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Hydrolysis

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Description of key information

Hydrolysis half-life: In contact with water, the substance reacts slowly with half-lives of 3.6 h at pH 4, 2.3 h at pH 5, 631 h at pH 7 and 5.3 h at pH 9 and 20-25°C (QSAR). The stated half-life is for removal of siloxane group in the parent substance. Complete reaction to the ultimate end products will take longer.

Key value for chemical safety assessment

Half-life for hydrolysis:
631 h
at the temperature of:
20 °C

Additional information

The registration substance, 3-[3,3,3-trimethyl-1,1-bis[(trimethylsilyl)oxy]disiloxanyl]propyl methacrylate contains two types of hydrolysable groups: three siloxane groups (which react to the corresponding silanols) and one methacrylate ester group (which has the potential to react to the corresponding alcohol plus methacrylic acid). The chemical safety assessment of the substance assumes that these reaction processes will proceed independently based on their rates under the relevant conditions.

Hydrolysis of the siloxane groups:

Hydrolysis half-lives of 3.6 h at pH 4, 2.3 h at pH 5, 631 h at pH 7 and 5.3 h at pH 9 and 20-25°C were determined for the substance using a validated QSAR estimation method. The result is considered to be reliable and was selected as key study.

3-[3,3,3-Trimethyl-1,1-bis[(trimethylsilyl)oxy]disiloxanyl]propyl methacrylate is a linear siloxane chain with four silicon atoms, connected by three oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. The stated half-life is for removal of the registration substance due to hydrolysis. The products of this reaction are also unstable in water, and so further hydrolysis reactions will follow, the ultimate products being 3-(trihydroxysilyl)propyl methacrylate and trimethylsilanol per mole of parent substance.

As the hydrolysis reaction may be acid or base catalysed, the rate of reaction is expected to be slowest at around pH 7 and increase as the pH is raised or lowered. For an acid-base catalysed reaction in buffered solution, the measured rate constant is a linear combination of terms describing contributions from the uncatalysed reaction as well as catalysis by hydronium, hydroxide, and general acids or bases.

kobs= k0+ kH3O+[H3O+] + kOH-[OH-] + ka[acid] + kb[base]

At extremes of pH and under standard hydrolysis test conditions, it is reasonable to suggest that the rate of hydrolysis is dominated by either the hydronium or hydroxide catalysed mechanism.

Therefore, at low pH:

kobs≈kH3O+[H3O+]

 

At pH 4 [H3O+] = 10-4 mol dm-3 and at pH 2 [H3O+] = 10-2 mol dm-3; therefore, kobs at pH 2 should be approximately 100 times greater than kobs at pH 4.

The half-life of a substance at pH 2 is calculated based on:

t1/2(pH 2) = t1/2(pH 4) / 100

The calculated half-life of the substance at pH 2 is therefore 0.036 hours (2.2 minutes). 

Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions. Under ideal conditions, hydrolysis rate can be recalculated according to the equation:

DT50(XºC) = DT50(T) * e(0.08.(T-X))

Where T = temperature for which data are available and X = target temperature.

Thus, for the substance, the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 232 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half- life is calculated as 0.013 hours (approximately 47 seconds). At 37.5°C and pH 5.5 (relevant for dermal exposure), the hydrolysis half-life is expected to be between the values for pH 5 (0.85 h) and pH 7 (232 h).

The ultimate products of hydrolysis are 3-(trihydroxysilyl)propyl methacrylate and trimethylsilanol.

Note on hydrolysis of the methacrylate ester group:

Very limited evidence of the hydrolysis of the methacrylate ester group is available. The hydrolysis half-life of methyl methacrylate (a much more soluble substance) is reported to be >1 year at 25°C and pH 7 and 14.4 days at 25°C and pH 9 (WHO CICAD summary citing Howard, 1989). However, in the case of the registration substance 3-[3,3,3-trimethyl-1,1-bis[(trimethylsilyl)oxy]disiloxanyl]propyl methacrylate, the reaction may be slower and for the purposes of chemical safety assessment, in the absence of clear evidence it is considered that the methacrylate ester group reaction is not significant for the chemical safety assessment at any relevant pH. Therefore, where the chemical safety assessment considers hydrolysis products, these are identified as 3(trihydroxysilyl)propyl methacrylate and trimethylsilanol. The same assumptions are made for read-across substances containing methacrylate groups. This is a conservative position in the absence of clear evidence, as the methacrylate structural feature is likely to be more toxic than the corresponding alcohol.

The hydrolysis data for substances used in this dossier for read-across purposes for other endpoints are now discussed.

The hydrolysis half-lives of substances used for read-across in other areas are discussed below.

Hydrolysis of the read-across substance trimethylsilanol (CAS 1066-40-6)

Data for the substance trimethylsilanol (CAS No. 1066-40-6) are read-across to the submission substance 3-[3,3,3-trimethyl-1,1-bis[(trimethylsilyl)oxy]disiloxanyl]propyl methacrylate for appropriate endpoints. The submission substance undergoes slow hydrolysis in water to produce 3-(trihydroxysilyl)propyl methacrylate and trimethylsilanol. Trimethylsilanol is the second silanol hydrolysis product of the submission substance and data for this hydrolysis product are used in relevant sections.

Trimethylsilanol does not contain any chemical groups susceptible to hydrolytic degradation.

Hydrolysis of the read-across substance, 3-trimethoxysilylpropyl methacrylate CAS No. 2530-85-0

Data for the substance, 3-trimethoxysilylpropyl methacrylate (CAS No. 2530-85-0) are read-across to the submission substance 3-[3,3,3-trimethyl-1,1-bis[(trimethylsilyl)oxy]disiloxanyl]propyl methacrylate for appropriate endpoints. The silanol hydrolysis product of the two substances are relevant to this read-across, as discussed in the appropriate Sections for each endpoint.

Hydrolysis half-lives of 0.018 h at pH 4, 1.87 h at pH 7, and 0.068 h at pH 9 and 20°C were determined for the substance in accordance with OECD 111 Test Guideline and in compliance with GLP. Also, predicted hydrolysis values of 0.3 h at pH 4, 0.3 h at pH 5, 4.4 h at pH 7 and 0.1 h at pH 9 and 20 -25°C were obtained for the substance using a validated QSAR estimation method. Similarly, hydrolysis half-lives of 3 h at 25°C, 4 h at 20°C and pH 7 were determined in a preliminary study conducted in accordance with OECD 111 and in compliance with GLP. At pH 4 and pH 9, the test substance was observed to undergo rapid hydrolysis and as such half-lives at these pHs were estimated by the author of this record to be < 1 hour. In a secondary source to which reliability could not be assigned, a hydrolysis half-life of 121.6 minutes (approximately 2 hours) at pH 4.5 and 25°C was reported for the substance. The available data are in agreement that the substance undergoes rapid hydrolytic degradation.

The half-lives at pH 2 and 20°C, at pH 7 and 37.5°C and at pH 2 and 37.5°C may be calculated in the same way as for the registration substance above. This gives a half-life of 0.00018 h (approximately 1 second) at pH 2 and 25°C. However, it is not appropriate or necessary to attempt to predict accurately when the half-life is less than 5 -10 seconds. The half-life is therefore reported as 5 seconds as a worst case. At pH 7 and 37.5°C, the hydrolysis half-life is 0.46 hours. At pH 2 and 37.5°C; the half-life is approximately 5 seconds. At 37.5ºC and pH 5.5 (relevant for dermal exposure), the hydrolysis half -life will be in between the half-lives at pH 4 and pH 7 at 37.5ºC.

The hydrolysis products in this case are 3-(trihydroxysilyl)propyl methacrylate and methanol.

Hydrolysis of the read-across substance (1,1,3,3-tetramethyldisiloxane-1,3-diyl)dipropane-1,3-diyl dimethacrylate; CAS No. 18547-93-8

Data for the substance (1,1,3,3-tetramethyldisiloxane-1,3-diyl)dipropane-1,3-diyl dimethacrylate; CAS No. 18547-93-8 are read-across to the registration substance, 3-[3,3,3-trimethyl-1,1-bis[(trimethylsilyl)oxy]disiloxanyl]propyl methacrylate for appropriate endpoints. The silanol hydrolysis product and the rate of hydrolysis of the two substances are relevant to this read-across, as discussed in the appropriate sections for each endpoint.

Hydrolysis half-lives of 1.1 h at pH 4, 0.9 h at pH 5, 63 h at pH 7 and 0.8 h at pH 9 and 20-25°C were predicted for the substance using a validated QSAR estimation method for reaction of the siloxane group.

The half-lives at pH 2 and 25°C, at pH 7 and 37.5°C and at pH 2 and 37.5°C may be calculated in the same way as for the target substance, as described above. This gives a half-life of 0.011 h (approximately 40 seconds) at pH 2 and 25°C. At pH 7 and 37.5°C, the half-life is 23 hours and at pH 2 and 37.5°C, the half-life is 0.004 hours (approximately 14 seconds). At 37.5ºC and pH 5.5 (relevant for dermal exposure), the hydrolysis half-life will be in between the half-lives at pH 4 and pH 7 at 37.5ºC (0.4 - 23 hours).

The hydrolysis product in this case is 3-[hydroxy(dimethyl)silyl]propyl methacrylate (2 moles).