relationship between reduction potentials and - Le LEM

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RELATIONSHIP BETWEEN REDUCTION POTENTIALS
AND ANION RADICAL CLEAVAGE RATES
IN AROMATIC MOLECULES
C. P. Andrieux, J. M. Savéant and D. Zann
Laboratoire d'Électrochimie de l'Université de Paris- VI/,
2, place Jussieu, 75251 Paris Cedex 05 (France).
Received July 11, 1983.
ABSTRACT. - The rate constants for the cathodic cleavage of aryl chlorides and bromides exhibit an approximate
correlation with the standard potentials of the starting molecule-anion radical couple. For a given halogen
atom, the more negative the standard potential, the faster the cleavage. Extended Hückel calculations show
that the odd electron is transferred, upon increasing the carbon halogen distance from the 1t* orbital of the
aromatic system into the cr* orbital of the carbon-halogen bond. The latter varies little with the structure of the
aromatic residue whereas the level of the 1t* orbital governs bath the standard potential and the activation
energy. The correlation does not hold for small structural changes leading in sorne cases to inversions. Possible
reasons causing these behaviors are discussed.
Rt:SUMË. - Il existe une corrélation approximative entre les constantes de vitesse des réactions de dissociation
des dérivés aromatiques chlorés et bramés lors de leur réduction électrochimique et leur potentiel standard.
Pour un même halogène la coupure est d'autant plus rapide que le potentiel standard est négatif. Des calculs
quantiques effectués par la méthode « Hückel Étendu » montrent que l'augmentation de la distance entre le
carbone et l'halogène permet le passage de l'électron célibataire d'une orbitale 1t* délocalisée sur le groupement
aromatique dans l'orbitale cr* de la liaison carbone-halogène. L'énergie de cette dernière varie peu avec la
nature du groupement aromatique, au contraire de l'orbitale 1t* que l'on peut relier non seulement au potentiel
standard de la molécule de départ, mais aussi à l'énergie d'activation de la réaction de coupure. Cependant
cette corrélation est mise en défaut pour de faibles changements de structure. Des arguments pour rendre
compte des inversions observées sont proposés.
The electron transfer reduction of a large number of
aromatic molecules involving a carbon-heteroatom cr-bond,
ArX, produces a frangible anion radical which gives rise
upon cleavage to the corresponding aryl radical and an anion
containing the heteroatom. The most widely investigated
compounds in this connection have been the aryl halides.
Mechanistic and kinetic information is available for these
molecules from electrochemical 1 - 20 and pulse radiolysis
studies 21 • 22 • They show large variation of the cleavage rate
of the anion radical with the structure of the starting molecule, i. e., with the nature of the halogen (kA ri> k Ar Br> k ArcJ),
the location of the halogen substituent relative to other
non-cleavable substituents (kortho > kpara ~ kmera) and the nature
of the. aromatic residue (for example: kcJc6 H 4 cN > kc•c6 H 4 No 2).
In this respect, there appears to be a rough correlation
between the reduction potential of the starting molecule and
the rate of cleavage of the initially formed anion radical as
noted on the example of aromatic halides 14 and aromatic .
thiocarbonates 23 • In a series involving the same aromatic
residtie, the more positive the reduction potential, the faster
the cleavage when passing from one leaving group to the
other. This is typically observed for halo gens: l >Br> Cl.
Conversely, for the same leaving group, the more positive
the reduction potential, the slower the cleavage when passing
from one aromatic residue to the other.
One purpose of the work reported hereafter was to test .
this correlation more extensively in the series of aryl chlorides
and bromides using both previous and new determinations
of the standard potentials and cleavage rate constants. In
NOUVEAU JOURNAL DE CHIMIE, VOL.
this connection, we used two kinds of methods according to
the magnitude of the cleavage rate. For cleavage rate constants smaller than about 104 s-· 1, it is possible to use directly
electrochemical techniques such as cyclic voltam~etry and
double potential step chronoamperometry. For fàster processes, catalysis of the electrochemical reductive cleavage by
a redox mediator offers a means for determinin~ the rate
constant 14 • 24 • The procedure involves the generatipn of the
reduced form of the homogeneoùs catalyst, Q, by the electrochemical reduction of the oxidized form, P, at a potential
positive to the reduction of ArX. Q transfers an electron to
ArX regenerating P, .this step being followed by the cleavage
reaction:
.,
P+le~Q.
Q+ArX
~
( 1)
ArX:-+P,
(2)
kz
k
Arx:---. Ar' +X-.
(3)
The ensuing catalytic increase of the reduction current of P
reflccts the kinctics of reactions (2) and (3) 24 • By a proper
choice of the mediator couple and by taking advantage of
the effect of its concentration upon the interplay of reactions .
(2) and (3) it is possible to derive the value of the constant
k from the measurement of the catalytic enhancement of the
current using the available theoretical relationships 14 • 24,
A second question that we will address concerns the theoretical background underlying the observed correlation.
8, N" 2-1984, p. 107.- 0398-9836/84/0210710/$ 3.00/© CNRS-Gauthier-Villars