Reaction mass of 2-(3-ethylphenyl)oxirane and 2,2’-(1,3-phenylene)dioxirane and 2,2'-(1,4-phenylene)dioxirane

Hydrolysis

Description of key information

The determination was carried out using Method C7 Abiotic Degradation, Hydrolysis as a Function of pH of Commission Regulation (EC) No 440/2008 of 30 May 2008 with the following deviation: 
To maximise the efficiency of the procedure for the pH 4 testing at 10.0°C and 20.0°C, sample preparation having to be performed as rapidly as possible, samples were taken for analysis directly from a single stock solution incubated in the bath as opposed to using individual vessels for each time point. This was concluded not to impact on the validity of the data, as the use of individual vessels is required as a precautionary step against losses through volatility and biodegradation, neither of which were relevant over the temperature and time scale of the tests. When the procedure of splitting the sample solution between individual vessels was applied to the pH 4 preliminary test at 50°C, significant hydrolysis occurred even before sampling for the initial time point due to the delay introduced by this step.

Key value for chemical safety assessment

Half-life for hydrolysis:

68.3 h

at the temperature of:

25 °C

Additional information

Available literature indicated that oxirane functional groups present in the test substance were readily susceptible to both acid catalysed and base catalysed hydrolysis resulting in the formation of a common hydrolysis product, the relevant diol.

Complete hydrolysis of the test substance and the presence of corresponding anticipated hydrolysis products were investigated in a hydrolysed sample solution using high performance liquid chromatography and mass spectroscopy (HPLC-MS, Agilent Technologies 1200). Significant mass ions of m/z 221 and 237, representative of the sodium adduct and potassium adduct of the diols anticipated from oxirane, 2,2’-(1,3-phenylene)bis- and oxirane, 2,2’-(1,4-phenylene)bis- were successfully detected. Sodium and potassium ions are unavoidable contaminants in an HPLC-MS system and often these ion pair adducts are significantly more stable than the parent ion alone. For the investigation of the lesser anticipated hydrolysis products, the diols originating from the oxirane, 2-(3-ethylphenyl)- and oxirane, 2-(4-ethylphenyl)- components, additional analysis was performed specifically scanning for the ions of m/z 189 and 205. This again confirmed the presence of the mass ions of m/z 189 and 205, representative of the sodium adduct and potassium adduct of the diols anticipated from oxirane, 2-(3-ethylphenyl)- and oxirane, 2-(4-ethylphenyl)-.

The kinetics of the study have been determined to be consistent with that of a pseudo-first order reaction as the graphs of log₁₀concentration versus time are straight lines.

No significant peaks were observed at the approximate retention time of the test material on analysis of any matrix blank solutions.

It has been observed that the rate of hydrolysis increases with a decrease in pH.

For Tier 2 testing at pH 4, only two temperatures were investigated as opposed to three temperatures as required by the guideline. This was due to the extremely rapid rate of hydrolysis observed at this pH. Testing was initiated at 10°C, the minimum temperature recommended by the guideline. The guideline states that each temperature must differ by a minimum of 10°C and therefore a second test was performed at 20°C. However at 20°C, the half-life of each component was already significantly below 1 hour and thus it was concluded that insufficient data points would be generated at 30°C due to the further accelerated rate of hydrolysis. In addition, a suspected artefact peak was observed at the approximate retention time of peak 3 in a number of later timepoints during the 20°C test. As the almost complete hydrolysis of this component had been confirmed analytically prior to the development of this peak, these later samples were excluded from the definitive final calculations.

For Tier 2 testing at pH 7, it was strongly suspect that the 30°C and 40°C, 6 hour timepoint samples had been transposed in error prior to analysis. However as this could not be definitively confirmed after the event, the 6 hour timepoints results were excluded from the final calculations of rate constant and half-life for both temperatures as a precautionary step.Tentative peak assignment was made for the HPLC chromatograms obtained by monitoring the test material by UV detection based on percentage area normalisation data, elution order and compositional information supplied by the sponsor. Peak 1 was attributed to the two main bis(epoxyethyl) benzene components co-eluting. The supportive evidence for 2 components co-eluting in peak 1 was seen as a shoulder on the peak profile for a number of injections. However, it was not possible to satifactorily resolve the two peaks by a method suitable for analysing all peaks of interest. Peak 2 and peak 3 were attributed to the two remaining (ethylpenyl)oirane components. These peaks assignments were further suppported by data generated during the partition coefficient test(see Section 9), specifically the comparison between computer predicted and experimental partition coefficient values for each component.

All concentrations have been reported as equivalent test material concentrations, i.e. the concentration of any given component was equivalent to that concentration of the test material as a whole having been dissolved.

Under the physiologically relevant conditions of pH 1.2, 37.0 ± 0.5°C, the test material was determined to undergo essentially spontaneous hydrolysis.