6
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Reactions of Aminyl Radicals and Mechanisms of Amine Regeneration as Inhibitors of Oxidation E. T. DENISOV Institute of Chemical Physics, USSR Academy of Sciences, 142 432 Chernogolovka, USSR
Diphenylaminyl radicals, In•, produced in tetraphenylhydrazine decomposition, react with the formation of diphenyl amine and oligomeric semidienes. Diphenyl amine is formed in the reaction of In• with labile dimers, AmAm, with iminoquinone structure. The rate constant of Am•recombination measured by flash photolysis technique (FTP) was found to be 1.8x10 l.mole s (cyclohexane, 293°K). The combination of In• andRO •was studied by FPT using monitoring at two different wavelengths. It runs with the rate constant of 6x10 l.mole s (cyclohexane, 293°K) with formation of iminoquinone as the main product. RadicalsAm •react with cumylhydroperoxide very fast (k = 1.1x10 l.mole s , cyclohexane, 293°K, FTP). The mechanism of this reaction is complicated at [ROOH] > 5xl0 mole 1-1 by parallel consecutive reaction with proton transfer and InH+• formation and electron transfer from ROO to InH+•. Quinone imine retards the oxidation of n-heptadecane (393°K) with regeneration of 4-hydroxydiphenylamine in the presence of hydroperoxide. 7
-1
-1
2
8
-1
-1
5
-1
-1
5
-3
Each aromatic amine molecule, InH, terminates many free radical chains in autooxidation of alcohols and amines due to the ability of oxyperoxy and aminoperoxy radicals to oxidize InH as well as to reduce In to InH CO. However, the coefficient of inhibition, f > 2, can be very often observed in oxidizing hydrocarbons too (_2) . Therefore, some reduction of aminyl radicals to InH proceeds in oxidizing hydrocarbons. To ellucidate the ways of such reduction we have studied the products and kinetics of the reactions of diphenylaminyl radical In*. Recombination of Aminyl Radicals. A convenient source of In* is tetraphenylhydrazine, Inln. Diphenylamine and oligomeric semidienes were found to be the products of Inln decomposition at T > 400°K (.3*4) . The kinetics of their formation were studied by Varlamov C5). He found that InH and o-semidienes were primary stable products of 0097-6156/85/0280-0087$06.00/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
88
P O L Y M E R STABILIZATION A N D D E G R A D A T I O N
I n l n d e c o m p o s i t i o n and p - s e m i d i e n e s were formed a f t e r any p e r i o d o f induction. InH was proposed t o be formed i n t h e f o l l o w i n g sequence of r e a c t i o n s (4^,5^):
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In-
+ In'
-
-
In-C H =N-C H 6
5
6
—
5
InH + (
C
6
H
5
)
2
N C
6 4^ 6 5 H
C
H
As t h e r e s u l t o f such r e a c t i o n pathways, o l i g o m e r i c s e m i d i e n e s were formed. The y i e l d o f InH was found t o be 80% on decomposed I n l n (CCl^, 348°K). Such r e a c t i o n s may be t r e a t e d as r e g e n e r a t i o n o f InH from In* . The k i n e t i c s o f I n l n d e c o m p o s i t i o n a r e f i r s t o r d e r ( 6 ) ; t h e i n t r o d u c t i o n of a free r a d i c a l acceptor i n c r e a s e s the rate_^onst^ant. F o r example, I n l n decomposes w i t h t h e r a t e c o n s t a n t 6.3x10 s in C C l ^ at 348°K and i n t h e j j r e s e n c e o f N - p h e n y l - 2 - n a p h t h y l a m i n e t h e r a t e c o n s t a n t i s 14.8x10 s . This suggests that the decomposition o f I n l n i n c l u d e s an e q u i l i b r i u m s t a g e and may be d e s c r i b e d by t h e f o l l o w i n g scheme:
From t h e d a t a mentioned k
2
/
k
-L
=
° '
7
4
l ~
2In
above,
one can e s t i m a t e
k
Inln
2 —
k
Products
the r a t i o
,
The k i n e t i c s o f In* r e c o m b i n a t i o n were s t u d i e d by FPT i n c y c l o hexane s o l u t i o n (_7). The c o n c e n t r a t i o n o f In* was measured s p e c t r o p h o t o m e t r i c a ^ l y at X_= 77(j) nm. The e x t i n c t i o n c o e f f i c i e n t was found t o be 3.9x10 l.mole cm . I n . r a d i c a l s were found t o d i s a p p e a r i n a bimo^ecular r ^ a c | i o n with the o v e r a l l r a t e constant of 1.8x10 l.mole s , which i s t h e sum o f r a t e c o n s t a n t s on Inr e c o m b i n a t i o n i n J o _ J n I n and q u i n i n e i m i n e ^ f o y m a t i o n e q u a l t o 0.75x10 l.mole s and 1.05x10 l.mole s , respectively (cyclohexane, 293°K). R e a c t i o n o f In* With R O ' ^ The k i n e t i c s o f In- r e a c t i o n w i t h RO^ from c y c l o h e x a n e were a l s o s t u d i e d by FPT (1). D i - t - b u t y l p e r o x i d e was d e composed p h o t o c h e m i c a l l y , and t h e r e a c t i o n o f TCH„)^C0* r a d i c a l s w i t h cyclohexane produced c y c l o h e x y l r a d i c a l s . The l a t t e r were t r a n s formed i n t o p e r o x y r a d i c a l s R0£ a f t e r t h e a d d i t i o n o f o x y g e n . R a d i c a l s In* were formed by t h e p h o t o l y s i s o f I n l n . The k i n e t i c s o f RO' and In* d i s a p p e a r a n c e were m o n i t o r e d s i m u l t a n e o u s l y at X = 270 and 770 nm, r e s p e c t i v e l y . Under t h e e x p e r i m e n t a l c o n d i t i o n s when [R0 ] » [In* ] , In* d i s a p p e a r e d m a i n l y by t h e r e a c t i o n w i t h R0 » Because o f t h e slow r e c o m b i n a t i o n o f RO^* t h e r e i s a n e a r l y c o n s t a n t R0 concentration ( ~ 6 x l 0 mole.l ). The r a t e c o n s t a n t o f t h e r e a c t i o n w i t h ^ I n ^ w i t h ROl was e s t i m a t e d t o be e q u a l t o the 6x10 l.mole s i n t h e t e m p e r a t u r e range o f 2 8 3 - 3 0 3 ° K i n c y c l o hexane. The d e c o m p o s i t i o n o f I n l n i n t h e p r e s e n c e o f c u m y l h y d r o p e r o x i d e y i e l d e d d i p h e n y l a m i n e , quinone i m i n e , cumyl a l c o h o l and acetophenone ( 8 ) . D i p h e n y l n i t r o x i d e was d e t e c t e d by EPR t e c h n i q u e s . The comparison o f t h e c o n c e n t r a t i o n s o f decomposed ROOH and InH and ROH formed shows t h a t ROOH i s n o t t h e o n l y donor o f h y d r o g e n atoms but a p a r t o f InH and ROH was formed i n t h e r e a c t i o n s o f In* and R0" with l a b i l e intermediate products. The most p r o b a b l e p r o d u c t s a r e 2
q
2
2
l a b i l e quinone i m i n e s ° 2 6 5 6 4 ^ 6 5^2 6 5 6 5* w i t h I n and Ro* forming InH and ROH. Quinone imine a p p a r e n t l y R
C
H
C
H
a
n
d
C
H
N C
H
N C
H
w
h
i
c
h
In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
r
e
a
is
c
t
6.
DENISOV
formed from RO„C^H^NC H,_. So RO^ r e a c t s benzene rriinn gg o or r aminyl aminyl r a d i c a l : R0
+ In
2
RO! + In* 2
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89
Aminyl Radical Reactions and Amine Regeneration w i t h a n i t r o g e n atom and
—-
RO* + InO*
—
R0 C,H,NC.H, 2 o D b D o
The Mechanisms o f A m i n y l R a d i c a l R e a c t i o n w i t h ROOH. The k i n e t i c s o f In* r e a c t i o n w i t h c u m y l h y d r o p e r o x i d e were s t u d i e d i n c y c l o h e x a n e at 393°K u s i n g F P T . The c o n c e n t r a t i o n o f In* was measured s p e c t r o p h o t o m e t r i c a l l y at X = 812 nm. Aminyl r a d i c a l d i s a p p e a r e d i n t h e p r e s e n c e of ROOH i n two r e a c t i o n s , namely In* + In* and In* + ROOH. r e a c t i o n r a t e c o n s t a n t o f In* w i t h ROOH at [ROOH] = 1 . 4 - 5 . 1 x 1 0 was c a l c u l a t e d v i a computer from experimental_£in|tic curves o f In* d i s appearance and found t o be 1.1x10 l.mole s . A new a b s o r p t i o n was observed a t _ ^ m a x = 6^0 nm i n experiments w i t h [ROOH] > 10 mole.l . T h i s i s a t t r i b u t e d t o a r a d i c a l - c a t i o n InH* , which may be formed by p r o t o n a t i o n o f a m i n y l r a d i c a l : +
In*
+ HOOR
»
InH* + R0~ +
The k i n e t i c s o f In* and IriR' consumption were measured at X = 712 nm. It was found t h a t I n ' + InH* d i s a p p e a r i n t h e p r e s e n c e o f ROOH w i t h f i r s t o r d e r k i n e t i c s , t h e observed r a t e c o n s t a n t k k [ROOH] A l n D ^ ^ / t decreases with increasing hydroperoxide concentration. The f o r m a t i o n o f I n H . and dependence o f k [ROOH] may be e x p l a i n e d by t h e f o l l o w i n g k i n e t i c scheme: +
g
=
+
Q b s
In*
+ HOOR
InH
In*
+ HOOR
InH* + R 0
2
InH
2
+ InH* + R 0
o
+
o
n
R0'
+ R0
2
The treatment of t h e e x p e r i m e n t a l (lata according t o t h i s scheme^gives the f o l l o w i n g v a l u e s : k = 1.1x10 l.mole s , K = 42 l . m o l e , k^ = 1.0x10 s~" ( 2 9 3 ° K , c y c l o h e x a n e ) . When ROOH c o n c e n t r a t i o n i s h i g h enough ( 0 . 1 m o l e . l o r more) t h e r e a c t i o n runs v i a p r o t o n a t i o n of In* and e l e c t r o n t r a n s f e r . Quinoneimine as an I n h i b i t o r o f O x i d a t i o n R e a c t i o n s . Quinoneimine, C H NC H 0 , was found t o r e t a r d o x i d a t i o n f o r a l o n g p e r i o d o f t i m e . If quinoneimine i s introduced i n t o o x i d i z i n g n-heptadecane (393°K) containing hydroperoxide, i t retards oxidation. The h i g h e r t h e hydroperoxide c o n c e n t r a t i o n the stronger the r e t a r d i n g a c t i o n of quinoneimine. Aminophenol, HOC^H^NC^H , was found i n s m a l l c o n c e n t r a t i o n among t h e r e a c t i o n p r o d u c t s . T h i s means t h a t q u i n o n e i m i n e i s reduced t o aminophenol i n o x i d i z i n g h y d r o c a r b o n i n t h e p r e s e n c e o f hydroperoxide. S i n c e t h e aminophenol i s o x i d i z e d by peroxy r a d i c a l s to quinoneimine, the opposing c y c l i c o x i d a t i o n - r e d u c t i o n r e a c t i o n s proceed i n o x i d i z i n g h y d r o c a r b o n w i t h p a r t i c i p a t i o n o f R0« and ROOH.
In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
P O L Y M E R STABILIZATION
90
A N D DEGRADATION
Hence, the extra chain termination (f > 2) when aromatic amines are used as inhibitors of oxidation may be the result of the following reactions: In* + In-
> InC H NC H 6
5
6
5
RO: + InC^H NC^H > ROOH + InCJH. NC.H,, 2 6565 6465 In' + RO! > R0 C.H,.NC.H_ 2 2oD o 5 In + R0 C,H_NC H •> InH + RO* + 0C,H. NC H. 26565 6465 ROOH + 0C H,NC^H 5> . . . > HOC.H. NHC.H_ 6465 6 4 6 5 All the reactions mentioned above make the inhibiting coefficient f much more than 2, when amines are used as inhibitors. At present we do not know either the role of alkyl radicals in such reactions or the stages of quinoneimine reduction to aminophenol . Literature Cited c
c
o
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o
c
£
1. 2. 3. 4. 5. 6. 7. 8.
c
C
c
Denisov, E.T. in "Developments in Polymer Stabilization 3"; Scott, G., Ed.; Appl. Sci. Publ. LTD: London, 1980; pp. 1-20. Berger, H.; Bolsman T.A.B.M.; Brower, D.M. in "Developments in Polymer Stabilization 6"; Scott, G., Ed.; Appl. Sci. Publ. LTD: London, 1983; pp. 1-27. Musso, H. Chem. Ber. 1959, 92, 2881. Welzel, P. Chem. Ber. 1970, 103, 1318. Varlamov, V.T. Izv. Akad. Nauk SSSR, ser.khim. 1982, 1481. Varlamov, V.T. Izv. Akad. Nauk SSSR, ser.khim. 1982, 1629. Varlamov, V.T. Safiullin, R.L.; Denisov, E.T. Khimicheskaya Fizika 1983, 408. Varlamov, V.T.; Denisov, E.T. Kinetika i Kataliz 1983, 24, 547.
RECEIVED December 7,1984
In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
