Aluminium tri-sec-butanolate

Hydrolysis

Description of key information

For the aluminium alkoxides, there is a two-step process of degradation. The first step is the hydrolysis of the aluminum alkoxides to their constituent linear alcohols and alumina.  

The reaction mechanism is described in depth in Brinker and Scherer (1990), who indicate that under neutral, acid or base conditions it is expected that the hydrolysis and condensation reactions would be quite rapid. While no rates of hydrolysis for aluminum alkoxides are available in the literature, data for silicone alkoxide show rapid hydrolysis (log Kspont) in aqueous systems. Log Kspont values within the pH range 4 to 10 vary from approximately -4 at neutral pH to -2 at both acid and base pH. Since this reaction is acid catalyzed, the rate at pH 1 is anticipated to extend even further upward from the "acidic portion" of the curve. 

Based on reaction dynamics, aluminium alkoxide would be expected to be even faster than silicone alkoxide under both environmental and physiological conditions (Brinker and Scherer 1990).

Under neutral conditions, it is expected that both hydrolysis and condensation of aluminium alkoxides occur by nucleophilic addition, followed by proton transfer and elimination of either water or alcohol in a manner analogous to transition metal alkoxides. Likewise, both of these reactions are catalyzed by addition of either acid or base. Acids protonate OR or OH ligands creating good leaving groups and eliminating the requirements for proton transfer in the intermediate. Bases deprotonate water or OH ligands, creating strong nucleophiles. (Brinker and Scherer 1990).

The findings as described above are also observed in a limited reported hydrolysis study on the structural analogue aluminium tributanolate (Ineos 2018).

0.009 mol of aluminium tributanolate was added to 5.345 mol water (initial concentration 22.8 g/L) under continuous stirring. The resulting solution was hazy white. After further vigorously stirring for 4 hours, the solution was filtered to remove insoluble (aluminium compounds) and a sample of the resulting solution was analysed by gas-chromatography (no details available) for 1-butanol. The analyses showed that 1-butanol was present at 2.1%, which confirms almost complete hydrolysis. Temperature measurements did not show an increase in temperature of the initial solution during the stirring time, which would have been expected for the exothermic hydrolysis reaction. It seems, however, that the exothermic effect at the low concentration tested has been minimal.
No information on pH changes during the reaction is available, but in a similar limited reported test with the substance it self (Sasol 2000) no effects on pH were reported either (pH 8.5). It is shown that at this pH the aluminium species present are mainly Al(OH)₄^-, Al(OH)₃and Al(OH)²⁺(see Langmuir et al. 2004, Issue paper on the environmental chemistry of metals, US EPA Contract #68 -C-02 -060). The identity of aluminium^{3 +} species was not determined either of the experiments available.

Key value for chemical safety assessment

Half-life for hydrolysis:

4 h