Self-Reactions of Alkylperoxy Radicals in Solution (1) - ACS Publications

25 Self-Reactions of Alkylperoxy Radicals in Solution (1) J. A.

HOWARD

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D i v i s i o n of C h e m i s t r y , N a t i o n a l Research C o u n c i l of C a n a d a , O t t a w a , O n t a r i o , C a n a d a K1A 0R9

In 1967 Benson (2) noted that the most important area of d i s agreement, or perhaps u n c e r t a i n t y , which had been r a i s e d at the I n t e r n a t i o n a l Oxidation Symposium i n San F r a n c i s c o (3-5) was the nature of the termination r e a c t i o n of a l k y l p e r o x y r a d i c a l s . In the ten years that have elapsed s i n c e that meeting numerous papers have appeared on the k i n e t i c s and mechanisms of the s e l f - r e a c t i o n s o f these r a d i c a l s and it is t r u e t o say t h a t we are no c l o s e r t o a complete understanding of the mechanisms of these r e a c t i o n s than we were in 1967. P r i o r t o 1957 the termination r e a c t i o n f o r l i q u i d - p h a s e hydrocarbon a u t o x i d a t i o n at oxygen pressures above ca. 100 t o r r was g e n e r a l l y w r i t t e n as

K i n e t i c r e s u l t s were c o n s i s t e n t with a bimolecular termina t i o n r e a c t i o n whereas r e a c t i o n products and mechanisms were somet h i n g of a mystery. At t h a t time it was known that the termina t i o n r a t e constant f o r a u t o x i d a t i o n of cumene (6) i s about three orders of magnitude smaller than the t e r m i n a t i o n r a t e constant f o r a u t o x i d a t i o n of tetralin (7). I t was, however, g e n e r a l l y accepted that the termination r a t e constants f o r t e r t i a r y (8) and secondary (9) alkylperoxy r a d i c a l s are i n s e n s i t i v e t o the s t r u c t u r e of the hydrocarbon residue i n the r a d i c a l . R u s s e l l (10) was the first t o propose an acceptable mechanism f o r the termination of primary and secondary a l k y l p e r o x y r a d i c a l s while Blanchard (11) made the important discovery that cumylperoxy r a d i c a l s are capable of undergoing non-terminating as w e l l as terminating i n t e r a c t i o n s during a u t o x i d a t i o n of cumene. These two pieces of work stimulated a great deal of f u r t h e r research on the s e l f - r e a c t i o n of alkylperoxy r a d i c a l s . The r e s u l t s of t h i s work, which are reviewed here, have provided compelling evidence that t e r t i a r y and secondary (and primary) alkylperoxy r a d i c a l s can terminate by d i f f e r e n t mechanisms i n the l i q u i d - p h a s e . For t h i s reason these two types of r a d i c a l s w i l l be discussed s e p a r a t e l y .

©0-8412-0421-7/78/47-069-413$05.00/0

Pryor; Organic Free Radicals ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

ORGANIC F R E E

414

RADICALS

T e r t i a r y a l k y l p e r o x y r a d i c a l s w i l l be discussed f i r s t because the mechanism f o r these r a d i c a l s i s reasonably w e l l understood. On the other hand the mechanism f o r s e l f - r e a c t i o n of secondary and primary a l k y l p e r o x y r a d i c a l s i s s t i l l i n doubt and these r a d i c a l s are considered i n the second p a r t of t h i s review. Tertiary Alkylperoxy Radicals


t - B u t y l p e r o x y . The s i m p l e s t t e r t i a r y a l k y l p e r o x y r a d i c a l i s t - b u t y l p e r o x y and there i s considerable experimental evidence i n support of the mechanism g i v e n i n Scheme I f o r s e l f - r e a c t i o n of this radical. Scheme I 2t-Bu0 '

(2)

2

^

^

£-BuOi»Bu-t

t-BuO^Bu-t

> rt-BuOOa'OBu-t] L J cage (U) i t - B u 0 0 * 0 B u - t ! — * t-BuOOBu-t + 0 ·Jcage

(3)

e

(5)

k 3

2

it-Bu0 0 '0Bu-t! — L u cage e

2

2t-BuO* + 0

•

2

2

The alkoxy r a d i c a l s formed i n (5) may r e a c t w i t h the s o l v e n t SH t o g i v e , i n the presence of oxygen, s o l v e n t d e r i v e d peroxy r a d i c a l s or scavenge a t-Bu0 * 2

(6)

t-BuO* + SH

(7)

t-BuO + t - B u 0 '

9

• t-BuOH + S0

^

2

e

• t-Bu0 Bu-t

#

2

3

The i n i t i a l r e a c t i o n between £-Bu02* (2) must i n v o l v e a headto-head i n t e r a c t i o n because i f peroxy r a d i c a l s l a b e l l e d w i t h oxygen-18 ( t - B u 0 0 ) are allowed t o r e a c t w i t h normal peroxy r a d i c a l s ( t - B u 0 0 * ) the oxygen evolved has a t o t a l mass of 3^ 1 8

1 6

1 8

e

1 6

(12).

The e x i s t e n c e of d i - t - b u t y l t e t r o x i d e was deduced from s t u d i e s of the i n f l u e n c e of temperature on the c o n c e n t r a t i o n of t - b u t y l p e r o x y r a d i c a l s by e l e c t r o n s p i n resonance spectroscopy (13, 1*0. Thus i t was shown t h a t at temperatures below 193K the r a d i c a l c o n c e n t r a t i o n can be i n c r e a s e d by r a i s i n g the temperature and decreased by lowering the temperature w i t h no apparent l o s s i n r a d i c a l c o n c e n t r a t i o n . The i n f l u e n c e of temperature on the c o n c e n t r a t i o n of t-Bu02 i s shown i n F i g u r e 1. I f the t e t r o x i d e i s completely d i s s o c i a t e d at the h i g h e s t temperature e q u i l i b r i u m constants f o r r e a c t i o n (2), K2, can be c a l c u l a t e d from „ K 2

_ "

[t-Bu0itBu-£] |>-Bu02?


HOWARD

25.

Self-Reactions

of

Alkylperoxy

415

Radicals

because B

[t-BuO^Bu-t] = h { [ * - u ° 2 ]

- [t-BuOS]}

m a x

where [t-BuOi] i s the measured r a d i c a l concentration and [t-BuC>2] i s the maximum r a d i c a l concentration. E q u i l i b r i u m constants obtained at d i f f e r e n t temperatures are given i n Table I. P l o t s of l n K against the r e c i p r o c a l of the absolute temperature yielde'd values of ΔΗ2 and AS ° between -8.0 and -8.8 k c a l mol" and -27 and -3^ c a l deg" m o l " , r e s p e c t i v e l y , depending on the method of r a d i c a l p r e p a r a t i o n ( l U , 15_). It would, t h e r e f o r e , appear t h a t although ΔΗ ° i s known with a f a i r degree of accuracy there i s some u n c e r t a i n t y about the magnitude of AS °,probably because of e r r o r s i n v o l v e d i n measuring [t-BuOifBu-t]. max

2

2

1

1

1


2

2

Table I

E q u i l i b r i u m constants f o r t-Bu0

Temperature/K l

xr\K /yr ) 2

- t-BuOitBu-t e q u i l i b r i u m

2

183

173

163

153

0.1

o.u

1.8

10

Heats of formation of t-BuOifBu-t and t-Bu02* have been e s t i mated (l6_) t o be -UT ± 8 and -21.5 ±2.5 k c a l m o l " , r e s p e c t i v e l y , which give a c a l c u l a t e d Δ Η ° — 5 k c a l mol" which i s ~3 k c a l mol" smaller than the measured v a l u e . This d i f f e r e n c e i s probably because the t e t r o x i d e i s more s t a b l e than was p r e d i c t e d ( l 6 ) . Above 193K t-Bu02* decay i r r e v e r s i b l y with second-order k i n e tics , i.e., 1

1

1

2

(β) =£ψοη

2

m

_ -]

2kaCt

Bu02

The maximum value of 2 k i s UkaK^ksAkit + ks ) and i s obtained i f a l l £-BuO* are removed by (7). Values of t h i s r a t e constant have been obtained f o r r a d i c a l s prepared by p h o t o l y s i s of 2,2 -azoisobutane i n oxygenated CF2CI2 and by complete o x i d a t i o n of t-Bu00H with a l a r g e excess o f Ce(lV) i n CH3OH u s i n g k i n e t i c e . s . r . spectroscopy (.17). Values of 2 k at 303K from 2.5 10 t o 2.5x10** M" have been r e p o r t e d with a "best" value of ~10 M"" s " . The most r e l i a b l e Arrhenius equation f o r t h i s r a t e constant (lU) appears t o be a

f

x

2

a

1

1

u

1

1

logUkg/^s- )

=

9.7

-

8.7/Θ

1

where θ = 2.303 RT k c a l m o l " . In the presence of l a r g e concentrations of t - b u t y l hydro peroxide the t-butoxy r a d i c a l s formed i n (5) a b s t r a c t the hydrop e r o x i d i c hydrogen t o regenerate t-Bu02* . (9)

t-BuO* + t-Bu00H

t-BuOH + £-Bu0 ' 2


ORGANIC

416

FREE

RADICALS

Consequently i r r e v e r s i b l e r a d i c a l decay i s slower than i t i s i n the absence of t-BuOOH with a second-order r a t e constant k = 2k K W(ki + k ). There have been many determinations of t h i s composite r a t e constant by KESR and the hydroperoxide method (17). Absolute values at 303K vary from 7 10 t o 3 * 1 0 M" s" ", values of logiA^/M" s " ) from 5.5 t o 1 2 , and a c t i v a t i o n energies from U.5 t o 10 k c a l mol" (17)· I t has, however, been concluded t h a t the "best" Arrhenius equation i s (l8). b

3

2

f

5

x

1

5

1

1

1

1

log(2k /M" Downloaded by FUDAN UNIV on April 13, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0069.ch025

b

1

s " ) = 9.2 1

8.5/Θ

-

Product and k i n e t i c s t u d i e s of the i n i t i a t e d decomposition of t-Bu00H ( l £ , 2 0 ) have been p a r t i c u l a r l y u s e f u l i n e l u c i d a t i n g the r e l a t i v e importance of (h) and (5). Induced hydroperoxide decom p o s i t i o n can be described by r e a c t i o n s ( 2 ) t o (6) plus an i n i t i a t i o n r e a c t i o n such as the decomposition of d i - t - b u t y l p e r o x y oxalate ( 1 0 ) . (10)

£-Bu00C(0)C(0)00Bu-t

• 2t-Bu0' + 2C0

2

A k i n e t i c a n a l y s i s of t h i s r e a c t i o n gives ks_ k»,

-d[t-Bu00H]/dt

=

2d[0 ]/dt Ri

=

2

λ

K

~

±

where i s the r a t e of chain i n i t i a t i o n . These equations have been v e r i f i e d experimentally and H i a t t , Clipsham, and V i s s e r (19) obtained values of (-d[t-Bu00H]/dt)/R i n the .range 6-10 at U5°, implying ks/k^-7. The r a t i o of t-Bu0H t o t-Bu00Bu-t ( c o r r e c t e d f o r a l c o h o l produced from the i n i t i a t o r ) was c o n s i s t e n t with t h i s r a t i o . F a c t o r , R u s s e l l and T r a y l o r (20_) confirmed these r e s u l t s and found that dEO^/dt/Ri i s -10 i n chlorobenzene. The r a t i o ks/ki* increases with an increase i n temperature (18,21 ) with E -Ei» = 5.3 t o 6.6 k c a l mol" and l o g ( A / A i J = k.6 t o 5.2. I f i t i s assumed t h a t the r e a c t i o n of t - B u 0 i n the cage r e q u i r e s no a c t i v a t i o n energy, E 5 , the energy r e q u i r e d t o d i f f u s e out of the cage, must be about 6 k c a l m o l " . The d i f f e r e n c e i n Af a c t o r s f o r (5) and {k) of about 5 orders of magnitude i s c o n s i s tent with a bimolecular cage r e a c t i o n and f i r s t - o r d e r d i f f u s i o n out of the cage. These d i f f e r e n c e s i n a c t i v a t i o n parameters are, however, not c o n s i s t e n t with the "best" values f o r k and k^ given above. Thus 2 k / 2 k = 2 k / k i and l o g ( k / k ) = 0 . 5 and E5 - Ει» = 0.2 k c a l m o l ' . C l e a r l y , there are d i s c r e p a n c i e s i n the k i n e t i c data f o r s e l f - r e a c t i o n of t-Bu02* which w i l l r e q u i r e f u r t h e r i n v e s t i g a t i o n before they are r e s o l v e d . Adamic, Howard and Ingold (ih) have used the thermodynamic parameters f o r the t - B u 0 -t-BuOi^Bu-t e q u i l i b r i u m and a c t i v a t i o n parameters f o r i r r e v e r s i b l e r a d i c a l decay t o c a l c u l a t e a c t i v a t i o n parameters f o r i r r e v e r s i b l e t e t r o x i d e decomposition. Decay ±

1

5

5

e

1

a

a

b

5

t

a

b

1

e

2


25.

HOWARD

Self-Reactions

of

Alkylperoxy

Radicah

417

constants f o r £-Bu02* prepared by p h o t o l y s i s of 2,2'-azoisobutane i n CF2CI2 were used and i t was assumed that ks/iki^+ks) i s equal t o 1 at the temperature of the experiments. The r a t e constant 2k i s , t h e r e f o r e , equal t o UK2k and a

3

E

a Aa ~ RT

-, ι, =

l n U

+

A

AS ° AH ° ~ΊΓ~ - ~RT~ 2

2

+

l

n

A

_ . E - RT

3

3

Using = 1 0 * NT s " , E = 8.7 k c a l m o l " , ΔΗ ° = -8.8 kcal m o l " , and AS ° = - 3 U c a l deg" mol" values of A = 1 0 s " and E = 17.5 k c a l mol" were c a l c u l a t e d . I t was concluded (lU) from the magnitude of the Α-factor t h a t only one bond i n the t e t r o x i d e i s cleaved i n the r a t e determining step. 9

7

1

1

1

a

1

2

1

1

1 6 , 6

2

1

3

1


3

(11)

y £-Bu0

t-BuOi^Bu-t

e 3

+

t)Bu-t

• t-BuCf + 0

+

2

OBu-t

In support of the intermediacy of £-Bu0 * there i s some evidence that i t can be prepared from t-butoxy and oxygen (22,23) while CF 0 * has been unambiguously i d e n t i f i e d (2k). I t should, however, be noted that B a r l e t t and G u a r a l d i (13) and M i l l and Stringham (25) obtained low A - f a c t o r s ( 1 0 * and 10 s " , r e s p e c t i v e l y ) f o r i r r e v e r s i b l e decomposition of t-BuOt* Bu-t and concluded that decomposition must be concerted. There have been s e v e r a l r e p o r t s of t-butylperoxy r a d i c a l s undergoing s e l f - r e a c t i o n with f i r s t - o r d e r k i n e t i c s (21,26). Now i t i s w e l l known that decay of c e r t a i n r a d i c a l s can be f i r s t - o r d e r i f the r a d i c a l i s i n e q u i l i b r i u m with a diamagnetic dimer and s u b s t a n t i a l concentrations of dimer are present at the decay temperature (27-28). t-Butylperoxy r a d i c a l s do not f a l l i n t o t h i s category because the t e t r o x i d e i s completely d i s s o c i a t e d before i r r e v e r s i b l e r a d i c a l decay occurs. Other l e s s p e r s i s t e n t t-R02* do, however, decay i r r e v e r s i b l y i n the presence of t e t r o x i d e and i n these cases f i r s t - o r d e r decay k i n e t i c s are observed (29) because r a d i c a l decay monitors t e t r o x i d e decomposition. 3

3

3

9

1 2

3

1

Cumylperoxy. The i n i t i a l r e a c t i o n between cumylperoxy r a d i c a l s (RO2*) i n v o l v e s a head-to-head i n t e r a c t i o n t o give d i cumyl t e t r o x i d e (30) (12)

R0 * ^ F = ^ r 2

R0i|R

The e q u i l i b r i u m constants f o r t h i s process, K12, relation l o g ( K i / M ) = (-7 t o -10.5) + (9.2 _ 1

2

(17) f i t the

to 11.2)/θ

Product analyses (11, 31, 32) have shown that the cumylperoxy r a d i c a l undergoes non-terminating and terminating r e a c t i o n s during autoxidation of cumene.


ORGANIC FREE RADICALS

418

(13)

RO*R

ilk) TRO' 0 «_ (15) |R0' 0

>[R0OR"' J cage * OR J 1

2

2

L

J

c a

ê

O2 - C B j

• ROOR + 0

c a g e

2

• 2R0* + 0

2

e


The cumyloxy r a d i c a l s produced by non-terminating i n t e r a c t i o n s e i t h e r a b s t r a c t a Η-atom from cumene or undergo β-scission at ambient temperatures t o g i v e acetophenone and methyl r a d i c a l s , the l a t t e r being converted t o methylperoxy by r e a c t i o n with oxygen.

(.16)

R0 + RH

(17)

RO- — ^

y ROH + R'

e

e

• GH Qz

+ C H C(0)CH

3

6

5

3

Methylperoxy r a d i c a l s e i t h e r propagate a u t o x i d a t i o n by r e a c t i n g with cumene or terminate the r e a c t i o n by r e a c t i n g with cumyl peroxy r a d i c a l s . (18)

CH Q2*

+ RH

(19)

CH 0 *

+ R0 '

3

3

2

• CH 00H + R' 3

2

• ROH + CH 0 + 0 2

2

The absolute t e r m i n a t i o n r a t e constant, 2 k , obtained from r o t a t i n g s e c t o r s t u d i e s o f a u t o x i d a t i o n o f neat cumene i s , t h e r e f o r e , an overall r a t e constant and i s given by t

(.20)

2k = 2 f k t

1 3

K

1 2

+ 2(l-f)k

1

3

K

1

2

^

i

7

^

7 6

[

R

H

]

.

Uk [R0 '] 1 ^k [R0 ']+k [RH]J 19

1 9

2

2

1 8

where f = k ^ / t k ^

+ kis).

The magnitude of t h i s r a t e constant depends on k i , K i , f , the f r a c t i o n o f cumyloxy r a d i c a l s which undergo 3 - s c i s s i o n , and the f r a c t i o n of methylperoxy r a d i c a l s which are consumed i n the termination r e a c t i o n (19). The absolute value o f 2 k at 30° = 1.5 10** M" s"" with log(2k /M~ s " ) = 10.1 - 9.2/Θ (.17). The value o f E i s much l a r g e r than the value o f -0 k c a l m o l " found by M e l v i l l e and Richards (6j and Thomas (21). This high a c t i v a t i o n energy has, however, been confirmed by KESR (ik). T r a y l o r and R u s s e l l (32) made the important discovery i n 1965 that the r a t e o f o x i d a t i o n o f cumene i s increased by the a d d i t i o n of cumene hydroperoxide. Thus the r a t e depends on the hydro peroxide concentration u n t i l a l i m i t i n g r a t e i s reached whereupon a d d i t i o n o f more hydroperoxide has no e f f e c t on the r a t e 3

x

2

1

1

t

1

1

t

t

1


25.

HOWARD

Self-Reactions

of Alkylperoxy

419

Radicals

(Figure 2). This increase i n r a t e was a t t r i b u t e d to reaction of cumyloxy and methylperoxy r a d i c a l s with the hydroperoxide thus preventing CH3O2* from undergoing chain termination r e a c t i o n s .

ROH (21)

}

+ ROOH

+

R0 * 2

CH3OOH

e

CH 02 3

Measurement o f the t e r m i n a t i o n r a t e constant 2 k under these conditions gave a value of 6 x 10 M" s " with log(2k- /M" s " ) = b

3


10.Τ

1

1

1

1

b

- 9.5/Θ (IT).

Values o f and i n conjunction with AS?2 and ΔΗ?2 have been used t o c a l c u l a t e the a c t i v a t i o n parameters l o g ( A i 3/s" )=1T.1 and E i 3 = l 6 . 5 k c a l mol" f o r i r r e v e r s i b l e decomposition o f d i cumyl t e t r o x i d e (lU). Although there i s l i t t l e d i f f e r e n c e between the a c t i v a t i o n parameters f o r di-cumyl- and d i - t - b u t y l - t e t r o x i d e s i t would appear t h a t the former i s somewhat l e s s s t a b l e towards i r r e v e r s i b l e decomposition. Fukuzumi and Cno have very r e c e n t l y concluded t h a t the t e r m i n a t i o n r e a c t i o n f o r o x i d a t i o n o f cumene with manganese dioxide or cobalt oxide supported on s i l i c a (33) and during a u t o x i d a t i o n o f cumene i n i t i a t e d by r e a c t i o n o f cumene hydroper oxide with l e a d oxide (3k) i s s t r i c t l y f i r s t - o r d e r with respect t o the concentration o f cumylperoxy r a d i c a l s . These workers proposed an unprecedented 1,3-methyl s h i f t followed by 0-0 bond cleavage t o account f o r these unusual k i n e t i c s , 1

1

R0 * 2

• C6H (CH )C00CH3 5

3

• C H C(0)CH 6

5

3

+ CH 0

#

3

whereas a p s e u d o - f i r s t order r e a c t i o n i s more p l a u s i b l e . Other t - a l k y l p e r o x y s . The s e l f - r e a c t i o n s o f a wide v a r i e t y of other t - a l k y l p e r o x y r a d i c a l s have been examined by hydrocarbon a u t o x i d a t i o n and KESR (lU, IT, 35_, 36) and they a l l e x i s t i n e q u i l i b r i u m with t-RO^R-t. U n f o r t u n a t e l y , most of these r a d i c a l s are l e s s p e r s i s t e n t than t-Bu02* and values o f AS° could not be determined with any degree o f accuracy because [t-RO^R-t] could not be measured. Estimates o f AS° and ΔΗ° have, however, been made (IT, 36) and i t has been concluded that the nature o f R has very l i t t l e i n f l u e n c e on the magnitude of these parameters. In a d d i t i o n , the nature o f R appears t o have l i t t l e or no i n f l u e n c e on the r a t i o o f r a t e constants f o r non-terminating and terminating i n t e r a c t i o n s (,3T). Absolute values of o v e r a l l t e r m i n a t i o n r a t e constants (2k^) and r a t e constants f o r self-terminâtion ( 2 k ) have been measured and t y p i c a l values are presented i n Table I I . b



420

ORGANIC F R E E

100

120

140

160

180

200

TEMPERATURE / Κ Figure 1. Variation of the concentration of (CH ) C0 · with temperature in the range where irreversi ble decay does not occur 3 S

2

ο-

I -Δ ^λϊ

ο

ι

θ !

ο

ι

03

o~

ι

04

ι

05

I

[CUMENE HYDROPEROXIDE]/M Figure 2. Rate of oxidation of cumene (3.6M) in chlorobenzene as a function of cumene hydroper oxide concentration at 330.2 Κ (SI)


RADICALS

25.

HOWARD

Table I I

Self-Reactions

of

Alkylperoxy

O v e r a l l and s e l f - t e r m i n a t i o n r a t e constants f o r some t e r t i a r y a l k y l p e r o x y r a d i c a l s (17 )· 1

Peroxy r a d i c a l

10- *(2k )

a

I

10"" *(2k )

t

(M" Cumylperoxy


421

Radicah

1

1

s" )

1.5

(NT

1

1

s" )

0.6-0.8

2-Phenyl-2-butylperoxy

18

3.2

1,1-Diphenylethylperoxy

13

6.h

1- MethyIcyclopentylperoxy

-

0.6-23

2- Cyano-2-propylperoxy

-

100

a

b

b

From r o t a t i n g sector studies of hydrocarbon a u t o x i d a t i o n . From r o t a t i n g sector studies of hydrocarbon a u t o x i d a t i o n i n the presence of hydroperoxide or by KESR i n the presence of hydro peroxide.

Values of 2k f o r these r a d i c a l s w i l l be given by an equation s i m i l a r t o (20) and as might be expected 2k depends on the nature of R because the t-RO* produced by non-terminating i n t e r a c t i o n s e x h i b i t d i f f e r e n t s u s c e p t i b i l i t i e s t o β-scission. Somewhat more s u r p r i s i n g l y values of 2k a l s o depend on the nature of R. For instance the s e l f - t e r m i n a t i o n r a t e constant f o r 1,1-diphenylethylperoxy i s over an order of magnitude l a r g e r than 2k f o r £-Bu0 * · Since Κ and f do not depend on R i t has been concluded that d i f f e r e n c e s i n k are due t o d i f f e r e n c e s i n the r a t e constants f o r unimolecular decomposition of t-ROt^R-t. t

t

b

b

2

b

Acylperoxy r a d i c a l s T r a y l o r and co-workers have r e c e n t l y provided evidence from a u t o x i d a t i o n of acetaldehyde (38-UP) and induced decomposition of p e r a c e t i c a c i d (Ul) f o r a mechanism f o r the s e l f - r e a c t i o n o f the acetylperoxy r a d i c a l which i n v o l v e s a non-terminating i n t e r a c t i o n v i a a tetroxide. (22)

2CH C(0)0 3

e 2

• ΟΗ θ(θ)θι*θ(θ)αί 3

3

• 2CH '+ 2C0 3

2

+ 0

2

A c e t y l peroxide, methyl a c e t a t e , dimethyl peroxide and ethane are not produced i n these r e a c t i o n s i n d i c a t i n g that there i s no cage c o l l a p s e of the r a d i c a l s . Instead the methyl r a d i c a l s d i f f u s e out of the cage and r e a c t with oxygen t o g i v e methylperoxy. This means that a u t o x i d a t i o n of acetaldehyde i s terminated e i t h e r by r e a c t i o n of methylperoxy with acetylperoxy or by s e l f - r e a c t i o n of methyl peroxy (23) and (2h).


422

ORGANIC F R E E RADICALS

(23)

CH 0 '

(2k)

2CH 0 *

3

+ CH C(0)0

2

3

3

•

2

2

• CH C(0)OH + C H 0

e

2

3

CH 0 2

+ CH OH + 3

0

2

2

Termination r a t e constants f o r a u t o x i d a t i o n o f some aldehydes (k2) are given i n Table I I I . The value f o r acetaldehyde must be a composite one c o n t a i n i n g c o n t r i b u t i o n s from 2 k and 2 k i * . I f r e a c t i o n s ( 2 2 ) , ( 2 3 ) , and (2k) apply t o the other aldehydes a l l the r a t e constants i n t h i s t a b l e w i l l be composite ones. Benzoyloxy r a d i c a l s do not undergo decarboxylation as r e a d i l y as other acylperoxy r a d i c a l s . The t e r m i n a t i o n r a t e constant i s , however, c l o s e t o the d i f f u s i o n c o n t r o l l e d l i m i t . This apparent anomaly has been a t t r i b u t e d (k2) t o i r r e v e r s i b l e formation o f the t e t r o x i d e f o l l o w e d by complete d i m e r i z a t i o n o f benzoyloxy r a d i c a l s i n the cage. 2


+ 0

(25)

2C H C(0)0 ' 6

C H C(0)0 6

5

Table I I I .

• CeHsCtOjO^CKoJCeHs

2

5

#

0

2

'OC(0)C H 6

5

3

2

•

• C H5C(0)0 C(0)C H5 6

2

6

Termination r a t e constants f o r aldehyde a u t o x i d a t i o n

l(T (2k ) 7

Aldehyde

1

/M"

t

s"

1

10 Λ

Acetaldehyde

a

8.

Heptaldehyde

5·k

Octaldehyde

7.0

Cyclohexanecarboxaldehyde

0.7

Pivaldehyde

0.7

Benzaldehyde

176

At 273°K.

Primary and secondary

alkylperoxy r a d i c a l s .

Two mechanisms have been considered f o r the s e l f - r e a c t i o n o f primary and secondary a l k y l p e r o x y r a d i c a l s , the R u s s e l l mechanism (10) r e a c t i o n s (26), (27), and (28) and a mechanism i n v o l v i n g the intermediacy o f alkoxy r a d i c a l s (U3, kk) r e a c t i o n s (29), (30), (31), and (32). The r e a c t i o n s i n v o l v e d i n these two mechanisms are presented i n Scheme I I .


25.

HOWARD

Self-Reactions

of Alkylperoxy

423

Radicals

Scheme I I

O*SN> R'

Bf

β/

O-CH I

R R 1 . 2 6 1

2RC02- ^ Downloaded by FUDAN UNIV on April 13, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0069.ch025

H

f

R

r

^28

1

« v

RO\CR H

H

C « 0 + 0 +HOCH 2

29

R 1

R*

R !

I

I

R

!

f

!

e

RCO*+0 + OCR !M-^2RC0*+0 2

I

I

! H J

H

J

2

I

Η

cage

Xl

3

Rt R»

R(j!00(j:R + 0 Η

2

Η

R u s s e l l (10) s u g g e s t e d t h a t t h e b i m o l e c u l a r s e l f - r e a c t i o n o f S-R02* i n v o l v e s t h e c o n c e r t e d d e c o m p o s i t i o n o f a c y c l i c t e t r o x i d e formed b y combination o f the r a d i c a l s . T h i s mechanism was deduced from a c o n s i d e r a t i o n o f the r e s u l t s o f a k i n e t i c and p r o d u c t study o f t h e a u t o x i d a t i o n o f e t h y l b e n z e n e . Thus R u s s e l l f o u n d t h a t a l m o s t one m o l e c u l e o f a c e t o p h e n o n e i s p r o d u c e d p e r t w o k i n e t i c c h a i n s a n d t h a t C6H5CH(CH ) 02* i n t e r a c t t o f o r m n o n - r a d i c a l p r o d u c t s n e a r l y t w i c e a s f a s t a s C H C D ( C H ) 0 * . The f o r m e r r e s u l t i s o n l y c o m p a t i b l e w i t h (29) i f a l l t h e a l k o x y r a d i c a l s d i s p r o p o r t i o n a t e i n t h e s o l v e n t cage (30) w h i l e t h e d e u t e r i u m i s o t o p e e f f e c t r e q u i r e s a Η-atom t r a n s f e r r e a c t i o n t o b e r a t e c o n t r o l l i n g , w h i c h i s u n l i k e l y f o r t h e r a d i c a l pathway. S u p p o r t f o r t h e R u s s e l l mechanism came f r o m Howard a n d I n g o l d ( Î 5 ) who f o u n d t h a t some o f t h e o x y g e n e v o l v e d f r o m s e l f - r e a c t i o n of s-butylperoxy r a d i c a l s i s i n the e l e c t r o n i c a l l y e x c i t e d s i n g l e t d e l t a s t a t e (*Ag) a s r e q u i r e d b y t h e Wigner s p i n c o n s e r v a t i o n r u l e f o r concerted decomposition t o g i v e s i n g l e t ketone and s i n g l e t alcohol. K e l l o g g (h6) h a s , h o w e v e r , s u g g e s t e d t h a t t e t r o x i d e d e c o m p o s i t i o n g i v e s t r i p l e t ketone and t r i p l e t oxygen and t h a t s i n g l e t o x y g e n i s f o r m e d b y cage e n c o u n t e r s o f t h e s e t w o s p e c i e s . On t h e o t h e r h a n d B e u t e l (hj) h a s a r g u e d t h a t k e t o n e can b e produced e i t h e r i n a v i b r a t i o n a l l y e x c i t e d s i n g l e t s t a t e from w h i c h i t may u n d e r g o an i n e f f i c i e n t a d i a b a t i c t r a n s i t i o n t o t h e t r i p l e t manifold o r i n an e l e c t r o n i c a l l y e x c i t e d s i n g l e t s t a t e followed e i t h e r by fluorescence o r intersystem crossing. Although 3

6

5

3

2



424

ORGANIC F R E E

RADICALS

the exact mechanism f o r the production of chemiluminescence during a u t o x i d a t i o n i s debatable there i s no doubt t h a t emission s p e c t r a from t r i p l e t ketones have been observed and the i n t e n s i t y o f chemiluminescence during non-stationary s t a t e a u t o x i d a t i o n has been used t o provide k i n e t i c data f o r t e r m i n a t i o n r e a c t i o n s ( I T ) . ι + Production o f s i n g l e t oxygen (both Σβ and *Ag) by s e l f r e a c t i o n o f s-butylperoxy r a d i c a l s and the peroxy r a d i c a l s d e r i v e d from l i n o l e i c a c i d has very r e c e n t l y been confirmed by a n a l y s i s o f the emission s p e c t r a observed during e e r i e ammonium n i t r a t e oxida t i o n of the. appropriate hydroperoxide (U8)« Howard and Ingold {k9) confirmed R u s s e l l ' s c o n c l u s i o n t h a t the r a t e constant f o r t e r m i n a t i o n of s - a l k y l p e r o x y r a d i c a l s depends on the s t r e n g t h o f the α-C-H bond. Thus the average isotope e f f e c t f o r t h i s r e a c t i o n at 303K, (2k )n/(2k )D» i s 1.37 ± O.lk which provides compelling evidence f o r a b s t r a c t i o n o f t h i s hydrogen i n the r a t e c o n t r o l l i n g step (Table I V ) . Furthermore 2k f o r eyelohexenylperoxy i s 2.8 times l a r g e r than 2k f o r c y c l o hexylperoxy which i s c o n s i s t e n t with the former r a d i c a l having the weaker α-C-H bond. t

t

t

t

Table IV.

Deuterium isotope e f f e c t s on r a t e constants f o r s e l f reaction

Peroxy r a d i c a l 5

C2H (CH )CH(D)(V 5

3

c y c l o - C H 9 ( D )0 5

9

(2k ) /(2k )

Temperature/K

(C H )2CH(D)02" 6

of S-R02*

2

e

t

H

t

D

1.36 1.3T

303 303 180

k.O

H i a t t and Zigmund (50_) p r o v i d e d f u r t h e r evidence f o r the R u s s e l l mechanism when they d i s c o v e r e d t h a t the i n t e r a c t i o n of two s - b u t y lperoxy r a d i c a l s does not give d i - s - b u t y l peroxide at 3l8K, a product that might be expected i f the r e a c t i o n i n v o l v e s the intermediacy o f s-butoxy r a d i c a l s . Diaper (51.),however, r e p o r t e d t h a t 1-methoxy- and 1-t-butoxy-nonane-l-hydroperoxides are o x i d i z e d t o d i - ( l - a l k o x y a l k y l ) peroxides by 1 equiv o f Ce (IV) i n methanol at 2T3K. Since Ce(lV) o x i d i z e s a l k y l hydroperoxides t o the corresponding a l k y l p e r o x y r a d i c a l i n high y i e l d these r e s u l t s are not compatible with H i a t t and Zigmund s f i n d i n g s or t h e R u s s e l l mechanism. Other product s t u d i e s (25_, 52-5*0, mainly at ambient tempera t u r e s , have shown t h a t almost equal y i e l d s of a l c o h o l and ketone are formed from the s e l f - r e a c t i o n o f S - R O 2 ' as p r e d i c t e d by (27) and (28) or by (.29) and (30). In a d d i t i o n H i a t t and co-workers (55.) found chain lengths o f 0.7 t o 1.0 f o r d i - t - b u t y l peroxyoxalate induced decomposition of η-butyl-, s - b u t y l - , and α-tetralyl-hydroperoxides a t 3l8K i n d i c a t i n g t h a t i n t e r a c t i o n s o f the S - R 0 2 * from these hydroper oxides are almost always t e r m i n a t i n g at t h i s temperature. 1


25.

HOWARD

Self-Reactions

of Alkylperoxy

425

Radicab

Lindsay et ai (5*5) made a thorough study o f the products of the s e l f - r e a c t i o n o f 1-ethoxyethylperoxy and 1 , 2 - d i p h e n y l e t h y l peroxy r a d i c a l s a t ambient temperatures, r a d i c a l s which would be expected t o g i v e the products o u t l i n e d i n Schemes I I I and IV. Scheme I I I CH

Non-radical^

^

O

C

(

O

)

C

H

3

+

C

3

H CH 0C0H 3

2

I ?H

Η 2CH CH 0C0 ' —} Downloaded by FUDAN UNIV on April 13, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0069.ch025

3

2

CH

3

3

^CH CH 0C00C0CH CH

2

3

Κ

CH

Radical

e

2

Η

3

2CH CH 0C0 3

2

3-Scission

2

CH CH 0C(0)H 3

2

CH

H

+H"

3

Η

3

•CH CH 0C0H 3

2

Η Scheme IV Ph

Non-radical Ph

) p h C H 2

l

Q H

+

p h C H 2 C ( o ) p h

Ph Ph

t Η

I

2PhCH C0 H 2

I

2

2

Η Radical

>2PhCH C0 2

β-Scission

I

PhCH C00CCH Ph t ! Η Η PhCH + PhCHO 2

e

2

Η

Ph I PhCH C0H 2

Η 1-EthoxyethylperoxyIs were prepared by a v a r i e t y o f methods and r e a c t t o g i v e e t h y l a c e t a t e , e t h a n o l , acetaldehyde and e t h y l formate. (Table V ) . There was no evidence f o r the formation of d i - ( 1 - e t h o x y e t h y l ) p e r o x i d e , a r e s u l t t h a t i s not e n t i r e l y i n compatible w i t h Diaper's work i n view o f the l a r g e d i f f e r e n c e i n the s i z e the peroxy r a d i c a l s s t u d i e d by the two groups o f workers. The absolute y i e l d s of the products were d i f f i c u l t t o a s c e r t a i n w i t h any degree o f c e r t a i n t y because of secondary r e a c t i o n s . The h i g h y i e l d of e t h y l formate i s , however, d i a g n o s t i c f o r the intermediacy o f alkoxy r a d i c a l s and i n d i c a t e s t h a t a t l e a s t of the s e l f - r e a c t i o n s o f 1-ethoxyethylperoxyIs occur v i a the r a d i c a l mechanism.


ORGANIC FREE RADICALS

426 8

T a b l e V.

Products - from o x i d a t i o n o f 1-ethoxyethyl

Oxidant

Solvent

Ethyl formate

Ce(lV)

CH3CH

0.20

0.3

0.U6

0.23

CCln-CeHe

O.OU-0.32

0.U3

0.18

0.2

C H

0.12

0.12

0.52

0.36

t-BuO

, C

Ag 0 2

6

6

Ethyl acetate

a


c

Ethyl alcohol

hydroperoxide

Acetaldehyde b

0

2

O.k

b mol p e r m o l o f h y d r o p e r o x i d e , from d i - t - b u t y l h y p o n i t r i t e .

as 1 , 1 - d i m e t h o x y e t h a n e ,

1,2-Diphenylethylperoxy r a d i c a l s undergo s e l f - r e a c t i o n t o g i v e a l m o s t e q u a l y i e l d s o f b e n z a l d e h y d e , b e n z y l a l c o h o l , 1,2d i p h e n y l e t h a n o l , and b e n z o i n . Benzaldehyde i s produced by 3 - s c i s s i o n o f 1 , 2 - d i p h e n y l e t h o x y l s w h i l e 1 , 2 - d i p h e n y l e t h a n o l and b e n z o i n a r e formed by a n o n - r a d i c a l p r o c e s s . The w o r k o f L i n d s a y et at p r o v i d e s good e v i d e n c e t h a t n o t a l l s-R0 ' undergo b i m o l e c u l a r s e l f - r e a c t i o n a t ambient temperatures e n t i r e l y by t h e R u s s e l l mechanism. I t would, however, appear from t h e i r w o r k t h a t t h e y i e l d o f a l k o x y r a d i c a l s i s v e r y d e p e n d e n t on the s t r u c t u r e o f t h e peroxy r a d i c a l . This l e a d these workers (56) t o p r o p o s e t h a t t h e t e t r o x i d e may decompose b y m u l t i p l e b o n d s c i s s i o n b y a c o n c e r t e d b u t n o n - c y c l i c mechanism. 2

M

R

R"

I

I

slow

R"

Reaction products + c = o + RC0 * RCO^CR I V R R» R S t r u c t u r a l f a c t o r s may, t h e r e f o r e , have a p r o f o u n d i n f l u e n c e n o t o n l y on t h e r a t e c o n s t a n t s f o r t e r m i n a t i o n o f s - R 0 * b u t a l s o on t h e r e a c t i o n mechanism. P r o d u c t s t u d i e s h a v e i n d i c a t e d (57.) t h a t t h e f r a c t i o n o f s-R0 * i n t e r a c t i o n s t h a t t e r m i n a t e d u r i n g a u t o x i d a t i o n o f neat η-butane i s 0.3-0.6 a t 373K a n d 0.18-0.35 a t 398K. Furthermore, t h e n o n - r a d i c a l c o n c e r t e d r e a c t i o n p r o v i d e s most o f t h e t e r m i n a t i o n f o r s-Bu0 a t 373K w h e r e a s a t t h e h i g h e r t e m p e r a t u r e a m a j o r f r a c t i o n o f t h e i n t e r a c t i o n s g i v e f r e e s-BuO*. These r e s u l t s a r e q u i t e compatible w i t h a l l s-Bu0 * s e l f - r e a c t i o n s b e i n g t e r m i n a t e d v i a a c o n c e r t e d n o n - r a d i c a l p r o c e s s a t 303K. V e r y r e c e n t l y B e n n e t t a n d Summers (58) r e p o r t e d a p r o d u c t study o f t h e low temperature s e l f - r e a c t i o n o f s - b u t y l p e r o x y , s h e x y l p e r o x y e y e l o h e p t y l p e r o x y , and e y e l o p e n t y l p e r o x y r a d i c a l s . A l t h o u g h a l c o h o l t o k e t o n e r a t i o s c l o s e t o 1.0 a r e p r o d u c e d a t a m b i e n t t e m p e r a t u r e s t h i s r a t i o decreases as t h e t e m p e r a t u r e i s reduced. For instance the r a t i o of cyclopentanol t o cyclopentanone i s 0.06 a t 173K. F u r t h e r m o r e s i g n i f i c a n t y i e l d s o f h y d r o g e n 3

f

1

2

2

2

2


25.

HOWARD

Self-Reactions

of Alkylperoxy

427

Radicals

peroxide areobtained. These r e s u l t s p r o m p t e d B e n n e t t a n d Summers t o propose an a l t e r n a t i v e t r a n s i t i o n s t a t e t o t h e R u s s e l l mechanism w h i c h i n c r e a s e s i n i m p o r t a n c e as t h e t e m p e r a t u r e i s lowered.

2R2CH02*

I

R -Cf 2

/ M * 2 — > R O=0 + H 0 2

2

2

K i n e t i c d a t a f o r s e l f - r e a c t i o n o f p-RCV a n d s - R 0 ' h a v e b e e n o b t a i n e d b y EPR s p e c t r o s c o p y a n d r o t a t i o n s e c t o r s t u d i e s on h y d r o c a r b o n a u t o x i d a t i o n ( 1 J ). These r e a c t i o n s u s u a l l y obey t h e k i n e t i c expression


2

-d[R0 '] 2

= 2k [R0 *] t

dt

2

2

where 2k^. i s t h e b i m o l e c u l a r t e r m i n a t i o n r a t e c o n s t a n t . I t h a s g e n e r a l l y b e e n f o u n d t h a t p-RCV h a v e t e r m i n a t i o n r a t e c o n s t a n t s i n e x c e s s o f 10 M ^ s " w h i l e S-RO2* e x h i b i t a w i d e v a r i a t i o n i n 2k^ w h i c h , w i t h a f e w e x c e p t i o n s , c a n b e c l a s s i f i e d according t o whether t h e r a d i c a l i s d e r i v e d from a b e n z y l i c , a l l y l i c , c y c l i c , o r a l k y l system. I n t h e b e n z y l i c group a r e compounds s u c h a s e t h y l b e n z e n e , n - b u t y l b e n z e n e , a n d s t y r e n e , w i t h 2k t f s o f 2-6x10 7 M " s " . S e c o n d a r y a l k y l p e r o x y r a d i c a l s f r o m a l k e n e s u n d e r g o t e r m i n a t i o n more s l o w l y t h a n b e n z y l i c p e r o x y radicals. T h i s i s p a r t l y due t o s t e r i c e f f e c t s a s s o c i a t e d w i t h t h e a l k y l m o i e t y o f t h e p e r o x y r a d i c a l s i n c e 2k^»s i n c r e a s e a s t h e s i z e o f t h e o l e f i n i s d e c r e a s e d a n d p a r t l y due t o a n i n c r e a s e i n t h e s t r e n g t h o f t h e α-C-H b o n d . C y c l i c h y d r o c a r b o n s s u c h as t e t r a l i n and c y c l o h e x e n e have t e r m i n a t i o n r a t e c o n s t a n t s i n t h e 6 r a n g e 2-8x10 M " s ~ . A d d i t i o n o f s e c o n d a r y a l k y l hydroperoxides, e.g., a - t e t r a l i n h y d r o p e r o x i d e t o t h e p a r e n t h y d r o c a r b o n h a s n o e f f e c t on t h e m a g n i t u d e o f 2k i n d i c a t i n g t h a t i n t h e s e c a s e s n o n terminating s e l f - r e a c t i o n s arenot s i g n i f i c a n t . V a l u e s o f 2k f o r 8

1

1

1

1

1

t

t

some p - R 0 2 *

and S-R02*

are given

i n Table V I .


ORGANIC

428

FREE

RADICALS

Table VI. Termination r a t e constants at 303K f o r some t y p i c a l primary and secondary a l k y l p e r o x y r a d i c a l s . 6

1

10" (2k )/M~ s~

Alkylperoxy r a d i c a l .

500

Methylperoxy

80

Ethylperoxy

2-Propylperoxy

3

59 59 11.59

a

a

1*0°, 300

n-Butylperoxy


Réfère

1

t

b

59 11.59 11.59

b

2-Butylperoxy

1.5°,

9

Cyclohexylperoxy

2.0°,

10

b

b

i i i i i i

e

Cyclohexenylperoxy

5.6

α-Tetralylperoxy

7.6,

Benzylperoxy

300

7.2

d

e

1-Phenylethylperoxy

1*0°

Poly(peroxystyrylperoxy)

U2

Tetrahydrofuranylperoxy

31°

C

l l l l l l

^Determined i n the gas phase using molecular modulation s p e c t r o scopy (5£). ^Determined i n the l i q u i d - p h a s e by KESR (59). R o t a t i n g s e c t o r study of a u t o x i d a t i o n (17). I n the presence of 0.2 M α-tetralin hydroperoxide (17)· c

d

Rate constants f o r methylperoxy and ethylperoxy were obtained i n the gas-phase by molecular modulation spectroscopy (59.) and i t was concluded i n t h i s work t h a t changing from the l i q u i d t o the gas phase has only a minor i n f l u e n c e on the magnitude of 2kj.. The order of r e a c t i v i t y , CH 0 *> C H s 0 >(CH ) CH0 i s contrary t o the previous c o n c l u s i o n that 2k^- increases as the α-hydrogen becomes weaker. I t would, t h e r e f o r e , appear f o r these r a d i c a l s t h a t the r e l a t i v e s t a b i l i t y of the i n c i p i e n t carbonyl compound overshadows a bond s t r e n g t h e f f e c t . I t should be noted t h a t there i s a s i g n i f i c a n t discrepancy between the values of 2k determined by the r o t a t i n g s e c t o r method and KESR f o r some of the r a d i c a l s i n Table VI. At the present time we are i n c l i n e d to favour the values obtained from hydro carbon a u t o x i d a t i o n because values o f the propagation r a t e constant are a l s o obtained by t h i s method, the magnitude of which gives an i n t e r n a l check on the value of 2k . Accurate Arrhenius parameters have been determined f o r a few primary and secondary R0 s and examples are given i n Table V I I . e

3

2

2

2

e

3

2

2

t

t

e

f

2


HOWARD

25.

Self-Reactions

of

Alkylperoxy

429

Radicah

Table V I I . Arrhenius parameters f o r s e l f - r e a c t i o n o f some primary and secondary alkylperoxy r a d i c a l s . Reference

1

t

8.7

0

59

Ethylperoxy

7.9

-0.5

59

s-Butylperoxy

9.0

2.7

60

10.0

3.1

60

Methylperoxy

Cyclopentylperoxy


E / k c a l mol"

1

log^/M^s" )

Alkylperoxy

The pre-exponential f a c t o r s f o r these r a d i c a l s are c l o s e t o the normal values f o r l i q u i d - p h a s e bimolecular r e a c t i o n s while the a c t i v a t i o n energies are small and positive. There i s evidence from e.p.r. s t u d i e s (29,36) t h a t g-R02* e x i s t i n e q u i l i b r i u m with a t e t r o x i d e below 173K and i n t h i s respect behave analogously t o t-R02*. Unfortunately S-R02* decay i r r e v e r s i b l y before the t e t r o x i d e i s completely d i s s o c i a t e d and values o f the e q u i l i b r i u m constant cannot be estimated. I t would, however, appear t h a t the thermodynamic parameters f o r S-RO2* are s i m i l a r t o those f o r t-R0 *,e.g., ΔΗ° --7 k c a l m o l " and AS°

Self-Reactions of Alkylperoxy Radicals in Solution (1) - ACS Publications

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