ACID-BASE CATALYZED HYDROLYSIS OF AZIMSULFURON
259
Table V Effect of Ionic Strength on Alkaline
are 35 [22], 27 [10], 15.4 [19], and 9.5 [23], respec-
tively. In view of these results it seems that the pKa of
9CONH9 group of AZIM should be Ͻ 4.
The measured pKa value of 4Hϩ is 4.37 at 39ЊC
[24]; hence the kinetically determined pKa (ϭ 4.37)
may be assigned for pyrimidinium ion of AZS.
Hydrolysis of AZSa
Ϫ1
Ionic Strength/M
104 kobs/min
0.6
0.6
1.1
1.6
2.6
3.1
3.6
47.6 Ϯ 1.4b
44.7 Ϯ 0.4
48.2 Ϯ 0.6
50.6 Ϯ 0.9
52.6 Ϯ 0.6
52.7 Ϯ 0.7
56.3 Ϯ 1.5
a Conditions: 1.5 ϫ 10Ϫ4 M AZS, ϭ 285 nm, 1% (v/v) ace-
tonitrile in the aqueous solvent, 0.6 M NaOH, 50ЊC, ionic strength
was maintained by the use of NaCl.
b Error limits are standard deviations.
kinetic data on the alkaline hydrolysis of trifluoroac-
etanilide and N-methylanilides [15]. These authors
proposed, for the first time, the existence of highly
unstable oxydianionic tetrahedral (ODT) intermedi-
ates in the hydrolysis of anilides under highly alkaline
medium. The existence of ODT intermediates have
been proposed in many related studies [16]. In an ear-
lier study on alkaline hydrolysis of formamide it was
concluded that the existence of an ODT intermediate
is not an implicit function of either alkalinity of the
reaction medium or the structural features of the sub-
strate but rather is a composite function of both effects.
The existence of an ODT intermediate was also pro-
posed in the alkaline hydrolysis of urea [4]. The ex-
istence and non-existence of trianionic tetrahedral in-
termediates in the alkaline hydrolysis of saccharin [14]
and phthalimide [17], respectively, were attributed to
the stronger electron withdrawing ability of ͘SO2
group compared to the ͘CO group [18]. The stronger
electron withdrawing nature of the ͘SO2 group com-
pared to that of the ͘CO group could also be seen in
the difference between the pKa of saccharin (1.31) [8a]
and that of phthalimide (8.3) [19], (9.5) [20].
The previous discussion reveals that the pKa value
of 4.37 cannot be assigned with certainty to either
9SO2NHCO9, 9CONH9, or pyrimidinium ion
of AZS. However, nearly 25-fold larger value of 1
than that of ␣1 creates doubt about pyrimidinium ion
being an acidic group of pKa 4.37, because the acidic
site is quite far from the expected reaction site.
The authors thank the National Scientific Research and De-
velopment Council of Malaysia under the IRPA program for
financial support (Grant No. 01-02-03-0040) and Mr. Sa-
budin Ali from Du Pont Far East Inc. for the samples of
Azimsulfuron.
The approximate value of pKa of 9SO2NH9
group may be estimated as follows. The pKa of sac-
charin is 1.3 [8a] and pKa values of phthalimide are
8.3 [19] and 9.5 [20a]. Thus, the replacement of CO
by SO2 causes the reduction in pKa by nearly 7–8 pK
units. The reported value of pKa of diacetylamine
(CH3CONHCOCH3) is 12.9 [21]; hence, the expected
pKa value for CH3SO2NHCOCH3 is 5–6. The kineti-
cally determined value of pKa (ϭ 4.37) therefore ap-
pears to be for the 9SO2NHCO9 group of AZS.
The values of pKa of protonated triazine (3Hϩ) and
the side chain amino group of 3 are 1.5 and 8.4, re-
spectively [11]. Thus, the pKa of the primary amino
group of 2 should be nearly 8–9. The values of pKa
of NH3, C6H5NH2, CH3CONH2, and CF3CONHC6H5
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