Reactions of 1, 1-diphenyl-2-picryl-hydrazyl with NO2, Cl2 and F2

Radiat. Phys. Chem. Vol. 40, No. 6, pp. 461-468, Int. 1. RaaYat. A@. Instnm.. Part C Printed in Great Britain. All rights reserved

0146-5724/92 SS.00 + 0.00 1992 Peraamon pms Ltd

1992

Copyright 0

REACTIONS OF 1,l -DIPHENYL-2-PICRYL-HYDRAZYL WITH N02, Cl, AND F2 L. GILLE, U. PR&CH~ and R. Sroessn~ Institut fiir Anorganische Chemie and Institut fur Analytische Chemie der Humboldt,

Universitgt zu Berlin, Hess&he Str. l-2, O-1040 Berlin, Germany (Received 26 May 1992)

Abstract-The reactions of DPPH with Cl,, NO2 and F, were studied. In the pora-phenyl position substituted dinhenylnicrvlhydrazines were formed. Besides this, the reaction of halogens led to products which were n&o-sub&inn&l in thepura-phenyl position. Thus, it was shown that the &actions of halogens and halogen radicals with DPPH include a substitution at the picryl group of DPPH and its derivatives under liberation of NO,. By ESR in situ speckoscopy and computational analysis of spectra, nitro-, dinitro-, fluoro- and difluoro-diphenylpicrylhydraxyls were identified as transient intermediates. In the formation of these species nitrogen dioxide and fluorine act as oxidants for the corresponding substituted hydraxines.

INTRODUCl-ION

other, since the reactions proceed via free radicals, Gille (1991).

The

degradation of chlorofluorocarbons (CFC) in the stratosphere, induced by u.v.-radiation is of actual interest. A method to study the stability of chlorofluorocarbons quantitatively, is the estimation of radical yields in condensed y-irradiated CFCs by a radical scavenger. For common hydrocarbons, l,l-diphenyl-2-picryl-hydrazyl (DPPH) is a convenient radical scavenger. However, the first applications of DPPH (SchulteFrohlinde and Erhardt, 1964; Magat et al., 1958; Ciborowski et al., 1961) in a totally chlorinated hydrocarbon like Ccl, gave widely differing radical yields from 4 to 22 radicals per 1OOeV absorbed radiation energy. This suggests remaining uncertainties concerning the reaction mechanism of DPPH with halogens and halogen radicals. The reaction of DPPH with molecular bromine was carefully studied by Currie et al. (1980). They found bromine and nitro-substituted diphenyl-picryl-hydrazines (RDPPH-H, R = NOz, Br; R means a substituent in the para-phenyl position, -H a hydraxine and a formula a hydraxyl). The reaction products without -H of chlorine with DPPH were the subject of an early work by Bouby et al. (1991). They found just one chlorine-substituted DPPH-H derivative. The reaction of molecular fluorine with DPPH was hitherto not investigated. In an approach to understand the mechanism of scavenging halogen radicals, the reactions of DPPH with molecular chlorine, fluorine and nitrogen dioxide were studied in detail and compared with each tAl1 correspondence should be addressed to: Dr U. Prcisch, HU-Berlin, FB Chemie, Institut fir Anorganishe Chemie, Hessische Str. l-2, O-1040 Berlin, Germany.

RESULTSAND DISCUSSION DPPH in solution was allowed to react with chlorine, fluorine or nitrogen dioxide (see Experimental section). The reaction products were separated by reversed phase-HPLC and identified by mass spectrometry, u.v.-vis. and ESR spectroscopy, directly or after an oxidation. In ESR spectroscopic measurements the fact was used that the unpaired electron in DPPH is a sensitive monitor to changes in the molecule. Reaction products of DPPH

with nitrogen dioxide

The reaction of DPPH with NOI gave two main products which were found to be the mono- and dinitro-substituted DPPH-H-derivatives (I) and (II) in agreement with Weil et al. (1980) (Scheme 1) (equations given in this text depict overall reactions). The disubstituted product was eluted first from the chromatographic column. Reaction products of DPPH

with chlorine

Chlorination of DPPH leads to a more complex product mixture (Scheme 2). The expected product Cl-DPPH-H (HI) and the mono-nitro-substituted derivative O,N-DPPH-H (I) were the main products. Furthermore it was found that substitution of NO1 by chlorine takes place in the 2 position of the picryl substituent (IV). The product (IV) was identified by coupling constants in the ESR spectra of the oxidized fraction containing nitro-substituted DPPH-H-derivatives. The constants (A, = 7.05 G, A2 = 9.8 1 G) of this fraction are similar to that of the products, which have

461

L. GILLE et

462

+

al.

O2N

NO;

-fYJ

N - N - pit /\ CYO2N

(I)

;

fJ

N - N - pit O2N

pic=C,H,(NO,),

(II)

ii

a

Scheme I

Cl,

+

(0

011)

y - pit H 0,N

0 (IV) /\ CY

pic=C,H,(NO,), Scheme 2

lost one O,N-group or have been substituted at the picryl group (Kozyrev et al., 1963). After the chlorination an excess of free NO2 was detected in the solution. Reaction products of DPPH

with fluorine

The chromatograms show four distinct peaks on an unresolved broad band. This suggests that besides

0/\

/\ o/-

R _ IY _ pit ’

+

F2

+

F2

the four well-defined main products (Scheme 3), there are other degradation products of similar retention behaviour. The products in the main fractions were fluorinated degradation products: fluorobenzene (V), fluorinated diphenylamine (VI), nitro-substituted DPPH-H (I) and DPPH-H together with small amounts of fluorine-substituted DPPH-H (VII).

09 -w

-

(0

l-

+

F2

F-Qi

N - N - pit /\ CY Scheme 3

;

PI0

Reactions of DPPH with NO,, Cl, and F,

463

where

-“-

F = ESR intensity of the first derivation of ESR spectrum Z, , I, = nuclear spin (I “N = 1) H-HO = difference of magnetic induction at a point of the spectrum and magnetic induction in the centre of ESR signal A,, A, = coupling constants of hydraxyl nitrogen atoms in gauss (G) dH = line width in gauss (G); distance between maximum and minimum of the tirst derivation of the ESR spectrum.

z

111 m

Fig. 1. ESR spectra of DPPH, Cl-DPPH, Cl,-DPPH and OzN-DPPH.

ESR in situ spectroscopy Intemediate radical species during the reaction of DPPH with NO,, Cl, and Fr were found by ESR spectroscopy in situ. The ESR spectra of DPPH and its derivatives are similar in shape, but the differences are sufficient for identification. The nitrosubstituted DPPH displays a markedly different ESR spectrum (Koqrev et al., 1963; Chen et al., 1961) (Fig. 1). The coupling constants (B-N: A,; a-N: A*) of hydraxyl nitrogen atoms, which cause the typical five-line signal by interaction with the free electron, are influenced by substituents in the pura-phenyl position. Their values (A, and A,) and the line width (dH) were calculated, corresponding to the following equation (Kozyrev et al., 1963) using the method of least squares:

F$ t m,--l,m*--1~

H-H,+A,m,fA,m,

rizi*

0/\ /\ CY

N-t-pit

+ NO:---,

O2N 0

O2N a

During the reaction a monotone decrease of the radical concentration was observed (Fig. 2). It is not only due to the simple scavenging of NO* by DPPH. Simultaneously, substituted hydraxyls are formed, as may be seen from a systematical change of the coupling constants in the course of the reaction. This means that the difference in spin density on the a- and 8-N atom is increased. Analysis and simulation of the spectra at different reaction times shows the formation of OrN-DPPH (VIII) and (O,N),-DPPH (III) in the last part of the reaction. An oxidation of the primarily formed substituted hydrazines to the corresponding hydraxyls is apparently occurring:

WJ 0

N - N - pit i

02N 0 N-N-pit A

+ NO:-

N-!-pit

+ NO:-

N-!-pi” ‘Lb CY 02N 0

/\ CY 02N -8\

(a) Reaction of nitrogen dioxide with DPPH

/\ u-

OZN 0 /\ u-

During the reaction of nitrogen dioxide, chlorine or fluorine with DPPH, ESR spectra were recorded. From these spectra coupling constants, line width and ESR intensity were numerically evaluated (see Experimental section).

O,N a w

N-N-pit h

(VIII)

N - N - pit A

0

N-N-pit

+ NO:----, ofi

+ HNOp

Q/

{x)

t

HN02

L.

464

GILLE er al.

0.20 rz I-m\.

.+z 0.15 2

, .2 Fge

1 0

Range c

‘%

i

I 50

100

--m-m.*-..-g+ 150

200

I 250

300

t (min)

!

Range b

!

a (I) 0.05 I Lu

a,

‘,

+J 0.10

0

20

40

$L--+~~~_l:+ 0

50

100

150

200

250

60

60

100

60

00

100

t (min)

300

t (min)

0

Fig. 2. The ESR intensity (top) and coupling constants (A, and Ar) (bottom) US reaction time during tlte nitroxylation of DPPH [A, and A, of DPPH (l), O,N-DPPH (2) and (O,N),-DPPH (3), assignments according to Kozyrev er al. (1963) and Chen et al. (1961)].

20

40

t

(min)

The ESR intensity (top) and coupling constants (A, and AZ) (bottom) US reaction time during tlte chlorination of DPPH [A, and A, of DPPH (l), O,N-DPPH (2) and (O,N),-DPPH (3), assignments according to Kozyrev et al. (1963) and Chen et al. (1961)].

(b) Reaction of chlorine with DPPH The change of radical concentration during this reaction is not describable by a simple function (Fig. 3). After a fast decay (range a), which is caused by the interception of chlorine by DPPH, there is an unexpected reincrease of the overall radical concentration (range b), corresponding to an oxidation of substituted hydraxines. In range c the radical concentration is decreased again by scavenging reactions. The measured change in coupling constants shows the formation of sec-

/\ l--l \--/

ondary hydrazyls. In range a the ESR spectrum is dominated by the DPPH signal. One of the resulting hydrazines is oxidized in range b to the corresponding hydraxyl 02N-DPPH (VII). In the last period of the reaction the spectrum of (O,N),-DPPH (IX) builds up. These experiments suggest a set of consecutive and competitive reactions: In the reaction of DPPH with chlorine the radical character is finally transferred to a chlorine atom:

Cl q--y ‘N-N-pie

+

Cl,

-

u

.N- N - pit

+

Cl’

The resulting chlorine atoms and molecular chlorine may substitute NO, at the picryl group of DPPH or other derivatives actually present (hydra&es and hydrazyls):

/\ CY

N - N - pit ;

+

Cl,

-

The chlorine radicals are also scavenged by DPPH, which requires both combination

Cl N-N-pit /\ CY

l

+

a

Cl’/\ CY

N - N - pit ;

and H-transfer:

Reactions of DPPH with NOz, Cl, and F,

465

The liberated NO2 reacts either with DPPH or oxidizes the hydra&es:

N-N-pit /\ CY

2

l

R-Q, N-N-pit /\ CY

+ NOI-

2

A.

Therefore in range b O,N-DPPH (VIII) and in range c (O,N),-DPPH (IX) are formed. It is expected from the investigations of Goldschmidt and Euler (1922), that the NO and Hz0 products do not play an active role in the mechanism when oxygen is absent. This reaction mechanism explains the observed reincrease of spin concentration. The oxidation of hydrazines by chlorine radicals is not connected with a reincrease of the radical concentration, since HCl quickly reduces hydrazyls:

+ Cl’-

R-Ql N - N - pit /\ CY

(c) Reaction of DPPH

with fluorine

Again during this reaction the radical concentration does not simply decrease (Fig. 4). Analogous to the chlorination at first a fast decay of the ESR intensity (range a) dominates, followed by a reincrease of radical concentration up to 90% of the original value (range b). This strong ESR signal finally decreases if the concentration of fluorine in the solution is enhanced (range c). In this system the expected reaction products [DPPH, F-DPPH (X) and F,-DPPH (XI) besides nitro-substituted DPPH-derivatives] are nearly indistinguishable by their coupling constants. Therefore, the line width and line shape had to be included in the identification procedure, as F-DPPH has a significantly greater line width than all the other DPPH-derivatives (DPPH: dH = 5.29 G; F,--DPPH: dH = 5.45 G; F-DPPH: dH = 6.55 G). In range a (Fig. 4) of the reaction the spectrum of DPPH dominates. Its intensity is lowered by radical scavenging. The following minimum (range b) in radical concentration and a strong increase of the line width is associated with the formation of F-DPPH. In the third part of the reaction (range c) a product with ESR coupling constants and line-width forms, which is attributable to DPPH or F,-DPPH as well, but under the reaction conditions the latter is favoured.

+ HCI

’

At the end of the reaction, the final product identified is 0, N-DPPH. These results are explained by a modified reaction mechanism, compared with the chlorination process:

20

0

40 t

60

60

(min)

+

IO A++’

+*.---------+

9

0

20

40

60

A2

-2 4-1

60

t (min) Fig. 4. The ESR intensity (top) and coupling constants (A, and AZ) (bottom) US reaction time during the fluorination of DPPH [A, and A2 of DPPH, FrDPPH (1) and O,N-DPPH (2); dII of F-DPPH (3) and DPPH. F,-DPPH (4), assignments according to Kozyrw et ul. (1963) and Chen et al. (l%l)].

L. GILLE

466

et al.

DPPH transfers its radical character to a fluorine atom:

N-N-

pit

+

F,

N-N-pit+ A

/\ CY

Simultaneously a substitution DPPH-H-derivatives:

N-N-pit

F

l

of nitro groups by fluorine radicals proceeds at the picryl group of DPPH- and

+

F,

-

These radicals react with hydrazyls forming substituted

+

hydrazines:

R’----+-

These hydrazines are oxidized. In chlorination the predominant oxidant is NO,, liberated by substitution at the picryl group. However, in fluorination the fluorine itself acts as the main oxidant. The substituted hydrazyls F-DPPH and F,-DPPH are formed:

/\ CY

N-N-pit+ A

F”

N-!-pie

/ \ CY

N-N-pit+

Fe

-

F-a

N-y-pie

+

HF

+

HF

(Xl

F-CJ N-i-pie

These hydrazyls are also radical scavengers. Back reactions of hydrazyls with HF are thermodynamically

+

(XI)

improbable:

HF 1

The strong and usually non-specific oxidizing property of elementary fluorine leads to multiple fragmentation products, but experimentally well-defined radical intermediates (F-DPPH, F,-DPPH) were also observable.

CONCLUSIONS

The reaction of molecular fluorine and chlorine with DPPH implies the generation of halogen atoms. They and the radical NO, are scavenged in the order

Reactions of DPPH with NO,, Cl, and F,

of increasing efficiency NO* < Cl < F but also react with rising complexity. The reaction of halogens with DPPH includes a substitution at the picryl group of DPPH and its derivatives under liberation of NO*, which acts as oxidizing agent in the formation of new hydrazyls. In the fluorination of DPPH the usually non-specific oxidizing properties of fluorine are accompanied by specific radical scavenging processes. The outlined reaction features of DPPH towards the halogens Cl, and F, improve the possibility of evaluating radical yields in various CFCs by application of DPPH. EXPERIMENTAL

DPPH (Aldrich) and fluorine (Kali Chemie) were used as received. NO2 was prepared from Pb(NO,), by thermal decomposition and adjacent oxidation by passing over PbOZ and drying over P,O,,. Chlorine was prepared by reaction of H,SO, with NaCl and MnO, and was bubbled through Ccl, until the solution was saturated. The solvents Ccl, and C, Cl, F, were dried over CaH, and distilled. Chromatographic analysis and separation

These procedures were carried out with a HPLC system from Knauer with an analytical pump and variable wavelength detector and an integrator CR6A from Schimadzu. A reversed phase column (i.d. = 4 mm, 1 = 250 mm) filled with Nucleosil C,, (particle size 5 pm) as stationary phase from Knauer was used. The mobile phase was a mixture of 70% acetonitrile (far U.V. quality from Ferak) and 30% double-distilled water. The solvent flow was adjusted to 0.8 ml/min and substances were detected by their absorption at a wavelength of 33Omn. Samples were dissolved in the eluent and injected using a 20~1 sample loop. For analysis, about lo-’ M solutions, and for separations, 1O-3-1O-2 M

461

solutions were used. Single products were isolated by repeating the separation procedure 15-20 times. The products were characterized by u.v.-vis spectroscopy (Perkin-Elmer Lambda 2), after oxidation with Pb02 by ESR spectroscopy and after removing the solvent by mass spectrometry (HP 5995 A). (a) Reaction of nitrogen dioxide with DPPH 0.2 ml gaseous NO2 were injected into 3 ml of lo-’ M DPPH solution in Ccl’. The reaction proceeds quickly. The reaction products were chromatographed after removal of Ccl,. Products were identified as (O,N),-DPPH-H (th, = 0.77) and 02N-DPPH-H (t&, =0.88). For DPPH-H t &, = 1.00. (b) Reaction of chlorine with DPPH

0.1 ml of a saturated solution of Cl, in Ccl, were diluted with 3 ml Ccl4 and added to 3 ml of a lo-’ M DPPH solution in Ccl’. After a reaction time of 5 min the remaining CIZwas removed by degassing in a vacuum. The reaction products were separated and identified as described. The chromatographic separation yielded three fractions which were identified as O,N-DPPH-H (t&, = 0.89), DPPH-H and Cl-DPPH-H (th, = 1.19). Free NO2 in solution was detected by bubbling nitrogen gas through the mixture during reaction. The gas was passed through a solution of sulfanilic acid and a-naphthylamine and NO, identified by the development of a red colour. (c) Reaction offluorine with DPPH Fluorine, diluted with N, (1: 3, flow 5 ml/min) was bubbled through a saturated solution of DPPH in CzC13F3 (5 x lo-’ M). When the reaction began, the gas flow was interrupted and the solution flushed with pure nitrogen. Product fractions were obtained after I

Fig. 5. The facility of ESR in situ spectroscopy during the reactions of DPPH with NO,, Cl, or F,.

468

L. GILLE et nl.

chromatographic separation. The separation yielded four fractions: ffuorobenzene (t&, = OSO), fluorinated diphenylamine (l,, = 0.71), O*N-DPPH-H (rare,= 0.89) and DPPH-H (tRrc,= 1.0) together, with traces of F-DPPH-H.

spectra by the “ESR-SIMU 7.02” program (Gille, 1990) on a personal computer. The ESR intensity was measured from the outermost peak in the first derivation of the ESR spectrum in arbitrary units.

ESR in situ spectroscopy during the reaction of DPPH with N02, Cl, or F_,

The reactions were carried out in a facility shown in Fig. 5. A solution of DPPH (lo-‘M) in Ccl, (nitroxylation and chlorination) or CZC13F3(fluorination) was poured into an ESR tube (c) and placed in the cavity of the ESR spectrometer (d). The facility and the solution were flushed with nitrogen until the ESR spectrum of DPPH in solution reached a stationary state. Then the condensed reactants (NOz, Cl, and F2) from the cooling trap (a) were added to the nitrogen stream by raising the temperature and passed through a mixing flask (b). The diluted reactants were flushed into the DPPH solution. Spectra were recorded at intervals of 1 min. Parameters used were a microwave power of 20 mW and a modulation amplitude of 0.5 G. The coupling constants and line width were calculated from ESR

REFERENCES

Bouby L., Chapiro A. and Chapiro E. (1916)J. Chem. Phys. 58,442. Chen M. M., Sane K. V., Walter R. I. and Weil J. A. (1961) J. Phys. Chem. 65, 713. Ciborowski S., Coleboume N., Collinson E. and Dainton F. S. (1961) Trans. Faraday Sot. 9, 1123. Currie P. F., Quail J. W. and Weil J. A. (1980) Can. J. Gem. 58, 723. Gille L. (1990) Quick Basic Program for Simulation and Analysis of ESR Spectra. Gille L. (1991) Dissertation A, Humboldt Universitit zu Berlin. Goldschmidt S. and Euler K. (1922) Ber. 55, 616. Kozyrev B. M., Yablokov Yu. V., Matevosian R. O., Ikrina A. W., Iljazov A. W., Reischmanov Yu. M., Staschkov L. I. and Schatrukiv L. F. (1963) Oprik. Spektrosk. 15, 625.

Magat M., Bouby A., Chapiro A. and Gislon N. (1958) Z. Elekfrochem. 62, 307. Schulte-Frohlinde D. and Erhardt F. (1964) Ann. 671, 88