Dihexyl fumarate

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Study period:

Experimental start date 03 July 2018. Experimental completion date 03 September 2018.

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 111 (Hydrolysis as a Function of pH)

Deviations:

no

GLP compliance:

yes (incl. QA statement)

Analytical monitoring:

yes

Details on sampling:

Preliminary test
The following procedure was carried out at each of pH 4, 7 and 9:
Aliquots (50 μL) of a stock solution of Dihexyl Fumarate (EC 242-833-4) in acetonitrile (7 mg/L) were added to separate Wheaton vials containing buffer solution (5 mL), which had been purged with nitrogen and pre-equilibrated at test temperature (50 ± 0.5°C).
The samples, of nominal concentration 0.07 mg/L, were placed in a water bath at 50°C in the dark until sampling was required (immediately, and then after 2.4, 48 and 120 hours).
At each sampling time, two samples were removed from the water bath and the entire volume diluted by addition of acetonitrile (5 mL) for analysis by high performance liquid chromatography (HPLC).
Sample pH and incubation temperature were monitored over the period of the test.

Extended testing
Separate tests were conducted at three different temperatures (50, 60 and 70˚C at pH 4 and pH 7; 20, 30 and 40°C at pH 9) in the dark, and the procedure followed was as detailed for the preliminary test. The temperature ranges selected were governed by the approximate degradation rates observed in the preliminary test and allowed practical sampling times to be adopted for the definitive testing. Duplicate samples were taken immediately to serve as t0 samples, and then several pairs of samples were analysed in the range 10 to 90% hydrolysis to test for pseudo-first order behaviour.

Buffers:

Preparation of buffer solutions
Buffer solutions were typically prepared as follows:
pH 4: 0.2M aqueous potassium dihydrogen orthophosphate (220 mL) was mixed with 0.07M aqueous disodium hydrogen orthophosphate dodecahydrate (500 mL) and purified water (1280 mL). The pH was adjusted to 4.0 ± 0.05 with orthophosphoric acid.

pH 7: 0.2M aqueous potassium dihydrogen orthophosphate (1250 mL) was mixed with 1M sodium hydroxide (150 mL) and purified water (3600 mL). The pH was adjusted to 7.0 ± 0.05 with 1M hydrochloric acid.

pH 9: 0.1M boric acid in 0.1M aqueous potassium chloride (2500 mL) was mixed with 1M sodium hydroxide (105 mL) and purified water (2395 mL). The pH was measured as 9.0 ± 0.05.

Duration:

120 h

pH:

4

Temp.:

50 °C

Initial conc. measured:

0.081 mg/L

Remarks:

Preliminary Study

Duration:

120 h

pH:

7

Temp.:

50 °C

Initial conc. measured:

0.078 mg/L

Remarks:

Preliminary Study

Duration:

120 h

pH:

9

Temp.:

50 °C

Initial conc. measured:

0.57 mg/L

Remarks:

Preliminary Study

Duration:

120 h

pH:

4

Temp.:

50 °C

Initial conc. measured:

0.068 mg/L

Duration:

46 h

pH:

4

Temp.:

60 °C

Initial conc. measured:

0.066 mg/L

Duration:

7 h

pH:

4

Temp.:

70 °C

Initial conc. measured:

0.07 mg/L

Duration:

96 h

pH:

7

Temp.:

50 °C

Initial conc. measured:

0.067 mg/L

Duration:

24 h

pH:

7

Temp.:

60 °C

Initial conc. measured:

0.073 mg/L

Duration:

20 h

pH:

7

Temp.:

70 °C

Initial conc. measured:

0.074 mg/L

Duration:

24 h

pH:

9

Temp.:

20 °C

Initial conc. measured:

0.059 mg/L

Duration:

19 h

pH:

9

Temp.:

30 °C

Initial conc. measured:

0.059 mg/L

Duration:

6 h

pH:

9

Temp.:

40 °C

Initial conc. measured:

0.064 mg/L

Number of replicates:

2

Positive controls:

no

Negative controls:

no

Statistical methods:

Calculations
The concentration of Dihexyl Fumarate (EC 242-833-4) in the analysed solution (CA) was calculated from standards introduced before and after samples (bracketing standards) by the following equation:
CA (mg/L) =
(sample peak area × standard concentration (mg/L)) / (mean peak area of bracketing standards)

The concentration of Dihexyl Fumarate (EC 242-833-4) in each test solution (CB) was then determined thus:
CB (mg/L) = CA (mg/L) × dilution factor
where the dilution factor was 2 for all samples.

For each experimental condition investigated in the definitive testing, plots of the logarithms (base 10) of the concentrations, Ct, versus time were prepared. From the slopes of the resulting straight lines (assuming pseudo-first order behaviour), the observed rate constant, kobs, was obtained:
kobs = -slope × 2.303

The rate constant at 25°C was estimated using the Arrhenius relationship. Plots of the logarithms (base 10) of the rate constants at the selected temperatures versus inverse temperature (in units of Kelvin) were prepared. The rate constant (k) at 25°C was then calculated by interpolation or extrapolation, as appropriate.
The environmental half-life, te½ (time for 50% of the test item to react at 25°C), was then calculated as follows:
te½ = 0.693/ k

Preliminary study:

The preliminary study showed that at each of pH 4, pH 7 and pH 9 and 50 ± 0.5ºC, more than 10% hydrolysis had occurred after 120 hours (5 days), indicating a te½ value of less than 1 year. This necessitated proceeding to definitive testing at all pH values.

Transformation products:

not measured

Key result

pH:

4

Temp.:

25 °C

Hydrolysis rate constant:

0.002 h-1

DT50:

383 h

Key result

pH:

7

Temp.:

25 °C

Hydrolysis rate constant:

0.001 h-1

DT50:

809 h

Key result

pH:

9

Temp.:

25 °C

Hydrolysis rate constant:

0.377 h-1

DT50:

18 h

pH:

4

Temp.:

50 °C

Hydrolysis rate constant:

0.014 h-1

DT50:

50.4 h

Type:

(pseudo-)first order (= half-life)

pH:

4

Temp.:

60 °C

Hydrolysis rate constant:

0.032 h-1

DT50:

21.9 h

Type:

(pseudo-)first order (= half-life)

pH:

4

Temp.:

70 °C

Hydrolysis rate constant:

0.057 h-1

DT50:

12.1 h

Type:

(pseudo-)first order (= half-life)

pH:

7

Temp.:

50 °C

Hydrolysis rate constant:

0.017 h-1

DT50:

40.9 h

Type:

(pseudo-)first order (= half-life)

pH:

7

Temp.:

60 °C

Hydrolysis rate constant:

0.064 h-1

DT50:

10.8 h

Type:

(pseudo-)first order (= half-life)

pH:

7

Temp.:

70 °C

Hydrolysis rate constant:

0.142 h-1

DT50:

4.9 h

Type:

(pseudo-)first order (= half-life)

pH:

9

Temp.:

20 °C

Hydrolysis rate constant:

0.023 h-1

DT50:

30.4 h

Type:

(pseudo-)first order (= half-life)

pH:

9

Temp.:

30 °C

Hydrolysis rate constant:

0.058 h-1

DT50:

11.9 h

Type:

(pseudo-)first order (= half-life)

pH:

9

Temp.:

40 °C

Hydrolysis rate constant:

0.182 h-1

DT50:

3.8 h

Type:

(pseudo-)first order (= half-life)

Extended testing

At pH 4, definitive tests were conducted at temperatures of 50, 60 and 70°C (Tables 3 to 5, Figures 2 to 4), and the hydrolysis rate constant (k) and te_½at 25°C were extrapolated from the measured values of log₁₀k at the selected temperatures using the Arrhenius relationship (Table 6, Figure 5). A te½ value of 16 days was obtained at this pH value.

At pH 7, definitive tests were conducted at temperatures of 50, 60 and 70°C (Tables 7 to 9, Figures 6 to 8), and the hydrolysis rate constant (k) and te_½at 25°C were extrapolated from the measured values of log₁₀k at the selected temperatures using the Arrhenius relationship (Table 10, Figure 9). A te½ value of 34 days was obtained at this pH value.

At pH 9, definitive tests were conducted at temperatures of 20, 30 and 40°C (Tables 11 to 13, Figures 10 to 12), and the hydrolysis rate constant (k) and te_½at 25°C were interpolated from the measured values of log₁₀k at the selected temperatures using the Arrhenius relationship (Table 14, Figure 13). A te_½value of 0.8 days was obtained at this pH value.

The hydrolysis reactions for Dihexyl Fumarate (EC 242-833-4) at all pH values were shown to follow pseudo-first order behaviour.

Table 1       Standard calibration for Dihexyl Fumarate (EC 242-833-4)

Standard concentration (mg/L)	Peak area
0.5532	200.71
0.2766	98.917
0.1383	49.903
0.02766	9.6571
0.01383	4.7514
0.006915	3.1645
0.003458	1.5152
0.001729	0.91433

Linear regression:       y = 362x - 0.00432                  

(including x=0, y=0)       r = 1.0000

x = concentration

y = peak area

Table 2       Preliminary results for hydrolysis of Dihexyl Fumarate (EC 242-833-4)

	Ct (mg/L)
pH	t0h		t2.4h		t48h		t120h
	Measured	Mean	Measured	Mean	Measured	Mean	Measured	Mean
4	0.082, 0.080	0.081	0.066, 0.066	0.066	0.031, 0.042	0.036	0.021, 0.016	0.018
7	0.079, 0.077	0.078	0.045, 0.072	0.059	0.032, 0.027	0.030	0.014, 0.012	0.013
9	0.060, 0.054	0.057	0.016, 0.022	0.019	ND, ND	ND	ND, ND	ND

Ct = concentration of Dihexyl Fumarate (EC 242-833-4) in solution at time th (in hours)

ND = not detected

Table 3       Results for hydrolysis of Dihexyl Fumarate (EC 242-833-4) at pH 4 and 50°C

Time	Concentration, Ct	Mean Ct	log10Ct
(hours)	(mg/L)	(mg/L)
0	0.06743, 0.06930	0.06837	-1.1652
6	0.06267, 0.05372	0.05820	-1.2351
24	0.03200, 0.04285	0.03743	-1.4268
48	0.02530, 0.02655	0.02593	-1.5863
72	0.02365, 0.02032	0.02199	-1.6579
96	0.01444, 0.01471	0.01458	-1.8364
120	0.01315, 0.03179*	0.01315	-1.8811

*anomalously high value excluded from calculations

Exponential regression, log10Ct = -1.229 - 0.00597t

                             r = -0.9802

rate constant, kobs = -slope x 2.303 = 0.01375 hour-1

Table 4       Results for hydrolysis of Dihexyl Fumarate (EC 242-833-4) at pH 4 and 60°C

Time	Concentration, Ct	Mean Ct	log10Ct
(hours)	(mg/L)	(mg/L)
0	0.06496, 0.06731	0.06614	-1.1796
3.5	*   , 0.05457	0.05457	-1.2630
6.75	0.05540, 0.05192	0.05366	-1.2703
17	0.04101, 0.02919	0.03510	-1.4547
24	0.02926, 0.02587	0.02757	-1.5596
41	0.02575, 0.02464	0.02520	-1.5987
46	0.009921, 0.01278	0.01135	-1.9450

*sample lost during processing

Exponential regression, log10Ct = -1.196 - 0.01373t

                             r = -0.9481

Rate constant, kobs = -slope x 2.303 = 0.03163 hour-1

Table 5       Results for hydrolysis of Dihexyl Fumarate (EC 242-833-4) at pH 4 and 70°C

Time	Concentration, Ct	Mean Ct	log10Ct
(hours)	(mg/L)	(mg/L)
0	0.06991, 0.06995	0.06993	-1.1553
1	0.06584, 0.06341	0.06463	-1.1896
2	0.05980, 0.05959	0.05970	-1.2241
3	0.05659, 0.05989	0.05824	-1.2348
4	0.05220, 0.05473	0.05347	-1.2719
5	0.05209, 0.05011	0.05110	-1.2916
6	0.04755, 0.04843	0.04799	-1.3188
7	0.04765, 0.04634	0.04700	-1.3279

Exponential regression, log10Ct = -1.165 - 0.02493t

                             r = -0.9923

Rate constant, kobs = -slope x 2.303 = 0.05742 hour-1

Table 6       Arrhenius relationship for Dihexyl Fumarate (EC 242-833-4) at pH 4

Temperature		1/T	kobs	log10kobs
°C	K	(K^-1)	(hour^-1)
50	323.15	0.003095	0.01375	-1.862
60	333.15	0.003002	0.03163	-1.500
70	343.15	0.002914	0.05742	-1.241

Exponential regression: log10kobs = 8.818 - 3447 (1/T)

                    

When        T = 25ºC = 298.15K

             

                 k = 0.001809 hour-1

              te½ = 0.693/k

                   = 383 hours (16 days)

Table 7       Results for hydrolysis of Dihexyl Fumarate (EC 242-833-4) at pH 7 and 50°C

Time	Concentration, Ct	Mean Ct	log10Ct
(hours)	(mg/L)	(mg/L)
0	0.06651, 0.06721	0.06686	-1.1748
6	0.05624, 0.04347	0.04986	-1.3023
18	0.03763, 0.03635	0.03699	-1.4319
24	0.02922, 0.02878	0.02900	-1.5376
48	0.02019, 0.02203	0.02111	-1.6755
72	0.01373, 0.02368	0.01871	-1.7280
96	0.01006, 0.01121	0.01064	-1.9733

Exponential regression, log10Ct = -1.269 - 0.00736t

                             r = -0.9699

rate constant, kobs = -slope x 2.303 = 0.01696 hour-1

Table 8       Results for hydrolysis of Dihexyl Fumarate (EC 242-833-4) at pH 7 and 60°C

Time	Concentration, Ct	Mean Ct	log10Ct
(hours)	(mg/L)	(mg/L)
0	0.07331, 0.07347	0.07339	-1.1344
3	0.05377, 0.05099	0.05238	-1.2808
4	0.04901, 0.04438	0.04670	-1.3307
5.5	0.04619, 0.04327	0.04473	-1.3494
6.75	0.03780, 0.03582	0.03681	-1.4340
17	0.02196, 0.004340*	0.02196	-1.6584
24	0.01611, 0.01221	0.01416	-1.8489

*anomalously low value excluded from calculations

Exponential regression, log10Ct = -1.194 - 0.02783t

                             r = -0.9898

Rate constant, kobs = -slope x 2.303 = 0.06409 hour-1

Table 9       Results for hydrolysis of Dihexyl Fumarate (EC 242-833-4) at pH 7 and 70ºC

Time	Concentration, Ct	Mean Ct	log10Ct
(hours)	(mg/L)	(mg/L)
0	0.07231, 0.07509	0.07370	-1.1325
1	0.05828, 0.05371	0.05600	-1.2519
3	0.03740, 0.04224	0.03982	-1.3999
5	0.03213, 0.03166	0.03190	-1.4963
6	0.02964, 0.03097	0.03031	-1.5185
7	0.02852,    *	0.02852	-1.5449
17	0.004905, 0.005688	0.005297	-2.2760
20	0.004593, 0.004144	0.004369	-2.3597

*unstable chromatography, sample rejected

Exponential regression, log10Ct = -1.168 - 0.06157t

                             r = -0.9954

Rate constant, kobs = -slope x 2.303 = 0.1418 hour-1

Table 10       Arrhenius relationship for Dihexyl Fumarate (EC 242-833-4) at pH 7

Temperature		1/T	kobs	log10kobs
°C	K	(K^-1)	(hour^-1)
50	323.15	0.003095	0.01696	-1.771
60	333.15	0.003002	0.06409	-1.193
70	343.15	0.002914	0.1418	-0.8483

Exponential regression: log10kobs = 14.12 - 5125 (1/T)

                    

When        T = 25ºC = 298.15K

             

                 k = 0.000856 hour-1

              te½ = 0.693/k

                   = 809 hours (34 days)

Table 11       Results for hydrolysis of Dihexyl Fumarate (EC 242-833-4) at pH 9 and 20ºC

Time	Concentration, Ct	Mean Ct	log10Ct
(hours)	(mg/L)	(mg/L)
0	0.05952, 0.05885	0.05919	-1.2278
3	0.05254, 0.05294	0.05274	-1.2779
5	0.05177, 0.05165	0.05171	-1.2864
6	0.04955, 0.04930	0.04943	-1.3061
18	0.03659, 0.03838	0.03749	-1.4261
24	0.03480, 0.03307	0.03394	-1.4694

Exponential regression, log10Ct = -1.240 - 0.00990t

                             r = -0.9955

rate constant, kobs = -slope x 2.303 = 0.02281 hour-1

Table 12       Results for hydrolysis of Dihexyl Fumarate (EC 242-833-4) at pH 9 and 30ºC

Time	Concentration, Ct	Mean Ct	log10Ct
(hours)	(mg/L)	(mg/L)
0	0.05984, 0.05866	0.05925	-1.2273
1.5	0.04930, 0.05104	0.05017	-1.2996
3.5	0.04572, 0.04613	0.04593	-1.3380
5	0.04551, 0.04197	0.04374	-1.3591
7	0.04170, 0.03930	0.04050	-1.3925
17	0.02127, 0.01658	0.01893	-1.7230
19	0.02045, 0.02111	0.02078	-1.6824

Exponential regression, log10Ct = -1.240 - 0.02536t

                             r = -0.9873

Rate constant, kobs = -slope x 2.303 = 0.05840 hour-1

Table 13       Results for hydrolysis of Dihexyl Fumarate (EC 242-833-4) at pH 9 and 40ºC

Time	Concentration, Ct	Mean Ct	log10Ct
(hours)	(mg/L)	(mg/L)
0	0.06384, 0.06406	0.06395	-1.1942
1	0.04029, 0.04103	0.04066	-1.3908
2	0.02774, 0.02956	0.02865	-1.5429
3	0.02512, 0.02420	0.02466	-1.6080
4	0.02346, 0.02356	0.02351	-1.6287
5	0.02110, 0.02231	0.02171	-1.6634
6	0.01898, 0.01916	0.01907	-1.7196

Exponential regression, log10Ct = -1.299 - 0.07844t

                             r = -0.9289

Rate constant, kobs = -slope x 2.303 = 0.1816 hour-1

Table 14       Arrhenius relationship for Dihexyl Fumarate (EC 242-833-4) at pH 9

Temperature		1/T	kobs	log10kobs
°C	K	(K^-1)	(hour^-1)
20	293.15	0.003411	0.02281	-1.642
30	303.15	0.003299	0.05840	-1.234
40	313.15	0.003193	0.1816	-0.7410

Exponential regression: log10kobs = 12.43 - 4130 (1/T)

                    

When        T = 25ºC = 298.15K

             

                 k = 0.03767 hour-1

              te½ = 0.693/k

                   = 18 hours (0.8 days)

Validity criteria fulfilled:

yes

Conclusions:

Dihexyl Fumarate (EC 242-833-4) was determined to be hydrolytically unstable under acidic, neutral and basic conditions with half-lives of 16, 34 and 0.8 days, respectively.

Description of key information

Dihexyl Fumarate (EC 242-833-4) was determined to be hydrolytically unstable under acidic, neutral and basic conditions with half-lives of 16, 34 and 0.8 days, respectively.

Key value for chemical safety assessment

Half-life for hydrolysis:

34 d

at the temperature of:

25 °C