1,1,5,5,5-hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane

Hydrolysis

Description of key information

Hydrolysis half-life: >200 hours at pH 7, >1.9 hours at pH 4, >2.0 hours at pH 9 and 20-25°C (QSAR). The stated half-life is for removal of parent. Complete reaction to the ultimate end products will take longer.

Additional information

Hydrolysis half-lives of >1.9 hours at pH 4, >200 hours at pH 7 and >2.0 hours 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.

Tris(trimethylsiloxy)phenylsilane is a linear branched siloxane chain with four silicon atoms, connected by three oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. All silicon atoms present are fully substituted with methyl and phenyl groups.The stated half-life is for removal of the registration substance due to hydrolysis of the first Si-O-Si bond. 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.

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.

k_obs= k₀+ k_H3O+[H₃O⁺] + k_OH^-[OH^-] + k_a[acid] + k_b[base]

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 >70 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half- life is calculated as approximately >25 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 4 (>0.7 h) and pH 7 (>70 h).

The ultimate products of hydrolysis are phenylsilanetriol and trimethylsilanol.

The hydrolysis of read-across substances for other endpoints are now discussed:

 

Hydrolysis of the read-across substance 3-[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane (CAS 60111-47-9)

 

Data for the substance 3-[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane (CAS 60111-47-9) are read across to the registration substance tris(trimethylsiloxy)phenylsilane

for appropriate endpoints (see the read-across justification attached in Section 13).The hydrolysis half-lives and silanol hydrolysis products of the two substances are relevant to this read-across, as discussed in the appropriate sections for each endpoint.

 

For 3-[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane, hydrolysis half-lives at 20-25°C of >1.9 hours at pH 4, >200 hours at pH 7 and >2.0 hours at pH 9 were determined using a validated QSAR estimation method.

 

The ultimate products of hydrolysis are dimethylvinylsilanol and phenylsilanetriol.

Hydrolysis of the read-across substance 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS No. 3390-61-2)

Data for the substance 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS No. 3390-61-2) are read across to the submission substance tris(trimethylsiloxy)phenylsilane for appropriate endpoints. The hydrolysis half-lives and the silanol hydrolysis products of the two substances are relevant to this read-across, as discussed in the appropriate sections for each endpoint.

For 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane, hydrolysis half-lives at 20-25°C of 2.0 hours at pH 4, >200 hours at pH 7 and 2.0 hours at pH 9 were determined using a validated QSAR estimation method.

The half-lives at pH 4 and 25°C, 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 registration substance above. This gives a half-life of 0.019 h at pH 2 and 20-25°C, 25 seconds at pH 2 and 37.5°C and approximately >70 h at pH 7 and 37.5°C.

The ultimate products of hydrolysis are methyl(diphenyl)silanol and methyl(phenyl)silanediol. 

Hydrolysis of the read-across substance 1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane (CAS No. 3982-82-9)

Data for the substance 1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane (CAS No. 3982-82-9) are read-across to the submission substance tris(trimethylsiloxy)phenylsilane for appropriate endpoints. The hydrolysis half-lives and the silanol hydrolysis products of the two substances are relevant to this read-across, as discussed in the appropriate sections for each endpoint.

For 1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane, hydrolysis half-lives at 20-25°C of 1.9 hours at pH 4, >200 hours at pH 7 and 2.0 hours at pH 9 were determined using a validated QSAR estimation method.

The half-lives at pH 4 and 25°C, 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 registration substance above. This gives a half-life of 0.019 h at pH 2 and 20-25°C, 25 seconds at pH 2 and 37.5°C and >70 h at pH 7 and 37.5°C.

The ultimate products of hydrolysis are methyl(diphenyl)silanol and dimethylsilanediol. 

Hydrolysis of the read-across substance decamethyltetrasiloxane (L4, CAS No. 141-62-8)

Data for the substance, decamethyltetrasiloxane (L4, CAS No. 141-62-8) are read across to the submission substance tris(trimethylsiloxy)phenylsilane for appropriate endpoints. The hydrolysis half-lives and the silanol hydrolysis products of the two substances are relevant to this read-across.

For decamethyltetrasiloxane (L4), hydrolysis half-lives at 25°C of 14 h at pH 5, 728 h (30.3 days) at pH 7 and 21.1 h at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation, 2009). The measured values are supported by predicted hydrolysis half-lives of 3.6 h at pH 4, 2.3 h at pH 5, 630 h at pH 7 and 5.3 h at pH 9 and 20 -25°C using a validated QSAR estimation method.

Thus, for L4 the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 270 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half- life is calculated as 90 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 (5 h) and pH 7 (270 h).

The half-lives at pH 4 and 25°C, 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 registration substance above. This gives a half-life of 3.6 h at pH 4 and 25°C, 0.036 h at pH 2 and 25°C, 19 seconds at pH 2 and 37.5°C and 270 h at pH 7 and 37.5°C.

The ultimate products of hydrolysis are dimethylsilanediol and trimethylsilanol.