N-1,3-dimethylbutyl-N'-phenyl-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 has been performed according to OECD 111.

However, a full OECD 111 study 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 Table below, 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: 3.4	pH4: 200 pH7: 5 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	4,4-Quinone-diimine (44QDI) N-1,4 dimethyl- pentyl-p-phenol p-Benzoquinone p-Hydroquinone 1,4 dimethyl- pentylamine

The half-lives of the PPDs at 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 (Allmendinger 2016).

The first step of hydrolysisis the cleavage of the alkyl chain yielding the phenolic compound (e.g. 4-hydroxydiphenylamine) and the alkylamine (e.g. 1,3 -dimethylbutylamine). It should be noted that the alkylamine has never been monitored in hydrolysis studies probably due to problems in chromatography. Only in the ready biodegradation test (MITI 1995) the alkylamine could be determined, but only after derivatisation.

In parallel, a further primary hydrolysis product is formed: the quinone-monoimine. It is not quite clear whether the formation occurs via hydrolysis of the oxidized PPD (e.g. 6QDI) or via oxidation of the phenolic compound (e.g. 4 -hydroxydiphenylamine). It should be noted that the formation of the quinone-monoimine can be explained in case of 6PPD as well as in case of 7PPD (R₁=phenyl) as the formation of the substance is preferred by mesomeric stabilisation. In case of 77PD (R₁=1,4-dimethylpentyl) the formation of the quinone-monoimine was not observed as no mesomeric-stabilised aromatic system can be formed.

 

The second step of hydrolysis is the cleavage of the second amino group which is linked to R₁=phenyl (in case of 6PPD, 7PPD) The secondary hydrolysis products are p-benzoquinone and p-hydroquinone as well as the respective amine. In case of phenylenediamines linked to phenyl groups (6PPD, 7PPD), the amine formed is aniline.

 

As explained above, the formation of the primary hydrolysis products occurs rapidly (half-life 3.4 to 8 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 secondary hydrolysis productsp-benzoquinone, p-hydroquinone and the relevant amines are known to be readily biodegradable.

Photolysis

6PPD degrades rapidly by indirect photolysis and by hydrolysis.

In the atmosphere a half-life of 1.7 hours for N-(1,3-dimethylbutyl)-N'-phenylbenzene-1,4-diamine (6PPD) is estimated due to reaction with photochemically produced hydroxyl radicals, considering an OH-concentration of 500,000 radicals/cm³ as a 24-h average (Currenta, 2009).