J. Agric. Food Chem. 1997, 45, 4495
−4499
4495
Kin etics a n d Hyd r olysis Mech a n ism of Tr ia su lfu r on
Ilaria Braschi,† Luca Calamai,‡ Mauro Andrea Cremonini,† Paolo Fusi,‡ Carlo Gessa,†
Ottorino Pantani,‡ and Alba Pusino*,§
Istituto di Chimica Agraria, Universita` di Bologna, via Berti Pichat 11, 40127 Bologna, Italy, Dipartimento
di Scienza del Suolo e Nutrizione della Pianta, Universita` di Firenze, Piazzale Cascine 28, 50144 Firenze,
Italy, and Dipartimento di Scienze Ambientali Agrarie e Biotecnologie Agro-Alimentari, Universita` di
Sassari, Viale Italia 39, 07100 Sassari, Italy
The hydrolysis of the sulfonylurea herbicide triasulfuron [(2-(2-chloroethoxy)-N-[[4-methoxy-6-
methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzenesulfonamide] was studied in aqueous buffers of pH
values 2, 3, 4, 5, 6, 7, and 9. The reaction was of first-order and pH-dependent. Triasulfuron was
more persistent in neutral or weakly basic than in acidic solution. Five metabolites have been
isolated and identified. At all pH values studied, the primary pathway of degradation was the
cleavage of the sulfonylurea bridge. However, minor degradation pathways have also been observed
like O-demethylation and opening of the triazine ring. The product distribution was pH-dependent.
Keyw or d s: Sulfonylureas; herbicides; triasulfuron; hydrolysis; degradation
INTRODUCTION
Saronno, Italy. Its purity was checked by HPLC. All the
solvents were of HPLC grade (Carlo Erba Reagenti, Milano,
Italy) and were used without further purification.
Triasulfuron is a selective sulfonylurea herbicide used
at very low rates (10-25 g of active ingredient/ha) for
weed control in cereals (Amrein and Gerber, 1985).
Sulfonylurea herbicides degrade in soil by chemical
hydrolysis and microbial degradation (Beyer et al.,
1988). Generally, hydrolysis involves the breakdown of
the sulfonylurea bridge to give the corresponding sul-
fonamide and heterocyclic amine. Nevertheless, alter-
native pathways were observed for chemical hydrolysis.
Rimsulfuron hydrolyzes easily through the contraction
of the sulfonylurea bridge (Schneiders et al., 1993),
whereas the cleavage of the sulfonylurea bridge and
O-demethylation of the methoxy group of the triazine
ring occur for thifensulfuron methyl (Cambon and
Bastide, 1996). Several studies were devoted to the
degradation of triasulfuron in soil (Iwanzik and Amrein,
1988; Martin and Blair, 1988; Walker and Welch, 1989).
In particular, Oppong and Sagar (1992) observed tria-
sulfuron degradation in both sterile and nonsterile soils.
Triasulfuron was more susceptible to chemical hydroly-
sis at acidic than neutral or basic pH (Dinelli et al.,
1993; Berger and Wolfe, 1996). Dinelli et al. (1995),
using capillary electrophoresis, detected five metabolites
in the hydrolysis of triasulfuron at pH 4. However, no
study concerning the degradation mechanism of tria-
sulfuron and the identity of its byproducts is available.
This work describes the kinetics of the chemical hy-
drolysis of triasulfuron in aqueous buffers in the pH
range 2-9 and the nature of degradation products. On
the basis of the results, a degradation pathway is
proposed.
2-Am in o-4-m eth oxy-6-m eth yltr ia zin e (2). Triasulfuron
(0.5 g) was dissolved in DMSO (dimethyl sulfoxide, 3 mL). The
solution was left at room temperature for 3 days. The
precipitate formed was filtered, washed three times with
CHCl3, and then dried in a desiccator. Compound 2 was
obtained as white crystals (Huffinan et al., 1963) (mp 256-
257 °C; 45% yield). MS (m/e): 140 (M)+, 110, 69, 42; IR (KBr
drift) ν cm-1: 1660, 1565, 1372, 1346, 1021.
2-(2-Ch lor oeth oxy)ben zen esu lfon a m id e (3). To a solu-
tion of triasulfuron (0.5 g) in acetonitrile (5 mL) was added 1
N HCl (1 mL). The solution was stirred at room temperature
overnight, and then the crude reaction mixture was concen-
trated under vacuum. The residue was separated by column
chromatography on silica gel (70-230 mesh, Merck) using
cyclohexane plus ethyl acetate (1 + 4 by volume) as eluant.
By collecting the eluate fractions, each of 50 mL, metabolite 3
was present in the pure state from the third to fifth fraction.
These fractions were gathered and evaporated under vacuum
to dryness. Compound 3 was obtained as white crystals (mp
118-119 °C; 53% yield). MS (m/e): 235 (M)+, 186, 173, 156;
IR (KBr drift) ν cm-1: 1589, 1481, 1324, 1159, 756.
2-(2-Ch lor oet h oxy)-N-[[(4-h yd r oxy-6-m et h yl-1,3,5-t r i-
a zin -2-yl)a m in o]ca r bon yl]ben zen esu lfon a m id e (4). Me-
tabolite 4 was obtained from triasulfuron according to the
procedure proposed by Sabadie (1992) for the analogous
byproduct of chlorosulfuron, modified as follows. To a suspen-
sion of triasulfuron (0.5 g) in water (50 mL) NaOH (0.1 g, pH
) 12.5) was added under stirring. After 16 h, more NaOH
(0.1 g) was added and the mixture was stirred at room
temperature until clear (about 24 h). Then 1 N HCl was added
to the solution up to pH 2.5. After precipitation, metabolite 4
was filtered, washed twice with distilled water, and dried in
a desiccator. Compound 4 was obtained as white crystals
(Adams et al., 1952) (mp 177-185 °C dec; 90% yield). MS
(m/e): 235, 219, 199, 186, 173, 156; IR (KBr drift) ν cm-1: 1737,
1690, 1591, 1471, 1432, 1361.
2-Am in o-4-h yd r oxy-6-m eth yltr ia zin e (5). Metabolite 2
(0.2 g) was dissolved in DMSO (2 mL) and aqueous 1 N NaOH
(2 mL) was added. The solution was left overnight under
stirring at room temperature. The clear reaction mixture was
added with 1 N HCl up to neutrality and then concentrated
under vacuum. The residue was recovered with water, filtered,
washed twice with distilled water, and dried in a desiccator.
The byproduct 5 was obtained as white crystals (mp >320 °C;
90% yield). MS (m/e): 127 (M - H)+, 109, 99, 85, 81, 79; IR
(KBr drift) ν cm-1: 1698, 1682, 1611, 1462.
MATERIALS AND METHODS
Ch em ica ls. Triasulfuron, 2-(2-chloroethoxy)-N-[[(4-meth-
oxy-6-m et h yl-1,3,5-t r ia zin -2-yl)a m in o]ca r bon yl]ben zen e-
sulfonamide (1) (purity 99.5%) was supplied by Ciba-Geigy,
* Author to whom correspondence should be ad-
dressed (fax, ++39-79-229276; e-mail, a.pusino@
antas.agraria.uniss.it).
† Universita` di Bologna.
‡ Universita` di Firenze.
§ Universita` di Sassari.
S0021-8561(97)00299-9 CCC: $14.00
© 1997 American Chemical Society