1,1-dimethyl-N,N'-bis(1-methylpropyl)silanediamine

Ecotoxicological Summary

Additional information

The registration substance 1-dimethyl-N,N'-bis(1-methylpropyl)silanediamine (CAS 93777-98-1) will hydrolyse very rapidly in contact with water and atmospheric moisture (half-life <2 minutes at pH 7, 25 °C) to form dimethylsilanediol (1 mole, CAS 1066-42-8) and sec-butylamine (2 moles, CAS 13952-84-6). 

 

No test data are available with the registration substance. Data have been read across from the structurally similar substance 1-methyl-N,N',N''-tris(1-methylpropyl)silanetriamine (CAS 37697-65-7) to assess the toxicity of the parent substance.  

Short-term tests have been conducted with 1-methyl-N,N',N''-tris(1-methylpropyl)silanetriamine (CAS 37697-65-7). Fish were tested up to a maximum concentration of 100 mg/l, where turbidity of test solutions was observed throughout the test. However, no effects were observed. Also short-term toxicity to Daphnia and algal tests have been conducted with the parent substance. In these studies a design was selected to avoid overloading of the test system and to determine the maximum achievable concentration in test media considered equivalent to the occurrence of turbidity. The maximum achievable concentrations were found as <3.75 mg/l for Daphnia and <2.5 mg/l for algae in a pretest. 3.75 mg/l and 2.5 mg/l were therefore set as the highest test concentrations for Daphnia and algae, respectively. No effects on the test organisms were observed.

Classification and Labelling under Regulation (EC) No 1272/2008 (as amended) (CLP) for the registration substance is derived from the results of these tests with 1-methyl-N,N',N''-tris(1-methylpropyl)silanetriamine (CAS 37697-65-7), based on the maximum technically achievable concentration of the parent substance.

 

The silanol hydrolysis product of the registration substance, dimethylsilanediol (CAS 1066-42-8), has a predicted water solubility of 1000000 mg/l and a log K_ow of -0.4. Dimethylsilanediol may undergo condensation reactions in water. The silanol hydrolysis product of the read-across substance, methylsilanetriol, may also undergo condensation reactions and this obviously became important in the Daphnia and Algae test system and limited the concentration of test substance dissolved in water. 

The non-silanol hydrolysis product, sec-butylamine (CAS 13952-84-6), is miscible in water and has a log K_ow  of 0.74.

Amines are basic, therefore once dissolved they will increase the pH of the test media. The un-ionised form of the amine is more toxic than the ionised form, therefore toxicity of the amine increases with increasing pH. The difference in toxicity is attributed to bioavailability, where the un-ionised form can cross cell membranes more readily than the ionised form. Consequently, pH plays an important role in the toxicity of amine substances. 

 

READ-ACROSS JUSTIFICATION

To reduce testing REACH recommends to make use of a read-across approach where appropriate based on the high accordance in properties relevant for the specific endpoint. In the case of short-term aquatic testing relevant properties are structural similarity, hydrolysis rate and the physical-chemical parameters in the same range. In the following paragraphs the proposed read-across from 1-methyl-N,N',N''-tris(1-methylpropyl)silanetriamine (CAS 37697-65-7) to 1,1-dimethyl-N,N'-bis(1-methylpropyl)silanediamine (CAS 93777-98-1; target substance) is evaluated point by point.

 

The measured hydrolysis half-lives of the registration target substance, 1,1-dimethyl-N,N'-bis(1-methylpropyl)silanediamine (CAS 93777-98-1), at 25°C, are <2 minutes at pH 4, 7 and 9. The products of hydrolysis are sec-butylamine (2 moles; CAS 13952-84-6) and dimethylsilanediol (1 mole; CAS 1066-42-8).

 

The source substance 1-methyl-N,N',N''-tris(1-methylpropyl)silanetriamine (CAS 37697-65-7) has measured hydrolysis half-lives at 25°C of < 2 minutes at pH 4 and pH 7 and 5.0 min at pH 9. The hydrolysis products are sec-butylamine (3 moles; CAS 13952-84-6) and methylsilanetriol (1 mole; CAS 2445-53-6).

 

No water solubility or K_ow values are available for the parent substances; measurement would not be technically feasible as they hydrolyse extremely rapidly in water. Both substances hydrolyse rapidly to produce an amine, sec-butylamine. This substance (as for other amines) is basic, with a pKa of >9.5 (SIDS INITIAL ASSESSMENT PROFILE for C1 -13 Primary Amines; SIAM 32, 19-21 April 2011). Therefore, solutions of sec-butylamine will be of very high pH (for example a 50% solution of the structurally related substance n-butylamine has a pH of 13 according to the ECHA disseminated dossier for n-butylamine, CAS 109-73-9).

Read-across from 1-methyl-N,N',N''-tris(1-methylpropyl)silanetriamine (CAS 37697-65-7) to 1,1-dimethyl-N,N'-bis(1-methylpropyl)silanediamine (CAS 93777-98-1) for aquatic toxicity is valid on the basis of structural similarity, similarity in physicochemical properties, and very rapid hydrolysis to a common hydrolysis product and structurally similar hydrolysis products.

 

Read-across approach justification

(a) Structural similarity 

The target and source substances are structurally similar silylamines. They each have one or two methyl groups and two or three 1-methylpropylamine groups bound to a central silicon atom. The difference between the target and source substance is that the target substance has two methyl groups and two 1-methylpropylamine groups bound to silicon, whereas the source substance has one methyl group and three 1-methylpropylamine groups.  Both substances hydrolyse extremely rapidly (half-life for full hydrolysis <2 mins at pH 4 and 7, ≤5 mins at pH 9) to produce the silicon-containing hydrolysis products, dimethylsilanediol or methylsilanetriol (1 mole) and the common non-silicon hydrolysis product sec-butylamine (2 moles for the target substance, 3 moles for the source substance). Dimethylsilanediol and methylsilanetriol are structurally similar, being low molecular weight alkylsilanols with one or two methyl groups and two or three hydroxy groups bound to a central silicon atom.

 

(b) Similar physicochemical characteristics

The similarity of the physicochemical properties and hydrolysis rates at pH relevant to aqueous exposure is important. Both parent substances hydrolyse very rapidly to give a common product, sec-butylamine, and silanols that are structurally similar and have similar physicochemical properties. The silanols are not expected to show aquatic toxicity. The key physicochemical parameters are summarised below.

 Table: Key physicochemical properties of the source and target substances

	Source	Target
Name	1-Methyl-N,N',N''-tris(1- methylpropyl)silanetriamine	1,1-Dimethyl-N,N'-bis(1- methylpropyl)silanediamine
CAS	37697-65-7	93777-98-1
EC	253-634-7	298-130-8
Amine leaving group	sec-butylamine (CAS 13952-84-6) 3 moles	sec-butylamine (CAS 13952-84-6) 2 moles
Si hydrolysis product	methylsilanetriol (CAS 2445-53-6) 1 mole	dimethylsilanediol (CAS 1066-42-8). 1 mole
Molecular weight	259.51	202.42
Hydrolysis t1/2 at pH 7 and 25°C	< 2 min	< 2 min
Hydrolysis t1/2 at pH 4 and 25°C	< 2 min	< 2 min
Hydrolysis t1/2 at pH 9 and 25°C	5 min	< 2 min
Vapour pressure amine leaving group	24000 Pa at 25°C	24000 Pa at 25°C
Water solubility amine leaving group	1.1E+05 mg/l	1.1E+05 mg/l
Log Kow amine leaving group	0.74	0.74
pKa amine leaving group	>9.5	>9.5
Vapour pressure silanol hydrolysis product	0.05 Pa at 25°C (QSAR)	7 Pa at 25°C (QSAR)
Water solubility silanol hydrolysis product	1E+06 mg/l (limited to around 1000 mg/l by condensation reactions)	1E+06 mg/l (QSAR) (limited to around 1000 mg/l by condensation reactions)
Log Kow silanol hydrolysis product	-2.4 (QSAR)	-0.4

 

(c) Effects of the hydrolysis product, sec-butylamine

Both the target and source substances hydrolyse extremely rapidly to generate sec-butylamine. This substance is Classified in Annex VI of Regulation (EC) No 1272/2008 (CLP Regulation) as Aquatic Acute 1, H400. The source substance produces 3 moles of sec-butylamine per mole of parent substance (85% by weight). The target substance produces 2 moles of sec-butylamine per mole of parent substance (72% by weight). Therefore, the read-across from the source to the target substance can be considered conservative.

Conclusion on classification

Data read-across from the structurally similar substance, 1-methyl-N,N',N''-tris(1-methylpropyl)silanetriamine (CAS 37697-65-7), determine short-term E(L)C₅₀ values of >100 mg/l in fish, >3.75 mg/l in invertebrates and >2.5 mg/l in algae. It has a reliable NOEC of 2.5 mg/l in algae.

The classification is based on the maximum technically achievable concentration of the parent substance.

The substance hydrolyses rapidly in water. The products of hydrolysis are soluble and have low log K_ow. The silanol hydrolysis product is not readily biodegradable; sec-butylamine is 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.