, a full OECD 111 study is available for 44PD which is a similar substance. It contains 2 butyl chains whereas 77PD contains 2 heptyl chains. Hydrolysis rates of the parent substance, identification of hydrolysis products as well as their quantification has been performed.

Another full OECD 111 test is available for 6QDI (N-(1,3-dimethylbutyl)-N'-(phenyl)-1,4-benzoquinonediimine, CAS No. 52870-46-9) which is the oxidized form of 6PPD. As the PPDs are sensitive to oxidation, 6QDI is regarded as the primary oxidation product of 6PPD which is expected also to be formed under hydrolytic conditions in the presence of oxygen. For this reason, this substance has been included in the read-across approach.

In the following table, the key values for the hydrolysis half-lives and the information from all available hydrolysis tests concerning the occurrence of hydrolysis products are summarized:

 

Table: Results of hydrolysis tests

 

	6PPD	6QDI	7PPD	77PD	44PD
Half-life (h)	pH7: 8	pH4: 19 pH7: 4.7 pH9: 4.3	pH7: 5.15	pH7: 3.6	pH4: 200 pH7: 5.3 pH9: 1.5
Hydrolysis products identified	4-Hydroxy-diphenylamine	4-Hydroxy-diphenylamine         p-Benzoquinone p-Hydroquinone Aniline	4-Hydroxy-diphenylamine   N-Phenyl-p-benzoquinone mono-imine	N-1,4 dimethylpentyl-p-phenol       p-Benzoquinone	44-Quinone-diimine (44QDI)     N-1,4 dimethylpentyl-p-phenol p-Benzoquinone p-Hydroquinone sec. Butylamine

The half-lives of the PPDs in the environmental relevant pH of 7 are in the same range (3.4 to 8 hours). The primary step of the hydrolysis is considered to occur rapidly.

 

Considering all information given in the various reports, the hydrolysis pathway of all PPDs follows the same pattern. The picture of the hydrolysis scheme is outlined in the read-across justification document attached to the IUCLID in chapter 13.

The first step of hydrolysis is the oxidation to a quinone-diimine structure followed bycleavage of the alkyl chain yielding the phenolic compound (in case of 77PD 1,4 -dimethylpentylamino-p-phenol) and the alkylamine (1,4-dimethylpentylamine).

 

The second step of hydrolysis is the cleavage of the second amino group which is linked either to R₁=phenyl (in case of 6PPD, 7PPD) or to R₁=1,4-dimethylpentyl (as for 77PD). (The data of 44PD are in preparation). The secondary hydrolysis products are p-benzoquinone and p-hydroquinone as well as the respective amine.

 

As explained above, the formation of the primary hydrolysis products occurs rapidly (half-life 5.3 hours). The half-life of the secondary hydrolysis products has not been described in detail. However, after having assessed the available information one can conclude that the primary hydrolysis products of 6PPD and 7PPD, namely 4-hydroxydiphenyl-amine and benzoquinone-monoimine, are rather stable, and the secondary hydrolysis reaction occurs more slowly. The primary hydrolysis products of 77PD and 44PD are less stable, and secondary hydrolysis to p-benzoquinone and p-hydroquinone occurs faster. In the test with 44PD (pH7, 20°C) parent substance and primary metabolites have disappeard completely after 3 days. After that time, only p-benzoquinone and p-hydroquinone are observed. p-Benzoquinone is further decreasing and is transformed to p-hydroquinone. After 14 days, the concentration of p-hydroquinone (expressed in mole/Litre) represents about 30% of the start concentration of the parent substance.

 

The secondary hydrolysis products p-benzoquinone,p-hydroquinone and the relevant amines are known to be readily biodegradable.

Direct and indirect photolysis

A photolysis screening test of 77PD in water shows a half life of 2h with radiation and of 4h in the dark.

For indirect photolysis in air a calculated half life of 3.7 hours is available showing that the substance is rapidly decomposed in air.

Biotic degradation

A test on ready biodegradability was conducted according to OECD Guideline 301 C (Ready Biodegradability: Modified MITI Test (I)). After 28 days 77PD showed 12 % degradation. Therefore 77PD is not ready biodegradable (Bayer,1990).

Some primary and secondary hydrolysis products (p-benzoquinone, p-hydroquinone and the relevant amines) are readily biodagradble. The question arises why the PPDs are not readily biodegradable, although some of their hydrolysis products (p-benzoquinone, p-hydroquinone, aniline and 1,4 -dimethylpentylamine) are known to biodegrade quite rapidly. Thus, limited biodegradation could have been expected. A possible explanation for this contradiction is the fact that p-hydroquinone is highly toxic against bacteria: Based on an EC50 of 71 mg/L and a NOEC of 1 mg/L the resulting PNEC(STP) is 0.71 mg/L. Biodegradation tests according to the OECD 30x series are typically performed with initial concentrations of 100 mg/L.