1996 Caffeic Acid J Electroanal Chem 405 169

8/15/2019 1996 Caffeic Acid J Electroanal Chem 405 169

1/8

E L S E V I E R J o u r n a l o f E lec t r n an a lyt i ca l C h emis t r y 4 0 5 ( 1 9 9 6 ) 1 6 9 - 1 7 6

JOURN L OF

x idat ion o f ca f fe ic ac id and re la ted hydroxy c innam ic ac ids

P h i l i p p e H a p i o t a A n d r e a s N e u d e c k a J e a n P i n s o n

a

H 6 1 ~ ne F u l c r a n d b

P e d a t s u r N e t a c C h r i s ti a n R o l a n d o d

a L a b o r a t o i r e d ' E l e c t r o c h i m i e M o l ~ c u l a i r e d e l " U n i v e r s i t ~ P a r i s 7 , U n i t ~ d e R e c h e r c h e A s s o c i ~ e a u C N R S N o . 4 3 8 , 2 p l a c e J u s s i e u ,

7 5 2 5 1 P a r i s C e d e x 0 5 , F r a n c e

b L a b o r a t o i r e d e s P o l y m b r e s e t d e s T e c h n i q u e s P h y s i c o - c h i m i q u e s , l n s t i t u t d e s P r o d u i t s d e l a V i g n e , 9 , p l a c e V i a l a , 3 4 0 6 0 M o n t p e l l i e r C e d e x , F r a n c e

c C h e m i c a l K i n e t i c s a n d T h e r m o d y n a m i c D i v i s io n , N a t i o n a l I n s t i t u te o f S t a n d a r d s a n d T e c h n o l og y , G a i t h e r sb u r g , A I D 2 0 8 8 9 , U S A

a L a b o r a t o i r e d e l ' A c t i v a t i o n M o l ~ c u l a i r e , E c o l e N o r m a l e S u p ~ r i e u r e , U n i t ~ A s s o c i ~ e a u C N R S N o . 1 1 1 0 ,

U n i t~ A s so c i ~ e ~ l ' U n i v e r si t ~ P i e r r e e t M a r i e C u r i e, 2 4 r u e L h o m o n d , 7 5 2 3 1 P a r i s C e d e x 0 5 , F r a n c e

Receiv ed 29 Septem ber 1995; in revised form 16 Octobe r 1995

A b s t r a c t

Oxidation of caffeic acid (3,4-dihydroxycinnamic acid) 1H a has been studied by electrochemical methods and by pulse radiolysis in

aqueous and organic solvents. The results h ave been c omp ared with the behaviour o f 4-coumaric acid 2 H 2 and ferulic acid 3H 2. The first

oxidative intermediates have b een characterised by the ir UV spectra and oxidation potentials. In the ca se o f 2H 2 and 3H 2, the initial

radicals d ecay by a second order process indicating a radic al-rad ical coupling mechanism. On the contrary, fo r caffeic acid the oxidation

leads to the form ation of the corresponding o-quinone through disproportionation of the initial semiquinone radical.

K e y w o r d s : Electrooxidat ion; Pulse radiolys is ; Caffeic acid ; 4-Couma ric acid ; Ferul ic acid

1 I n t r o d u c t i o n

P o l y p h e n o l s a x e t h e m a i n s e c o n d a r y m e t a b o l i t e s i n

p l a n ts . T h e y p a r t l y d e t e r m i n e t h e q u a l i t y o f t h e p l a n t - d e -

r i v e d f o o d p r o d u c t s , c o n f e r r i n g u p o n t h e m b e n e f i c i a l o r

d e t r i m e n t a l e f f e c t s . I n p a r t i c u l a r , t h e b r o w n i n g p r o c e s s

w h i c h a l s o t a k e s p l a c e i n w i n e s , m a y b e d e s i r a b l e f o r s o m e

o f t h e m ( s h e r r y , M a d e i r a , M a r s a l a ) w h e r e a s i t m a y b e

u n d e s i r a b l e f o r o t h e r s ( w h i t e w i n e s ) [ 1 ]. T h e b r o w n i n g i s

r e l a t e d t o t h e e n z y m a t i c o x i d a t i on o f p h e n o l s t o t h e c o r r e -

s p o n d i n g h i g h l y r e a c t i v e o - q u i n o n e s [ 2 ,3 ] . E n z y m a t i c o x i -

d a t i o n i s t h e m o s t i m p o r t a n t r e a c t i o n i n t h e p r e s e n c e o f

p o l y p h e n o l o x i d a s e [ 4 , 5 ] . H o w e v e r , n o n - e n z y m a t i c a u t o -

o x i d a t i o n c a n a l s o c a n t a k e p l a c e i n t h e p r e s e n c e o f

o x y g e n , m o r e e s p e c i a l l y w h e n t h e m e d i u m i s a l k a l i n e [ 6 ] .

T h e d i f f e r e n t s t e p s l e a d i n g t o t h e f o r m a t i o n a n d t o f u r t h e r

r e a c t i o n o f q u i n o n e s t o p r o d u c e c o n d e n s a t i o n p r o d u c t s a r e

n o t f u l l y u n d e r s t o o d a n d o n l y a f e w h y p o t h e s e s h a v e b e e n

p r o p o s e d c o n c e r n i n g t h e i r m e c h a n i s m [ 7 , 8 ] .

E l e c t r o c h e m i s t r y [ 9 - 1 1 ] a n d p u l s e r a d i o l y s i s [ 9 , 1 2 ]

t e c h n i q u e s p r o v i d e p o w e r f u l t o o l s f o r th e s t u d y o f r e a c t i o n

m e c h a n i s m s i n v o l v i n g e l e c t r o n t r a n s f e r a n d a f f o r d c o m p l e -

m e n t a r y i n f o r m a t i o n [ 9 , 1 3 - 1 6 ] . T h e f i r s t t e c h n i q u e a l l o w s

t h e c h a r a c t e r i s a t io n o f t h e i n t e r m e d i a t e s f o r m e d u p o n e l e c -

t r o n t r a n s f e r b y t h e i r r e d u c t i o n - o x i d a t i o n p a t t e r n [ 1 0 , 1 1 ] .

0 0 2 2 - 0 7 2 8 /9 6 / 1 5 . 0 0 © 1 9 96 E l s ev ie r S c ien ce S .A . A l l r ig h t s r e s e r v ed

S S D 0 0 2 2 - 0 7 2 8 ( 9 5 ) 0 4 4 1 2 -4

I t a l s o c a n p r o v i d e s p e c t r o s c o p i c i n f o r m a t i o n o n i n t e r m e d i -

a t e s w i t h l i f e t i m e s f r o m 1 s t o m i n u t e s b y c a r r y i n g o u t

e l ec t ro ly s i s i n a t h in l ay e r sp ec t ro e l ec t ro ch em ica l ce l l [1 7 ] .

I n s p i t e o f th e w i d e r a n g e o f l i f e t i m e s c o v e r e d b y c o m b i n a -

t i on o f t h e a b o v e e l e c t r o c h e m i c a l t e c h n i q u e s , i t i s d e s i ra b l e

t o h a v e a d d i t i o n a l i n d e p e n d e n t m e t h o d s t h a t a l lo w v e r i f i c a -

t i on a n d c o m p a r i s o n o f t h e r e s u l ts . F o r t h i s p u r p o s e , w e

u t i l i s ed p u l se r ad io ly s i s w h ich p e rm i t s t h e ch a rac t e r i sa t i o n

o f s h o r t - l i v e d i n t e r m e d i a t e s b y r e c o r d i n g t h e i r U V - v i s

s p e c t r a . T h e i r r e a c t i o n k i n e t i c s c a n a l s o b e f o l l o w e d s t a rt -

i n g in t h e m ic ro seco n d r an g e [1 2 ] .

W e h a v e u s e d t h e s e t e c h n i q u e s t o i n v e s ti g a t e t he o x i d a -

t i o n o f c a f f e i c a c i d 1 H 3 ( 3 , 4 - d i h y d r o x y c i n n a m i c a c i d ), a n

a n a l o g u e o f c a f t a r i c a n d c o u t a r i c a c i d s w h i c h a r e r e s p o n s i -

b l e f o r t h e b r o w n i n g o f w h i t e w i n e s . W e a l s o h a v e e x a m -

i n e d t h e o x i d a t i o n o f 4 - c o u m a r i c a c i d 2 H 2 ( 4 - h y d r o x y c i n -

n a m i c a c i d ) a n d f e r u l ic a c i d 3 1 -1 2 ( 3 - m e t h o x y - 4 - h y d r o x y -

c i n n a m i c a c i d ) . C o m p o u n d s o b t a i n e d b y c h e m i c a l o x i d a -

t i o n ( s o d i u m p e r i o d a t e ) o f c a f f e i c a c i d i n a c i d i c c o n d i t i o ns

( p H = 2 - 7 ) h a v e b e e n p r e v i o u s l y c h a r a c t e r i s e d a s t w o

s t e r e o i s o m e r s o f 2 , 5 - ( 3 , 4 - d i h y d r o x y p h e n y l ) t e t r a h y d r o -

f u r a n - 3 , 4 - d i c a r b o x y l i c a c i d [ 1 8 ,1 9 ] . T h e h a l f - w a v e p o t e n -

t ia l o f t h e o - q u i n o n e f o r m e d i n t h e o x i d a t i o n o f c a f f e ic

a c i d h a s b e e n d e t e r m i n e d b y p o l a r o g r a p h i c r e d u c t i o n i n

w a t e r b e t w e e n p H 1 a n d 9 [ 2 0 ] . T h e o n e - e l e c t r o n r e d o x


2/8

17 P. Hapiot et al. / Journal o f Electroanalytical Chemistry 405 1996) 169-176

p o t e n t i a l o f c a f f e i c a c i d h a s b e e n m e a s u r e d b y p u l s e

rad io ly s i s 0 . 0 8 4 V S H E ) ) [2 1] an d th e r eac t i o n o f 4 -h y -

d r o x y c i n n a m i c a c i d w i t h s e v e r a l r a d i c a l s h a s b e e n e x a m -

ined [22].

H O

H O ~ C O O H

C a f f e i c a c i d 1 H 3

H O ~ C O O H

C o u m a r i c a c i d 2 H z

H C o H ~ C O O H

F e r u l ic a c i d 3 I t 2

p u r p o s e - b u i l t p o t e n t i o s t a t e q u i p p e d w i t h a p o s i t i v e f e e d -

b a c k c o m p e n s a t i o n d e v i c e [ 2 3 ] . T h e d a t a w e r e a c q u i r e d

w i th a 3 1 0 N ico l e t o sc i l l o sco p e .

S p e c t r o e l e c t r o c h e m i c a l e x p e r i m e n t s w e r e p e r f o r m e d

w i t h a c a p i l l a r y - s li t - c e l l f o r U V - v i s s p e c t r o s c o p y u s in g

g o l d - L I G A - s t r u c t u r e s a s a n o p t i c a l t r a n s p a r e n t e l e c t r o d e

[ 1 7] . T h e g o l d - L I G A - f o i l s u s e d w e r e 1 0 0 / x m t h i c kn e s s

a n d o f a h o n e y c o m b s t r u ct u re w i t h 2 0 / z m d i a m e t e r h e x a g -

o n a l h o l e s w i th a w a l l t h i ck n ess b e tw een th e h o l e s o f 1 5

/ z m . T h e g o l d - L I G A - s t r u c t u r e w a s f i x e d i n a c a p i ll a r y s li t

o f 1 5 0 / z m . T h e c o m p l e t e e l e c t r o c h e m i c a l c o n v e r s i o n i n

th e cap i l l a ry s l i t o ccu r r ed i n l e s s t h an 2 s . A s i l v e r w i r e

c o a t e d w i t h s i l v e r c h l or i d e w a s u s e d a s r e f e r e n c e e l e c t r o d e .

I n o r g a n i c s o l v e n t s , t h e c o r r e s p o n d i n g p h e n o l a t e s w e r e

p r e p a r e d i n s i t u b y n e u t r a l i s a t i o n a f t e r r e m o v a l o f o x y g e n

b y b u b b l i n g a r g o n t o p r e v e n t a i r o x id a t i o n o f t h e p h e n o l a t e

[ 24 ]. T h e b a s e w a s a 2. 7 M s o l u t io n o f N M e a O H - 5 H 2 0

i n m e t h a n o l .

2.3 . Radiolyt ic experiments

2 Ex p er im en ta l

2.1. Chemicals

A c e t o n i t r il e A C N ) w a s U v a s o l q u a l i ty M e r c k ) a nd

w a s u s e d a s - r e c e i v e d . D i m e t h y l s u l p h o x i d e w a s f r o m

A l d r i c h . T h e s u p p o r t i n g e l e c t r o l y t e N E t 4 B F 4 an d th e b ase

N M e 4 O H - 5 H 2 0 w e r e f r o m F l u k a p u ri s s) . C a f f e ic a c id

a n d t h e d i f f e r e n t h y d r o x y c i n n a m i c a c i d s w e r e c o m m e r -

c i a l l y a v a i l a b l e A l d r i c h ). C a f f e i c a c i d w a s p u r if i e d b y

f l a s h c h r o m a t o g r a p h y b e f o r e u s e . p H s i n h y d r o - o r g a n i c

s o l v e n ts w e r e m e a s u r e d w i t h a g l a s s e l e c t ro d e s t a n d a r d i s e d

i n a q u e o u s b u f f e r s .

2.2. Cyclic voltamm etry an d spectroelectrochemical exper

iments

A l l t h e c y c l ic v o l t a m m e t r y e x p e r i m e n t s w e r e c a r r i e d

o u t a t 2 0 + 0 .1 ° C u s i n g a c e l l e q u i p p e d w i t h a j a c k e t

a l l o w i n g c i r c u l a t i o n o f w a t e r f r o m t h e t h e r m o s t a t . T h e

c o u n t e r e l e c t r o d e w a s a P t w i r e a n d t h e r e f e r e n c e e le c t r o d e

a n a q u e o u s s a t u r a t e d c a l o m e l e l e c t r o d e E ° S C E ) = E °

S H E ) - 0 . 2 4 1 2 V ) w i th a s a l t b r id g e co n ta in in g t h e su p -

p o r t i n g e l e c t r o d e . T h e S C E e l e c t r o d e w a s c h e c k e d a g a i n s t

t h e f e r r o c e n e / f e r r o c e n i u m c o u p l e E ° = + 0 . 40 5 V S C E ) )

b e f o r e a n d a f t e r e a c h e x p e r i m e n t .

F o r l ow s c a n r a te c y c l ic v o l t a m m e t r y 0 . 0 5 - 5 0 0 V

s - 1 , t h e w o r k i n g e l e c t r o d e w a s e i t h e r a g l a s s y c a r b o n d i s k

3 m m d i a m e t e r T o k a i C o r p .) o r a 1 m m d i a m e t e r g o l d

d i s k . T h e y w e r e c a r e f u l l y p o l i s h e d b e f o r e e a c h v o l t a m m o -

g r a m w i t h 1 / x m d i a m o n d p a s t e a n d u l t r a s o n i c a l l y r i n s e d

i n a b s o l u t e e t h a n o l . E l e c t r o c h e m i c a l i n s t r u m e n t a t i o n c o n -

s i s te d o f a P A R M o d e l 1 75 U n i v e r s a l p r o g r a m m e r a n d a

F o r r a d i o l y s i s e x p e r i m e n t s , f r e s h s o l u t i o ns o f t h e p h e n o -

l i c c o m p o u n d s u n d e r a n a e r o b i c c o n d i t i o n s w e r e p r e p a r e d

b y b u b b l i n g N 2 0 i n w a t e r p u r i f ie d w i th a M i l li p o r e

S u p e r - Q s y s t e m ) a n d t h e n a d d i n g N a N 3 a nd K O H a n d

f i n a l l y t h e p h e n o l . T h i s p r o c e d u r e a v o i d s c o n t a c t b e t w e e n

th e p h en o la t e a n d 0 2 an d p rev en t s au to -o x id a t io n . T h e

so lu t i o n w as t h en t r an s fe r r ed i n to t h e i r r ad i a t i o n ce l l w i th -

o u t b e i n g e x p o s e d t o a i r . T h e p u l s e r a d i o l y s i s a p p a r a t u s

h as b een d esc r ib ed b e fo re [2 5 ] . I t u t i l i s e s 5 0 n s p u l ses o f 2

M e V e l e c t r o n s f r o m a F e b e t r o n 7 0 5 a c c e l e r a t o r . T h e d o s e

p e r p u l s e w a s v a r i e d b e t w e e n 8 a n d 8 0 G y , t o p r o d u c e

b e t w e e n 5 a n d 5 0 m M o f o x id i s i n g r a d i c a ls , a s d e t e rm i n e d

b y S C N - d o s i m e t r y [ 26 ]. T h e k in e t i c s p e c t ro p h o t o m e t r i c

d e t e c t i o n s y s t e m c o n s i s t e d o f a V a r i a n 3 0 0 W x e n o n l a m p ,

a 2 cm o p t i ca l p a th l en g th i r r ad i a t i o n ce l l , a K ra to s h ig h

i n t e n s i t y m o n o c h r o m a t o r , a n R C A 4 8 4 0 p h o t o m u l t i p l i e r ,

an d th e ap p ro p r i a t e sh u t t e r s , l en ses , an d o p t i ca l f i l t e r s . T h e

s ig n a l s w ere d ig i t i s ed w i th a T ek t ro n ix 7 6 1 2 t r an s i en t

r e c o r d e r a n d a n a l y s e d b y c o m p u t e r . A l l e x p e r i m e n t s w e r e

car r ied ou t a t ro om tem per atu r e 22 __+ 2°C.

3 Resu l t s

3.1. O xidation of caffeic acid

3.1 .1 . Cycl ic vol tammetry

T h e o x i d a t i o n o f c a f f e i c a c i d 1 H 3 w a s f i r s t s t u d ie d b y

c y c l i c v o l t a m m e t r y i n w a t e r a t d i f f e r e n t p H v a l u e s a n d i n

sev e ra l w a te r + o rg an ic so lv en t m ix tu res . A ty p i ca l

v o l t a m m o g r a m o f t he o x i d a t i o n o f a 1 0 - 3 m o l 1 - 1 c a f f e i c

ac id so lu t i o n i n w a te r a t p H = 4 a t a s can r a t e o f 0 . 2 V s -

i s s h o w n i n F ig . 1 . A c h e m i c a l l y r e v e r s i b l e v o l t a m m o g r a m

E p~ = + 0 . 2 7 ,

E p a =

+ 0 . 3 9 V S C E ) ) i s o b s e r v e d t h a t


3/8

P . Ha p io t e t a l . / J o u rn a l o f E l e c t ro a n a ly t i c a l C h e mis t ry 4 0 5 1 9 96 ) 1 6 9 - 1 7 6 1 71

20 ~.A

~ r

I

- 0 . 4 0 . 0 0 . 4

E / V SC E)

F i g . 1 . C y c l i c v o l t a m m e t r y o f 1 0 - 3 t o o l 1 - J s o l u t i o n o f c a f f e i c a c i d in

w a t e r a t p H = 4 o n a 3 m m g l a s s y c a r b o n e l e c t r o d e . S c a n r a t e v =

0 . 2 V s - I .

can be cycled without noticeable decrease of the current.

The number of electrons per molecule was determined to

be two by comparison with the intensity of the peak

current of ferrocenemethanol under the same conditions.

The same general pattern is observed in (50/50) water

(pH = 4) + dimethyl formamide (Er~ = + 0.21, E v a =

+ 0.36 V (SCE)) and (50/50) water (pH = 4) + methanol

(E ~ = +0. 15, Eva = +0 .2 6 V (SCE)) mixtures. The elec-

trochemical behaviour was studied as a function of pH in a

(5 0/5 0) water + methanol mixture. By increasing the pH

(of the water component) above four, the oxidation poten-

tial becomes gradually more positive and the peak poten-

tial separation increases. This behaviour is in agreement

with what is expected for a quinone + hydroquinone sys-

tem. At pH > 10 the reverse reduction peak disappears,

indicating that under these conditions the generated o-

quinone is not stable.

In dimethylsulphoxide (D MSO) solutions containing 0.1

M NEtaBF4 as supporting electrolyte, on a glassy carbon

electrode, 1H 3 shows an irreversible oxidat ion peak at 0.2

V s -l (E p= + 0.8 5 V (SCE)) (see Fig. 2(a)). Upon

addition of two equivalents o f base (NMe4OH • 5H2 0) to

neutralise both the COOH group and one of the phenol

groups, to form 1 a 2 - , the oxidation wave appears at a less

positive potential (Fig. 2(b)). This wave becomes totally

reversible at scan rates higher than 0.5 V s- l and irre-

versible for scan rates lower than 0.05 V s -~. The height

of the anodic peak for the oxidation of 1H 2- corresponds

to the transfer of one electron, as shown by comparison

with t r i - t e r t - b u t y l phenolate under the same experimental

conditions. The E°potential for this oxidation can be deter-

mined from the reversible voltammogram as the midpoint

between the anodic and cathodic peak potentials (E ° =

-0 .3 1 V (SCE)) which can be ascribed to the reversible

couple 1 HE -/ 1H - . After the addition of a third equiva-

lent of base in order to achieve complete neutralisation of

the acid, the trianion 13- behaves as a quasi-reversible

system on a gold or G.C. electrode A E p = 90 mY, I v c / l p a

= 1), with E ° = - 0 .8 6 V (SCE), down to a scan rate of

50 mV s -~ (Fig. 2(c)). This first reversible system is

followed by a second partially reversible one (see Fig.

2(c)). These two peaks were found to be monoelectronic

and were ascribed to the reversible couples 1 3 - / 1 2 - . a n d

1 2 - / 1 -. Similar studies performed in ACN exhibit the

same general features with minor differences: the oxida-

tion of 1H a presents an irreversible peak around + 1.2 V

(SCE) (more positive than in DMSO) and the second

oxidation of 13- is completely irreversible. The electro-

chemical characteristics are summarised in Table 1.

I ~ (a)

I I I

- 0 . 7 0 . 1 0 . 9

o.5~ ~ b)

- 0 . 7 - 0 . 3 0 . 1

o . 4 ~ j ~

(c)

I [ I

- 1 . 2 - 0 . 4 0 . 4

E / V S C E )

F i g . 2 . C y c l i c v o l t a m m e t r y o f t h e o x i d a t i o n o f 1 0 - 3 m o l 1 - ~ s o l u t i o n o f c a f f e i c a c i d i n d i m e t h y l s u l p h o x i d e o n a 1 m m g l a s s y c a r b o n e l e c t r o d e + 0 . I M

N E t 4 B F 4 a s s u p p o r t i n g e le c t r o l y t e ) . a ) I H 3 , b ) I H 2 - , c ) 1 3 - . S c a n r a t e v = 0 . 2 V s - t .


4/8


5/8

P . Ha p io t e t a l . / J o u rn a l o f E l e c tro a n a ly t ic a l C h e mis try 4 0 5 1 9 9 6 ) 1 6 9 - 1 7 6

173

O . 2

0.16

0.12

0 . 0 8 •

• I

0 . 0 4

0 . . . . | * i i . . . . i . . ~ - . . ~

3 8 0 4 2 0 4 6 0 5 0 0 5 4 0

~ , / n m

Fig. 4. D ifferential absorption spectra after pulse radiolysis of

1 0 - 3

mol

1-t of caffeic acid in 0.1 M NaN3 aqueous solution saturated with N20

(pH = 5.7). Monitored af ter 1 gs ( I ) and 10 ms (0) .

c o m p l e t e i n 1 / xs o r l e s s . D i f f e r e n t i a l a b s o r p t i o n s p e c t r a

m o n i t o r e d a t v a r i o u s t i m e s a f t e r t h e p u l s e a r e p r e s e n t e d i n

F i g . 4 f o r s l i g h t l y a c i d i c p H ( p H = 5 .7 ) . T h e i n i ti a l s p e c -

t r u m ( 1 / z s a f te r t h e p u l s e ) c a n b e a s s i g n e d t o th e c o r r e -

s p o n d i n g o - s e m i q u i n o n e ; t h e s p e c t r u m o b t a i n e d a f te r 1 m s

i s a s c r i b e d t o s u b s e q u e n t p r o d u c t s . T h e o - s e m i q u i n o n e

e x h i b i t s a b r o a d p e a k w i t h a m a x i m u m a r o u n d 4 8 0 n m

( 6 = 1 9 0 0 1 m o l - ] c m - 1 ) s i m il a r t o t he s p e c tr a o f o t h e r

c o n j u g a t e d p h e n o x y l r a d i c a l s [ 1 5 , 1 6 ] . T h i s a b s o r p t i o n i s i n

t h e s a m e w a v e l e n g t h r a n g e a s w e f o u n d b y s p e c t r o e l e c t r o -

c h e m i s t r y f o r th e s a m e i n t e r m e d i a t e i n A C N . H o w e v e r , i n

t h e r a d i o l y s i s e x p e r i m e n t s a t p H = 6 - 1 2 , t h e s p e c t r u m

s h o u l d c o r r e s p o n d t o t h a t o f t h e d e p r o t o n a t e d f o r m o f t h e

s e m i q u i n o n e 1 2 - . a s th e p K a o f o - s e m i q u i n o n e r a d i c a l s

a r e k n o w n t o b e a r o u n d f i v e [ 2 8 ] . A f t e r 1 0 m s a b r o a d

p e a k i s o b s e r v e d w i t h A n ~x = 4 1 0 n m , w h i c h c a n b e

a s c r i b e d t o t h e o - q u i n o n e 1 H b y c o m p a r i s o n w i th a n

a u t h e n t i c s a m p l e u n d e r i d e n t i c a l c o n d i t i o n s [ 1 9 ] . B y v a r y -

i n g t h e d o s e p e r p u l s e , t o p r o d u c e d i f f e r e n t i n i t i a l c o n c e n -

t r a t io n s o f t h e s e m i q u i n o n e r a d i c a ls , 1 2 - ' , i t w a s f o u n d t h a t

t h e d e c a y o b e y e d a s e c o n d - o r d e r r a t e l a w , w i t h 2 k ] = 8 . 4

X 10 7 1 m o l - 1 s - 1 . ( s e e F i g . 4 ). A t t h is p H , s o m e o t h e r

a b s o r p t i o n s c o r r e s p o n d i n g t o d i f f e r e n t s p e c i e s w i t h d i f f e r -

e n t k i n e t i c e v o l u t i o n s w e r e a l s o o b s e r v e d a t l o w w a v e -

l e n g t h s ( A < 4 0 0 n m ) . T h e s e a r e a s c r i b e d t o t h e i n t e r m e d i -

a t e f o r m e d b y t h e r e a c t i o n o f c a f f e ic a c i d w i t h a s m a l l

f r a c t i o n o f t h e s o l v a t e d e l e c t r o n s ( i n c o m p e t i t i o n w i t h t h e

r e a c t i o n o f e~q w i t h N 2 0 ) . T h i s w a s d e m o n s t r a t e d b y

a d d i t i o n o f i s o p r o p a n o l t o t h e s o l u t i o n t o r e a c t w i t h t h e

O H r a d i ca l , t o p r e v e n t f o r m a t i o n o f t h e o x i d i s i n g ra d i c a l

N 3 r a d i c a l , a n d t o l e a v e t h e s o l v a t e d e l e c t r o n a s th e o n l y

r e a c t i v e s p e c i e s r e m a i n i n g i n t h e s o l u t i o n .

S i m i l a r s p e c t r a w e r e o b s e r v e d a t h i g h e r p H b u t , b e c a u s e

o f t h e s t r o n g a b s o r b a n c e o f t h e p a r e n t m o l e c u l e s a t t h e s e

p H v a l u e s , i t w a s n o t p o s s i b l e t o r e c o r d s p e c t r a a t w a v e -

l e n g th s b e l o w 4 2 0 - 4 3 0 n m . T h e a b s o r b a n c e w a s t h e r ef o r e

m o n i t o r e d a t 4 8 0 n m t o s t u d y t h e d e c a y o f t h e in i ti a l

s e m i q u i n o n e . L a r g e v a r i a t i o n s o f t h e s e m i q u i n o n e r a d i c a l

l i f e ti m e w i t h p H w e r e o b s e r v e d ( s e e T a b l e 2 ) . I n b a s i c

s o l u t i o n s ( p H = 1 1 . 3 ) th e s e m i q u i n o n e w a s r e l a t iv e l y s t a-

b l e , w i t h a l i f e - t i m e l o n g e r t h a n 1 0 0 m s , w h e r e a s a t

p H = 5 . 7 t h e r a d i c a l d e c a y e d a b o u t 2 0 0 0 t i m e s m o r e

r a p i d l y . T h e s e f i n d i n g s a r e i n a g r e e m e n t w i t h t h e u s u a l

b e h a v i o u r o f q u i n o n e / h y d r o q u i n o n e c o u p l e s an d c o rr e -

s p o n d t o a d i s p r o p o r t i o n a t i o n r e a c t i o n l e a d i n g t o t h e f o r -

m a t i o n o f th e o - q u i n o n e f r o m t h e s e m i q u i n o n e . F o r e x a m -

p l e , f o r t h e w e l l k n o w n

p - b e n z o q u i n o n e / p - h y d r o q u i n o n e

c o u p l e , t h e s e m i q u i n o n e i n t e r m e d i a te b e c o m e s u n s t a b le a t

p H < 1 0 in w a t e r [ 2 9 ]. A s i m i l a r b e h a v i o u r w a s f o u n d f o r

t h e s e m i q u i n o n e f o r m e d b y o x i d a t i o n o f t h e m e t h y l e s t e r

o f t h e c a f f ei c a c i d , w h i c h h a s a n a b s o r b a n c e p e a k a t 5 2 0

n m ( 6 = 2 8 0 0 1 m o l - t c m - ] ) a n d a l a rg e a b s o r b a n ce i n

t h e U V b e l o w 4 2 0 n m . R a d i o l y t i c o x i d a t i o n s w e r e a l s o

p e r f o r m e d b y c o n t i n u o u s i r r a d i at i o n o f a q u e o u s s o l u t i o n s

t o m o n i t o r s p e c t r a o f s t a b le p r o d u c t s . S o l u t i o n s s i m i l a r t o

t h o s e u s e d i n t h e p u l s e r a d i o l y s i s e x p e r i m e n t s w e r e i r r a d i -

a t e d i n a g a m m a s o u r c e a n d t h e s p e c t r a w e r e m o n i t o r e d

a f t e r v a r i o u s i r r a d i a t i o n t i m e s . T h e r e s u l t s o f th i s t e c h n i q u e

w e r e c o m p a r a b l e w i t h t h o s e o f s p e c t r o e l e c t r o c h e m i s t r y : i n

t h e r a d i o l y s i s t h e o x i d i s i n g s p e c i e s a r e p r o d u c e d c o n t i n u -

o u s l y i n a n h o m o g e n e o u s f a s h i o n , w h i l e i n t h e s e c o n d c a s e

a c o n t i n u o u s o x i d a t i o n i s p r o v i d e d b y t h e e l e c t r o d e . I n

b o t h m e t h o d s t h e r e a c t i o n i s f o l l o w e d b y r e c o r d i n g a b s o r p -

t i o n s p e c t r a . T h e s p e c t r u m o b t a i n e d u p o n i r r a d i a t i o n o f

c a f f e i c a c i d a t p H = 6 . 3 ( F i g . 5 ) s h o w s a d e c r e a s e o f t h e

3 1 0 n m p e a k a n d a n i n c r e a s e o f a b r o a d a b s o r p t i o n a t

3 4 0 - 3 5 0 n m . A f t e r l o n g e r i r r a d i a t i o n t i m e s a l o n g t a i l

g r o w s a t h i g h e r w a v e l e n g t h , a p p a r e n t l y i n d i c a t in g p o l y m e r

f o r m a t i o n .

Table 2

Pulse radiolysis: characteristics of the radicals in w ater

Compound Radical

hma t

¢rnax pH 2k/1.m ol- 1 s- 1

/ n m / 1 m o l -

1 c m i

Caffeic acid (1H 3) 12 . 480 1900 5.7 8.4 × 107

1 1 . 6 2 . 6 X

104

12.6 stable

Caffeic acid methyl ester (4H 2) 4 - 520 2800 8.0 (8 + 3) × 107

4-Coum aric acid (2H 2) 2 - 600 1900 6.3 7.3 × 10s

7.3 6.8 × lOs

12.05 7.5 × l0 s

Ferulic acid (3I-I2) 3 - 550 900 7.1 7.0 × 108


6/8

1 7 4 P. Hapiot et al . /Jo ur na l of Electroanalytical Chemistry 405 1996) 169-1 76

3 2 Oxidation of monohydroxycinnamic acids

3 2 1 Cyclic voltammetry

F o r c o m p a r i s o n w i t h t h e r e s u l t s o f c a f f e i c a c id , t w o

m o n o h y d r o x y c i n n a m i c a c i d s w i th t h e s a m e g e n e r a l s k e le -

t o n w e r e e x a m i n e d b y c y c l i c v o lt a m m e t r y u n d e r t he s a m e

c o n d i t io n s : 2 H 2 ( 4 - c o u m a r i c a c i d ) a n d 3 H 2 ( f e r u l ic a c i d ).

T h e c y c l i c v o l t a m m e t r y o f 2 H 2 a n d 3 H 2 i n 5 0 / 5 0 w a t e r

+ m e t h a n o l m i x t u r e s w a s i n v e s t i g a t e d a t d i f f e r e n t p H

v a lu es . A t n eu t r a l o r s l i g h t ly ac id i c p H , t h ey ex h ib i t an

i r r e v e r s ib l e o n e - e l e c t r o n o x i d a t i o n p e a k . T h e p e a k p o s i t i o n

v a r i e s l i n e a r l y w i t h t h e p H w i t h a s l o p e c l o s e t o 5 8 m V

p H - l ( t h e t h eo r e t ic a l s l o p e f o r 1 H ÷ a n d 1 e - ) . A t h i g h e r

p H , t h e v a l u e o f t he p e a k p o t e n t i a l r e m a i n s c o n s t a n t u p t o

p H = 1 3 . T h i s p H d ep e n d en ce in d i ca t e s t h a t i t i s t h e

d e p r o t o n a t e d f o r m s 2 2 - o r 3 2 - w h i c h a r e o x id i s e d , e v e n

a t p H b e l o w t h e p K a , to g i v e t h e p h e n o x y l r a d i c a ls

[ 9 , 1 3 - 1 6 ] . T h i s e l e c t r o c h e m i c a l b e h a v i o u r i s v e r y s i m i l a r

to t h e r e su l t s o b t a in ed fo r t h e a l co h o l s [9 , 1 6 ] o r a ld eh y d es

[ 1 5 ] d e r i v e d f r o m h y d r o x y c i n n a m i c a c i d .

I n n e u t r a l A C N , 2 H 2 a n d 3 H 2 e x h i b i t i r r e v e r s ib l e

b r o a d p e a k s a t 1 .0 a n d 0 .9 5 V ( S C E ) r e s p e c t i v e l y . O n t h e

r e v e r s e s c a n ( i f th e o x i d a t i o n o f 3 H 2 h a s b e e n s c a n n e d

b e f o r e h a n d ) a b r o a d r e d u c t i o n p e a k i s v is i b l e a r o u n d 0 . 5 V

( S C E ) w h i c h c a n b e a s c r i b e d t o t h e f o r m a t i o n o f d i m e r s o r

h i g h e r o l i g o m e r s . T h e o x i d a t i o n p e a k p o t e n t i a l w a s f o u n d

to in c rease l i n ea r ly w i th t h e l o g a r i t h m o f t h e scan r a t e ,

w i t h a s l o p e a r o u n d 4 0 m V p e r l o g ( v ) , w h i c h i n d i c a t e s a

m ix ed co n t ro l o f t h e g lo b a l k in e t i c s , i .e . b y th e e l ec t ro n

t r a n s f e r s t e p a n d b y a s u b s e q u e n t c h e m i c a l r e a c t i o n . T h i s

i s i n ag reem en t w i th t h e o x id a t io n o f 3 I - I 2 t o t h e p h e n o x o -

n i u m c a t i o n 3 H 2 + w h i c h i s l i k e l y t o b e a s t r o n g a c i d a n d

to d ep ro to n a t e r ap id ly .

I n o r d e r t o a v o i d t h e r e a c t i o n s o w i n g t o t h e p r o t o n

t r a n s f e r, t h e o x i d a t io n o f t h e c o r r e s p o n d i n g p h e n o l a t e s w a s

i n v e s ti g a t e d . A f t e r a d d i t io n o f t w o e q u i v a l e n t s o f b a s e

( N M e 4 O H - 5 H 2 0 ) , t he p h e no l a te 2 2 - e x h i b i t s i n D M S O

0

0.5

. ~ t

i

|

300 400 500

k / n m

F i g . 5 . U V - v i s s p e c t ra l c h a n g e s d u r i n g i rr a d i a ti o n o f a 4 . 4 × 1 0 . 4 m o l

- 1 o f c a f f e i c a c i d i n 0 . 0 3 M N a N 3 a q u e o u s s o l u t i o n s a t u r a t e d w i t h N 2 0

( p H = 6 . 3 ). N u m b e r s o n e a c h s p e c t r u m a r e t h e i r r a d i a t i o n t i m e s i n

m i n u t e s .

- 0 .0 8

O.S ~tA

I

-0 .8 -0 .4 0 .0

E / V ( S C E )

a )

~'~ -0 .12

- 0 . 1 4

(b)

-0. 16 , 11 t t , i

- 1 .5 - - 0 .5 0 0 .5

l o g (v )

F i g . 6 . C y c l ic v o l ta m m e t r y o f b a si c 3 2 - i n D M S O ( + 0 . 1 m o l 1 - 1

N E t 4 B F 4 ) o n a 1 m m d i a m e t e r g o l d e l e c t r o d e . ( a ) c ° = 1 0 - 3 t o o l 1 - i ,

v = 0 . 2 V s - ~ . ( b ) V a r i a t i o n o f t h e p e a k p o t e n t i a l a s a f u n c t i o n o f t h e

l o g a r i t h m o f t h e s c a n r a t e ( v ) .

a n d i n A C N a w e l l - d e f i n e d m o n o e l e c t r o n i c i r r e v e r s i b l e

o x i d at i o n p e a k ( E p = - 0 . 0 2 V ( S C E ) in D M S O a n d

- 0 . 0 3 V ( S C E ) i n A C N a t 0 . 2 V s - l ) . S i m i la r r e s ul ts

w e r e f o u n d f o r t he o x i da t i on o f 3 2 - ( E p = - 0 . 1 5 V

( S C E ) i n D M S O a n d i n A C N a t 0 .2 V s - 1 ). F o r b o t h

p h e n o l a t e s , a r e d u c t io n p e a k c o u l d b e o b s e r v e d o n t h e

r e v e r s e s c a n a r o u n d - 0 . 4 V ( S C E ) ( F i g . 6 ( a )) . T h e v a r i a -

t i o n o f t h e o x id a t io n p eak p o ten t i a l w i th t h e scan r a t e w as

i n v e s t i g a t e d i n D M S O w h e r e r e p r o d u c i b l e r e s u l t s c a n b e

o b t a i n e d . T h e p e a k p o t e n t i a l w a s f o u n d t o v a r y l i n e a r l y

w i th t h e scan r a t e b e tw een 0 . 0 5 a n d 1 0 V s - ~ w i th a s lo p e

o f 2 1 m V d e c a d e - ~ o f t h e s c a n r a t e . T h i s s l o p e i s c o n s is -

t e n t w i t h a m e c h a n i s m i n v o l v i n g a f a s t e l e c t r o n t r a n s f e r

b e t w e e n t h e e l e c t r o d e a n d t h e p h e n o l a t e f o l l o w e d b y a n

i r r e v e r s i b l e c o u p l i n g o f t w o r a d i c a l s ( D I M 1 m e c h a n i s m )

[111.

3 2 - , . 3 - - + e - ( E o )

2 k l

3 - + 3 - ~ d i m e r

S i m i l a r r e s u l t s l e a d in g t o t h e s a m e m e c h a n i s t i c c o n c l u -

s i o n s w e r e o b t a i n e d w i t h t h e n e u t r a l i s e d 4 - c o u m a r i c a c i d

2 2 - . T h e s e r e s u l ts a r e v e r y s i m i l a r t o t h o s e p r e v i o u s l y

o b t a i n e d f o r t h e c o r r e s p o n d i n g a l c o h o l s [ 16 ] a n d a l d e h y d e s

[ 1 5 ] w h e r e a c o u p l i n g b e t w e e n t w o p h e n o x y l r a d i c a l s w a s

f o u n d t o b e t h e k e y s t e p i n t he f o r m a t i o n o f t h e d i m e r .


7/8

P. Hapiot et al. / Journal of Electroanalytical Chemistry 405 1996) 169-176 1 7 5

0 . 0 2

0 . 0 1 5

~ O . O 1

<

0 . ~ 5

. . . . i . . ° . - . . . . i . . . L

3 9 0 4 3 0 4 7 0 S l 0 5 5 0 5 9 0 6 3 0

k / n m

0 . 0 2 .

0 . 0 1 5 [ ( b )

0 . 0 0 5

0 - * - - ' - - * - • ' - - * - '

3 9 0 4 3 0 4 7 0 5 1 0 5 5 0 5 9 0 6 3 0

k / n m

F i g . 7 . D i f f e r e n t i a l a b s o r p t i o n s p e c t r a a f t e r p u l s e r a d i o l y s i s o f 1 0 - 3 m o l

I - I o f a ) 2 H z a n d b ) 3 H z a c i d s i n 0 . 1 M N a N 3 a q u e o u s s o l u t i o n

s a t u r a t e d w i t h N 2 0 p H = 1 2 ) . M o n i t o r e d 1 / . ~ s a f t e r t h e p u l s e .

3 2 2 P u l s e r a d i o l y s i s

P u l s e r a d i o l y s i s e x p e r i m e n t s w e r e p e r f o r m e d w i t h t h e

m o n o h y d r o x y c i n n a m i c a c i d s 2 H 2 a n d 3 H 2 u n d e r t h e s a m e

c o n d i t i o n s a s t h o s e u s e d f o r t h e o x i d a t i o n o f c a f f e i c a c i d .

T h e s p e c t r a o f t h e f i rs t i n te r m e d i a t e s r e c o r d e d 1 g s a f t e r

t h e p u l s e , a s c r i b e d t o t h e g e n e r a t e d p h e n o x y l r a d i c a l , a r e

p r e s e n t e d i n F i g . 7. T h e p h e n o x y l r a d i c a l o f 4 - c o u m a r i c

a c i d s h o w s a l a r g e a b s o r b a n c e b e l o w 4 0 0 n m a n d a n o t h e r

o n e i n t h e v i s i b l e r a n g e , w i t h a p e a k a t h = 6 0 0 n m

( 6 = 1 9 0 0 1 m o l - 1 c m - 1 ) . T h e p h e n o x y l r a d ic a l o f f e r u l ic

a c i d s h o w s a b r o a d w e a k e r a b s o r b a n c e i n t h e v i s i b l e r a n g e

w it h a m a x i m u m a t A = 5 1 0 - 5 5 0 n m ( 6 = 9 0 0 1 t oo l - 1

c m - l ) . B o t h r a d i c a l s e x h i b i t e d a n o th e r in t e n s e a b s o r p t i o n

a t l o w e r w a v e l e n g t h s w h i c h w e r e n o t d e t e r m i n e d b e c a u s e

o f i n t e r f e r e n c e b y t h e a b s o r p t i o n o f t h e p a r e n t p h e n o l . T h e

d e c a y o f t h e p h e n o x y l r a d i c a l d e r i v e d f r o m 4 - c o u m a r i c

a c i d w a s s t u d ie d i n d e t a il a t d i f f e r e n t p H v a l u e s , p h e n o l a t e

c o n c e n t r a t i o n s ( 0 . 7 t o 7 m M ) , a n d d o s e s ( r a d i c a l c o n c e n -

t r a t i o n s b e t w e e n 5 a n d 5 0 m M ) . I n a l l c a s e s , t h e d e c a y o f

t h e r a d i c a l f o l l o w e d a s e c o n d - o r d e r r a t e l a w , w i t h 2 k = 7

× 1 0 8 1 m o l - 1 s - 1 ( c a l c u l a t i o n b a s e d o n t h e m o l a r a b -

so rp t iv i ty i n T ab le 2 ) . T h e f in d in g th a t t h e d ecay r a t e

c o n s t a n t o f t h e p h e n o x y l r a d i c a l d o e s n o t c h a n g e w i t h t h e

in i t i a l co n cen t r a t i o n o f p h en o la t e i n d i ca t e s t h a t t h e r eac t io n

p r o c e e d s v i a a r a d i c a l - r a d i c a l m e c h a n i s m a n d t h a t t h e

p o ss ib l e a d d i t i o n o f t h e r ad i ca l t o t h e p a ren t m o le cu le i s

t o o s l o w t o b e o b s e r v e d u n d e r t h e s e c o n d i t i o n s . F e r u l i c

a c i d e x h i b i t e d a s i m i l a r b e h a v i o u r a n d i ts s e c o n d o r d e r

d e c a y r a t e c o n s t a n t i s r e p o r t e d i n T a b l e 2 . ( T h e s e d i m e r i s a -

t i o n r a te c o n s t a n t s a r e o f t h e s a m e o r d e r o f m a g n i t u d e a s

th o se r ep o r t ed in t h e l i t e r a tu re [9 , 1 5 , 1 6 ] . )

4 . D i s c u s s i o n a n d c o n c l u s i o n s

T h e e l e c t r o c h e m i c a l o x i d a t i o n o f 4 - c o u m a r i c a n d f e r u l i c

a c i d s i n w a t e r + o r g a n i c s o l v e n t m i x t u r e s t a k e s p l a c e b y

e l e c t r o n t r a n s f e r f r o m t h e c o r r e s p o n d i n g p h e n o l a t e i o n

e v e n a t p H v a l u e s l o w e r t h a n t h e p K a . T h e p h e n o l a t e i s

o x i d i s e d t o t h e p h e n o x y l r a d i c a l w h i c h t h e n d i m e r i s e s b y

r a d i c a l - r a d i c a l c o u p l i n g , a c c o r d i n g t o t h e g e n e r a l p a t t e r n :

P h O H . P h O - + H +

P h O - • . P h O + e -

P h O + P h O ~ d i m e r

R a d i o l y t i c o x i d a t i o n b y t h e a z i d y l r a d i c a l i n a q u e o u s

so lu t io n s i s r ap id w i th b o th t h e p h en o la t e i o n an d th e

n e u t r a l p h e n o l s u n d e r s t u d y . T h e d i m e r i s a t i o n o f t h e p h e -

n o x y l r ad i ca l s i s f a s t , w i th r a t e co n s t an t s o f th e o rd e r o f

1 0 8 1 m o 1 -1 s -1 , s im i l a r t o t h e v a lu e s fo u n d fo r t h e

a l c o h o l s a n d a l d e h y d e s d e r i v e d f r o m h y d r o x y c i n n a m i c

acid .

T h e c h e m i c a l b e h a v i o u r o f c a f f e i c a c i d i s v e r y d i ff e r e n t .

C y c l i c v o l t a m m e t r y i n w a t e r s h o w s a r e v e r s i b l e t w o e l e c -

t r o n w a v e , s p e c t r o e l e c t r o c h e m i s t r y a t p H - ~ 4 s h o w s t h e

s p e c t r u m o f o - q u i n o n e , a n d d e c a y o f th e r a d i c al o b s e r v e d

b y p u l s e r a d i o l y s i s a l s o r es u l t s i n f o r m a t i o n o f t he o -

q u in o n e . In n eu t r a l o r ac id i c p H th e o -q u in o n e i s r ap id ly

f o r m e d b y d i s p r o p o r t i o n a t i o n o f t h e o - s e m i q u i n o n e w i t h

r e a c t i o n t i m e s l o w e r t h a n 1 0 - 4 s .

Q - ' + Q - ' + 2 H + , . Q + Q H 2

T h e o b s e r v a t i o n o f a r e v e r s i b l e t w o - e l e c t r o n t r a n s f e r i n

e l e c t r o c h e m i s t r y c o n f i r m s t h a t t h e f o r m a t i o n o f t h e o -

q u i n o n e i s f a s t a n d a l s o e q u i l i b r a t e d . T h e s a m e g e n e r a l

b e h a v i o u r i s o b s e r v e d w i t h t h e m e t h y l e s t e r o f c a f f e i c a c i d.

I n a p r o t i c s o l v e n t s , t h e s a m e o - q u i n o n e i s a l s o o b ta i n e d b y

a t w o - s t e p o x i d a t i o n i n v o l v i n g p r o t o n e x c h a n g e p r e - e q u i -

l i b r i u m p r i o r t o t h e e l e c t r o n t r a n s f e r . T h e s e m i q u i n o n e

i n t e r m e d i a t e s h a v e b e e n c h a r a c t e r i s e d b y t h e i r r e d o x p o -

t e n t i a l s a n d U V s p e c t r a .

T h e f a c t t h a t t h e o - q u i n o n e i s f o r m e d d u r i n g t h e o x i d a -

t i o n o f c a f f e i c a c i d i s i n a g r e e m e n t w i t h w h a t w a s p r e v i -

o u s l y d e s c r i b e d f o r t h e h o m o g e n e o u s o x i d a t i o n o f c a f f e i c

ac id b y d i f f e ren t o x id an t s [1 8 , 1 9 an d r e fe ren ces c i t ed

t h e re i n ]. T h e f o r m a t i o n o f t h e c o u p l i n g p r o d u c t s i s o b -

s e r v e d i n w a t e r a n d o r g a n i c s o l v e n t s d u r i n g c o n t i n u o u s

r a d i o l y s i s o r e l e c t r o l y s i s e x p e r i m e n t s ( e x p e r i m e n t a l t i m e

o f t h e o r d e r o f s e v e r a l m i n u t e s ) . T h i s i n d i c a t e s t h a t t h e

o - q u i n o n e i s f o r m e d p r i o r t o t h e c o u p l i n g p r o d u c t s a n d i s

i n v o l v e d i n t h e f o r m a t i o n o f t h e s e p r o d u c t s .

I n a p r e v i o u s p u b l i c a t i o n , i t w a s p r o p o s e d t h a t t h e

o x i d a t i v e c o u p l i n g o f c a f f e i c a c i d a t p H = 4 - 5 i n v o l v e d


8/8

176 P. Hapiot et aL / Journal o f E ectroanalytical Chemistry 405 1996) 169-176

t h e d i r e c t d i m e r i s a t i o n o f t h e s e m i q u i n o n e r a d i c a l [8 ]. T h i s

d i r e ct m e c h a n i s m i s u n l ik e l y t o o c c u r b e c a u s e o f t h e f a st

a n d d i re c t f o r m a t i o n o f t h e o - q u i n o n e f r o m t h e s e m i q u i n o n e

a t t h e s e p H v a l u e s ( d i s p r o p o r t i o n a t i o n r a t e c o n s t a n t 2 k = 8

X 1 0 7 1 m o l - 1 s - 1 . A v a r i a n t o f t h is m e c h a n i s m h a s b e e n

p r o p o s e d w h i c h c o n s i d e r s t he c o m p r o p o r t i o n a t i o n e q u i l ib -

r i u m b e t w e e n t h e o - q u i n o n e a n d t h e c a f fe i c a c i d t o g iv e

s e m i q u i n o n e , w h i c h t h e n l e a d s to t h e f o r m a t i o n o f t he

d i m e r . T h e a b o v e c o m p r o p o r t i o n a t i o n e q u i l i b r i u m i s c e r -

t a i n l y fa s t a n d r e v e r s i b l e , a s i n d i c a t e d b y t h e e l e c t r o c h e m i -

c a l d a ta , b u t t h e f o r m a t i o n o f t h e s e m i q u i n o n e i s n o t

t h e r m o d y n a m i c a l l y f a v o u r e d [ 2 9] a t s l ig h t l y a c i d ic p H . I n

a d d i t io n , u n d e r t h e c o n d i t i o n s w h e r e t h e s e m i q u i n o n e d o e s

n o t r a p i d l y d i s p r o p o r t i o n a t e t o t h e o - q u i n o n e ( i n w a t e r a t

b a s i c p H o r i n o r g a n ic s o l v e n t s ) n o e v i d e n c e f o r a c o u p l i n g

r e a c t i o n w a s f o u n d . O n t h e c o n tr a r y , u n d e r s u c h c o n d i t i o n s

t h e s e m i q u i n o n e w a s u n r e a c t i v e .

I t t h e r e f o r e a p p e a r s t h a t th e f o r m a t i o n o f d i m e r s ( o r

o l i g o m e r s ) f o l l o w s v e r y d i f f e re n t p a t h w a y s s t a rt i n g f ro m

t h e m o n o p h e n o l s ( d e r i v e d f r o m 4 - c o u m a r i c o r f e r u li c a c id s )

a n d f r o m c a f f e i c a c i d . I n t h e fi r s t ca s e , t h e p h e n o l a t e i s

o x i d i s e d t o a p h e n o x y l r a d i c a l w h i c h u n d e r g o e s f u r t h e r

d i m e r i s a t i o n , w h i l e i n t h e s e c o n d c a s e , c a f f e i c a c i d i s

o x i d i s e d to th e c o r r e s p o n d i n g o - q u i n o n e , w h i c h u n d e r g o e s

f u r th e r c o n d e n s a t i o n s l e a d i n g t o b r o w n i n g p r o d u c t s .

A c k n o w l e d g e m e n t

M s . B ~ n ~ d i c t e G r e n e t te i s g r a t e fu l l y a c k n o w l e d g e d f o r

h e r h e l p w i t h t h e c y c li c v o l t a m m e t r y e x p e r i m e n t s .

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1996 Caffeic Acid J Electroanal Chem 405 169