React. Kinet. Catal. Lett., Vol. 21, No. 4,491-496 (1982)

T R A P P I N G O F t ~ - P H E N Y L E T H Y L R A D I C A L S BY

9 , 1 0 - A N T H R A Q U I N O N E A N D 2 , 2 - D I P H E N Y L - 1 - P I C R Y L H Y D R A Z Y L

G. M6ger

Central Research Institute for Chemistry of the Hungarian Academy of Sciences, Budapest, Hungary

Received August 13, 1982 Accepted September 20, 1982

The rate constant ratio k 4/k s of the reactions of ~-phenylethyl radicals (I~I) with DPPH (k 4 ) and with 9,10-anthraquinone (ks) was calculated by measuring the quantum yields of the trapping of RH by DPPH: k4/k 5 = 44-+16 (at 25 ~

BI, u/ nzMepeH KBaI-ITOBBII~ BIaIXO~ peaKtl~rl C~-d~eI-Ikl.rl-3TH/IBI-IbIX pa/~KartoB fiSH)c ~tl}IIF B nprlcyTeramt BTOpO~I pa4~rcansaol~ noByturdi 9,10-aHTpammorta (Q). Ha OCHOBarmH aaMepeHm~ 6btno paee~a~arto oa~aomerme KoHeraaT cKopoerei~ peamu~ I~H c ~Ol-IF (k4) rt e Q (k s ) : k4/k s = 44-+16 (npH 25 ~

Radical addition to quinones has been known for a long time. Szwarc and Rem- b a u m / 1 / , Roginskii and Belyakov /2/, Meada and Ingold /3/ have published rate

constants for the addition of alkyl radicals to quinones. ESR measurements indicate

that secondary alkyl radicals are added exclusively to the carbonyl group of hindered

quinones, resulting in stable alkoxy.phenoxy radicals/2, 3/.

On the other hand, according to Rosantsev/4/ , secondary alkylaryl radicals are scavenged very effectively by the nonhindered quinoidal ring of 2,2-diphenyl-l-picryl.

hydrazyl (D') , but the rate constant of the reaction has not been measured so far. In previous works / 5 - 9 / D" as well as other inhibitors have been proved to be

excellent acceptors of secondarY arylalkyl radicals, among others of a-phenylethyl

radicals (HI~) produced in the reaction of triplet anthraquinone (QT) with ethyl- benzene (RH2) in air-free benzene solutions saturated by Ar /5/:

I o - I Q ,,, , QT (1)

X = 380 nm

491 ~

MOGER: TRAPPING OF RADICALS

k_l QT " , Q ( -1 ) C6H6

k2 QT + RH2 , (~H + HI~ (2)

2QH 'k3, Q + QH~ (3)

1~I + I) k4 ~ products (4)

where I0 - I means the light intensity absorbed, (~H is the semiquinone radical and QH2 refers to 9,10-dihydroxyanthracene.

In the present work we calculated the ratio of the rate constants of two com- petitive reactions, k4/ks, taking into account the interaction between 1~I radicals and Q:

HI~ + Q ks> HR0 (5)

Based on literature d a t a / 1 - 3 / , it can be assumed that radicals HI~ attack the car- bonyl group of Q and HR(~ corresponds to 9~-phenylethoxy-10-anthroxy radicals.

The initial rate of the disappearance of 1) has been measured and this value gives directly the quantum yield of the trapping:

al) = (Wl))o (Io - I) -1 (6)

All measurements were carried out at 25 ~ The concentration of D in benzene solutions saturated with Ar was determined by spectrophotometry /6/; solutions were illuminated by Hg lamp HBO-220, using UG-1 and acetophenone as filters. The values of Io - I were determined as described previously/7/.

Assuming /4/ a steady state for I~H and QT, and that I) reacts only with HI~, and

W4 + W5 >~ WRH + QH + WI~H + HRt~ (7)

we obtain from eqs. ( 1 ) - (5)

(al)) -1 -~- N l1 + ks [Q]o; ) (8) 1~ [O]o

492

MOGER: TRAPPING OF RADICALS

5 i I J I

0 20 60

[O]o/ [D' . ] o

-1 against the ratio [Q]o [Ib]o'; [I)10 = 2.5 x 10 -s M ; I R H = ] = 0 . 0 7 5 M; Fig. 1. Values o f aD

I o - I = 6.10 -6 tool pho ton dm -3 s - t

Table 1

, [ W D ' 1o Quantum yields ct D. = - - measured at various light

I o - I

intensities absorbed (I o - I ) [Qlo = 10-4 M; [D'lo = 2 X 1 0 - S M

[ R H 2 ] = 0 . 0 3 M ;

(I o - I * ) X 107 tVD.

5.0 0.048 21 0.048 24 0.050 26 0.054

0.050-+0.0025

[ R H : t ] = 0 . 1 1 M

(I o - - I ) * X l 0 T r D "

5.0 0.184 10.5 0.162 17.0 0.165 26 0.148

L16-+0.01

*(Mol photon) dm -3 s -1

where N = I + k - l k ~ 1 [RH2]-I (9)

Data of Table 1 indicate that the values of t~fi are practically constant and inde- pendent of Io - I, that is eq. (7) is valid and eq. (8) can be used for the calculation of the rate constant ratio k4/ks.

Values of a ~ 1 plotted vs. [Q]o [I)]o 1 are represented in Figs 1-3.

Table 2 summarizes the corresponding calculated k4/k s and k.1/k2 values. Accordingly,

k4/ks = 44-+16

s 493

MOGER: TRAPPING OF RADICALS

0 I I i 0 5 0 100 150

[ Q ] o / [ D ' ] o

Fig. 2. Dependence of the values ~)1 on the ratio [Q]0 [l)]ot; [I)]o = 1.1 x 10 -s M; [I 0 - I ] = 1.8X 10 -9 tool photondm "3 s-~; [RH2] = 0.70 M

"6 O

O

J 0 , l I I I L I I

0 500

[O]o/[O'] o -I

Fig. 3. Values of al) against the ratio [Ql0 [Dlo ' ; [Q]o = 10-3 M; IRH2I = 7,8 M; 0.75 • 10 -6 mol photon dm -s s -1 (o)

I o - I = 1.25 X 10 -6 mol photon dm -3 s -I (a)

The average value o f the rate cons tan t s / 1 - 3 / measured for reactions be tween

alkyl radicals and various hindered qu inones is k s ~ l0 s M "1 s -1 at 4 0 - 7 0 ~

that is, assuming that Es -~ 0 /2, 4, 6/

k 4 "~ 5 X 106 M -1 s -1 (at 25 ~

494

MOGER: TRAPPING OF RADICALS

Table 2

Ratios of rate constants calculated from data represented in Figs. 1 - 3

[RH2] (M)

0.075 0.70 7.8

N*

5.4-+0.4 1.9+-0.5 1.0 -+ 0.55

k s ** k 4 * *

9.2• 59.-17 7.2_+0.6 26-+ 9 2.4-+0.15 42-*26

~- I ***

0.33 0.63

*N= 1 +k-i �9 k~ 1 �9 ([RH2]) -1 **using eq. (8)

***using eq. (9)

Reaction (4) represents the addition of secondary arylalkyl radicals I~H to the

nonhindered quinonoidal ring of I ) / 4 / :

HR' - I -~ /~ N -lq - r -(NO 2 )3 �9 H R - - - c p - - ( N 0 2 } 3

It is known from the results of Citterio /10/ that primary alkyl radicals are added to the ring of unhindered quinone with a rate constant 2 • 10 7 M -x s -1

(69 ~ Thus we can conclude that both primary and secondary radicals add rapidly to the unhindered quinonoidal ring. From our data

killS, + quinonoidal ring >~ killS, + > C-O

and it can be assumed that secondary alkyl radicals react exclusively with the CO group of hindered quinones because of steric effects as was claimed by Maeda and Ingold /3/.

s* 495

MOGER: TRAPPING OF RADICALS

REFERENCES

1. R. Rembaum, M. Szwarc: J. Am. Chem. Soc., 77, 4468 (1955). 2. V. A. Roginsldi, V. A. Belyakov: Dokl. Akad. Nauk USSR, 237, 1404 (1977). 3. Y. Maeda, K. U. lngold: J. Am. Chem. Sot., 101, 4975 (1979). 4. E. G. Rozantsev: Free Iminoxyl Radicals, p. 91. Khimiya, Moskva 1970. 5. G. M6ger, P. Simon: VIII. Symp. Photochemistry, Seefield, Austria, p. 258, 1980. 6. G. V. Putirskaya, I. Matus: Aeta Chim. Acad. Sci. Hung., 92, 169 (1974). 7. G. M6ger, D. G~I: React. Kinet. Catal. Lett., 5, 289 (1976). 8. T. Vid6czy, D. Gill: Z. Phys. Chem., 106, 269 (1977). 9. G. V. Putirskaya, L. P. Kovhcs: Radiochem. Radioanal. Lett., 49, 771 (1981).

10. A. Citterio: Tetrahedron Lett., 2701 (1978).

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