810
Keerti M Naik and Sharanappa T Nandibewoor
compound. Hence the title reaction was investigated in KOH, and KIO4. The progress of the reaction was fol-
detail.
lowed spectrophotometrically at 415 nm by monitoring
the decrease in absorbance due to DPC with the molar
absorbancy index, ε to be 6230 100 dm3 mol−1 cm−1.
It was verified that there is a negligible interference
from other species present in the reaction mixture at this
wavelength.
2. Experimental
2.1 Materials and methods
The pseudo first-order rate constants, kobs, were
determined from the log (absorbance) versus time plots
and were the average of duplicate runs (table 1). The
plots were linear up to 80% completion of reaction
under the range of [OH−] used and the rate constants
were reproducible to within 5%. The orders for vari-
ous species were determined from the slopes of plots of
log kobs versus log respective concentration of species
except for [DPC] in which no variation of kobs was
observed as expected to the reaction condition. During
the kinetics, a constant concentration viz. 1.0 × 10−5
mol dm−3 of KIO4 was used throughout the study unless
otherwise stated. Since periodate is present in excess
in DPC, the possibility of oxidation of L-leucine by
periodate in alkaline medium at 25◦C was tested. The
progress of the reaction was followed iodometrically.
However, it was found that there was no significant
reaction under the experimental conditions employed
compared to the DPC oxidation of L-leucine. The total
concentrations of periodate and OH− was calculated
by considering the amount present in the DPC solu-
tion and that additionally added. Kinetic runs were also
carried out in N2 atmosphere in order to understand
the effect of dissolved oxygen on the rate of reaction.
No significant difference in the results was obtained
under a N2 atmosphere and in the presence of air. In
view of the ubiquitous contamination of carbonate in
the basic medium, the effect of carbonate was also stud-
ied. Added carbonate had no effect on the reaction rates.
The spectral changes during the reaction are shown in
figure 1. It is evident from the figure that the concentra-
tion of DPC decreases by observing the absorbance at
415 nm.
All chemicals used were of reagent grade, and dou-
bly distilled water was used throughout the work. A
solution of L-leucine was prepared by dissolving an
appropriate amount of L-leucine (S. D. Fine Chemi-
cals) in doubly distilled water. The required concentra-
tion of L-leucine was prepared from its stock solution.
The copper (III) periodate complex was prepared by
standard procedure.10 Existence of copper (III) com-
plex was verified by its UV–vis spectrum, which
showed an absorption band with maximum absorption
at 415 nm. The aqueous solution of copper (III) was
standardized by iodometric titration and gravimetrically
by the thiocyanate11 method. The copper (II) solution
was prepared by dissolving the known amount of cop-
per sulfate (BDH) in distilled water. Periodate solu-
tion was prepared by weighing the required amount
of sample in hot water and used after keeping it for
24 h. Its concentration was ascertained iodometrically12
at neutral pH by phosphate buffer. KOH and KNO3
(BDH, AR) were employed to maintain the required
alkalinity and ionic strength, respectively, in reaction
solutions.
2.2 Instruments used
(i) For kinetic measurements, a Peltier Accessory (tem-
perature control) attached Varian CARY 50 Bio UV-vis
Spectrophotometer (Varian, Victoria-3170, Australia)
was used. (ii) For product analysis, a QP-2010S
Shimadzu gas chromatograph mass spectrometer,
Nicolet 5700-FT-IR spectrometer (Thermo, U.S.A.) and
for pH measurements, an Elico pH meter model LI120
were used.
Regression analysis of experimental data to obtain
the regression coefficient r and standard deviation S
of points from the regression line was performed using
Microsoft 2003 Excel program.
2.3 Kinetic measurements
The kinetic measurments were performed on a Peltier
Accessory (temperature control) attached Varian CARY
50 Bio UV-vis Spectrophotometer (Varian, Victoria-
2.4 Stoichiometry and product analysis
3170, Australia). The kinetics was followed under Different sets of reaction mixtures containing various
pseudo first-order condition, where [L-leucine]>[DPC] ratios of DPC to L-leucine in the presence of constant
at 25 0.1◦C, unless specified. The reaction was initi- amounts of OH− and KNO3 were kept for 6 h in a closed
ated by mixing the DPC with L-leucine solution, which vessel under nitrogen atmosphere. The remaining con-
also contained the required concentration of KNO3, centration of DPC was estimated by spectrophotomet-