Tetrapropyl orthosilicate

Endpoint summary

Description of key information

In an acute oral toxicity study conducted to OECD 423 and to GLP (LPT, 2002a) the oral LD50 value for tetrapropyl orthosilicate in CD rats was established to exceed 2000 mg/kg bw. There were no clinical signs of toxicity, affects on body weights or macroscopic findings. 
There are no acute inhalation data on tetrapropyl orthosilicate therefore good quality data have been read-across from the structurally-related tetraethyl orthosilicate. 
The LC50 value of 10.0 mg/l tetraethyl orthosilicate (aerosol) in male rats and >16.8 mg/l in female rats was determined in a reliable study (Hoechst AG, 1991) conducted according to an appropriate test protocol, and in compliance with GLP.
There are no acute dermal data. 

Key value for chemical safety assessment

Acute toxicity: via oral route

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed

Dose descriptor:

LD50

Value:

2 000 mg/kg bw

Acute toxicity: via inhalation route

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed

Dose descriptor:

LC50

Value:

10 000 mg/m³ air

Acute toxicity: via dermal route

Endpoint conclusion

Endpoint conclusion:

no study available

Additional information

There is one reliable acute oral study on tetrapropyl orthosilicate. There are no acute inhalation data on tetrapropyl orthosilicate therefore good quality data have been read-across from the structurally-related tetraethyl orthosilicate (CAS 78-10-4).

In the absence of measured data for tetrapropyl orthosilicate, it is considered appropriate to use this result in support of the acute inhalation toxicity endpoint. The registered substance and read-across substance are both moderately lipophilic substances (log K_ow 3.4 and 3.0, respectively) for which uptake across the respiratory tract is favourable. The vapour pressures of the substance (0.26 Pa (predicted) and 110 Pa (measured)) indicate that exposure by the inhalation route is potentially more significant for the read-across substance and, since the study used an aerosol, it can be considered as a worst-case in respect of likely exposure.

After uptake both substances hydrolyse rapidly with half-lives of 1.6 and 2.5 hours respectively at pH 7 and 37.5°C forming propanol or ethanol and silicic acid. Neither propanol nor ethanol is acute harmful by the inhalation route, with reported LC₅₀ values of:

42 mg/l for n-propanol (Union Carbide Corp. 1992, cited in ESR Risk Assessment Report for Propan-1 -ol, EC, 2008).

>114 mg/l for ethanol (1-hour, mouse, cited in SIAR, OECD, 2004).

Since the two substances share the common hydrolysis product, silicic acid, and no enhanced toxity would thus be expected from hydrolysis to propanol rather than ethanol at test concentrations relevant to current guideline limit values, it is considered appropriate to read-across the available inhalation result. Read-across is discussed in more detail in Section 7.5.

Justification for selection of acute toxicity – oral endpoint
The selected study is the only acute oral toxicity study available for the registered substance. It was conducted in accordance with an appropriate OECD test guideline and in compliance with GLP.

Justification for selection of acute toxicity – inhalation endpoint
The selected study is the only acute inhalation toxicity study available for an appropriate surrogate substance. It was conducted in accordance with an appropriate OECD test guideline and in compliance with GLP.

Justification for classification or non-classification

The oral LD₅₀ is greater than 2000 mg/kg bw/day, and the inhalation LC₅₀ (aerosol) is 10 mg/l (based on read-across). There are therefore no data to suggest that tetrapropyl orthosilicate should be classified on the basis of its lethality according to Regulation (EC) No 1272/2008.