Registration Dossier

Administrative data

Hazard for aquatic organisms

Hazard for air

Hazard for terrestrial organisms

Hazard for predators

Additional information

Trimethylsilane (CAS 993-07-7) is a gaseous silane with a low molecular weight (74.2), a low log Kow(2.2), and a high predicted water solubility (1400 mg/l, although this concentration in water is not expected to be achieved due to the very high volatility of the substance). The substance is expected to undergo oxidation reactions rather than hydrolysis, although this is not certain. The registration substance will react in contact with water and atmospheric moisture to form trimethylsilanol (CAS 1066-40-6) and hydrogen with a half-life of about 3 days at 24.7°C. The fact that trimethylsilane is a gas means that it is likely to volatilise before it has the chance to dissolve in water, and its more stable silanol reaction product is likely to be present in the water. It is not feasible to maintain significant concentrations of the registration substance in aqueous solution in an open (particularly aerated) system. The environmental hazard assessment is therefore based on the properties of the silanol reaction product trimethylsilanol. As described in Section 4.8 of IUCLID, the silanol reaction products may be susceptible to condensation reactions.

READ-ACROSS JUSTIFICATION

In order to reduce testing read-across is proposed to fulfil up to REACH Annex VII requirements for the registered substance from substances that have similar structure and physicochemical properties. Ecotoxicological studies are conducted in aquatic medium or in moist environments; therefore the reactivity of the substance in water is particularly important, because after reaction in water occurs the resulting product has different structural features, physicochemical properties and behaviour.

 

The registration substance and the substances used as surrogate for read-across are part of a class of low functionality compounds acting via a non-polar narcosis mechanism of toxicity. The group of organosilicon substances in this group contain alkyl, aryl, alkoxy or hydroxy groups attached to the silicon atom when present in aqueous solution. Secondary features may be present in the alkyl chain (e.g. halogen, nitrile, unsaturated bonds) that do not affect the toxicity of the substances. Additional information is given in a supporting report (PFA 2016y) attached in Section 13.

 

The registration substance reacts in water, and therefore the selection of surrogate substances is based on the properties of the silanol reaction product. As discussed in Section 5.1.2, an abiotic stability in water study was conducted and the half-life in water of trimethylsilane was reported as 4.2 days by the study author. A review of the study showed that the reaction sped up throughout the test and a half-life of about 3 days at 24.7°C has since been determined. However, it is unclear whether the reaction measured during the test is a result of hydrolysis or oxidation. Oxygen levels were not reported in the study, and if the reaction is due to oxidation, the amount of oxygen present could affect the rate of reaction to trimethylsilanol as well as the ability to maintain concentrations of the test substance. In addition, the highly volatile nature of the substance could play a role in the behaviour of the substance in water. It is therefore considered likely that the majority of the parent substance will volatilise during its residence in the Waste Water Treatment Plant (WWTP). For these reasons, the hazard assessment is based upon the toxicity of the silanol reaction product trimethylsilanol.

 

A structural analogue of trimethylsilane is triethylsilane (CAS 617-86-7) and this substance is due to be tested for future registration purposes. When these data become available it may be useful to read-across to trimethylsilane and review the dataset. Structural information and physicochemical properties for triethylsilane are therefore discussed.

 

In the following paragraphs the read-across approach is assessed for the surrogate substances taking into account structure, reaction rate and physicochemical properties. Table 1 presents relevant physicochemical properties and the available ecotoxicological data.

 

Table 1: Summary of physicochemical and ecotoxicological properties of the registered and surrogate substances.

CAS Number

993-07-7

1066-40-6

617-86-7

Chemical Name

Trimethylsilane

Trimethylsilanol

Triethylsilane

Si reaction product

Trimethylsilanol

n/a (hydrolytically stable)

Triethylsilanol

Molecular weight (parent)

74.20

90.2

116.28

Molecular weight (reaction product)

90.2

n/a

132.28

log Kow (parent)

2.2

1.2

3.6 (20°C, QSAR prediction)

log Kow (silanol reaction product)

1.2

n/a

2.6 (20°C, QSAR prediction)

Water sol (parent)

1400 mg/l (QSAR prediction)

995 mg/l (measured), limited by condensation reactions

6 mg/l

Water sol (silanol reaction product)

995 mg/l (measured),

limited by condensation reactions

n/a

1E+06 mg/l (QSAR prediction), limited to approximately 30 mg/l due to condensation reactions.

Vapour pressure (parent)

>100000 Pa (20°C)

1290 Pa (20°C)

1900 Pa (25°C)

2700 Pa (25°C) (QSAR prediction)

Vapour pressure (reaction product)

1290 Pa (20°C)

1900 Pa (25°C)

n/a

20 Pa at 25°C (QSAR prediction)

Reaction t1/2 at pH 7 and 25°C

3 days at 24.7°C (pH unknown)

n/a

377 h (20°C)

Reaction t1/2 at pH 4 and 25°C

3 days at 24.7°C (pH unknown)

n/a

53.8 h (20°C)

Reaction t1/2 at pH 9 and 25°C

3 days at 24.7°C (pH unknown)

n/a

56.8 h (20°C)

Short-term toxicity to fish (LC50)

n/a

271 mg/l

n/a

Short-term toxicity to aquatic invertebrates (EC50)

n/a

124 mg/l

No data available

Algal inhibition (ErC50 and NOEC)

n/a

EC50: >750 and NOEC: 50 mg/l

No data available

Long-term toxicity to fish (NOEC)

n/a

n/a

n/a

Long-term toxicity to aquatic invertebrates (NOEC)

n/a

n/a

n/a

Long-term sediment toxicity (NOEC)

n/a

n/a

n/a

Short-term terrestrial toxicity (L(E)C50)

n/a

n/a

n/a

Long-term terrestrial toxicity (NOEC)

n/a

n/a

n/a

 

 

Read-across from trimethylsilanol to trimethylsilane

Trimethylsilane (CAS 993-07-7) will react in water to form trimethylsilanol (CAS 1066-40-6) and hydrogen. As discussed previously, the mechanism is uncertain and may be due to oxidation reactions. The reaction rate half-life is about 3 days. The environmental hazard assessment is based on the silanol reaction product as it is more likely be present in the environment in the longer term due to the high volatility of the parent substance. Trimethylsilanol is part of a category of organosilicon substances containing only alkyl, halogen, alkoxy or silanol groups attached to the Si atom. The category has a low hazard profile and data are consistent with a simple polar narcosis mechanism. Trimethylsilanol is soluble in water (measured value of 995 mg/l), has a low molecular weight (90.2) and low log Kow (1.2). The E(L)C50 values for trimethylsilanol indicate that it is not toxic to freshwater fish, invertebrates and algae, as shown in Table 1. Trimethylsilanol is unlikely to biodegrade.

 

Read-across from triethylsilane to trimethylsilane

The registration substance, trimethylsilane (CAS 993-07-7) is a silane with one hydrogen and three methyl groups bound to the silicon atom. It reacts in water to form trimethylsilanol and hydrogen, has low molecular weight (74.2) and low log Kow (2.2). It is soluble in water (1400 mg/l (QSAR prediction)), however, the registration substance is a gas and is likely to evaporate and therefore the concentrations in aquatic media may be low. The read-across substance triethylsilane (CAS 617-86-7) is also a silane but with three ethyl groups bound to the silicon atom. Triethylsilane will react in water to form triethylsilanol. Triethylsilane also has a low molecular weight (116.28) and low predicted log Kow (3.6). It has a predicted water solubility of 6 mg/l at 20 - 25°. There are no aquatic toxicity data available for this substance at the moment but testing is planned and may be used for read-across to trimethylsilane when available.

Consideration of the non-silanol hydrolysis product hydrogen

The other hydrolysis product, hydrogen gas, is a ubiquitous element present in the atmosphere at 0.55 ppmV. Any anthropogenic contribution of hydrogen gas to the atmosphere from hydrolysis of trimethylsilane is negligible therefore the substance is not considered further.

Conclusion on classification

The substance has reliable short-term E(L)C50 values of 124 mg/l for invertebrates and >750 mg/l for algae as well as a NOEC value of 50 mg/l for algae, based on read-across from the silanol reaction product trimethylsilanol, upon which this hazard assessment is based.

Trimethylsilanol is not readily biodegradable.  

These data are consistent with the following classification under Regulation (EC) No 1272/2008 (as amended) (CLP):

Acute toxicity: Not classified.

Chronic toxicity: Not classified