284 Bull. Chem. Soc. Jpn., 76, No. 2 (2003)
Acid-Catalysed Reactions of Dihydroxamic Acids
Scheme 2.
cured from Sigma. Benzohydroxamic acid (BHA) was prepared
by standard methods.20
1.5%.
Products. Product studies were carried out in solutions iden-
tical with those used in the kinetic measurements, except that the
concentrations of the substrates employed were higher. In moder-
ately concentrated acids, 1.5 g of the substrate was dissolved in
100 ml of acid and heated at 85 °C using a water bath. After reac-
tions had reached at least 90% completion, aliquots were removed
and chilled for product identification.
Hydrolysis products, i.e. oxalic, malonic and succinic acids and
hydroxylamine hydrochloride, were identified qualitatively for di-
hydroxamic acids. For desferal succinic acid, acetic acid and 5-
amino pentyl hydroxylamine were identified qualitatively by usual
organic tests. These products were separated by fractional crystal-
lization and purified by recrystallization. The UV spectra of iso-
lated products were compared with those of authentic samples.
Prepared hydroxamic acids were characterized by mp (ODHA
= 160 °C, MDHA = 155 °C, SDHA = 180 °C and BHA = 126
°C), elemental analysis and UV and IR spectral data. All the sol-
vents used in spectroscopic analyses were of HPLC grade. The
mineral acids (HCl, H2SO4, HClO4) and other solvents and salts
used were of analytical reagent grade.The concentrations of acids
were determined by titration with standard alkali. Deuterium ox-
ide, D2O (isotopic purity > 99.8%) and DCl (isotopic purity >
95%) were procured from Bhabha Atomic Research Centre, Bom-
bay, India. Iron (Ⅲ) chloride (Qualigens) solution used in the col-
orimetric procedure was prepared by the standard method.8
Kinetic Measurements. For each kinetic run, two reaction
vessels were used. One of these contained appropriate volumes of
acid (catalyst) and water; the other one contained the hydroxamic
acid. After thermostating for about 30 minutes the acid solution
was transferred to the reaction vessel containing hydroxamic acid.
After the content of the reaction vessel was shaken, an aliquot of
the reaction mixture was withdrawn into a 10 mL volumetric flask
containing 2 mL of Iron(Ⅲ) chloride. A double purpose, quench-
ing of the reaction and colour development, was thus served. The
volume of the coloured solution was made up to 10 mL and its ab-
sorbance was measured at 500–520 nm using a reference solution
containing 2 mL of the same Iron(Ⅲ) chloride in 10 mL of water.
The kinetic runs were studied generally up to two half-lives. For
measuring absorbance, a Systronics UV-VIS Spectrophotometer
type 108 was used. For protonation studies a Unicam UV2 300
spectrophotometer was used.
For a pseudo first-order reaction, a plot of log absorbance vs t
will give a straight line of slope, −kψ /2.303 (where kψ is the pseu-
do first-order rate constant for the reaction). The rates of hydroly-
sis were determined spectrophotometrically by following the
decrease in the characteristic absorption of the hydroxamic acid–
ferric chloride complex. As Beer’s law is applicable to all the
Iron(Ⅲ) hydroxamic acid complexes, the concentration of reacting
species is proportional to the absorbance A. [log A ∝ log (a − x)].
To obtain the rate constant kψ, log (a − x) was plotted against time
t; from the slope of the plot, kψ was determined. The fact that a
straight line was obtained for all the plots of (log a − x) vs t mea-
sured in this investigation is, in itself, an indication that the reac-
tions are all first-order in substrate. The initial concentration of
the hydroxamic acid in reaction mixture is about 7.0 × 10−3 M.
The experimental errors in the respective runs were generally less
than 1.0% and the reproducibility of the rate constants was within
Results and Discussion
All the reactions followed pseudo first order kinetics:
d
−
[HA] = k2[HA][H+]
= kψ [HA].
dt
The observed pseudo-first-order rate constants of ODHA
and MDHA for the catalytic effects of hydrochloric (0.58 to
11.4 M), sulfuric (0.58 to 13.5 M) and perchloric acids (0.58 to
11.1 M) are given in Table 1. In order to explain the differenc-
es in the catalytic efficiencies of the different acids and to illus-
trate that the rate-acidity profiles go through maxima, these
data have been plotted in Fig. 1 against molarity (HCl). In the
weakly acidic solutions, linear plots were obtained. From the
intercept it could be inferred that hydroxamic acid did not react
with water (in the absence of catalyst) to a measurable extent
at 55 °C. Generally, hydroxamic acids are susceptible both to
acid catalysis and to base catalysis, but their spontaneous hy-
drolysis contributes relatively little to the overall reaction rate.
In higher acid regions, the dependence of hydroxamic acid is
characterised by an initial rate increase with the acid concen-
tration, passing through a maxima, followed by a rate decrease
with further increasing acid concentration. This non-linearity
is because the equilibrium between the reactants and the proto-
nated species of the rate-determining step does not correspond
only to a simple protonation, but also to addition of water mol-
Ghosh, Kallol K.
