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Mechanisms of Chromic Acid Oxidation.

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Investigations on Isopolytungstate and
Isopolymolybdate Anions in Aqueous Solution
By 0. Glemser and W. Holznagel, Gottingen (Germany)
Isopolytungstate and isopolymolybdate anions exist in
differing stages of condensation according to the pH value.
We have found :
1. For isomolybdate solutions : The octamolybdate anion,
Mo80i;.xH20, occurs as the highest stage of condensation
in “metamolybdate solutions” (pH rn 3; H+/MoO$- = 1.5).
The dependence of the degree of hydrolysis of the octamolybdate on concentration was calculated, and a complex
stability constant for the octa-ion arrived at. In “paramolybdate solutions” (PH FW 8; H+/MoOi- = 1.143), the occurrence
of a heptamolybdate anion M070$;.xH20 is verq probable.
The existence of hydrogen paramolybdate anions can be
established indirectly. There is a high probability that
aggregation stages with a degree of condensation under 6 are
excluded. Establishment of the equilibrium in polymolybdate
solutions takes place instantaneously.
2. For isopolytungstate solutions: In contrast to the earlier
statements of other authors, the degree of polymerisation of
isopolytungstate anions is not 6 but 12. Solutions in equilibriumat a degree ofprotonation < l S H+/WOf and pH >5
contain the so-called “paratungstate” anion, W120$ .xH2O,
and the so-called” metatung state” anion, W120g2xH20.
Solutions with a pH value < 5 contain only metatungstate as polyanion. Approximate stability constants were
obtained for both polyanions.
The protracted aging process in freshly prepared solutions
comprises the slow formation of metatungstate and an
associated decrease in the concentration of the already
existing protonated products. Even in solutions with pH >5 and in opposition to reports in the literature - the metatungstate ion is formed at the expense of the primarily formed
para-ion.
[Colloquium, Anorganisch-Chemihes Institut der
Universitat Gottingen (Germany), January 29 th. 19621
[VB 571116 IE]
fore, Li3ReO4 can be considered structurally as Liz(Re0.75LiO.25)03.
In experimental attempts to prepare rhenatesW) by thermal
methods, disproportionation of Re02 invariably occurred. It
is only in tne equilibrium system attained at fairly high
temperatures that the Na2Re03 coexists with Na-rhenate (VII)
and metallic rhenium. LiReOz containing trivalent Re has the
NaCl structure as do the corresponding compounds of
Mo(III) and W@II).
The potassium ferrate that was prepared by Klemm [4] is also
produced, together with KFe02, by the thermal decomposition
of K2Fe04. With a longer reaction time and at a higher
temperature pure KFeO2 is eventually produced as a result
of the evaporation of K20. The most useful temperatures for
the preparation of pure K3Fe04 from Fez03 and KO, in 0 2
are those in the range 750-780°C if at the same time the
reaction time is chosen to be as short as possible. The
K3Fe04 thus produced disproportionates in alkali and
65.6-66.1 % of the total Fe is present as Fe(v1) (calc.
66.67 %).K3Fe04 decomposes gradually into KFe02 and
potassium superoxide at 400-500 ‘C. According to the first
experiments K3Fe04 dissolves in cold, conc., sodium hydroxide evidently without disproportionation. The solution
gave the ratio Fe: 0 act. as 1: 1.50 within the limits of experimental error. A residue of Fe203.aq. was not observed
on filtration.
[GDCh-Ortsverband Hannover (Germany),
February 1st. 19621
[VB 568/18 IE]
[l] Cf. R. Scholder and M. Mansmann, Z. Naturforsch. 156, 681
(1960).
[2] R. Scholder, Angew. Chem. 70, 583 (1958).
[3] R. Wardet al., J. Amer. chern. SOC. 83, 1088,2816 (1961).
[4] K. Wahl, W . Klemm, and G. Wehrmeyer, Z. anorg. allg. Chem.
285, 322 (1956).
Mechanisms of Chromic Acid Oxidation
Kenneth B. Wiberg, Seattle, Washington (USA)
Recent Investigations on Oxometallates
and Double Oxides
R. Scholder, Karlsruhe (Germany)
Compounds of the so-called p-alumina type were formulated
as M2(1)0*11M2(nOO,’(M(o = alkali metal,
= Al, Ga,
Fe). X-Ray and chemical methods (action of acids on
M20aM203, volatalisation of K20 from K20-2.5 Fez03 at
1OOO-1100 “C) showed that p-alumina has the composition
MzO.6M203. Pure K20.6Fe203 can be prepared in a
coarse crystalline form from a K F melt and Fe203.A phase
M20.11 M2O3 was not observed in any of the oxide systems
that were investigated [l].
Alkaline earth rhenates
of the types M3Re06, MzReOs
and M3Re209 can be prepared from rhenatesvm [2],
rhenium metal and alkaline earth oxides by mixing together
in the correct proportions. The Li compounds, Li6ReO6 and
Li4ReO5, can be prepared similarly. The powder photograph
of M3Re209 is very similar to photographs of the alkaline
earth orthophosphates ; this clearly suggests formulation as
M3(ReO&)2 with a n excess of oxygen when compared with
M3(P04)2. Oxorhenate containing Re(v) was first observed
in the compounds BaReo.5(vLao.&m03 and BaReo.50Ino.s(III)03 which crystallise as super-lattices with the
Perovskite type of structure. Corresponding compounds
containing Re(VI) and M(II) were obtained by Ward [3] and
by us, using different methods, a t about the same time. The
easily obtainable rhenatec”) L i ~ R e 0 4
does not have the rocksalt structure of the corresponding osmatdv) Li3OsO4 but
the structure described by Lang for the A2CnB(IV)03. There-
220
The rates of oxidation of both aliphatic hydrocarbons and
the side chains of aromatic hydrocarbons parallel the rates
of free radical hydrogen abstraction from these compounds,
and are quite different from the relative rates of solvolysis of
the corresponding chlorides. This, and other evidence,
suggests that the first step involves the abstraction of a
hydrogen atom from the hydrocarbon giving the alkyl or
benzylic radical which reacts further.
The oxidation of a hydrocarbon with an optically active
tertiary center gave a tertiary alcohol with 80 % retention
of configuration. This suggests that the second step involves
recombination of the alkyl radical with chromium (V) in the
solvent cage to give a chromium (Iv) ester. The latter is then
hydrolyzed with chromium oxygen bond cleavage. This
is in accord with the mode of hydrolysis of tetra-t-butoxychromium.
The Etard reaction of the hydrocarbons is somewhat different.
The oxidation of n-propylbenzene gives propiophenone and
benzyl methyl ketone in a 1:3 ratio. The latter product
arises from an initial a-attack followed by a rearrangement,
as shown by deuterium labeling in the u- and @-positions.
The product ratio is affected by the chromyl chloride concentration. High concentrations favor the formation of
propiophenone. This indicates that the propiophenone is
formed by two successive normal oxidations, and that benzyl
methyl ketone arises by elimination following the initial
oxidation, and then, finally, reaction of the alkene with
chromyl chloride.
[GDCh-Ortsverband Darmstadt (Germany),
-
January 16th. 19621
[VS570117 IE]
Ansew. Chem. internot. Edit.
1 Val. 1
(1962) I No. 4
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