2
BALADO ET AL.
3
Ϫ
reaction between the hexacyanoferrate (III) ion and
the butane-2,3-diol in aqueous alkaline medium, us-
ing ruthenium trichloride as catalyst. We have found
that the reaction involves the formation of free radi-
cals. This fact seems to suggest a mechanism which
involves a one-electron-transfer given the one-elec-
tron nature of the Ru(III).
CH3 !CHOH!CHOH!CH3 ϩ 2Fe(CN)6
ϩ 2OHϪ 9: CH3 !CHOH!CO!CH3
4
ϩ 2Fe(CN)6 Ϫ ϩ 2H2O
RESULTS
Characterization of Ionic Forms of the
Catalyst
EXPERIMENTAL
3
2
ϩ
ϩ
The spectra of Ru(H2O)6 and Ru(H2O)5OH con-
tain absorption peaks at 225 and 290 nm, respec-
tively, which are assigned to ligand to metal charge-
transfer transitions. Both spectra also contain a very
weaker absorption peak at 392 nm assigned to an al-
lowed d-d transition [7].
Aqueous solutions of potassium hexacyanoferrate
(III) (Merck), sodium hydroxide (Merck), butane-
2,3-diol (Merck), and ruthenium trichloride (John-
son-Matthey) were prepared and used. All the
reagents used were of A.R. grade, except for the cata-
lyst which had an acceptable grade of purity.
When an aqueous solution 4 ϫ 10Ϫ5 M of RuCl3
is brought to an alkaline medium three absorption
peaks at 212, 280, and 390 nm are observed. Firstly,
as the [OHϪ] increases the intensity of the three
peaks increases. From pH Ӎ 10.5 to pH Ӎ 13.5 the
intensity of peaks at 212 and 390 nm increases as
[OHϪ] increases and the intensity of the peak at 280
nm decreases. Moreover the absorption peaks are
slightly shifted. Also, in the interval of the pH used
three isobestic points, no very well defined, appear
located about 219, 357, and 429 nm. The optical ab-
sorbance at 390 nm against pH is showed in Figure 1.
This optical behavior could involve the existence
3
A stock solution of RuCl3 3 H2O (2.4 ϫ 10Ϫ M)
in hydrochloric acid 16.38 ϫ 10Ϫ2 M was prepared
and used. All solutions were prepared in water
obtained by a Millipore-Milli Q Water Purifica-
tion System. The ionic strength was kept constant at
0.5 M by adding the appropriate amount of sodium
perchlorate.
The progress of the reaction has been followed by
monitoring the optical absorbance of hexacyanofer-
rate (III) ion at 420 nm on a Shimadzu UV-160 spec-
trophotometer, equipped with a thermostated cell
holder. NMR spectra were obtained by using a Varian
200 Gemini spectrometer.
It has been identified by means of UV-Vis spec-
trophotometry that the reduction product of hexa-
cyanoferrate (III) is hexacyanoferrate (II).
The identification of 3-hydroxy-2-butanone as
product of the oxidation reaction was made directly
in the reaction mixture by the formation of a
2,4-dinitrophenylhydrazone derivative [6]. The exper-
imental conditions in a typical experiment were:
x
Ϫ
of several ionic forms, [Ru(H2O)x (OH)x ](3
in
)
ϩ
6
Ϫ
equilibrium. Thus the following equilibria may exist.
3
Ru(H2O)6 ϩ ϩ OHϪ EF
Ru(H2O)5(OH)2ϩ ϩ H2O
K0 (1)
K1 (2)
Ru(H2O)5OH2ϩ ϩ OHϪ EF
Ru(H2O)4(OH)2ϩ ϩ H2O
[Butane-2,3-diol] ϭ 0.13 M,
[NaOH] ϭ 0.27 M,
[RuCl3 ] ϭ 2 ϫ 10Ϫ6 M, [K3Fe(CN)6 ] ϭ 2 ϫ 10Ϫ
M, and T ϭ 30°C. After completing the reaction, a
saturated solution of 2,4-dinitrophenylhydrazine in
2 M hydrochloric acid was added slowly to the reac-
tion mixture, previously neutralized. The precipitate
obtained was identified by NMR spectrum as 2,4-
dinitrophenylhydrazone derivative of 3-hydroxy-2-
butanone.
3
Also, the reaction mixture was extracted with di-
ethyl ether. The ether solution was analyzed by gas
chromatography and found that 3-hydroxy-2-bu-
tanone was the only oxidation product of butane-2,3-
diol.
On the basis of the aforesaid results the stoichiom-
etry of the reaction may be written as:
Figure 1 Optical
10Ϫ5 M.
absorbance-pH.
[RuCl3 ] ϭ 4 ϫ