Propyltriacetoxysilane

Endpoint summary

Description of key information

Short description of key information on bioaccumulation potential result: 
Based on the experimental results obtained with the supporting substance sodium acetate in male and female rats, the read-across approach was applied and similar results on excretion were expected with propyltriacetoxysilane hydrolysis product acetic acid:
Most of orally administered acetic acid-14C is expected to be recovered in the form of14CO2. Only a small amount of the radioactivity would be found in the urine and faeces. The radioactivity recovered from excreta during the first 24-h period is expected to be considerably less from the males than from the females. Male rats apparently tend to fix more of the radioactivity in the body tissues, and the acetate carbons were not turned over as rapidly as in the females.

Additional information

It is well known that triacetoxisilane undergoes rapid hydrolysis in aqueous or moist environments to acetic acid and trisilanol.

The confirm that hydrolysis of acetoxysilanes is fast, the test of hydrolysis of propyl triacetoxysilane in water was performed. It was measured, that the process was very fast. The half-life at different pH of propyltriacetoxysilane was determined to be < 37.5 seconds since the test item was completely hydrolysed at 150 seconds after the initial contact with water.

As it is stated in different publications, silanols hydrolyse well in water and the carbon- bounded substituents can have profound effects on the rate of hydrolysis. (Arkles B., Chemtech 1977; Pluddemann E.P., Plenum Press NY, 1982; Kay, B.D. and Assink R.A, J. Non-Cryst. Solids, 1988).

The rates of hydrolysis of the alkoxy groups are generally related to their steric bulk: CH₃O>C₂H₅O> t-C₄H₉O and a methoxysilane hydrolyzes at 6-10 times rate of an ethoxysilane. Smith (Smith K. J. Org. Chem 1986) proved that increased organic substitution enhances the hydrolysis rate Me₃SiOMe> Me₂Si(OMe)₂> MeSi(OMe)₃.

During the performed hydrolysis test, the condensation and polimerysation of the molecules formed in hydrolysis were observed too. It was observed as the phase separation. Unfortunately, this phase separation caused the technical difficulties of the determination of the molecular weight of larger condensation products. It was possible to determined MW of smaller condensates which still are in solutions. Their average MW were between 604-695.

This phase separation as a result of condensation was described by Arkles. The hydrolysis of propyltrimetoxysilane showed that oligomers are formed and branched structures presages phase separation (Arkles B. et al, Silanes and Coupling Agents, 1992).

Taking in account both, the hydrolysis and condensation, it is expected that the observed in the hydrolysis test phase changed product contains large chain polymers with MW>1000.

Authors showed that molecules of MW>1000 cannot be biologically available (Van Gestel et a, Reg. Toxicol. and Pharmacol., 1985, 5, 422-31 and Zitko V, Handbook of Environmental Chemistry, v. 2 221-29).

A stated before, triacetoxysilane is not stable when exposed to water or moisture and undergo rapid hydrolysis. The hydrolysis produces 3 moles of acetic acid and the silanetriol which condensate to higher weight siloxanes. The polymerization products are considered biologically unavailable and the toxicity is driven by acetate anion.

The toxicity of propyltriacetoxysilane should be evaluated as the toxicity of acetic acid.

Supporting study:

Read-across from experimental results with sodium acetate:

In the study by Philleo et. al., 1967 (reliability 2) male and female rats were used for an excretion study with sodium acetate-C¹⁴oral administration. CO₂was trapped by sodium hydroxide solution. The trapped CO₂was periodically analyzed for radioactivity after conversion of CO₂to BaCO₃(urine and faeces samples).

Based on the experimental results obtained with the supporting substance sodium acetate in male and female rats, the read-across approach was applied and similar results on excretion were expected with propyltriacetoxysilane hydrolysis product acetic acid:

Most of orally administered acetic acid-14C is expected to be recovered in the form of¹⁴CO₂. Only a small amount of the radioactivity would be found in the urine and faeces. The radioactivity recovered from excreta during the first 24-h period is expected to be considerably less from the males than from the females. Male rats apparently tend to fix more of the radioactivity in the body tissues, and the acetate carbons were not turned over as rapidly as in the females.

There are two separate rates for the elimination of¹⁴CO₂. The initial rate of elimination is very rapid and comes followed by a much slower rate of elimination. The appearance of two separate rates of¹⁴CO₂elimination could represent two routes of metabolism of the acetate carbons. The initial rate of elimination is probably due to the direct reaction of acetate with CoA-SH to form acetyl-CoA, which subsequently is oxidised to CO₂through the TCA cycle. In the intact animal, the CO₂would be present in the form of blood bicarbonate and as the blood bicarbonate is turned over, there is a subsequent release of CO₂from the lungs. The slower, secondary rate of elimination is probably derived from acetate carbons which are incorporate into other metabolites, such as fatty acids and amino acids and are subsequently catabolised to CO₂.

Discussion on bioaccumulation potential result:

Supporting study:

Read-across from experimental results with sodium acetate:

In the study by Philleo et. al., 1967 (reliability 2) male and female rats were used for an excretion study with sodium acetate-C¹⁴oral administration. CO₂was trapped by sodium hydroxide solution. The trapped CO₂was periodically analyzed for radioactivity after conversion of CO₂to BaCO₃(urine and faeces samples).

Based on the experimental results obtained with the supporting substance sodium acetate in male and female rats, the read-across approach was applied and similar results on excretion were expected with propyltriacetoxysilane hydrolysis product acetic acid:

Most of orally administered acetic acid-14C is expected to be recovered in the form of¹⁴CO₂. Only a small amount of the radioactivity would be found in the urine and faeces. The radioactivity recovered from excreta during the first 24-h period is expected to be considerably less from the males than from the females. Male rats apparently tend to fix more of the radioactivity in the body tissues, and the acetate carbons were not turned over as rapidly as in the females.

There are two separate rates for the elimination of¹⁴CO₂. The initial rate of elimination is very rapid and comes followed by a much slower rate of elimination. The appearance of two separate rates of¹⁴CO₂elimination could represent two routes of metabolism of the acetate carbons. The initial rate of elimination is probably due to the direct reaction of acetate with CoA-SH to form acetyl-CoA, which subsequently is oxidised to CO₂through the TCA cycle. In the intact animal, the CO₂would be present in the form of blood bicarbonate and as the blood bicarbonate is turned over, there is a subsequent release of CO₂from the lungs. The slower, secondary rate of elimination is probably derived from acetate carbons which are incorporate into other metabolites, such as fatty acids and amino acids and are subsequently catabolised to CO₂.