tert-butyl-dimethyl-piperazin-1-ylsilane

Endpoint summary

Description of key information

The substance, 1-(tert-butyldimethylsilyl)piperazine is not stable in water, which affects the approach to the determination of physicochemical properties.

1-(Tert-butyldimethylsilyl)piperazine is a liquid at ambient temperature and pressure. It has a predicted density of 0.81 g/cm³ at 20°C and a predicted vapour pressure of 3.2 Pa at 25°C.

1-(Tert-butyldimethylsilyl)piperazine is not explosive and not oxidising on the basis of chemical structure.

In contact with water, the substance hydrolyses very rapidly at 25°C to form tert-butyldimethylsilanol and piperazine according to the following equation:

C₁₀H₂₄N₂Si + H₂O → C₆H₁₆OSi + C₄H₁₀N₂

Therefore, requirements for testing of water-based physicochemical properties for the substance are waived on the basis of instability in water. The properties of the silanol hydrolysis product, tert-butyldimethylsilanol and piperazine are assessed instead.

Piperazine is freely soluble with water (O'Neil M J (ed) (2001)), has a low log K_ow of -1.5 (Hansch et al 1995) and vapour pressure value of 21 Pa at 20°C (Kirk-Othmer 1978). Piperazine is positively charged at neutral pH and it is expected to bind theoretically to negatively charged soil particles and humus.

The silanol hydrolysis product; tert-butyldimethylsilanol, may undergo condensation reactions in solution to give siloxane dimers and a dynamic equilibrium is established. The overall rate and extent of condensation is dependent on nominal loading, temperature, and pH of the system, as well as what else is present in the solution. It may also change over time.

The condensation reactions of monosilanols may be modelled as an equilibrium between monomer and dimer. The reaction is reversible unless the dimer concentration exceeds its solubility; in this case, the dimer forms a separate phase, driving the equilibrium towards the dimer. For tert-butyldimethylsilanol, a solution at 100 mg/l (the highest concentration often used in ecotoxicity tests) is predicted to contain >99.9% monomer. At loadings above about 80 mg/l the concentration of the dimer is predicted to exceed its solubility, resulting in the formation of a separate phase. In addition, the dimer is expected to have a high volatility from water and this may cause losses from water under some conditions. Further information is given in a supporting report (PFA 2016am) attached in Section 13.

The water solubility of the silanol hydrolysis product (tert-butyldimethylsilanol) has been calculated using a validated QSAR estimation method to be 970 mg/l. However, the saturation concentration in reality may be limited by the formation of less soluble dimer rather than the solubility of the monomer itself. It has a predicted log K_ow of 2.5 at 20°C and predicted vapour pressure of 64 Pa at 25°C.

References:

Hansch, C., Leo, A., D. Hoekman. Exploring QSAR - Hydrophobic, Electronic, and Steric Constants. Washington, DC: American Chemical Society., 1995., p. 10

Kirk-Othmer (1978). Kirk-Othmer Encyclopedia of Chemical Technology. 3rd ed., Volumes 1-26. New York, NY: John Wiley and Sons, 1978-1984., p. V2: 296 (1978)

O'Neil M J (ed.). (2001). The Merck Index-An Encyclopedia of chemicals, Drugs and Biologicals. 13th Edition, Whitehouse Station, NJ: Merck and Co., Inc. 2001. pp 1337)

PFA (2016am). Peter Fisk Associates, Silanols and aquatic systems, 404.105.003