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Hydrolysis

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Description of key information

Hydrolysis half-life: >200 -2000 hour at pH 7, 1.9 -6.6 h pH 4, and 2.0 -13.9 h pH 9 at 20 - 25°C (QSAR)

Key value for chemical safety assessment

Additional information

The requirement to conduct a hydrolysis study for the substance is waived in accordance with column 2 of REACH Annex VIII because the substance is insoluble in water. A measured water solubility of <10 ng/l at 20°C was determined for the substance.

Half-life values at 20-25°C of > 200 - 2000 h at pH 7, 1.9 – 6.6 h at pH 4, and 2.0 – 13.9 h at pH 9 were obtained for 3,3-bis[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-1,5-divinyltrisiloxane using accepted validated QSAR methods. The result is considered to be reliable and has been used for assessment purposes.

As the hydrolysis reaction may be acid or base catalysed, the rate of reaction is expected to be slowest ataround 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 uncatalyzed 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+]=10-4mol dm-3 and at pH2 [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 0.019 hours (approximately 68 seconds). 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) x e(0.08.(T-X))

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

Thus, for 3,3-bis[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-1,5-divinyltrisiloxane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 74 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), hydrolysis half-life is 25 seconds.

The products of hydrolysis are dimethylvinylsilanol and silicic acid.

Hydrolysis of the read-across substance Silsesquioxanes, Phenyl (CAS 70131-69-0)

Data for the substance Silsesquioxanes, Phenyl(CAS 70131-69-0)are read-across to the submission substance for appropriate endpoints (see Section 1.4).The structural similarity and the siloxane hydrolysis half-life of the read-across substance to the submission substance is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

No measured hydrolysis data is available for Silsesquioxanes, Phenyl (CAS 70131-69-0); predicted half-lives of 1.9 -3.3 h at pH 4, 200 - 630 h at pH 7 and 2.0 - 5.3 h at pH 9 and 20 -25°C are calculated for this substance.

Under laboratory conditions, the likely final products of hydrolysis are phenylsilanetriol and trimethylsilanol.

Hydrolysis of the read-across substance 1,1,1,5,5,5-hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 2116-84-9)

Data for the substance1,1,1,5,5,5-hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 2116-84-9) are read-across to the submission substance for appropriate endpoints (see Section 1.4 of the CSR). Hydrolysis half-lives of 1.9 -3.6 h at pH 4, 200 - 630 h at pH 7 and 2.0 -5.3 h at pH 9 and 20-25°C were determined for the substance using a validated QSAR estimation method. 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 phenylsilanetriol (1 mole) and trimethylsilanol (3 moles) per mole of parent substance.

 

Hydrolysis of the read-across substance decamethylcyclopentasiloxane (CAS 541-02-6)

Data for the substance decamethylcyclopentasiloxane (CAS 541-02-6) are read-across to the submission substance,3,3-bis[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-1,5-divinyltrisiloxane for appropriate endpoints (see Section 1.4).The rate of hydrolysis of the two substances is relevant to this read-across.

For decamethylcyclopentasiloxane, hydrolysis half-lives at 25°C of 9.3 h at pH 4, 1590 h at pH 7 and 24.8 – 31.6 h at pH 9 were determined in accordance with OECD 111 (Durham 2006). 

The ultimate products of hydrolysis are dimethylsilanediol and trimethylsilanol. 

 

Hydrolysis of the read-across substance dodecamethylcyclohexasiloxane (CAS 540-97-6)

Data for the substance dodecamethylcyclohexasiloxane (CAS 540-97-6) are read-across to the submission substance for appropriate endpoints (see Section 1.4).The rate of hydrolysis of the two substances is relevant to this read-across.

For dodecamethylcyclohexasiloxane (CAS 540-97-6), hydrolysis half-lives at 25°C of 22.3 h at pH 4, >1 year at pH 7 and 96 h at pH 9 were determined. 

The ultimate products of hydrolysis are dimethylsilanediol and trimethylsilanol.