Tributyl borate

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: data from peer-reviewed publication

Qualifier:

no guideline followed

Principles of method if other than guideline:

Test substances were prepared in mixtures of dioxane and water and the rate of degradation of the test substances were followed colormetrically. The dioxane was added to increase the solubility of the test substances.

GLP compliance:

no

Analytical monitoring:

not required

Preliminary study:

not applicable

Test performance:

1) Hydrolysis in 60% (Vol.) aqueous Dioxane at 21° C:
The following procedure was developed for the measurement of the hydrolysis rates. A weighed sample of ester (0.4 to 1.2 mmoles) was dissolved in 15 mL of purified dioxane. A solution of 10 mL of dioxane, 3 drops of phenolphthalein, 15 mL of a 9.09 weight % solution of mannitol in water, and one half of the amount of 0.2457 or 0.1130N sodium hydroxide (approximately 2 mL) necessary for neutralization of the boric acid resulting from complete hydrolysis were added at zero time. The resulting solution was swirled to effect homogeneity and the time for fade of the indicator was recorded as the half life. A blank determination with 1 mmol of boric acid and 0.5 meq. of sodium hydroxide under the above conditions was too fast to measure.

2) Hydrolysis in water at 21° C (heterogeneous):
In order more closely to approximate actual conditions of possible applications of the boric acid esters, the esters were subjected to hydrolysis by agitation in
water. A solution of 50 mL of water, 5 g of mannitol, 4 drops of phenolphthalein, and one half the amount of 0.2457 or 0.1130N sodium hydroxide necessary to neutralize the boric acid resulting from complete hydrolysis was added to a weighed sample of ester. The mixtures were agitated, and the time for fade of the indicator was recorded as the half time.

Transformation products:

not specified

No.:

#1

No.:

#2

Details on hydrolysis and appearance of transformation product(s):

Tributyl borate is rapidly hydrolyzed to boric acid and n-butanol in the presence of water.

Data source:
(1) Lewis RJ, Sr, ed; Hawley's Condensed Chemical Dictionary. 13th ed. NY, NY: John Wiley & Sons, Inc. p. 1122 (1997);
(2) Docks EL; in Kirk-Othmer Encycl Chem Technol. 4th ed. NY, NY: John Wiley and Sons, 4: 414 (1992).

Key result

Temp.:

21 °C

DT50:

18.3 s

Type:

not specified

Remarks on result:

other: pH not specified

Validity criteria fulfilled:

not applicable

Conclusions:

Tributyl borate hydrolyzes very rapidly in water with a half-life of 18.3 sec at 21 °C.

Executive summary:

Boric acid esters (i.a. Tri-n-butyl borate) were prepared in mixtures of dioxane and water and the rate of degradation was followed colormetrically. Sodium hydroxide necessary for neutralization of the boric acid resulting from complete hydrolysis were added at zero time. The dioxane was added to increase the solubility of the test substances.

Tri-n-butyl borate hydrolyzes very rapidly

- in water with a half-life of 18.3 sec at 21 °C.

- in 60% aqueous Dioxane with a half-life of less than 1 sec at 21 °C (and it was too fast to measure, respectively).

Description of key information

- Tributyl borate hydrolyzes very rapidly in water with a half-life of 18.3 sec at 21 °C.

- Hydrolysis products: n-Butanol and Boric acid.

- Intermediates / hydrolysis products monobutyl and dibutyl borate: It is safe to say that the two potential hydrolysis products have a very low hydrolysis stability as well. Borate esters are in general not very hydrolysis-resistant since stability is easily attacked by water due to the lack electrons of the B atom. In conclusion it should be sufficient to solely consider the final hydrolysis products n-butanol and boric acid.

Key value for chemical safety assessment

Half-life for hydrolysis:

18.3 s

at the temperature of:

21 °C

Additional information

Borate esters, like tributyl borate, are in general not very hydrolysis-resistant.

The relative rates of the aliphatic esters are in the order predicted by the steric requirements in the nucleophilic attack of water (or hydroxyl ion) on the central boron atom. The greater the bulk of the alcohol the slower the rate of hydrolysis. Thus in the normal alkyl borates: methyl~ethyl~propyl~butyl > amyl > hexyl > octyl > dodecyl > stearyl; branching of the alkyl chain produces a further decrease in rate:

No.	Borate Ester	Half Time
1	Trimethyl borate	too fast to measure
2	Triethyl borate	7.8 sec
3	Tri-n-hexyl borate	11.8 sec
4	Tri-n-amyl borate	7.7 sec
5	Tri-n-butyl borate	18.3 sec
6	Tri-n-octyl borate	21.7 sec
7	Tri-n-propyl borate	38.8 sec
8	Tri-(2-ethyIhexyI) borate	70.0 sec
9	Triisobutyl borate	82.5 sec
10	Tri-n-dodecyl borate	9.60 min
11	Triisopropyl borate	9.68 min
12	Tri-n-octyl borate	16.1 min
13	Tri-sec-butyl borate	53.7 min
14	Tristearyl borate	58.0 min
15	Tri-tert-butyl borate	2.81 hr

Table: Hydrolysis Borate esters, examples

Accordingly, hydrolysis would be expected to be the dominate fate process for (aliphatic) borate esters, like tributyl borate, in the environment.

Tributyl borate is rapidly hydrolyzed to boric acid and n-butanol in the presence of water, whereas

- the hydrolysis product Boric acid is an inorganic compound that is not subject to hydrolysis (and bio-/photodegradation),

- the hydrolysis product n-butanol is rapidly biodegraded in water and does not persist in the environment.