738
MOHAMED, SHOUKRY, AND SHOUKRY
the Cu(Me4en)2+. The kinetic data, the volume of base
added to keep the pH constant versus time, could be
fitted by one exponential as shown in Fig. 1. Various
other kinetic models were tested without leading to
satisfying fits of the data. Plots of kobs versus the hy-
droxide ion concentration is linear (Figs. 2–4). The rate
expression can therefore be given in the form (Eq. (1))
EXPERIMENTAL
All reagents were of Analar grade. N,N,Nꢁ,Nꢁ-
Tetramethylethylenediamine was obtained from
Sigma Chemical Co. The glycine-, histidine-, and
methionine methyl esters were purchased from Fluka.
Cu(NO3)2 · 3H2O was provided by BDH. The copper
content of solutions was determined by complexomet-
ric EDTA titrations [7]. Carbonate-free NaOH was
prepared and standardized against potassium hydro-
gen phthalate solution. All solutions were prepared in
deionized H2O.
kobs = k0 + kOH[OH−]
(1)
The term k0 arises because of the water attack on the
mixed-ligand complex and is expressed by the relation
(2) [15]
The kinetics of hydrolysis was monitored us-
ing a Metrohm 751 Titrino operated with the SET
mode. The titroprocessor and electrode were calibrated
with standard buffer solutions according to NIST
specifications [8]. Hydrolysis kinetics of glycine-,
methionine-, and histidine methyl esters in the presence
of [Cu(Me4en)(H2O)2]2+ was investigated by using
pH-state techniques. The kinetics of hydrolysis of the
complexed esters was investigated using an aqueous
solution (40 cm3) containing a mixture of copper(II)
(6.25 × 10−3 M), (Me4en) (6.87 × 10−3 M), methyl es-
ter (1.25 × 10−3 M), and NaNO3(0.1 M). In this mix-
ture, the [Cu(Me4en)2+]:[ester] ratio was adjusted to
5:1, so as to maximize the amount of complexed ester
present. A 10% excess of Me4en over copper(II) was
used to ensure coordination of all copper(II), which is
itself an excellent catalyst. In all cases, the solutions
were equilibrated at the desired temperature under a
constant nitrogen flow. The ester solution was then
added, and the pH of the mixture was progressively
raised to the desired value by the addition of 0.05 M
NaOH as described previously [9–11]. The hydrolysis
was then followed by the automatic addition of 0.05 M
NaOH to maintain the desired pH. The data fitting was
performed with OLIS KINFIT set of programs [12]
as described previously [13]. Values of the hydroxide
ion concentration were estimated from the pH using
pKw = 13.997, and an activity coefficient of 0.772 was
determined from the Davis equation [14]. At the vari-
able temperature studies, the following values of pKw
and γ were employed [15]: at 15◦C (pKw = 14.35,
γ = 0.776), at 20◦C (pKw = 14.16, γ = 0.774), at 25◦C
(pKw = 14.00, γ = 0.772), at 30◦C (pKw = 13.83,
γ = 0.770), and at 35◦C (pKw = 13.68, γ = 0.768).
k0
kH O
=
(2)
2
55.5
where 55.5 mol dm−3 is the molar concentration of
water. The value of k0 can be determined from the
intercept of Fig. 2, while the value of kOH can be deter-
mined from the slope of the respective plot. The rate
constant values kobs and kOH are given in Tables I–III.
Table I Kinetics of Hydrolysis of Coordinated Glycine
Methyl Ester at Different Temperatures in Aqueous
Solution
104 × k
OH
Temperature
(◦C)
104 × [OH−]a
k
(s−1
(dm3 mol−1
obs
pH
(mol dm−3
)
)
s−1
)
15
20
25
30
6.00
6.25
6.50
6.75
7.00
6.00
6.25
6.50
6.75
7.00
6.00
6.25
6.50
6.75
7.00
5.75
6.00
6.25
6.50
6.75
7.00
5.50
5.75
6.00
6.25
6.50
6.75
0.45
0.79
1.41
2.51
4.47
0.68
1.20
2.14
3.80
6.76
1.00
1.78
3.16
5.62
10.00
0.83
1.48
2.63
4.68
8.32
14.79
0.66
1.17
2.09
3.72
6.61
11.75
0.14
0.61
1.36
2.38
4.11
0.21
0.80
1.93
4.12
7.49
0.40
1.10
3.40
6.95
13.64
0.81
2.11
3.70
6.88
14.76
24.60
1.09
2.52
4.87
7.25
13.32
24.50
0.97
1.21
1.49
1.73
35
2.07
RESULTS AND DISCUSSION
The hydrolysis of the coordinated esters was monitored
over the pH ranges 5.5–7.0 for glycine- and methio-
nine methyl esters and 8.0–9.0 for histidine methyl
ester. Throughout these pH ranges, the rate of hydrol-
ysis of the free ester is negligible in the presence of
a pKw 14.35 at 15◦C; 14.17 at 20◦C; 14.00 at 25◦C, 13.83 at
30◦C, and 13.68 at 35◦C. These data were taken from ref. [22].
International Journal of Chemical Kinetics DOI 10.1002/kin