Chemosphere, Vol.16, No.I, pp 7 - 17, 1987 0045-6535/87 $3.OO + .00 Printed in Great Britain Pergamon Journals Ltd.

PHOTOCHEMISTRY OF" DIBENZO.-P-DIOXlN IN ORGANIC 501_VENTS AT 253.7 NM:

GC/M5 CHARACTERIZATION OF" TiE PHOTOTRANSFORI'M, TION PRODUCTS

Robert Maas~ and Bruno Pelletier

Institut National de la Recherche Scientifique (INRS-Sant~)

Univerait~ du Qu4bec

7401 rue Hochelaga

Montr4aI, Quebec HIN 3M5, Canada

AB5TRACT

Dibenzo-p-dioxin (DBD), a model compound of the chlorinated dibenzo-p-dioxin was irradiated

at 253.7 nm in acetonitrile, hexane and methanol solutions. In each solvent, DBD was rapi-

dly transformed through a cascade of hydroxylated compounds to yield 2-hydroxybenzoic acid

which appears to be its ultimate aromatic photodegradation product. Mass spectrometric

characterization of the photoproducts are presented and some mechanistic aspects of the

photoreactions are discussed.

INTROI)UCTION

The photochemistry of organic pollutants has become nowadays an integrated part in the stu-

dy of their environmental fate. The polychlorinated dibenzo-p-dioxins (PCDDa) which result

mainly from the improper disposal of chlorinated chemical wastes, combustion processes and

fires and malfunctions in PCB filled transformers [I-7], have received a great deal of at-

tention in recent years over a growing concern that their presence in the environment may

pose a potential health hazard to human and other living organisms [8]. Among the many

PCDD isomers detected in the environment 2,3,7,8-tetrachlorodibenzo-p-dioxin has been the

most extensively studied [9].

In the environment, PCDDs are not readiIy destroyed by the routes of decomposition effecti-

ve for other polIutants. Of these, photochemical degradation seems to be the most effecti-

ve route by which chiorinated dibenzo-p-dioxins are decomposed [I0-12]. Most photodegrada-

Lion studies have been performed in Iaboratory model systems at wavelenghta above 290 nm

[13]. Photodegradation of PCDOs at wavelenghts shorter than 290 nm has received much less

attent ion [12] since sunlight reaching the earth contains wavelenghts greater than 286.) nm

[13]. However, photolysis of PCDDB with radiat ion of shorter wsvelenghts usually proceeds

at faster rates [12]. This could be used with advantage par t icu lar ly for the destruction

of contaminated materials. We wish to report here the photolysis of dibenzo-p-dioxin, a

model compound of PCDOs in various organic solvants under high energy emission at 253.7 nm.

Gas chromatography-mass spectrometry was used for the characterization and quantitation of

the photoproducts.

EXPERIHENTAL

Reagents aria Solvents

Dibenzo-p-dioxin was synthetized according to the method described by Gibman and Dietrich

[14]. Its purity (99.9%) was determined by GC/H5 analysis. Acetonitrile, hexane and me-

thanol were HPLC grade and obtained from Caledon Laboratories Ltd. (Ontario, Canada), N,O-

bia(trimethyIsiIyi)-trifluoroacetamide (BSTFA) was purchased from Pierce Chemical Co.

(Rockford, lilinois, U.S.A.) and used as received.

Photolysie Experiments

Irradiation experiments with dibenzo-p-dioxin were carried out in a Rayonet merry-go-round

photochemical reactor (The Southern New England Co., Middletown, Connecticut, U.S.A.)

equipped with sixteen RPR 2537A ° lamps. The temperature of the reactor chamber was kept

approximately at 40°C with fan in operation. The photoiysis solutions containing 2 mg of

DBD dissolved in 20 ml of acetonitriIe, hexane or methanol were pIaced in quartz tubes and

irratiated for various period of time (i, 2, 5, iO, 20, 30, 60, 120 and 180 min) so as to

monitor the photolytic transformation of the substrata. The solvents were not deoxygenated

prior to irradiation.

I so la t ion of Photoproducta

After irradiation, the organic solutions w e r e transferred in 15 ml vials and evaporated to

dryness under a nitrogen stream at room temperature. The residues were dissolved in 500~L

of acetonitriie and the resuIting soIutions were stored in the dark at -IO°C in one ml Re-

acti-Vials R untii analysed.

Derivatization

IO0~L of the acetonitrile solutions containing the photoresidues were placed in Reacti-Vi-

ais R and evaporated to dryness under a nitrogen stream. The residues were dissolved in

50~L of a mixture of aeetonitrile-BSTFA (3/2, v/v). The resulting mixture was heated at

70"C for 20 min.

Gas Chromatography-Haas Spectrometry

One ~L of the derivatization mixtures was injected in the gas chromatograph for GC/HS ana-

lysis. GC/H5 was performed with a Kratos H5-25 double focusing mass spectrometer (Kratos

Analytical Instruments, Manchester, England) interfaced to a Data General Nova 5 (D5-55)

data system and coupled to s Sigma ) gas chromatograph (Perkin Elmer, Norwalk, Connecticut,

U.S.A.) equipped with a 50 m (0.25 mm I.D.) DB-5 fused silica capillary column. Injections

were done in the splitless mode. The capillary column was introduced directly in the mass

spectrometer source. Helium was the carrier gas at a flow rate of I ml/min GC injector

and GC/H5 interface were maintained st 290°C and 250"C respectively. The analysis were

perfomed at an initial temperature of 60°C maintained for two minutes and programmed at

IO°C min -I to 140°C and at 5°C min -1 up to 280°C. The operating conditions for the mass

spectrometer were the following: electron energy was 70eV, ion acceleration voltage was

set at 2 KV, scan speed was I sec/dec , ion source temperature was 250°C and ion source

pressure was 5.0 x I0 "7 Torr.

RESULTS AND DISCUSSION

Photolysis in Methanol

Table I lists the photoproducts of DBD (2--9) detected in the photolysis solutions. Typical

total ion-current chromatograms of the trimethylsilylated residues of the 5, 30, 60 and 120

min photolyzates in methanol are shown in Fig. I (a-d). Contrary to results reported in

previous studies (lO, ll) no insoluble polymeric product was formed upon irradiation of DBD

in methanol, but the solutions progressively yellowished to a final orange-yellow colora-

tion after 3 hr of exposure. As can be seen from Fig. l, irradiation of DBD resulted in

the rapid formation of products 4 and ~ (Fig. Ib) followed by subsequent production of com-

pound 7a which was apparently the ultimate aromatic product of the photodegradation cascade

of DBD. Figure 2 shows the plots of the relative concentration of DBP and that of the ma-

jor photoproducts 4, 5 and 7a against the irradiation time. It is clear that the photoche-

mistry of products ~ and ~ follow similar courses. Their rapid formation in the first 50

min of reaction is accompanied by the concomitant disappearance of 1. Also shown in Fig. 2

is the formation of o-hydroxybenzoic acid 7a which was first observed only after 15 min of

reaction at a time where about 60% of the initial concentration of DBD had dissappeared and

where the rates of formation of 4 and 5 were at their highest values. This and the GC/HS

data shown in Fig. 1 indicate that DBD was transformed into 7a through a specific pathway

in which 4 and 5 were the prominent intermediate photoproducts. Although the relative con-

centration of 7a steadily increases with exposure time (Fig. 2), its absolute concentration

starts to decrease after about 90 min of reaction, thereby indicating that it was further

degraded upon irradiation, probably into non aromatic photoproducts which were not charac-

terized h e r e .

Also formed, but in minute amounts were mono and dihydroxylated dibenzo-p-dioxinis which

THS derivatives showed characteristic molecular ions at m/z 272 and 360 respectively. The-

lO

T I M E

7 l e l f i l l 9 l a I m i . . . . . . . . . i . . . . . . . . . i

l l m x -~JIBP l I

J : . , .:;::, I . . . . . . ! . . . . . +"

( m i n u t e s ) T I M E ( m i n u t e s )

I 9 1 n I P I " 2 4 1 | ~ ~ I d 0 P i I l 1 1 1 1 9 1 5 1 4 2 0 1 I I 2 4 1 2 2 2 8 1 4 3 . , I . . . . . . . . . i . . . . . . . . . i . . . . . . . . . I I . . . . . . . . . i . . . . . . . . . I . . . . . . . . . + . . . . . . . . . i . . . . . . . . . I

1

3

I . . . . I N a m

8 C & N

T I M E

, . ° , : , . , , , , . , , | C O I . . . . . . . . . . . . . . . . . . I . . . .

i m . l ~ m

el l

so

se

3O

-J L i: 9 ,,,, ~', .' J.k.~,..

A

7 . 1

. . . . . . i . . . . . . . . . . . . .

5 2

. . . . . . . . . . . . . t ~ . . . . . . . . +

N U M B E R

( m l n u t e n }

~ t l l 24133 ~ + q T191 11119

. . . . . s . . . . . . . . . I . . . . . . . . . I , l l * J * ' ' ' * . . . .

i m - 5 1 3 1 |

4

5 8

o_

4

8 }R

i . . . . . . . . . i . . . . . . . . . i . . . . . . . . . i m s 3 o i . , t lm s n o

S ¢ A N N U M B E R

T I M E ( m i n u t e n )

1B ias ze~ez 24s2S 2O,44 . . . . # . . . . . . . . . i . . . . . . . . . i . . . . . . . . . i

_Do 7.

4 1 9 7b 9 I

. . . . . . . i ~ . - " . . . . . . i . . . . i . . . . . . . . . i . . . . . . . . . # . . . . . . . . . i . . . . . . . . . i . . . . . . . . . m 48e ~ I m m 3 o e 4 Q I sI)o

B C A N N U M B E R 8 C A N N U M B E R

Figure I: Typical total ion-current chromatograms of the (a) 5 min (b) }0 min (c) 60 min and (d) 120 min photolyzates of dibenzo-p-dioxin irradieted at 254 nm in metha- nol. Conditions for GC/MS analysis are given in the text and mass spectra of the photoproducts are partially described in Table i.

i i

TABLE I

PARTIAL MASS SP£CTRA OF THE MA3OR PHOTOPROIXICTS OF DIBENZO.-DIOXIN[ l AS THEIR TMS DERIVATIVES

PHOTOPRODUCTS M +'a CHARACTERISTIC IONS IN MASS SPECTRUMb

2 4-hydroxydibenzofuran

3 2-phenoxyphenol

4 2,2'-dihydroxybiphenyl

5 2,2',3-trihydroxybiphenyl

6 2,2',3,3'-tetrahydroxybiphenyl

7a o-hydroxybenzoic acid

256(ioo)

258(ioo)

330(76)

418(loo)

506(74)

282(1)

241(85),225(34),181(10),155(8),127(8),73(60)

243(35),227(4),150(15),135(6),121(5),73(I0)

315(8),242(4),227(7),147(2),73(100)

403(3),330(3),315(11),147(2),73(55)

418(9),330(5),147(8),73(68)

267(100),209(7),193(8),135(9),91(7),73(92)

7b o-hydroxybenzoic acid methyl 224(-) ester

209(100),193(8),179(23),161(18),135(15), 91(14),89(19),73(14)

4-cyclopenten-2-[£-hydroxy- 304(11) phenyl]-l-one-2-carboxylic acid methyl esterC

289(100),273(7),257(9),245(25),229(45),73(39)

o--hydroxybenzoyl formic acid methyl ester

252(1) 237(8),193(I00),175(4),151(6),135(7),91(6), 89(6),73(8)

a) molecular ions b) in parentheses, relative intensity as percentage of base peak c) proposed structure

se compounds were shown to possess retention times identical to those of hydroxylated di-

benzo-p-dioxins obtained from the bacterial metabolism of DBD [15]. Other photoproducts

including 2, 3, 6, 8 and 9 were also produced during the 3 hr exposure of DBD. Their mass

spectral features and their probable role in the photochemical transformation of DBD into

o_-hydroxybenzoic acid 7sara discussed below.

I r r a d i a t i o n in A c e t o n i t r i l e and Hexane

The photochemistry of DBD in aprotic and nonpolar solvents follows a course which is appa-

rently similar to that observed in methanol. Indeed, irradiation of DBD at 254 nm in ace-

tonitrile or hexane resulted in the formation of o_-hydroxy-benzoic acid 7ass the major and

ultimate aromatic photoproduct (Fig. 3). It is likely that the photodegradation of I into

7a proceeds through the same reactional pathway as well in acetonitrile and hexane as in

methanol since compounds ~ and ~ were the major intermediate photoproducts in all cases

(Fig. I and 3). It is noteworthy that the photolysis of DBD in acetonitrile and hexane

also resulted in the formation of a dark brown precipitate which was slightly more abundant

in the hexane photolysates. It is probable that this polymeric photoproduct originates

from the polymeriaation of the hydroxylated biphenyls 4 and ~ [lO,11]. As expected, very

few photoproducta were produced upon irradiation of DBD in these aprotic solvents (Fig. 3).

In the absence of a solvent such as methanol capable to quench radicalar intermediates, the

formation of methoxylated photoproducts such as 7b, 8 and ~ was impeded. As a result, the

degradation of DBD appears to proceed at a faster rate in these solvents than in methanol.

i 2

=1

70

Z 60

0

50 E

W

40 Z

0

e - - 1 0 - - 4 o ~ 5 0 - - 7

I I I I / I 30 60 90 120 150 160

I R R A D I A T I O N ( m l n u t s s )

Figure 2: Plot of reaction mixture composition against the irradiation time of a solution of dibenzo-p-dioxin I in methanol. The plots represent the reIative concentra- tions of compound I,--4, 5 and 7 in the soIution: [I]+[4]+[5]+[7]=I00%.

Ident i f ica t ion of the Photopcoducts

The major and minors photodegradation pathways of DBD are summarized in Scheme I.

S C H E M E 1

__2 OH 1 .=1,=

x ) ' - OH

OH OH HQ OH

7a R=H 5 6 7b R=GH,

i 3

Figure 3:

?Je t I t J l e lee r . . . . . . . . . . . . . *

I oex - lZ39e4

sa

ee

re

Ge

~e

~e

~e

2e

l e

e ~ , ~ l r , ~.,~. , ~ . . ~ , leQ

T I M E ( m i n u t e s )

issue Is,as 24, le 2e,ze , , , , , I . . . . . . . . . I . . . . . . . . . I . . . . . . . . .

5 & I

;zoo zoo ~6 seo

S C A N N U M B E R

Typical total ion-current chromatogrsm of the 120 min photolysate of dibenzo-p-

dioxin irradiated at 254 nm in aceteonitrile. GC/M5 conditions are as in Fig.

i.

The previously published works on the photochemistry of DBD found that its photodecomposi-

tion is initiated by the specific cleavage of one ether bond to yield 2-phenoxyphenol 3

[iO,II]. This first intermediate photoproduct in the photodegradation cascade of DBD is

highly photoreactive and was detected in minute amounts only during the first 5 min of ir-

radiation. It rearranges [I6] readily into 2,2'-dihydroxybiphenyi 4 (Tabie i and Scheme

i). Crosby et al. have proposed that 4 polymerized upon irradiation to yield insoluble po-

lymeric photoproducts [IO]. We have aiso obtained similar products by irradiation of DBD

in acetonitriie and hexane but not in methanol. However, these authors did not further in-

vestigated the photochemistry of DBD and 4-hydroxybenzofuran 2 and compounds 3 and 4 were

the sole photoproducts reported.

In the study presented here, new photoproducts of DBD are detected and characterized, thus

providing further insight into the nature and origin of its photointermediates. As can be

seen from Fig. I and 2, the formation of 4 was accompanied by the production of compound 5

which T|45 derivative mass spectrum is shown in Fig. 4a. The molecular ion at m/z 418 and

the presence of structuraIiy informative ions at m/z 330 (M-88)+ and 315 (M-I03) + indicate

the presence of three hydroxyi groups, two of which being vicinals. The ion at m/z 330 re-

sults from the loss of one molecuIe of tetramethyisiiane (Me45i) from the molecular ion

through an intramolecular rearrangement of the 2,3-vicinai THS ethers [I7-]9]. The ion at

m/z 315 originates from the consecutive Ioss of one methyi group from the nonvicinal TM5

group and one molecule of tetrsmethyisilane.

14

IIII

le

I'0

III

IIII

4e

IU

III

JJ

A

_5

311

Lr" [" n I j H H H H l J H HH I , I J , , , ; , I H I J H a H H H J H I l H I H H ' H ' H I I ' H ' T

In I z

all lee

M**

le

le

le

H

l |

I r r

B

oslK~,

CO, M, lla

OSIMe I COl

_Sb

M.Me. COiMe. H + ~ZJ

M-Me

IIi

2, 191 =io OM'

:, 1 llll I,I Ill I~II 2~

m / z

Figure 4: a) mass spectrum of 2,2',3-trihydroxybiphenyl TMS derivative and b) mass spec-

trum of photoproduct 8 TMS derivative.

Consequently, compound 5 was identified as 2,2',3-trihydroxybiphenyl. Interestingly, no

methyl ether analog of 5 was detected in the photolysates indicating that the radicalar

precursor of 5 was not trapped by methanol. Thus, it is likely that the selective hydroxy-

lation of 4 at C-3 result from the trapping of radicalar 2,2'-dihydroxybiphenyl by oxygen

and subsequent rearrangement of the peroxyde. As shown by the mass spectral data (Table

I), 5 was also hydroxylated, to yield minute amounts of a product which was tentatively

identified as 2,2',),3'-tetrahydroxybiphenyl 6 (Scheme I and Table I).

The formation of o_-hydroxybenzoic acid ?__aa implies that one of the aromatic ring of the hy-

droxylated biphenyls 4, 5 and/or ~ is cleaved upon irradiation. The mass spectral data in-

dicate that the involvement of compound ~ in the production of 7a and 7b seems highly pro-

bable. Indeed, the disappearance of 5 was accompanied by the formation of compounds 8 and

9 which both resulted from the cleavage of one aromatic ring of a hydroxylated biphenyl in-

termediate. The mass spectrum of compound 8 shown in Fig. 4b, is dominated by an intense

ion at m/z 289 (M-15)+and accompanied by several structurally informative ions at m/z 273

(M-OCH~ 257 (M-Me-MeOH)~ 245 (M-Me-CO) + and 229 (M-Me-CO-H)~

i5

The formation of 8 can be rationalized according to the pathway illustrated in Scheme 2.

S C H E M E 2

5 " - ' ~ R - . - ~ R

I 8_. a

R.C 0

The 2,3-dioi of 5 is probably transformed into the corresponding 2,3-quinone. Hemolytic

scission of the C -C bond gives a ketonic biradical. The latter rearranges and closure to

yield one of the two isomeric cyclopentenone biradicals which is readily trapped by metha-

nol to give 8a or 8b. Boule eL al. reported an analogous reaction in a study on the photo-

chemistry of chlorinated phenols [20]. They reported the formation of cyclopentadiene car-

boxylic acids from the irradiation 2,4-dichlorophenol in water. Moreover, our tentative

structure assignment for compound 8 is supported by the fact that dienones and quinones

usually rearrange to cyclopropyl compounds and/or cyclopentenones upon irradiation [21].

Due to the unavailability of an authentic standard we were unable to ascertain whether 8a

or 8b (fig. 5) is the most probable structure for compound 8. However, examination of the

mass spectrum of its TH5 derivative (Fig. 4b) suggests that structure 8b is the most like-

Iy. Indeed, the elimination of the carbomethoxy group from the benzyIic position in 8b i8

expected to give rise to the intense and characteristic ion at m/z 245. In structure 8~a,

the benzylic position is not substituted by this group and one would expect the correspon-

ding ion at m/z 245 to be much less stabiIized and consequentIy iess intense than that

shown in Fig. 4b.

It is worth nothing that compound 8 (expected from the photolysi8 of 2,2',3-trihydroxybi-

phenyl 5) was not detected among the photoproducts produced by the irradiation of DBD in

acetonitrile and hexane, thereby indicating that, in aprotic solvents, the radicalar spe-

cies generated upon irradiation of 5 (Scheme 2) are not trapped but transformed into labile

photoproducts, yielding ultimately o_-hydroxybenzoic acid 7a.

Finally, compound ~ (Fig. I and Table I) was identified as o__-hydroxybenzyl formic acid me-

thyl ester. It was not detected in irradiated acetonitrile and hexane solutions of DBD.

i6

The structural similarity existing between compound 9 and the ~-hydroxybenzylic moiety of

compound 8 and the parallel courses of their photochemistry indicate that ~ probably origi-

nates from the radicalar precursors of compound 8 and also from 8 itself. As shown in

Scheme 5, further irradiation of 9 resulted in the formation of 7a and 7b most probably

S C H E M E 3

OH OH hz, " ~ C h~ ~ C H,O 8 " ~ ~'~ -COOCHa "--~ " CH30 ~ 7a * 7b

~ - J O

9

through the formation of the corresponding ~-hydroxybenzoyl radical which was trapped by

water and methanol to yield the acid 7a and the ester 7b respectively. Upon, prolonged

irradiation, the latters are sIowly decomposed and minutes amounts of benzoic acid, 2-me-

thoxybenzoic acid and few nonaromatic photoproducts which were not characterized were de-

tected i~ the photoIysates.

In conclusion, the photodecomposition of dibenzo-p-dioxin i in acetonitrile, hexane and me-

thanol appears to proceed with similar efficiencies. In apretic as well as in protic sol-

vents, DBD was ultimately converted to o--hydroxybenzoic acid 7a through a photochemical

cascade involving the formation of 2,2'-dihydroxybiphenyl 4 and 2,2',3-trihydroxybiphenyl 5

as the major intermediate photoproducts. The data indicated that 5 is probably the first

product of the photodegradation cascade to undergo ring cleavage upon irradiation, yielding

the photoprecursors of~-hydroxybenzoic acid 7a.

ACKNOWLED~NI5

Financial support from Natural Sciences and Engineering Research Council bf Canada and Le

Fonds FCAR, Ministry of Education of the Province of Quebec is gratefuIly acknowledged.

REFERENCES

1.

2.

T.O. Tierman, M.L. Taylor, J.H. Garrett, G.F. VanNess, J.G. Solch, D.J. Wagel, G.L.

Ferguson and A. Schecter, Environ. Health Perspect. 59, 145 (1985).

R.R. Bumb, W.B. Crummett, S.S. Cutie, J.R. Glendhill, R.H. Hummel, R.O. Kagel, L.L.

Lamparski, E.V. Luoma, D.L. Mi l ler , T.J. Nestrick, L.A. Shadoff, R.A. Stehl and O.5.

Woods, Science 210, 385 (1980).

17

3. H.R. Buser, Chemosphere 6, 415 (1979).

4. B. Jansson and G. Sundstrom. In: Chlorinated dioxins and related compounds. Impact on the Environment (0. Hutzinger, R.W. Frei and F. Pocoohiari, eds.), Pergamon Press, Oxford, 1982, pp. 201-207.

5. A. 5ohecter, Chemosphere 12, 669 (1983).

6. C. Rsppe, S. Marklund, P.A. Bergqvist and M. Hsnsson, Chemioa Scripts, 20, 56 (1982).

7. J.W.A. Lustenhouwer, K. 0 l i e and O. Hutzinger, Chemosphere ~, 501 (1980).

8. E.E. McConnell, Environ. Health Perspect. 60, 29 (1985).

9. G. Reggiani. Regulatory l o x i o o l . Pharmacol. ~, 211 (1981).

10. D.G. Crosby, A.5. Wong, J.R. Plimmer and E.A. Woolson, Science 173, 749 (1971).

11. J.R. Plimmer, U.I . K1ingebiel, D.G. Crosby and A.S. Wong, Adv. Chem. Set. 120, 44 (19733.

12. G.G. Choudray and O. Hutzinger. Residue 844, 113 (1982).

13. G.G. Choudhry, A.A.M. Roof and O. Hutzinger, Toxiool. Environ. Chem. ~, 259 (1979).

14. M.P. Gilman and J.J. D ie t r ich, 3. Am. Chem. Soc. 79, 1439 (1957).

15. R. Mass~ and B. Pe l l e t i e r , Results to be published.

16. Y. Ogsta, K. Iakagi and I , Ishino, Tetrahedron 26, 2703 (1970).

17. R.J. Horvat and 5.D. Senter, Org. Mass Spectrom. 18, 413 (1983).

18. N. Narasimhaohari andn P. Vouros, Anal. Biochem. 45, 154 (1972).

19. K. Halpaaps, M.G. Horning and E.C. Homing, J. Chromatogr. 166, 479 (1978).

20. P. Boule, C. Guyon and J. Lemaire, Chemosphere 1_33, 603 (1984).

21. D.O. Cowan and R.L. Drisko. Elements of Organic photochemistry, Plenum Press, New York, 1976, pp. 307-335.

(Received in Germany 4 June 1986; accepted 30 June 1986)