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288 1
REVERSIBLE
HYDRATION
OF PYRUVIC
ACID
Table I: Fraction of Hydration, X , and Equilibrium Constant,
K = x/(l - x), for the Hydration of Pyruvic Acid as a
Function of pH at 25.0' and 0.0'
c----25.O0-
PH
B
0.35
1.13
1.50
1.95
2.53
2.92
3.47
4.29
4.51
6.22
K
1.55
1.39
1.06
0.711
0.294
0.163
0.0667
0.0726
0.0500
0.0572
X
-
0.608
0.583
0.514
0.416
0.227
0.140
0.0624
0.0677
0.0476
0.0540
F---o.oo---
K
4.42
4.42
3.27
1.64
0.593
0.303
0.213
0.207
X
0.816
0.816
0.764
0.622
0.372
0.233
0.176
0.172
"'I
2!2
2!4
2'.6
2!8
3!0
PPM
3!2
31.4
316
Figure 1. A, nmr spectrum of pyruvic acid in HtO at 25.0',
pH 0.35; B, nmr spectrum of pyruvic acid in HzO at 25.0', pH
4.22; C, nmr spectrum of pyruvic acid in HzO
at 25.0°, pH 6.22.
droxypropionate anion. Again, the forward process is
not favored for reasons stated above.
The nmr spectra of aqueous solutions of pyruvic acid
a t 25.0' are shown in Figure 1 at pH 0.35, 4.22, and
6.22. These spectra show the appearance of two peaks
at 2.58 and 3.44 ppm which represent the signals arising
from the methyl hydrogens associated with the hydrated and unhydrated forms, respectively. The relative positions of these peaks are consistent with this
assignment since the shielding of the methyl hydrogens
of the hydrate would be expected to be greater than that
associated with the corresponding methyl hydrogens in
the unhydrated form. It will be noted that a t pH 0.35
a considerable quantity of the hydrate exists whereas a t
pH 4.22 and 6.22 the extent of hydration has markedly
decreased following the deprotonation of pyruvic acid.
The variation in the fraction of hydration, x, with pH
was studied quantitatively as a function of pH in the
range pH 0.35 to pH 6.22 a t both 0.0' and 25.0'.
When x is plotted against pH, the data listed in Table I
give rise to sigmoid curves having points of inflection a t
pH 2.2 as shown in Figure 2. These curves show maxima at low pH of xrnax0.~e
= 0.82 and x ~ ~ =~ 0.61
~
~
while a t higher values of pH very low fractions of
. 0 0and xrnin2g.,p =
hydration are obtained: ~ ~ ~ ~ =0 0.17
0.06. These latter values appear to remain constant
with further increases of pH. Because the fraction of
hydration of pyruvic acid is so intimately related to the
Figure 2. Fraction of hydration, x, plotted against pH for the
hydration of pyruvic acid: 0, data at 0.0'; 0, data at 25.0'.
relative concentrations of the acid and its conjugate
base, these curves resemble the titration of a weak acid.
The points of inflection represent the maximum variation of the fraction of hydration with pH which in turn
is caused by the rapidly changing ratio of pyruvic acid
to its anion. Thus the value of pH 2.2 at the point of
inflection is in complete accord with the pK, of pyruvic
acid as obtained polarographically by Strehlowsb (pKa
= 2.18). The close agreement between the values of
pK, as obtained by the two methods is further substantiation of the quantitative nature of the data resulting from the nmr measurements.
The equilibrium constant for the hydration of pyruvic
acid, K = [hydrate]/ [pyruvic acid], was determined as
a function of temperature. These studies were conducted using solutions of pH 0.35 so that almost 99% of
the total concentration of pyruvic acid existed in its pro.tonated
~
o form.
Table I1 summarizes the data which
were obtained using a total concentration of pyruvic
acid of 1.3 M over a temperature range between 0.0" and
50.0'. These data allowed the evaluation of the thermodynamic parameters for the hydration. From the
linear plot of log K against 1/T we deduced the standard
Volume 73,Number 0 September 1060