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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis half-lives

Constituent 1: 0.3 h at pH 4, 4.0 h at pH 7 and 0.1 h at pH 9 and 20-25°C (QSAR)

Constituent 2: 0.2 h at pH 4, 10.0 h at pH 7, and 0.2 h at pH 9 and 20 -25°C (QSAR)

Key value for chemical safety assessment

Additional information

The submission substance is a multi-constituent substance containing two constituents, (trimethoxysilylpropyl)methylmethylphosphonate (Constituent 1) and bis(trimethoxysilylpropyl)methylphosphonate (Constituent 2). Constituent 1 has five hydrolysable groups; three methoxy groups and two phosphonate ester groups. Constituent 2 has eight hydrolysable groups; six methoxy groups and two phosphonate ester groups. The chemical safety assessment of the substance assumes that these reaction processes will proceed independently based on their rates under the relevant conditions. The hydrolysis reactions for both constituents are illustrated in the attached figure in Section 13.

Phosphonate esters are stable under neutral conditions and hydrolyse only slowly under alkaline conditions (Keay L 1965; Mabey and Mill 1978). The half-lives of the trimethoxy groups can be predicted using validated QSAR methods. The predicted half-lives for Constituent 1 are 4.0 h at pH 7, 0.3 h at pH 4 and 0.1 h at pH 9 and 20-25°C. The predicted half-lives for Constituent 2 are 10.0 h at pH 7, 0.4 h at pH 4 and 0.2 h at pH 9 and 20-25°C. Therefore, for the purposes of the environmental assessment, the hydrolysis products for assessment are:

• methyl 3-(trihydroxysilyl)propyl methylphosphonate (1 mole from Constituent 1; HP1)

• bis[3-(trihydroxysilyl)propyl] methylphosphonate (1 mole from Constituent 2; HP2)

• methanol (3 moles from Constituent 1; 6 moles from Constituent 2)

Over a long timescale, hydrolysis of the phosphonate ester groups may occur. The final hydrolysis products from both constituents are (3-hydroxypropyl)silanetriol (1 mole from Constituent 1; 2 moles from Constituent 2) and methylphosphonate (1 mole from each constituent).

Under acid conditions, hydrolysis of the phosphonate ester bonds is expected to be much faster (Keay L 1965) and could be comparable to or faster than the rate of the methoxy hydrolysis.

 

Reference:

Mabey W. and Mill T. (1978). Critical review of hydrolysis of organic compounds in water under environmental conditions. J. Phys. Chem. Ref. Data, 7, 383-415

Keay Leonard (1965). The preparation and hydrolysis of alkyl hydrogen metylphosphonates. Can J. of Chem, Vol 43, 2637-2639