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Electroanalytical investigations on the kinetics of reactions between electrogenerated superoxide ion and organic substrates
J. Electroanal. Chem., 97 (1979) 85--90 © Elsevier Sequoia S.A., Lausanne -- Printed m The Netherlands
85
ELECTROANALYTICAL INVESTIGATIONS ON THE KINETICS OF REACTIONS BETWEEN ELECTROGENERATED SUPEROXIDE ION AND ORGANIC SUBSTRATES
F. MAGNO, G. BONTEMPELLI and M.M. ANDREUZZI SEDEA Istituto di Chimica Analitica, Universit~ degli Studi di Padova, via Marzolo, 1, 35100 Padova (Italy) (Received 29th March 1978, in revised form 16th June 1978) ABSTRACT The reaction between electrogenerated superoxide ion and aromatic halides has been studied in DMF solution by electroanalytical procedure. Evidence of nucleophilic reactions has been gained. Following the procedure of Nicholson and Shain, which employs cyclic voltammetry with different potential scan rates, the second-order rate constants could be evaluated. For p-fluoronitrobenzene it resulted to be 9.0 + 1.5 M -l s -1 while for p-chloronitrobenzene only a superior limit, 2 M -1 s -1 , could be given. In addition a comparison among the kinetic constants relative to the reactions between O~ and some organic substrates is reported. INTRODUCTION In r e c e n t y e a r s c o n s i d e r a b l e a t t e n t i o n has b e e n d e v o t e d t o p r o c e s s e s in w h i c h t h e a c t i v a t i o n o f o x y g e n as s u p e r o x i d e ion, is t h e p r e l i m i n a r y s t e p in t h e i n s e r t i o n o f an o x y g e n a t e d f u n c t i o n in organic m o l e c u l e s [ 1 - - 6 ] . T h e m o t i v a t i o n o f t h e i n t e r e s t in t h e c h e m i s t r y o f t h e s u p e r o x i d e ion is d u e t o t h e well r e c o g n i z e d f u n c t i o n o f this species in several e n z y m a t i c r e a c t i o n s [ 7 - - 1 0 ] . In p a r t i c u l a r t h e r e a c t i o n b e t w e e n s u p e r o x i d e ion a n d an organic m o l e c u l e , can o c c u r e i t h e r as a c o n s e q u e n c e o f a charge t r a n s f e r r e a c t i o n [6] o r via a n u c l e o p h i l i c s u b s t i t u t i o n [ 11]. E l e c t r o a n a l y t i c a l m e t h o d s a p p e a r t o b e v e r y useful in this field o f research as t h e y allow b o t h t h e g e n e r a t i o n o f s u p e r o x i d e ion b y c a t h o d i c r e d u c t i o n o f m o l e c u l a r o x y g e n a n d t h e d e r i v a t i o n o f i n f o r m a t i o n o n r e a c t i o n kinetics involving this e l e c t r o g e n e r a t e d species [ 1 1 - - 1 3 ] . T o e x t e n d o u r investigations dealing w i t h t h e r e a c t i v i t y o f e l e c t r o g e n e r a t e d s u p e r o x i d e ion t o w a r d s organic m o l e c u l e s [ 1 2 , 1 3 ] , we h a v e s t u d i e d t h e a t t a c k o f O r on p - c h l o r o n i t r o b e n z e n e a n d p - f l u o r o n i t r o b e n z e n e in d i m e t h y l f o r m a mide medium with the aim of correlating the reactivity of carbon atoms with d i f f e r e n t c h e m i c a l e n v i r o n m e n t s t o w a r d s t h e s a m e n u c l e o p h i l i c species. EXPERIMENTAL Chemicals and reagents All c h e m i c a l s and r e a g e n t s w e r e p r e p a r e d a n d e m p l o y e d as d e s c r i b e d in a p r e v i o u s p a p e r [ 13].
86
Apparatus and procedure Unless noted otherwise all experiments were carried o u t at 20 -+ 0.1°C. The reference electrode was an aqueous SCE and all potential values are referred to it. The electrical apparatus employed in the voltammetric and coulometric tests has been described in previous papers [ 12,14]. Working electrodes were a mercury covered gold sphere in the voltammetric experiments and a mercury pool in the large scale electrolyses. The reactions between electrogenerated Or and the organic halides were studied under pseudo-first order conditions, the organic c o m p o u n d s being in large excess. The concentration of the organic substrates varied in the range 5 X 10 -3 to 5 X 10 -2 M. The p r o d u c t of the electrolyses, O2NC6H4OH, was identified b y thin-layer and gas chromatographic tests. RESULTS
Preliminary tests have shown that, according to literature data [15], both pfluoronitrobenzene and p-chloronitrobenzene undergo an one-electron reversible reduction process at --1.13 V and --1.08 V respectively, giving radical anions which decay slowly [15]. Figure 1 shows the effect of the presence of the organic substrate on the oxygen reduction: the cathodic current is markedly increased while the anodic
- 1.0,
E/v
SY /f ] ~x~.~0.5
25
.50
-75
Fig. 1. Cyclic v o l t a m m e t r i c curves recorded o n a m e r c u r y covered gold m i c r o e l e c t r o d e in 0.1 M TBAP, D M F solutions. ( . . . . . . ) Saturated with an o x y g e n / n i t r o g e n m i x t u r e (0.25 v/v); ( ~ ) saturated as above and containing 0.05 M p - f l u o r o n i t r o b e n z e n e . Scan rate 0.024 V 6-1.
87
peak relative to the oxidation of O~ to 0 2 is practically absent. In general by increasing the sweep rate the ratio ip/V '/2 (v = potential sweep rate) was progressively lowered approaching the value obtained in absence of the aromatic halide while a progressive increase of the ratio (ip)a/(ip)c up to unity could be observed. These results are typical of an e.c.e, mechanism in which the effect of the interposed chemical step progressively decreases as a consequence of the increased sweep rate. To gain information a b o u t the final products of the reaction under investigation, controlled potential electrolyses were performed under continuous passage o f oxygen at potential values corresponding to its reduction. Thin-layer and gas chromatographic tests carried o u t on the electrolyzed solutions after suitable acidification, indicated that p-nitrophenol was obtained in the investigated reaction. Its yield was found to be about 85% of the theoretical one. Therefore the voltammetric experiments and the large scale electrolyses suggest an e.c.e, mechanism quite similar to those found for the interaction of electrochemically generated Or with other organic species [ 11--13]. O2 + e - - + O~
(1)
O~ + O2NC6H4F~ O2NC6H4OO" + F-
(2)
O2NC6H4OO" + e- -* O:NC6H4OO-
(3)
O2NC6H4OO
- -~ 0 2 N C 6 H 4 0 -
+ '0 2
(4)
The experimental ik/i d values were fitted, as shown in Fig. 2, on a working curve constructed with the data reported by Nicholson and Shain [16]. Changes of 02 concentrations, obtained b y saturating the solutions with oxygen nitrogen mixtures of composition ranging from 0.25 to 0.75 (v/v), did n o t cause a shift of experimental ik/i d values from the reported working curve as required by a reaction of first order with respect to 02 molecule. The adimensional k / a values obtained (where k is the pseudo-first order kinetic constant and a is the function ( n F / R T ) • v plotted against 1/a (see Fig. 3), allowed the determination of the k value. The linear dependence o f the 1.0 Ik
t4
1.2
tO
o:s
~
i
Fig. 2. F i t t i n g o f e x p e r i m e n t a l ik/i d v a l u e s o n t h e w o r k i n g c u r v e d r a w n f r o m r e f . 1 6 .
88
04-
0.3-
0.2-
0.I
o
Fig. 3. Plot of k/a values against 1/a for the determination of k.
apparent kinetic constant on the concentration of the organic substrate in the range 10 -2 to 5 × 10-2'M indicated also for O2NC6H4F a reaction order o f one; at lower concentrations the chemical reaction was too slow, while at higher concentrations the reduction o f the aromatic c o m p o u n d occurred at potential values too near to the reduction peak of oxygen. The mean value of the second order kinetic constant was 9.0 -+ 1.5 M -~ s -~. No data could be obtained for the p-chloronitrobenzene because the kinetic constant of the reaction between O3 and O~NC6H4C1 is too small to be obtained by voltammetric techniques, in the experimental conditions described above. However the occurrence of this reaction could be checked by thin-layer and gas chromatographic tests on solutions previously electrolyzed, which revealed the formation of p-nitrophenol. Furthermore the presence of chloride ions was detected testing it with a silver nitrate-acetonitrile solution. DISCUSSION
The experimental data reported above indicate that electrogenerated O3 is able to carry out a nucleophilic attack (reaction 2) on the aromatic carbon
89 atom bound to the halide function. This statement is supported by the circumstance that a homogeneous charge transfer reaction must be disregarded on the basis of the E1/2 values relative to the reduction o f O~ and O2NC6H4F. However, Frimer and Rosenthal claim t h a t a redox reaction occurs between Of and some halonitrobenzenes [6]. This apparent disagreement can be understood taking into account that in their investigation aromatic compounds, whose reduction potentials are less negative than that of oxygen, were investigated. The formation o f p - n i t r o p h e n o l can be rationalized by assuming t h a t the peroxide anion, generated in the second charge transfer step, undergoes a decomposition (reaction 4) in which 02 is partially regenerated. However it must be pointed out that the good agreement o f our experimental results with the working curve based on the data of Nicholson and Shaln [16], together with the experimental i k / i d values smaller than two, strongly suggest that the last step must be much slower than the chemical reaction (2). The different reaction rate constants relative to O2NC6H4F and O2NC6H4C1 can be explained on the basis of the higher electron with-drawing character of a fluorine atom in respect to that of chlorine. The availability of several kinetic constant values for reactions of superoxide ion with different organic species (see Table 1) allows us to make a comparison among the reactivity of carbon atoms with different chemical environments towards the same nucleophilic agent. Literature data [17,18] suggest that the reaction rate sequence should be C(sp 2 carboxylic) > C(sp 3) > C(sp 2 aromatic) in dipolar protic solvents. On the contrary our experimental sequence of the reaction rates, obtained in DMF medium, is quite different because C(sp 3) > > C(sp 2 carboxylic) ~> C(sp 2 aromatic). The comparable values of the kinetic constants relative to the nucleophilic attack involving both carboxylic and aromatic C atoms can be accounted for by the "solvent e f f e c t " which makes these reactions faster in dipolar aprotic solvents than in protic ones; this effect appears to be more effective towards aromatic compounds than towards carboxylic ones because the susceptibility of SN2 transition-state anions to transfer from protic to dipolar aprotic solvents depends very much on the type of C atom being attacked [17]. TABLE 1 Kinetic constants for the reactions between O~ and organic substrates in DMF medium Organic substrate o II C6HsCOC6Hs O
T/K
Reference
3
293
12
25 11000 3000 500 9 <2
293 293 273 273 293 293
12 13 13 11 Present work Present work
k / M -1 s - l
Jl
C6HsCOC6H4CI C6HsCH2Br C6HsCH2Br C4HgBr O2NC6H4F O2NC6H4C1
90 However the "solvent e f f e c t " can not explain the relevant reactivities of alkyl bromides if compared with those exhibited by the other organic species investigated. A tentative explanation of this anomaly could be advanced by admitting that the reaction between OF and aliphatic halides is a charge transfer reaction instead of a nucleophilic one. In this connection it must be remarked that a redox reaction can be proposed only for the two aliphatic halides as only these compounds undergo irreversible electrochemical reductions; hence their E1/2 reduction values could be markedly inferior to the corresponding thermodynamic values as a consequence of overvoltage contributions. In an effort to ascertain whether aliphatic bromides can undergo homogeneous reduction in our experimental conditions, we tried to reduce ArCH2Br by a reducing agent with an electronic activity like that of O~. It is well known [ 15] that aromatic nitrocompounds undergo, in aprotic solvents, a reversible cathodic reduction giving stable radical anions which are not able to carry out a nucleophilic attack on an alkyl halide. Suitable choice of the nitro-compound, namely o-dinitrobenzene, allowed us to produce, in DMF solution in presence of ArCH2Br, a radical anion, via a reversible electrochemical process, at a potential value (Ep = --0.8) very close to that characteristic o f the reduction o f 02. As a matter o f fact the presence o f ArCH2Br did not affect the regular shape of the voltammetric peaks relative to the C6H4(NO:)2/C6H4(NO2)~ system so suggesting that also Or is not able to reduce it. Hence the rate constant sequence found by us can not at present be explained; it seems to indicate that superoxide ion does not behave as a regular a-nucleophilic agent [18]. ACKNOWLEDGEMENT We thank C.N.R. (Rome) for partial support. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
J.S. F l l i p P o , J r . , C.I. C h e r n a n d J . S . V a l e n t i n e , J. O r g . C h e m . , 4 0 ( 1 9 7 5 ) 1 6 7 8 . R . A . J o h n s o n a n d E G. N l d y , J . Org. C h e m . , 4 0 ( 1 9 7 5 ) 1 6 8 0 . E.J. C o r e y , K.C. N l c o l a o u , M. S h l b a s a k l , J . M a c h i d a a n d C.S. S h i n e r , T e t r a h e d r o n L e t t . ( 1 9 7 5 ) 3 1 8 3 . E . J . C o r e y , K . C . N i c o l a o u a n d M. S h i b a s a k l , J. C h e m . S o c . C h e m . C o m m u n . , ( 1 9 7 5 ) 6 5 8 . S.S. V a l e n t i n e a n d A.B. C u r t i s , J. A m e r . C h e m . S o c . , 9 7 ( 1 9 7 5 ) 2 2 4 . A. F r l m e r a n d I. R o s e n t h a l , T e t r a h e d r o n L e t t . ( 1 9 7 7 ) 2 8 0 9 . D.P. B a l l o u , J . P a l m e r a n d V. M a s s e y , B l o c h e m . B l o p h y s . Res. C o m m u n . , 3 8 ( 1 9 6 9 ) 9 0 5 . J . M . M c C o r d a n d I. F r l d o v l c h , J. Biol. C h e m . , 2 4 4 ( 1 9 6 9 ) 6 0 4 9 . I. F n d o v l c h , A c c o u n t s C h e m . Res., 5 ( 1 9 7 2 ) 3 2 1 . D. K l u g * R o t h , I. F r l d O v l c h a n d J . R a b a n i , J. A m e r . C h e m . S o c . , 9 5 ( 1 9 7 3 ) 2 7 8 6 . R. D l e t z , A . E . J . F o r n o , B . E . L a r c o m b e a n d M . E . P e o v e r , J . C h e m . S o c . (B), ( 1 9 7 0 ) 8 1 6 . F. M a g n o a n d G. B o n t e m p e l l i , J . E l e c t r o a n a l . C h e m . , 6 8 ( 1 9 7 6 ) 3 3 7 . F. M a g n o , R. S e e b e r a n d S. V a l c h e r , J. E l e c t r o a n a l . C h e m . , 83 ( 1 9 7 7 ) 1 3 1 . F. M a g n o , G. B o n t e m p e l l i , G . A . M a z z o c c h i n a n d I. P a t a n ~ , C h e m . I n s t r u m . , 6 ( 1 9 7 5 ) 2 3 9 . C.K. M a n n a n d K . K . B a r n e s , E l e c t r o c h e m i c a l R e a c t i o n s m N o n a q u e o u s S y s t e m s , M a r c e l D e k k e r , New York, 1970. 1 6 R.S. N l c h o l s o n a n d I. S h a m , A n a l . C h e m . , 3 7 ( 1 9 6 5 ) 1 7 8 . 17 R. A l e x a n d e r , E . C . F . K o , A . G . P a r k e r a n d T . J . B r o x t o n , J. A m e r . C h e m . S o c . , 9 0 ( 1 9 6 8 ) 5 0 4 9 . 1 8 N.J. F i n a a n d J . O . E d w a r d s , I n t . J , C h e m . K m e t . , 5 ( 1 9 7 3 ) 1.
85
ELECTROANALYTICAL INVESTIGATIONS ON THE KINETICS OF REACTIONS BETWEEN ELECTROGENERATED SUPEROXIDE ION AND ORGANIC SUBSTRATES
F. MAGNO, G. BONTEMPELLI and M.M. ANDREUZZI SEDEA Istituto di Chimica Analitica, Universit~ degli Studi di Padova, via Marzolo, 1, 35100 Padova (Italy) (Received 29th March 1978, in revised form 16th June 1978) ABSTRACT The reaction between electrogenerated superoxide ion and aromatic halides has been studied in DMF solution by electroanalytical procedure. Evidence of nucleophilic reactions has been gained. Following the procedure of Nicholson and Shain, which employs cyclic voltammetry with different potential scan rates, the second-order rate constants could be evaluated. For p-fluoronitrobenzene it resulted to be 9.0 + 1.5 M -l s -1 while for p-chloronitrobenzene only a superior limit, 2 M -1 s -1 , could be given. In addition a comparison among the kinetic constants relative to the reactions between O~ and some organic substrates is reported. INTRODUCTION In r e c e n t y e a r s c o n s i d e r a b l e a t t e n t i o n has b e e n d e v o t e d t o p r o c e s s e s in w h i c h t h e a c t i v a t i o n o f o x y g e n as s u p e r o x i d e ion, is t h e p r e l i m i n a r y s t e p in t h e i n s e r t i o n o f an o x y g e n a t e d f u n c t i o n in organic m o l e c u l e s [ 1 - - 6 ] . T h e m o t i v a t i o n o f t h e i n t e r e s t in t h e c h e m i s t r y o f t h e s u p e r o x i d e ion is d u e t o t h e well r e c o g n i z e d f u n c t i o n o f this species in several e n z y m a t i c r e a c t i o n s [ 7 - - 1 0 ] . In p a r t i c u l a r t h e r e a c t i o n b e t w e e n s u p e r o x i d e ion a n d an organic m o l e c u l e , can o c c u r e i t h e r as a c o n s e q u e n c e o f a charge t r a n s f e r r e a c t i o n [6] o r via a n u c l e o p h i l i c s u b s t i t u t i o n [ 11]. E l e c t r o a n a l y t i c a l m e t h o d s a p p e a r t o b e v e r y useful in this field o f research as t h e y allow b o t h t h e g e n e r a t i o n o f s u p e r o x i d e ion b y c a t h o d i c r e d u c t i o n o f m o l e c u l a r o x y g e n a n d t h e d e r i v a t i o n o f i n f o r m a t i o n o n r e a c t i o n kinetics involving this e l e c t r o g e n e r a t e d species [ 1 1 - - 1 3 ] . T o e x t e n d o u r investigations dealing w i t h t h e r e a c t i v i t y o f e l e c t r o g e n e r a t e d s u p e r o x i d e ion t o w a r d s organic m o l e c u l e s [ 1 2 , 1 3 ] , we h a v e s t u d i e d t h e a t t a c k o f O r on p - c h l o r o n i t r o b e n z e n e a n d p - f l u o r o n i t r o b e n z e n e in d i m e t h y l f o r m a mide medium with the aim of correlating the reactivity of carbon atoms with d i f f e r e n t c h e m i c a l e n v i r o n m e n t s t o w a r d s t h e s a m e n u c l e o p h i l i c species. EXPERIMENTAL Chemicals and reagents All c h e m i c a l s and r e a g e n t s w e r e p r e p a r e d a n d e m p l o y e d as d e s c r i b e d in a p r e v i o u s p a p e r [ 13].
86
Apparatus and procedure Unless noted otherwise all experiments were carried o u t at 20 -+ 0.1°C. The reference electrode was an aqueous SCE and all potential values are referred to it. The electrical apparatus employed in the voltammetric and coulometric tests has been described in previous papers [ 12,14]. Working electrodes were a mercury covered gold sphere in the voltammetric experiments and a mercury pool in the large scale electrolyses. The reactions between electrogenerated Or and the organic halides were studied under pseudo-first order conditions, the organic c o m p o u n d s being in large excess. The concentration of the organic substrates varied in the range 5 X 10 -3 to 5 X 10 -2 M. The p r o d u c t of the electrolyses, O2NC6H4OH, was identified b y thin-layer and gas chromatographic tests. RESULTS
Preliminary tests have shown that, according to literature data [15], both pfluoronitrobenzene and p-chloronitrobenzene undergo an one-electron reversible reduction process at --1.13 V and --1.08 V respectively, giving radical anions which decay slowly [15]. Figure 1 shows the effect of the presence of the organic substrate on the oxygen reduction: the cathodic current is markedly increased while the anodic
- 1.0,
E/v
SY /f ] ~x~.~0.5
25
.50
-75
Fig. 1. Cyclic v o l t a m m e t r i c curves recorded o n a m e r c u r y covered gold m i c r o e l e c t r o d e in 0.1 M TBAP, D M F solutions. ( . . . . . . ) Saturated with an o x y g e n / n i t r o g e n m i x t u r e (0.25 v/v); ( ~ ) saturated as above and containing 0.05 M p - f l u o r o n i t r o b e n z e n e . Scan rate 0.024 V 6-1.
87
peak relative to the oxidation of O~ to 0 2 is practically absent. In general by increasing the sweep rate the ratio ip/V '/2 (v = potential sweep rate) was progressively lowered approaching the value obtained in absence of the aromatic halide while a progressive increase of the ratio (ip)a/(ip)c up to unity could be observed. These results are typical of an e.c.e, mechanism in which the effect of the interposed chemical step progressively decreases as a consequence of the increased sweep rate. To gain information a b o u t the final products of the reaction under investigation, controlled potential electrolyses were performed under continuous passage o f oxygen at potential values corresponding to its reduction. Thin-layer and gas chromatographic tests carried o u t on the electrolyzed solutions after suitable acidification, indicated that p-nitrophenol was obtained in the investigated reaction. Its yield was found to be about 85% of the theoretical one. Therefore the voltammetric experiments and the large scale electrolyses suggest an e.c.e, mechanism quite similar to those found for the interaction of electrochemically generated Or with other organic species [ 11--13]. O2 + e - - + O~
(1)
O~ + O2NC6H4F~ O2NC6H4OO" + F-
(2)
O2NC6H4OO" + e- -* O:NC6H4OO-
(3)
O2NC6H4OO
- -~ 0 2 N C 6 H 4 0 -
+ '0 2
(4)
The experimental ik/i d values were fitted, as shown in Fig. 2, on a working curve constructed with the data reported by Nicholson and Shain [16]. Changes of 02 concentrations, obtained b y saturating the solutions with oxygen nitrogen mixtures of composition ranging from 0.25 to 0.75 (v/v), did n o t cause a shift of experimental ik/i d values from the reported working curve as required by a reaction of first order with respect to 02 molecule. The adimensional k / a values obtained (where k is the pseudo-first order kinetic constant and a is the function ( n F / R T ) • v plotted against 1/a (see Fig. 3), allowed the determination of the k value. The linear dependence o f the 1.0 Ik
t4
1.2
tO
o:s
~
i
Fig. 2. F i t t i n g o f e x p e r i m e n t a l ik/i d v a l u e s o n t h e w o r k i n g c u r v e d r a w n f r o m r e f . 1 6 .
88
04-
0.3-
0.2-
0.I
o
Fig. 3. Plot of k/a values against 1/a for the determination of k.
apparent kinetic constant on the concentration of the organic substrate in the range 10 -2 to 5 × 10-2'M indicated also for O2NC6H4F a reaction order o f one; at lower concentrations the chemical reaction was too slow, while at higher concentrations the reduction o f the aromatic c o m p o u n d occurred at potential values too near to the reduction peak of oxygen. The mean value of the second order kinetic constant was 9.0 -+ 1.5 M -~ s -~. No data could be obtained for the p-chloronitrobenzene because the kinetic constant of the reaction between O3 and O~NC6H4C1 is too small to be obtained by voltammetric techniques, in the experimental conditions described above. However the occurrence of this reaction could be checked by thin-layer and gas chromatographic tests on solutions previously electrolyzed, which revealed the formation of p-nitrophenol. Furthermore the presence of chloride ions was detected testing it with a silver nitrate-acetonitrile solution. DISCUSSION
The experimental data reported above indicate that electrogenerated O3 is able to carry out a nucleophilic attack (reaction 2) on the aromatic carbon
89 atom bound to the halide function. This statement is supported by the circumstance that a homogeneous charge transfer reaction must be disregarded on the basis of the E1/2 values relative to the reduction o f O~ and O2NC6H4F. However, Frimer and Rosenthal claim t h a t a redox reaction occurs between Of and some halonitrobenzenes [6]. This apparent disagreement can be understood taking into account that in their investigation aromatic compounds, whose reduction potentials are less negative than that of oxygen, were investigated. The formation o f p - n i t r o p h e n o l can be rationalized by assuming t h a t the peroxide anion, generated in the second charge transfer step, undergoes a decomposition (reaction 4) in which 02 is partially regenerated. However it must be pointed out that the good agreement o f our experimental results with the working curve based on the data of Nicholson and Shaln [16], together with the experimental i k / i d values smaller than two, strongly suggest that the last step must be much slower than the chemical reaction (2). The different reaction rate constants relative to O2NC6H4F and O2NC6H4C1 can be explained on the basis of the higher electron with-drawing character of a fluorine atom in respect to that of chlorine. The availability of several kinetic constant values for reactions of superoxide ion with different organic species (see Table 1) allows us to make a comparison among the reactivity of carbon atoms with different chemical environments towards the same nucleophilic agent. Literature data [17,18] suggest that the reaction rate sequence should be C(sp 2 carboxylic) > C(sp 3) > C(sp 2 aromatic) in dipolar protic solvents. On the contrary our experimental sequence of the reaction rates, obtained in DMF medium, is quite different because C(sp 3) > > C(sp 2 carboxylic) ~> C(sp 2 aromatic). The comparable values of the kinetic constants relative to the nucleophilic attack involving both carboxylic and aromatic C atoms can be accounted for by the "solvent e f f e c t " which makes these reactions faster in dipolar aprotic solvents than in protic ones; this effect appears to be more effective towards aromatic compounds than towards carboxylic ones because the susceptibility of SN2 transition-state anions to transfer from protic to dipolar aprotic solvents depends very much on the type of C atom being attacked [17]. TABLE 1 Kinetic constants for the reactions between O~ and organic substrates in DMF medium Organic substrate o II C6HsCOC6Hs O
T/K
Reference
3
293
12
25 11000 3000 500 9 <2
293 293 273 273 293 293
12 13 13 11 Present work Present work
k / M -1 s - l
Jl
C6HsCOC6H4CI C6HsCH2Br C6HsCH2Br C4HgBr O2NC6H4F O2NC6H4C1
90 However the "solvent e f f e c t " can not explain the relevant reactivities of alkyl bromides if compared with those exhibited by the other organic species investigated. A tentative explanation of this anomaly could be advanced by admitting that the reaction between OF and aliphatic halides is a charge transfer reaction instead of a nucleophilic one. In this connection it must be remarked that a redox reaction can be proposed only for the two aliphatic halides as only these compounds undergo irreversible electrochemical reductions; hence their E1/2 reduction values could be markedly inferior to the corresponding thermodynamic values as a consequence of overvoltage contributions. In an effort to ascertain whether aliphatic bromides can undergo homogeneous reduction in our experimental conditions, we tried to reduce ArCH2Br by a reducing agent with an electronic activity like that of O~. It is well known [ 15] that aromatic nitrocompounds undergo, in aprotic solvents, a reversible cathodic reduction giving stable radical anions which are not able to carry out a nucleophilic attack on an alkyl halide. Suitable choice of the nitro-compound, namely o-dinitrobenzene, allowed us to produce, in DMF solution in presence of ArCH2Br, a radical anion, via a reversible electrochemical process, at a potential value (Ep = --0.8) very close to that characteristic o f the reduction o f 02. As a matter o f fact the presence o f ArCH2Br did not affect the regular shape of the voltammetric peaks relative to the C6H4(NO:)2/C6H4(NO2)~ system so suggesting that also Or is not able to reduce it. Hence the rate constant sequence found by us can not at present be explained; it seems to indicate that superoxide ion does not behave as a regular a-nucleophilic agent [18]. ACKNOWLEDGEMENT We thank C.N.R. (Rome) for partial support. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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