Adsorption and Degradation of Rimsulfuron
J. Agric. Food Chem., Vol. 44, No. 2, 1996 621
the pyrimidine ring. Actually, the rimsulfuron molecule
is provided with a pyridine ring having a very low
electron density (δ+) (Figure 6), because of the presence
of two electron-withdrawing sulfone and sulfonamide
groups. As a consequence, the pyridine moiety is a well-
suited substrate for aromatic nucleophilic substitution.
However, such a type of reaction does not occur in neat
chloroform, while it takes place in the presence of clay.
This suggests that the coordination of the carbonyl
group to the clay exchangeable cation enables the
reaction. The coordination enhances the partial positive
charge on the carbonyl carbon, allowing the dissociation
of the NH group within the sulfonamide bridge, which
becomes remarkably more acidic. This group already
exhibits a pKa value of 4.1 in water solution (Worthing
and Hance, 1991). The negative charge following this
deprotonation can be delocalized onto the carbonyl
oxygen, which makes the lone electron pair of the
nitrogen atom adjacent to the pyrimidine ring (δ-) more
prone to act as a nucleophile (Figure 6). The whole
process leads to a nucleophilic migration, where sulfur
dioxide is the leaving group, known as the Smiles
rearrangement of arylsulfones (Cerfontain, 1968). In
the second step, metabolite 324 is formed from metabo-
lite 367 that is coordinated through the CdO group to
the exchangeable cation. Similarly as in rimsulfuron,
this coordination increases the partial positive charge
of the CdO carbon. This allows a nucleophilic attack
by the solvation water of the exchangeable clay cation,
leading to the formation of carbamic acid and metabolite
324. This latter remains adsorbed on clay by protona-
tion, while carbamic acid, being unstable, may decom-
pose to NH3 and CO2.
The possibility that the hydrolysis observed could be
due to water associated with hectorite without clay
assistance is ruled out by the isolation of metabolite 324
on the clay. In fact, the water content of Al hectorite,
determined by heating from room temperature to 990
°C, is 14.7%. This value includes also the structural
OH (Si-OH), which disappears at high temperature.
The actual free water content depends on temperature
and external relative humidity. Since experiments were
performed at room temperature, an average value of
5.5% can be reasonably assumed on the basis of litera-
ture data (Deer et al., 1962). Upon contact of the clay
with chloroform this small amount of water is strongly
polarized by interlayer cation and the pH is in the range
previously reported.
ica, Universita` di Firenze) and Prof. C. Luchinat (Isti-
tuto di Chimica Agraria, Universita` di Bologna) for the
execution and interpretation of NMR spectra. We thank
F. Bini and F. Filindassi for computer assistance and
preparation of the layouts.
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In aqueous solution, according to Schneiders et al.
(1993), metabolite 324 is formed, at short times, from
the intermediate 367 only at pH g7, probably because
of the need of a strong nucleophilic water. The water
of chloroformic clay suspension is a very poor nucleo-
phile, and the hydrolysis reaction can take place only if
assisted by CdO interaction with saturating ions.
Therefore, the observed distribution of metabolites
indicates that the different properties of the interlayer
solution, as compared to normal aqueous solution, affect
the degradation mechanism.
ACKNOWLEDGMENT
Received for review February 21, 1995. Accepted November
27, 1995.X This research was supported by National Research
Council of Italy, Special Project RAISA, Subproject 2, Paper
2488.
We [members of GRIFA (Gruppo Ricerca Italiano
Fitofarmaci e Ambiente] gratefully acknowledge Dr. B.
Casetta (Perkin Elmer, Italy) and Dr. Raffaelli (CNR,
Centro di Studio per le Molecole Stereordinate e Otti-
camente Attive, Pisa) for MS analysis; Dr. M. V. Naidu
(DuPont de Nemours, Wilmington, DE) for supplying
evidence for the determination of the structure of some
metabolites; and Prof. I. Bertini (Dipartimento di Chim-
J F950107J
X Abstract published in Advance ACS Abstracts, Janu-
ary 15, 1996.