KINETIC STUDY OF OXIDATION OF PROPAN-1-OL
665
order in catalyst concentration and the rate of the uncata-
lyzed reaction is negligible in comparison with the
catalyzed reaction.
Activation parameters
Under the conditions
[Fe(CN)63Ϫ ]=1·20ϫ10Ϫ3
[NaOH]=0·3 and I=0·5
parameters found were ⌬H - =56·07 kJ molϪ1 and
⌬S ≠ =Ϫ148·38 J KϪ1 molϪ1
[propan-1-ol]=0·5
M
,
,
M
,
[RuCl3]=1·21ϫ10Ϫ6
M
M
M
, the experimental activation
Absence of polymerization of RuCl3
.
No evidence for the existence of oxo-bridged ruthenium(III)
complexes was obtained from the following experiment. A
MECHANISM
1ϫ10Ϫ3
M
solution of 2,2Ј-bipyridine in toluene was
added to a 5ϫ10Ϫ5
M
solution of Ru(III) in 0·1
M
Before considering the probable oxidation mechanism, it
may be helpful to discuss the species of ruthenium(III) that
may act as catalysts in the reaction.
NaOH. The organic ligand was slowly extracted into the
aqueous phase. The spectrum of the aqueous phase was
then recorded. The absence of an intense band about 650 nm
indicates that oxo-bridged dimers such as [(bipy)2(H2O)
RuORu(H2O)(bipy)2]4+ are not formed.13
The electronic spectra of Ru(H2O)6OH3+ and
Ru(H2O)5OH2+ show absorption peaks at 225 and 290 nm,
respectively, which are assigned to ligand to metal charge-
transfer transitions. Both spectra display a much weaker
peak at 392 nm assigned to an allowed d–d transition.15
The absence of oxo-bridged Ru(III) complexes was also
confirmed as follows: the evaporation of a 2·4ϫ10Ϫ3
M
solution of RuCl3 in 0·04
M
NaOH to dryness yielded a
When the pH of a 4ϫ10Ϫ5
M
aqueous solution of RuCl3
green solid. The IR spectrum of this solid did not display
any absorption bands in the 800–900 cmϪ1 region, where an
asymmetric stretch for linear M–O–M species13 is normally
located.
is increased by stepwise addition of NaOH, three absorption
peaks at 212, 280 and 390 nm are observed. First, as [OHϪ ]
increases the intensity of the peaks at 280 and 390 nm
increases. In the interval [OHϪ ]=10Ϫ3–0·4
M
the intensity
of the peaks at 212 and 390 nm increases while the intensity
of the peak at 280 nm decreases slightly. Moreover, the
absorption peaks are slightly shifted. In the interval of
[OHϪ ] used, three unclearly defined isosbestic points
located at about 219, 357 and 429 nm are observed. This
optical behavior could involve the existence of several
hydroxo–aquo ruthenium complexes in equilibrium:
K0
Substituent effects
Under kinetic conditions, the tertiary alcohol 2-methylbu-
tan-2-ol was unreactive, implying that a hydrogen on the
␣-carbon of the alcohol is necessary for the reaction to
occur.
Ϫ
2+
Ru(H2O)63+ +OH )Ru(H2O)5OH +H2O
(6)
*
Reaction with alcohols
K1
*
2+
Ϫ
+
It is well known that methanol is more difficult to oxidize
than propan-2-ol by a one-electron (hydrogen atom ab-
straction) path: ⌬H0(R–H)=95·9 kcal molϪ1 (1 kcal=
4·184 kJ) for CH3OH and 90·7 kcal molϪ1 for (CH3)2CHOH
[⌬H0(R–H) is the bond dissociation energy at 298 K
for the gas-phase reaction RH→R• +H•].14 Thus, we
obtained v0 (methanol)=0·37ϫ10Ϫ5 and v0 (propan-
Ru(H2O)5OH +OH )Ru(H2O)4(OH)2 +H2O (7)
K2
*
+
Ϫ
Ru(H2O)4(OH)2 +OH )Ru(H2O)3(OH)3 +H2O (8)
where pK0 =Ϫ11·1.16
Literature data suggest that ruthenium trichloride exists in
aqueous alkaline media as [Ru(H2O)6Ϫx(OH)x](3Ϫx)+ , where
x is always <6.17–19 Anderson and McConnell20 observed
similar species for hydroxo–aquo ruthenium(IV) com-
plexes.
Given the tendency of Ru(III) complexes to form oxo-
bridged species, the existence of polynuclear species such
as (H2O)5Ru(OH)2Ru(H2O)5 is probable. Thus, an equation
similar to equation (7) may be written:
2-ol)=1·26ϫ10Ϫ5
[alcohol]=0·5
[NaOH]=0·2
M
minϪ1
for
[K3Fe(CN)6]=1·2ϫ10Ϫ3
and I=0·5 . This
the
conditions
M
,
M,
M
, [RuCl3]=1·21ϫ10Ϫ6
M
M
result suggests a hydrogen atom abstraction in the rate-
determining step.
Detection of free radicals
When acrylonitrile (0·01
mixture, in a typical kinetic experiment, the initial rate
decreased (25%). Also, when acrylonitrile (0·7 ) was
M
) was added to the reaction
K1Ј
*
Ϫ
3+
Ru2(OH)24+ +OH )Ru2(OH)3
(9)
M
added, large amounts of polymer were formed after a few
minutes, indicating the formation of free radicals in the
reaction. No polymeric species were formed when acryloni-
However, the behavior of ruthenium trichloride under the
experimental conditions seems to be similar to that of
Ru(IV), which has been shown to polymerize slowly.20
Furthermore, the oxo-bridged dimers have not been
detected by using the aforementioned 2,2Ј-bipyridine test.
trile (0·7 ) was added to the reaction mixture without
M
propan-1-ol.
© 1997 John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 662–668 (1997)