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Degradation and photodegradation of tetraacetylethylenediamine (TAED) in the presence of iron (III) in aqueous solution
Chemosphere,Vol. 34, No. 12, pp. 2637-2648, 1997
Pergamon PII: S0045-6535(97)00107-0
© 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0045-6535/97 $17.00+0.00
DEGRADATION AND PHOTODEGRADATION OF TETRAACETYLETHYLENEDIAMINE (TAED) IN THE PRESENCE OF IRON (III) IN AQUEOUS SOLUTION
N. Brand, G. Mailhot and M. Bolte*
Laboratoire de Photochimie Moi6culaire et Macromol~culaire URA CNRS 433 Universit~ Blaise Pascal 63177 AUBIERE Cedex (France) (Received in Germany 15 January 1997; accepted 5 February 1997)
ABSTRACT: The dark degradation of tetraacetylethylenediamine (TAED) was investigated. It is a slow process which is favored in acidic medium. There is a hydrolysis of an imide group with the scission of the C-N bond giving rise to the triaeetyl derivative (TAED'). When allowed to stand for longer times a second acetyl group is eliminated with the formation of the symetric diacetyl derivative (DAED). The degradation of TAED photoinduced by iron (III) was also investigated. It appears a faster degradation which does not lead to the same products. The process only involves "OH radicals formed upon photolysis o f aquocomplex of iron (III). They preferentially abstract a hydrogen from the methylene group. The degradation is then assisted by oxygen and leads to the formation of earbonylated oxidation products. © 1997 Elsevier Science Ltd
I) I N T R O D U C T I O N To improve the performance of household detergents, manufacturers have used sodium perborate as bleaching agent together with a bleaching activitor (up to 2% in the detergent formulation) whose role is to permit the bleaching to occur at lower temperature (above 30°C instead of above 80°C). Among the bleaching activators, TAED is one of the most widely used [1] and as a result is found in waste waters.
0
0
U U CH3-- CxN__CH2__CH2__N~C'- CH3
CH3--~ O
i~--CH3 O TAED 2637
2638 So far, no work has been reported on the fate of TAED in the aquatic compartment and on its possible degradation by photoactive species present in the environment. In previous works, we reported the degradation of pollutants photoinduced by iron (III) [2,3]. The attack by "OH radicals arising from iron (Ill) species in the excited state is a very efficient process. Because of our interest in compounds entering the formulation of detergents [4] we decided to investigate the degradation of TAED photoinduced by iron (III). To be representative of what can happen in the environment with solar emission, irradiation at 365 nm were performed. This wavelength is present in the solar emission and its energy is sufficient to promote photoredox process.
II) EXPERIMENTAL
SECTION
1) Reagents and solutions * Tetraacetylethylenediamine (TAED) was purchased from Acros (+99.5%) and used without further purification. * Ferric perchlorate nonahydrate (Fe(C104)3,9H20) was a Fluka product (97%) kept in a dessicator. The iron (III) solutions for the studies were prepared by diluting stock solutions (10 .3 mol.L q in Fe(CIO4)3,9H20), having aged for one day, to the appropriate iron (III) concentration. Under these conditions the percentage in FeOH 2+, the major monomeric species, was approximately 25 to 30% (cf analysis). * Isopropanol was a Prolabo product (HPLC grade chromanorm > 99.7%). * All solutions were prepared with deionized ultra pure water (Q=18.2 MfLcm). pH measurements were carried out with an ORION pH-meter to 0.01 unit. The ionic strength was not controlled. Deaerated solutions were obtained by bubbling with Argon for 40 minutes at room temperature.
2) Apparatus * UV-visible spectra were recorded on a CARY 3 double beam spectrophotometer. * HPLC experiments were carried out by using a liquid chromatography system Waters 540 equipped with a diode array UV-visible detector (Waters 990) giving the UV-visible spectrum corresponding to each peak. The eluent was a mixture of methanol/water (10/90). The flow rate was 1 mL.min -1 and the column was a Merck Lichrosphere 100 RP18 (51am) of 25cm length. * Preparative HPLC experiments were carried out by using a Gilson chromatograph equipped with a UV-visible detector and a C-18 reverse phase column (21.4 x 50mm). The flow rate was 5 mL.min ~. * 1H NMR spectra were recorded on a BRUKER AC 400 MHz Fourier transform spectrometer. * Liquid chromatography positive electrospray mass spectra (LC/ESMS) were obtained from "Service Central d'analyse", CNRS, Vernaison, France. * In order to measure the quantum yields, the monochromatic irradiations at 365 nm were carried out with a highpressure mercury lamp (Osram HBO 200 W) equipped with a grating monochromator (Bausch and Lomb). The
2639 beam was parallel and the reactor was a cylindrical quartz cell of lcm path length. The light intensity was measured by ferrioxalate actinometry (Io365nm-~2.4x1015photons.s l.cm- z) [5]. *
.A second irradiation set-up used for kinetic and preparative experiments was an elliptical stainless steel cylinder.
A high pressure mercury lamp (Philips HPW type 125 W), the emission of which at 365 nm was selected by an inner filter, was located at a focal axis of the elliptical cylinder. The reactor, a water-jacketed pyrex tube (diameter=2.8 cm), was centered at the other focal axis. The reaction medium was well stirred. The unit delivered an intensity Io=4.5x10 is photons.s~.cm -z, over a large volume (40 mL).
3) Analysis * Iron (II) concentration was determined by complexometry with o-phenanthroline, using es10=l.118x104 L.mol 1.cm-~ for the iron (II)-phenanthroline complex [5]. * Iron (III) monomeric species concentration was determined by complexometry with 8-hydroxyquinoline-5sulfonic acid (HQSA). The absorbance of the resulting complex Fe(HQS)3 was monitored at 572 nm as described by Faust and Hoign6. [6].
III) RESULTS
1) iron (III) in aqueous solution As already described in a previous paper [6], the iron (III) species present just after dissolving Fe(C1Q)s,9H20 are the monomeric species Fe 3÷, FeOH z÷ and Fe(OH)2 ÷. According to our experimental conditions (3.0 < pH < 5.0 and 3x10 -4 mol.L -1 > [Fe(III)] > 10.4 mol.Ll), FeOH z÷ appeared to be the major iron (lid species. However a continuous evolution of iron (III) solution was observed with the decrease in FeOH z÷. In this domain of concentration, Fez(OH)z "*+the dimeric species could be neglected.
2) TAED in aqueous solution TAED is soluble in water up to 10 -3 mol.L -~. The UV-visible spectrum presents a maximum at 213 nm with a molar extinction coefficient equal to 18000 L.mol~.cm 1. At pH=5.5, natural pH of
a 10 -4 mol.L -l
TAED solution,
there was a slow evolution of the spectral features with time; a decrease was observed at 213 nm together with a continuous absorption at longer wavelength. The HPLC chromatograms gave evidence for the formation of only one product P. After 20 days, approximately 30% of the starting TAED had been transformed. By plotting concentration of TAED as a function of time it appeared that the reaction was of zero order with a constant k=l.4xl06 mol.L~day -~ (figure la).
2640
10.0 I
8.0 "6
6.0
C,
4.0 <
2.0
0.0 0
5
1'0
1'5
2'0
2'5
30
Time (days) Figure 1 : Kinetics of TAED disappearance. (a): [TAED]0=104 mol.L "I (b): [TAED]0=I0 "4 mol.L q ; [iron(III)]0=3xl0 "4 mol.L q
The half-life estimated from this value is z=
[TAEDL "~ =35 days. 2k
The UV-visible spectrum of the transformation product P presents a maximum at 218 nm. P was separated by preparative HPLC and evaporation of the solution
to dryness. It was identified by NMR and mass
spectrometry. The molecular ion peak was found at m/z = 187. The 1H NMR spectrum of P (figure 2) is in agreement with the triacetyl derivative so called TAED'.
,q H, C--CH 3 N~-0-I2-- ~I2-- 1~
0-13--~ o
i(~--CI-13
o
Product P = TAED'
2641
4
(ppm)
Figure 2 : tH NMR (400 MHz) spectrum of the product P obtained in the first stage of TAED hydrolysis (CDCI3).
3) The mixture TAED.iron (III) in aqueous solution The addition of iron (III) with a concentration of 3xl04mol.L l to a solution 104 mol.L 1 of TAED made the pH decrease from 5.5 to 2.9. The UV-visible spectrum of the mixture roughly represented the sum of the two components. There was no detectable complexation between iron (III) and TAED. A transformation of TAED was still present leading to the formation of TAED' as already described in the absence of iron (III). However the process was accelerated (figure lb), We checked that the difference in the rates was not due to the presence of iron (III) but to the more acidic medium introduced by the protolytic equilibria of iron (III) in aqueous solution [7]. In addition, the concentration in iron(II) was very low (104-10 ,7 moI.L -1) strongly arguing against a redox process between TAED and iron (III). The rate of TAED transformation in aqueous solution acidified at pH=3.0 was similar to that obtained in the presence of iron (III) 3x10 4 mol.L ~ at that precise pH. The transformation of TAED was not of zero order any longer. A satisfactory linear relationship was obtained when plotting In(C0 / C) = fit), reflecting a reaction of first order. The formation of TAED' was also accelerated (figure 3).
2642 12,0
(b) 10,0~ 8,0~ (a)
6,0~
o 4,0~ <
2,0~ 0,0 0
5
1'0
I'5
2'0
2'5
30
Time (days) Fieure 3 : Kinetics of TAED' formation. v
(a): [TAED]o= 10.4 mol.L "l (b): [TAED]o=I0"4 moI.L "! ; [iron(III)]o=3xl0"4 moLLa Due to the higher rate of transformation, it was possible to detect the formation of DAED corresponding to the loss of one more aeetyl group and identified by mass spectrometry and NMR after separation by preparative HPLC (figure 4). I-l, H N---CH2--CH2--~ o
1 ^
DAED
o
1 (ppm)
Fi~ur¢ 4 : IH NMR (400 MHz) spectrum of DAED obtained in the second stage
of TAED hydrolysis (CDCI3).
2643 The hydrolysis process can be schematized as follows (scheme 1) :
o
CH3 - C,
C- CH3 N-- CHz - Ot z - N~ ot3-f ~-- a-13 o o
H, .,C- tit 3 N-- Clt2-- CI'tz - N ot3--f ~ - CH3 O O
TAED
TAED'
HI"1 H20 H
H,
N"-CHz - CH2 - N( cn3- ~ ~ - crl3 0 0
DAED
scheme 1
It is worth noting the absence of the non-symetric diacetyl derivative. The hydrolysis process seemed to be easier when two acetyl groups are bound to the same nitrogen atom.
4) Photodegr~__d~_6onof TAED Iron (III) aquocomplexes are known to undergo a photoredox process through an internal electron transfer giving rise to "OH radicals and iron
(II) [8]. Fe3+
hv~ H20
Fe2++.OH + H +
The quantum yield of "OH formation which measures the efficiency of a photon of given wavelength to produce "OH radicals were determined by different authors [9] : for FeOH 2+, • is equal to 0.073 and 0.288 upon irradiation at 360 and 290 nm respectively. Upon irradiation at 365 nm of a mixture of [TAED]=10 4 mol.L -I and [iron (III)]=3xl0-4mol.L -l, the complete degradation of TAED was observed together with the formation of iron (II). The disappearance of TAED appeared to be of first order, a good linear relationship was obtained when plotting In([TAED]0 / [TAED]) = f(irradiation time) (figure 5).
2644
1,4
1,21,00,80,6"
0,4-
0,2-
Irradiation time (hours) Fit, u r e 5 : Plot o f In( [TAED]0/[TAED] ) versus irradiation time (Zl~uanoo=365nm). [TAED]0=10 "4 moI.L "l ; [iron(llI)]0=3xl0 "4 mol.L "t The rate constant k calculated from the slope was equal to 0.17 min -1. The formation of iron (H) as a function of irradiation time is presented in figure 6.
6,
5.
~.
4.
u~
3-
~-,
2-
0 0
i Irradiation time (hours)
: F o r m a t i o n o f iron (II) as a function of irradiation time at ),=365 nm. [TAED]0=10 "4 mol.L "1 ; [iron(IH)]o=3xl0 "4 mol,L "t The concentration reached a plateau value of 6x10 s mol.L 1, giving evidence for the reoxidation of iron (II) into iron (liD.
2645 In the absence of oxygen, the reaction was strongly slowed down (table 1). Irradiation time 2h
4h
6h
39%
57%
68%
6%
9%
10%
~. = 365 nm % of TAED disappeared in aerated solution % of TAED disappeared in deaerated solution
Table 1 : T A E D d i s a p p e a r a n c e in a e r a t e d a n d deaerated solutions as a function of irradiation time
01.irr,~iation=365nm). [TAED]o=10 "4 mol.L '1 and [Fe(llD]o=3Xl0 "4 mol.L "1
The quantum yield of TAED disappearance and iron (II) formation were equal to 2.2x10 3 and 6.6x10 -3 respectively. There was no significant difference with the quantum yield of iron (II) formation under similar conditions ~Fe2+---8xl0 3 in the absence of TAED. In the presence of isopropanol used as a quencher of "OH radicals, the degradation of TAED was totally inhibited.
5) Identification of the photoproducts A typical HPLC chromatogram is represented in figure 7 : three main peaks were detected corresponding to compounds more polar than the starting TAED that was not eluted under these conditions.
4~
A
3-
2-
O.
time (min) : H P L C c h r o m a t o g r a m of a m i x t u r e of T A E D - i r o n ( l l I ) irradiated at k=365 nm (irradiation time=Th30) : [TAED]0=10 "4 moI.L "l ; [iron(lll)lo=3xl0 "4 mol.L a
2646 Photoproducts A, B e t C were separated by preparative HPLC. The tH NMR spectrum of photoproduct A was in agreement with N-formylacetamide. IH-NMR (DzO): CH3; 2.25ppm (s) and CHO; 9.05ppm (s)
H'N_~, (9 CH3--I~
H
o
This was further confirmed by mass spectrometry with a molecular ion peak at m/z = 87. Photoproduct B was identified as diacetamide by comparison with an authentic sample.
CH3--C, N--H
CH3--1~ o
All the attemps to isolate photoproduct C failed. From the mass spectrum, it was possible to put forward the following formula O
H, ~
H
C--CH3
N---C-- C~-Iz--b( o
o
which is easy to account for from a classical mechanism (cf discussion).
6) Mechanism of the photoreaction Iron (lid species were the only absorbing species when a mixture of TAED and iron (III) was irradiated at 365 nm in aqueous solution. The reactive species responsible of TAED degradation were "OH radicals arising from the photoredox process occurring in iron(III) species in the excited state. Among iron (III) species, FeOH z+ was reported to be the most photoreactive one in terms of "OH radicals formation [6].
Fe(OH) z+ .( ......... h v .) kd
[Fe(OH) z+] *
v ~
Fe 2+
+
"OH
Then "OH radicals react with TAED to initiate the degradation. The complete inhibition of the reaction when isopropanol was added to the solution implied the only involvement of "OH radicals in the process, In addition the strong decrease of the rate of TAED disappearance in the absence of oxygen evidenced a process assisted by oxygen.
2647
From all the experimentalresults, the degradation of TAED photoinducedby iron (HI) can be represented in scheme 2: Ac
= --
Ae,, /Ac Ac/N- O"I2--CH2- KNAc
Ct-- C~'I3
~1"OH AC'N'- ~I'- CI'12--N(Ac Ac/ l O, Ac
R"
v / O"
Ac,,, ~) /Ac /N-- Of-- C112--NN Ac AC TAED
ROO*
~
/ Oil Ac.., t~ ,-'Ac N--Or- Ot2-- K,,Ac Ac/ 2+ • ~Fe
Ac\ ~) .Ac radical Ac/N_ H~__CHz_I~.,,Ac atkoxy RO"
13scissiens
unstable CH3--C~N--c, OJ" CH3-- i~
0 acidic [ hydrolysis~
H
H,n_ o o
~ C~.I3--C~
N--H CH3--~O
x~
CH3--C,.N--C'--(J.Iz--N ~f ."C'--Ot3 CH3-- i~
0
unstable
i~- G[-I3
0 [ acidic ~[ hydrolysis
B
A
O
C
O
IV) DISCUSSION TAED degradation observed in the dark at room temperature is a slow process occuring in a far larger time scale when compared with the time scale of the photodegradation. In addition, the two degradations do not involve the same process and differents sites of attack are used : the CO-N bond is broken by acidic thermal hydrolysis whereas "OH radicals formed upon irradiation preferentially abstract a
2648 hydrogen from the methylene group of N methylated amides [10]. The alkoxy radical RO" is then formed after reaction of R ° with oxygen and through a Fenton like process in the presence of iron (II) [11]. From RO ° three different 13 scissions can be put forward [12]. The scission of the C-N bond leads to the formation of diaeetamide (B). The scissions of C-C and C-H bonds give rise to the formation of products A and C respectively. As a matter of fact, the mechanism of [5 scission leads to the formation of products with three carbonyl groups bound to one nitrogen atom. Such products are unstable and-immediately lead by acidic hydrolysis to the formation of more stable products with only two carbonyl groups bound to the nitrogen atom.
V) C O N C L U S I O N The present work illustrates the efficiency of TAED degradation when the process is photoinduced by iron (III). The photolysis of Fe(OH) 2+ produces "OH radicals and under these conditions, the degradation of TAED is only due to the attack by "OH radicals assisted by oxygen. The primary step of the TAED decomposition involves the hydrogen abstraction of the methylene group and leads to the formation of three main photoproducts. For longer irradiation times, we observe the total disappearance of TAED and of the photoproducts. TAED degradation was also observed in the dark at room temperature in a far larger time scale by comparison with the time scale of the photodegradation. There is a slow hydrolysis process which is favored in acidic medium and that gives rise to the triacetyl derivative TAED'. For longer period of time, a further hydrolysis yields the symetric diacetyl derivative DAED. As far as the projection to environment is concerned, this work provides information about the fate of TAED : the two processes described in the present paper can occur in the aquatic compartment.
REFERENCES [1] : L. Baini, G. Bolzoni, G. Carrer, E. Faccetti, L. Valtora, E. Guiati, C. Paccheti, C. Ruffo, O. Cozzoli and L.
Sedea, Riv. Ital. Sostanze Grasse, 1992, 69, 615-617. [2] : S.L. Andrianirinaharivelo, J.F. Pilichowski and M. Bolte, Transition Met. Chem., 1993, 18, 37-41. [3] : P. Mazellier, J. Jirkovsky and M. Bolte, Pestic. Sci., in press. [4] : G. Mailhot and M. Bolte, J. Adv. Oxid. Technol., submitted. [5] : J.G. Calvert and J.M. Pitts, Photochemistry, John Wiley & Sons : New York, 1966, 783-786. [6] : B.C. Faust and J. Hoign6, J. Atmospheric Environment, 1990, 24A, 79-89. [7] : R.J. Knight and A.N. Sylva, J. lnorg. Nucl. Chem., 1975, 37, 779-783. [8] : J.H. Baxendale and J. Magee, Trans. Faraday Soc., 1955, 51,205-213. [9] : H-J. Benkelberg and P. Wameck, J. Phys. Chem., 1995, 99, 5214-5221. [10] : E. Hayon, T. Ibata, N. N. Lichtin and M. Simic, J. Am. Chem. Soc., 1970, 92, 3898-3903. [11] : D. Swern, Organic Peroxides, Wiley interscience : New York, 1971, 153-269. [12] : D.J. Carlson and D.M. Wiles, Macromolecules, 1969, 2, 597-606.
Pergamon PII: S0045-6535(97)00107-0
© 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0045-6535/97 $17.00+0.00
DEGRADATION AND PHOTODEGRADATION OF TETRAACETYLETHYLENEDIAMINE (TAED) IN THE PRESENCE OF IRON (III) IN AQUEOUS SOLUTION
N. Brand, G. Mailhot and M. Bolte*
Laboratoire de Photochimie Moi6culaire et Macromol~culaire URA CNRS 433 Universit~ Blaise Pascal 63177 AUBIERE Cedex (France) (Received in Germany 15 January 1997; accepted 5 February 1997)
ABSTRACT: The dark degradation of tetraacetylethylenediamine (TAED) was investigated. It is a slow process which is favored in acidic medium. There is a hydrolysis of an imide group with the scission of the C-N bond giving rise to the triaeetyl derivative (TAED'). When allowed to stand for longer times a second acetyl group is eliminated with the formation of the symetric diacetyl derivative (DAED). The degradation of TAED photoinduced by iron (III) was also investigated. It appears a faster degradation which does not lead to the same products. The process only involves "OH radicals formed upon photolysis o f aquocomplex of iron (III). They preferentially abstract a hydrogen from the methylene group. The degradation is then assisted by oxygen and leads to the formation of earbonylated oxidation products. © 1997 Elsevier Science Ltd
I) I N T R O D U C T I O N To improve the performance of household detergents, manufacturers have used sodium perborate as bleaching agent together with a bleaching activitor (up to 2% in the detergent formulation) whose role is to permit the bleaching to occur at lower temperature (above 30°C instead of above 80°C). Among the bleaching activators, TAED is one of the most widely used [1] and as a result is found in waste waters.
0
0
U U CH3-- CxN__CH2__CH2__N~C'- CH3
CH3--~ O
i~--CH3 O TAED 2637
2638 So far, no work has been reported on the fate of TAED in the aquatic compartment and on its possible degradation by photoactive species present in the environment. In previous works, we reported the degradation of pollutants photoinduced by iron (III) [2,3]. The attack by "OH radicals arising from iron (Ill) species in the excited state is a very efficient process. Because of our interest in compounds entering the formulation of detergents [4] we decided to investigate the degradation of TAED photoinduced by iron (III). To be representative of what can happen in the environment with solar emission, irradiation at 365 nm were performed. This wavelength is present in the solar emission and its energy is sufficient to promote photoredox process.
II) EXPERIMENTAL
SECTION
1) Reagents and solutions * Tetraacetylethylenediamine (TAED) was purchased from Acros (+99.5%) and used without further purification. * Ferric perchlorate nonahydrate (Fe(C104)3,9H20) was a Fluka product (97%) kept in a dessicator. The iron (III) solutions for the studies were prepared by diluting stock solutions (10 .3 mol.L q in Fe(CIO4)3,9H20), having aged for one day, to the appropriate iron (III) concentration. Under these conditions the percentage in FeOH 2+, the major monomeric species, was approximately 25 to 30% (cf analysis). * Isopropanol was a Prolabo product (HPLC grade chromanorm > 99.7%). * All solutions were prepared with deionized ultra pure water (Q=18.2 MfLcm). pH measurements were carried out with an ORION pH-meter to 0.01 unit. The ionic strength was not controlled. Deaerated solutions were obtained by bubbling with Argon for 40 minutes at room temperature.
2) Apparatus * UV-visible spectra were recorded on a CARY 3 double beam spectrophotometer. * HPLC experiments were carried out by using a liquid chromatography system Waters 540 equipped with a diode array UV-visible detector (Waters 990) giving the UV-visible spectrum corresponding to each peak. The eluent was a mixture of methanol/water (10/90). The flow rate was 1 mL.min -1 and the column was a Merck Lichrosphere 100 RP18 (51am) of 25cm length. * Preparative HPLC experiments were carried out by using a Gilson chromatograph equipped with a UV-visible detector and a C-18 reverse phase column (21.4 x 50mm). The flow rate was 5 mL.min ~. * 1H NMR spectra were recorded on a BRUKER AC 400 MHz Fourier transform spectrometer. * Liquid chromatography positive electrospray mass spectra (LC/ESMS) were obtained from "Service Central d'analyse", CNRS, Vernaison, France. * In order to measure the quantum yields, the monochromatic irradiations at 365 nm were carried out with a highpressure mercury lamp (Osram HBO 200 W) equipped with a grating monochromator (Bausch and Lomb). The
2639 beam was parallel and the reactor was a cylindrical quartz cell of lcm path length. The light intensity was measured by ferrioxalate actinometry (Io365nm-~2.4x1015photons.s l.cm- z) [5]. *
.A second irradiation set-up used for kinetic and preparative experiments was an elliptical stainless steel cylinder.
A high pressure mercury lamp (Philips HPW type 125 W), the emission of which at 365 nm was selected by an inner filter, was located at a focal axis of the elliptical cylinder. The reactor, a water-jacketed pyrex tube (diameter=2.8 cm), was centered at the other focal axis. The reaction medium was well stirred. The unit delivered an intensity Io=4.5x10 is photons.s~.cm -z, over a large volume (40 mL).
3) Analysis * Iron (II) concentration was determined by complexometry with o-phenanthroline, using es10=l.118x104 L.mol 1.cm-~ for the iron (II)-phenanthroline complex [5]. * Iron (III) monomeric species concentration was determined by complexometry with 8-hydroxyquinoline-5sulfonic acid (HQSA). The absorbance of the resulting complex Fe(HQS)3 was monitored at 572 nm as described by Faust and Hoign6. [6].
III) RESULTS
1) iron (III) in aqueous solution As already described in a previous paper [6], the iron (III) species present just after dissolving Fe(C1Q)s,9H20 are the monomeric species Fe 3÷, FeOH z÷ and Fe(OH)2 ÷. According to our experimental conditions (3.0 < pH < 5.0 and 3x10 -4 mol.L -1 > [Fe(III)] > 10.4 mol.Ll), FeOH z÷ appeared to be the major iron (lid species. However a continuous evolution of iron (III) solution was observed with the decrease in FeOH z÷. In this domain of concentration, Fez(OH)z "*+the dimeric species could be neglected.
2) TAED in aqueous solution TAED is soluble in water up to 10 -3 mol.L -~. The UV-visible spectrum presents a maximum at 213 nm with a molar extinction coefficient equal to 18000 L.mol~.cm 1. At pH=5.5, natural pH of
a 10 -4 mol.L -l
TAED solution,
there was a slow evolution of the spectral features with time; a decrease was observed at 213 nm together with a continuous absorption at longer wavelength. The HPLC chromatograms gave evidence for the formation of only one product P. After 20 days, approximately 30% of the starting TAED had been transformed. By plotting concentration of TAED as a function of time it appeared that the reaction was of zero order with a constant k=l.4xl06 mol.L~day -~ (figure la).
2640
10.0 I
8.0 "6
6.0
C,
4.0 <
2.0
0.0 0
5
1'0
1'5
2'0
2'5
30
Time (days) Figure 1 : Kinetics of TAED disappearance. (a): [TAED]0=104 mol.L "I (b): [TAED]0=I0 "4 mol.L q ; [iron(III)]0=3xl0 "4 mol.L q
The half-life estimated from this value is z=
[TAEDL "~ =35 days. 2k
The UV-visible spectrum of the transformation product P presents a maximum at 218 nm. P was separated by preparative HPLC and evaporation of the solution
to dryness. It was identified by NMR and mass
spectrometry. The molecular ion peak was found at m/z = 187. The 1H NMR spectrum of P (figure 2) is in agreement with the triacetyl derivative so called TAED'.
,q H, C--CH 3 N~-0-I2-- ~I2-- 1~
0-13--~ o
i(~--CI-13
o
Product P = TAED'
2641
4
(ppm)
Figure 2 : tH NMR (400 MHz) spectrum of the product P obtained in the first stage of TAED hydrolysis (CDCI3).
3) The mixture TAED.iron (III) in aqueous solution The addition of iron (III) with a concentration of 3xl04mol.L l to a solution 104 mol.L 1 of TAED made the pH decrease from 5.5 to 2.9. The UV-visible spectrum of the mixture roughly represented the sum of the two components. There was no detectable complexation between iron (III) and TAED. A transformation of TAED was still present leading to the formation of TAED' as already described in the absence of iron (III). However the process was accelerated (figure lb), We checked that the difference in the rates was not due to the presence of iron (III) but to the more acidic medium introduced by the protolytic equilibria of iron (III) in aqueous solution [7]. In addition, the concentration in iron(II) was very low (104-10 ,7 moI.L -1) strongly arguing against a redox process between TAED and iron (III). The rate of TAED transformation in aqueous solution acidified at pH=3.0 was similar to that obtained in the presence of iron (III) 3x10 4 mol.L ~ at that precise pH. The transformation of TAED was not of zero order any longer. A satisfactory linear relationship was obtained when plotting In(C0 / C) = fit), reflecting a reaction of first order. The formation of TAED' was also accelerated (figure 3).
2642 12,0
(b) 10,0~ 8,0~ (a)
6,0~
o 4,0~ <
2,0~ 0,0 0
5
1'0
I'5
2'0
2'5
30
Time (days) Fieure 3 : Kinetics of TAED' formation. v
(a): [TAED]o= 10.4 mol.L "l (b): [TAED]o=I0"4 moI.L "! ; [iron(III)]o=3xl0"4 moLLa Due to the higher rate of transformation, it was possible to detect the formation of DAED corresponding to the loss of one more aeetyl group and identified by mass spectrometry and NMR after separation by preparative HPLC (figure 4). I-l, H N---CH2--CH2--~ o
1 ^
DAED
o
1 (ppm)
Fi~ur¢ 4 : IH NMR (400 MHz) spectrum of DAED obtained in the second stage
of TAED hydrolysis (CDCI3).
2643 The hydrolysis process can be schematized as follows (scheme 1) :
o
CH3 - C,
C- CH3 N-- CHz - Ot z - N~ ot3-f ~-- a-13 o o
H, .,C- tit 3 N-- Clt2-- CI'tz - N ot3--f ~ - CH3 O O
TAED
TAED'
HI"1 H20 H
H,
N"-CHz - CH2 - N( cn3- ~ ~ - crl3 0 0
DAED
scheme 1
It is worth noting the absence of the non-symetric diacetyl derivative. The hydrolysis process seemed to be easier when two acetyl groups are bound to the same nitrogen atom.
4) Photodegr~__d~_6onof TAED Iron (III) aquocomplexes are known to undergo a photoredox process through an internal electron transfer giving rise to "OH radicals and iron
(II) [8]. Fe3+
hv~ H20
Fe2++.OH + H +
The quantum yield of "OH formation which measures the efficiency of a photon of given wavelength to produce "OH radicals were determined by different authors [9] : for FeOH 2+, • is equal to 0.073 and 0.288 upon irradiation at 360 and 290 nm respectively. Upon irradiation at 365 nm of a mixture of [TAED]=10 4 mol.L -I and [iron (III)]=3xl0-4mol.L -l, the complete degradation of TAED was observed together with the formation of iron (II). The disappearance of TAED appeared to be of first order, a good linear relationship was obtained when plotting In([TAED]0 / [TAED]) = f(irradiation time) (figure 5).
2644
1,4
1,21,00,80,6"
0,4-
0,2-
Irradiation time (hours) Fit, u r e 5 : Plot o f In( [TAED]0/[TAED] ) versus irradiation time (Zl~uanoo=365nm). [TAED]0=10 "4 moI.L "l ; [iron(llI)]0=3xl0 "4 mol.L "t The rate constant k calculated from the slope was equal to 0.17 min -1. The formation of iron (H) as a function of irradiation time is presented in figure 6.
6,
5.
~.
4.
u~
3-
~-,
2-
0 0
i Irradiation time (hours)
: F o r m a t i o n o f iron (II) as a function of irradiation time at ),=365 nm. [TAED]0=10 "4 mol.L "1 ; [iron(IH)]o=3xl0 "4 mol,L "t The concentration reached a plateau value of 6x10 s mol.L 1, giving evidence for the reoxidation of iron (II) into iron (liD.
2645 In the absence of oxygen, the reaction was strongly slowed down (table 1). Irradiation time 2h
4h
6h
39%
57%
68%
6%
9%
10%
~. = 365 nm % of TAED disappeared in aerated solution % of TAED disappeared in deaerated solution
Table 1 : T A E D d i s a p p e a r a n c e in a e r a t e d a n d deaerated solutions as a function of irradiation time
01.irr,~iation=365nm). [TAED]o=10 "4 mol.L '1 and [Fe(llD]o=3Xl0 "4 mol.L "1
The quantum yield of TAED disappearance and iron (II) formation were equal to 2.2x10 3 and 6.6x10 -3 respectively. There was no significant difference with the quantum yield of iron (II) formation under similar conditions ~Fe2+---8xl0 3 in the absence of TAED. In the presence of isopropanol used as a quencher of "OH radicals, the degradation of TAED was totally inhibited.
5) Identification of the photoproducts A typical HPLC chromatogram is represented in figure 7 : three main peaks were detected corresponding to compounds more polar than the starting TAED that was not eluted under these conditions.
4~
A
3-
2-
O.
time (min) : H P L C c h r o m a t o g r a m of a m i x t u r e of T A E D - i r o n ( l l I ) irradiated at k=365 nm (irradiation time=Th30) : [TAED]0=10 "4 moI.L "l ; [iron(lll)lo=3xl0 "4 mol.L a
2646 Photoproducts A, B e t C were separated by preparative HPLC. The tH NMR spectrum of photoproduct A was in agreement with N-formylacetamide. IH-NMR (DzO): CH3; 2.25ppm (s) and CHO; 9.05ppm (s)
H'N_~, (9 CH3--I~
H
o
This was further confirmed by mass spectrometry with a molecular ion peak at m/z = 87. Photoproduct B was identified as diacetamide by comparison with an authentic sample.
CH3--C, N--H
CH3--1~ o
All the attemps to isolate photoproduct C failed. From the mass spectrum, it was possible to put forward the following formula O
H, ~
H
C--CH3
N---C-- C~-Iz--b( o
o
which is easy to account for from a classical mechanism (cf discussion).
6) Mechanism of the photoreaction Iron (lid species were the only absorbing species when a mixture of TAED and iron (III) was irradiated at 365 nm in aqueous solution. The reactive species responsible of TAED degradation were "OH radicals arising from the photoredox process occurring in iron(III) species in the excited state. Among iron (III) species, FeOH z+ was reported to be the most photoreactive one in terms of "OH radicals formation [6].
Fe(OH) z+ .( ......... h v .) kd
[Fe(OH) z+] *
v ~
Fe 2+
+
"OH
Then "OH radicals react with TAED to initiate the degradation. The complete inhibition of the reaction when isopropanol was added to the solution implied the only involvement of "OH radicals in the process, In addition the strong decrease of the rate of TAED disappearance in the absence of oxygen evidenced a process assisted by oxygen.
2647
From all the experimentalresults, the degradation of TAED photoinducedby iron (HI) can be represented in scheme 2: Ac
= --
Ae,, /Ac Ac/N- O"I2--CH2- KNAc
Ct-- C~'I3
~1"OH AC'N'- ~I'- CI'12--N(Ac Ac/ l O, Ac
R"
v / O"
Ac,,, ~) /Ac /N-- Of-- C112--NN Ac AC TAED
ROO*
~
/ Oil Ac.., t~ ,-'Ac N--Or- Ot2-- K,,Ac Ac/ 2+ • ~Fe
Ac\ ~) .Ac radical Ac/N_ H~__CHz_I~.,,Ac atkoxy RO"
13scissiens
unstable CH3--C~N--c, OJ" CH3-- i~
0 acidic [ hydrolysis~
H
H,n_ o o
~ C~.I3--C~
N--H CH3--~O
x~
CH3--C,.N--C'--(J.Iz--N ~f ."C'--Ot3 CH3-- i~
0
unstable
i~- G[-I3
0 [ acidic ~[ hydrolysis
B
A
O
C
O
IV) DISCUSSION TAED degradation observed in the dark at room temperature is a slow process occuring in a far larger time scale when compared with the time scale of the photodegradation. In addition, the two degradations do not involve the same process and differents sites of attack are used : the CO-N bond is broken by acidic thermal hydrolysis whereas "OH radicals formed upon irradiation preferentially abstract a
2648 hydrogen from the methylene group of N methylated amides [10]. The alkoxy radical RO" is then formed after reaction of R ° with oxygen and through a Fenton like process in the presence of iron (II) [11]. From RO ° three different 13 scissions can be put forward [12]. The scission of the C-N bond leads to the formation of diaeetamide (B). The scissions of C-C and C-H bonds give rise to the formation of products A and C respectively. As a matter of fact, the mechanism of [5 scission leads to the formation of products with three carbonyl groups bound to one nitrogen atom. Such products are unstable and-immediately lead by acidic hydrolysis to the formation of more stable products with only two carbonyl groups bound to the nitrogen atom.
V) C O N C L U S I O N The present work illustrates the efficiency of TAED degradation when the process is photoinduced by iron (III). The photolysis of Fe(OH) 2+ produces "OH radicals and under these conditions, the degradation of TAED is only due to the attack by "OH radicals assisted by oxygen. The primary step of the TAED decomposition involves the hydrogen abstraction of the methylene group and leads to the formation of three main photoproducts. For longer irradiation times, we observe the total disappearance of TAED and of the photoproducts. TAED degradation was also observed in the dark at room temperature in a far larger time scale by comparison with the time scale of the photodegradation. There is a slow hydrolysis process which is favored in acidic medium and that gives rise to the triacetyl derivative TAED'. For longer period of time, a further hydrolysis yields the symetric diacetyl derivative DAED. As far as the projection to environment is concerned, this work provides information about the fate of TAED : the two processes described in the present paper can occur in the aquatic compartment.
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Sedea, Riv. Ital. Sostanze Grasse, 1992, 69, 615-617. [2] : S.L. Andrianirinaharivelo, J.F. Pilichowski and M. Bolte, Transition Met. Chem., 1993, 18, 37-41. [3] : P. Mazellier, J. Jirkovsky and M. Bolte, Pestic. Sci., in press. [4] : G. Mailhot and M. Bolte, J. Adv. Oxid. Technol., submitted. [5] : J.G. Calvert and J.M. Pitts, Photochemistry, John Wiley & Sons : New York, 1966, 783-786. [6] : B.C. Faust and J. Hoign6, J. Atmospheric Environment, 1990, 24A, 79-89. [7] : R.J. Knight and A.N. Sylva, J. lnorg. Nucl. Chem., 1975, 37, 779-783. [8] : J.H. Baxendale and J. Magee, Trans. Faraday Soc., 1955, 51,205-213. [9] : H-J. Benkelberg and P. Wameck, J. Phys. Chem., 1995, 99, 5214-5221. [10] : E. Hayon, T. Ibata, N. N. Lichtin and M. Simic, J. Am. Chem. Soc., 1970, 92, 3898-3903. [11] : D. Swern, Organic Peroxides, Wiley interscience : New York, 1971, 153-269. [12] : D.J. Carlson and D.M. Wiles, Macromolecules, 1969, 2, 597-606.