Permanganate oxidation of unsaturated alcohols in alkaline media

Article abstract of DOI:10.1002/kin.10080

A study was made on permanganate oxidation of olefinic and acetylenic alcohols in aqueous alkali media. Deprotonation constants of alcohols can be calculated from the kinetic data. The rate constant of alkoxide group oxidation exceeded that of the unsaturated bond. For oxidation of the alcoholic group a mechanism based on hydride ion transfer is proposed.

Full text of DOI:10.1002/kin.10080

Permanganate Oxidation
of Unsaturated Alcohols
in Alkaline Media
´
´
MIKLOS JAKY, EDIT SIMON-TROMPLER
Institute of Chemistry, Chemical Research Center of Hungarian Academy of Sciences, P.O. Box 17, H-1525,
Budapest, Hungary
Received 20 June 2001; accepted 20 May 2002
DOI 10.1002/kin.10080
ABSTRACT: A study was made on permanganate oxidation of olefinic and acetylenic alcohols in
aqueous alkali media. Deprotonation constants of alcohols can be calculated from the kinetic
data. The rate constant of alkoxide group oxidation exceeded that of the unsaturated bond.
For oxidation of the alcoholic group a mechanism based on hydride ion transfer is proposed.
C
2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 561–567, 2002
In this paper we study the oxidation of olefinic and
acetylenic alcohols (and halides) with aqueous alka-
line permanganate. In this case two processes, i.e. the
oxidation of unsaturated bonds and that of alkoxide
anions, proceed simultaneously. According to experi-
ence so far, the rate constants of alkoxide anions were
found to be much lower than those of unsaturated bonds
(for methanol, ethanol, and n-butanol values of 3.9,
INTRODUCTION
As is known, permanganate ion is very reactive to un-
saturated bonds both in aqueous acidic and alkaline me-
dia, and these reactions are considered to be some of the
fastestreactionsinorganicchemistry. Thiscanbestated
in spite of the fact that the vast majority of rate con-
stants measured for double bonds vary in a rather wide
1
1
39.2, and 142 dm3 mol
s
were measured [13]).
range, between the values 250 and 3000 dm3 mol 1 s
1
Based on the analysis of kinetic results obtained in
this work, however, a surprising conclusion may be
drawn: in some cases the oxidation rates of alkox-
ide anions considerably exceed those of unsaturated
bonds.
Measurements were performed in the range of
0.1 [OH ] 2.0 mol dm 3. Under these conditions
permanganate is in the first step reduced only to man-
ganate, which then further reacts at a much lower
rate. In spite of the apparent one-electron reduction
of permanganate, instead of a radical mechanism, two-
electron oxidation steps are generally assumed in the
literature [20,26]. Namely the manganate (V) species
produced could not accumulate in detectable amount
owing to its very fast reaction with permanganate
[20,27–29].
[1–9]. For the acetylene bond somewhat lower values
were measured [10–12].
In the oxidation of aliphatic alcohols in alkaline
medium, alcoholate (alkoxide) anions are the reac-
tive species, which may be oxidized by permanganate
by means of direct electron [13–16] or hydride ion
(H ) abstraction [17–25]. The dissociation constant
of alcohols can be calculated from the kinetic results
[13–16].
Correspondence to: Edit Simon-Trompler; e-mail: Simone@
chemres.hu.
Contract grant sponsor: Hungarian Scientific Research Fund.
Contract grant number: T016211.
2002 Wiley Periodicals, Inc.
c

´
JAKY AND SIMON-TROMPLER
562
and therefore the k values measured could be related to
k1 (see Eq. (2)). Although the 1/[MnO4 ] vs. time plots
always showed a slight curvature, the k values deter-
mined from the initial section of the plots verified the
validity of = 2 for olefinic and = 4 for acetylenic
alcohols.
EXPERIMENTAL
Materials
The reagents KMnO4, NaClO4, NaOH, and HClO4
were of analytical grade (Merck). The substrates were
also of analytical grade (Fluka), further purified by
fractional distillation and dried on 4A molecular sieves
or by recrystallization in the case of solids as recom-
mended in the literature [30].
Products
For determination of the products, we applied a re-
3
action mixture containing the substrate in 1.0 10
mol dm 3, and permanganate in 2 10 3 (for allyl al-
Kinetics
3
3
cohol (1)) or in 4 10 mol dm (for propargyl al-
cohol (7)) initial concentration. After quenching with
hydrazine hydrate and filtering the MnO2, the products
were determined by means of the HPLC method (ISCO
instrument, model 2350, detector V4) as described in
detail in our earlier papers [16,31]. In order to evaluate
the quantity of products, we compared the HPLC peak-
areaswiththepeaksgivenbystandardsin1 10 3 mol
dm 3 concentration under the same conditions.
Kinetic measurements were carried out with at
least tenfold excess of substrates under pseudo-
first-order conditions. Ionic strength was maintained
with NaClO4. Alkali concentration was adjusted with
NaOH. In some supplementary measurements hydro-
gen ion concentration was adjusted with HClO4, and
pyrophosphate ions were applied in fivefold excess re-
lated to permanganate in order to avoid disproportion-
ation and fast reactions of the Mn3+ ions formed.
Permanganate consumption was monitored with
the use of a Hewlett-Packard spectrophotometer (type
8452A) coupled to a stopped-flow device (developed
RESULTS AND DISCUSSION
in our laboratory). The lg(A A ) vs. time plots (A
∞
The reactions were found to obey first-order with re-
spect to both permanganate and the substrate:
is the absorbance at 524 nm) showed a slight curva-
ture owing to consecutive reactions of the intermedi-
ates. Therefore, the rate constants were computed from
the initial slopes of the plots determined by means of
a Hewlett-Packard computer (type 9000-300) directly
coupled to the spectrophotometer. Each value repre-
sents the average of 5–9 individual measurements. Es-
timated accuracy was 3%.
d[MnO4
]
= k0[AOH]T[MnO4
]
(1)
dt
where AOH stands for substrate alcohol.
The k0 values measured (Table I) increase with the
concentration of hydroxide ions, which may be at-
tributed to alcoholate dissociation [13–16].
Let us denote the individual rate constants as fol-
lows:
The orders related to the reagents were determined
3
in 0.25 mol dm base concentration by varying sub-
2
strate concentration between 1.0 10 and 10.0
10 2 mol dm 3, or permanganate concentration in the
3
range of 1.0 10 3–5.0 10 mol dm 3. The kobs
k1
R CH CH CH2OH + MnO4
→ Intermediate
values were strictly proportional to both [Substrate]0
and [MnO4]0.
(R C C CH2OH)
(2)
k10
If the rate-determining step is followed by very
fast reactions, where further MnO4 molecules are
consumed, these reactions act as multipliers in the
rate constant of the first reaction. In order to deter-
mine this kinetic factor ( ) we applied the relatively
sensitive method of reacting reagents in the initial
concentration ratio of the presumed value. If this
ratio is correct, then the integrated kinetic equation
R CH CH CH2O + MnO4
→ Intermediate
(R C C CH2O )
(3)
R CH CH CH2O + MnO4
(R C C CH2O )
k2
→ R CH CH CHO(R C C CHO)
3
+ MnO4 + H
(4)
1/[MnO4
]
1/[MnO4 ]0 = kt must be valid, and
the k value gained from the slope must be equiva-
lent to the k value obtained under pseudo-first-order
conditions. Measurements were performed in acidic
medium, where only unsaturated bonds are oxidized,
The term Intermediate indicates the well-known short-
lived cyclic hypomanganate diester, which is assumed
to hydrolyze fast, resulting in MnO4 and further or-
ganic intermediates and products.
3

PERMANGANATE OXIDATION OF UNSATURATED ALCOHOLS IN ALKALINE MEDIA
563
1
Table I Apparent Second-Order Rate Constants (k0, dm3 mol
k0 (dm3 mol
s
1) as a Function of NaOH Concentration
1
3
s
1) at different [OH ] (mol dm
)
0.10
354
0.25
376
0.50
397
0.75
431
1.00
451
1.50
509
2.00
552
0.10a
258
1 2-Propen-1-ol
(allyl alcohol)
CH2 CH CH2OH
2 1-Propene, 3-bromo
(allyl bromide)
CH2 CH CH2Br
3 1-Propene, 3-chloro
(allyl chloride)
CH2 CH CH2Cl
4 2-Propen-1-ol, 2-methyl
(methallyl alcohol)
CH2 C(CH3) CH2OH
5 2-Buten-1-ol
179
175
336
743
182
173
374
786
180
176
443
831
174
181
178
177
254
534
513
938
583
714
835
1019
1153
1295
(crotyl alcohol)
CH3 CH CH CH2OH
6 2-Butene-1,4-diol
HOCH2 CH CH CH2OH
7 2-Propyn-1-ol
534
135
598
192
657
233
709
253
735
265
770
277
800
282
379
94
(propargyl alcohol)
CH C CH2OH
8 1-Propyne, 3-bromo
(propargyl bromide)
CH C CH2Br
9 1-Propyne, 3-chloro
(propargyl chloride)
CH C CH2Cl
48
46
47
47
49
46
48
49
48
45
10 2-Butyne-1,4-diol
HOCH2 C C CH2OH
218
254
275
277
283
285
287
151
2
3
[Substrate]0 = 1.00 10 mol dm 3; [MnO4]0 = 1.0 10 mol dm 3; I = 2.0 mol dm 3; T = 25 C.
a Ionic strength 0.10 mol dm
.
3
The oxidation rate of nondissociated hydroxyl
groups can be neglected on the basis of available data
[13–16].
All three processes were followed by the fast
reaction:
With the use of this equation we attain the kinetic
equation
d[MnO4
]
k1 + k10 KB[OH ] + k2 KB[OH ]
1 + KB[OH ]
= 2
dt
[AOH]T[MnO4
]
(7)
k4
3
2
MnO4 + MnO4
→ 2MnO4
(5)
Factor 2 appears as a result of the fast reaction (5).
By comparison of Eqs. (1) and (7), we obtain
where k1, k10 , k2
k4.
Let us define the equilibrium
2k1 + (2k10 + 2k2)KB[OH ]
KB
*
)
k0 =
1 + KB[OH ]
AOH + OH
AO + H O
(6)
2
2k1 + kKB[OH ]
(8)
From this, based on the consideration that [AOH]T =
[AOH] + [AO ], we obtain
1 + KB[OH ]
In the case of acetylenic compounds, reactions (2)
and (3) are followed by a fast attack of another per-
manganate molecule, since the remaining double bond
KBbOH c
[AO ] =
[AOH]T
1 + KB[OH ]

´
JAKY AND SIMON-TROMPLER
564
of the short-lived intermediate, or that of its hydrolytic
product, i.e. unsaturated glycol, is very reactive [10–
12,32]. This is also supported by the observation that
in these cases = 4. Therefore, for acetylenic alcohols
Eq. (8) changes as follows:
than the literature data [33–36], which is in accordance
with the trend expected due to high ionic strength.
Rate constants k10 and k2 cannot be separated by
kinetic methods, since both are comprised in k. An es-
timation, however, can be made: k10 cannot be higher
than k1. There are data available only for unsaturated
carboxylicacids, wheretheanionswithnegativecharge
are less reactive than their neutral nondissociated forms
[3,4,8,9]. This phenomenon may be due to electrostatic
effects. We think that no significant error is made by the
assumption of k10 k1. Based on this consideration k2
values could be calculated (Table II). As can be seen,
the rate constants of alkoxide anions (k2) are signifi-
cantly higher than those of the unsaturated bond (k1)
for substrates studied in this article. Moreover, these k2
values exceed most rate constants known for perman-
ganate oxidation of the unsaturated bond.
The k2/k1 ratios measured were also supported by
product analysis carried out for allyl (1) and propar-
gyl (7) alcohols. HPLC investigations justified that
glycerin and acrylic acid, or glycollic and propyolic
acids were formed. (Other products or intermediates
did not accumulate in measurable amount.) The un-
saturated carboxylic acids were the products of alkox-
ide group oxidation, while oxidation of the unsaturated
bond yielded glycerin or chain rupture products (see
Scheme 1).
4k1 + (4k10 + 2k2)KB[OH ]
k0 =
1 + KB[OH ]
4k1 + kKB[OH ]
(9)
1 + KB[OH ]
Considering the k1 values to be equal to the rate
constants measured in acidic media may be a good
approximation, since the short-lived intermediates i.e.
the cyclic hypomanganate diesters, are identical in both
media. This assumption is also supported by the fact
that identical rate constants were measured for unsatu-
ratedalkylhalidesinacidicandalkalinemedia[k = 2k1
for allyl (2, 3) and k = 4k1 for propargyl (8, 9) halides
(see Table II)].
After accomplishing the two-parameter fitting of
Eqs. (8) and (9), respectively, we obtain the values k
and KB shown in Table II.
It must be admitted that the KB and pKa (pKa =
14 lgKB) values measured cannot be regarded as
thermodynamic constants owing to 2.0 mol dm 3 ionic
strength. The pKa values gained are somewhat lower
Table II The Measured Dissociation and Rate Constants
k1
171
k
KB
pKa (Literature)
k2
1 2-Propen-1-ol
(allyl alcohol)
1934
0.076
15.120 (15.52a; 15.48b)
796
2 1-Propene, 3-bromo
(allyl bromide)
3 1-Propene, 3-chloro
(allyl chloride)
87
86
179
176
4 2-Propen-1-ol, 2-methyl
(methallyl alcohol)
5 2-Buten-1-ol
154
356
9620
8769
0.030
0.039
15.52
4656
4028
15.41 (15.80b)
(crotyl alcohol)
6 2-Butene-1,4-diol
7 2-Propyn-1-ol
240c
10e
910
304
1.45
5.62
13.84 (14.0d)
13.25 (13.55a; 13.57b)
215
132
(propargyl alcohol)
8 1-Propyne, 3-bromo
(propargyl bromide)
9 1-Propyne, 3-chloro
(propargyl chloride)
10 2-Butyne-1,4-diol
11.5e
48
47
11.5e
12.5e
292
22.91
12.64 (12.94 f )
121
a Ref. 33;
b Ref. 34;
c Ref. 5;
d Ref. 35;
e Ref. 10;
f
Ref. 36.

PERMANGANATE OXIDATION OF UNSATURATED ALCOHOLS IN ALKALINE MEDIA
565
Scheme 1
In the case of both substrates P1 stands for the prod-
ucts of unsaturated bond oxidation and P2 designates
those of the alkoxide group.
Measured and calculated product ratios are given
in Table III. For calculations made with the use of
Eq. (10) the assumption k1 = k10 was accepted.
may be due to the fact that in the case of aldehyde in-
termediates the oxidation rate of the unsaturated bond
related to the aldehyde group is not negligible.
In summary, the following conclusions may be
drawn:
On the basis of the KB values measured the fol-
lowing is the order of acidity: methallyl alcohol
(4) < crotyl alcohol (5) < allyl alcohol (1) < 2-butene-
1,4-diol (6) < propargyl alcohol (7) < 2-butyne-1,4-
diol (10), which agrees with the acidity scale expected
according to literature data. Owing to positive induc-
tive effect the alkyl group reduces the acidity of alco-
hols (decreasing KB and increasing pKa). On the other
hand, because of conjugation the unsaturated bond acts
as an electron-withdrawing group and accordingly, in-
creases acidity. This effect is enhanced in the case of
the acetylenic bond.
In this article we do not intend to deal with the mech-
anism of reactions (2) and (3) as it has been discussed
in detail in the literature [7–9,37–40]. We wish to make
some remarks, however, on the oxidation mechanism
of alkoxide anion (Eq. (4)).
For the oxidation rate constants of alkoxide anions
an opposite order was found: k2 increases with decreas-
ing acidity. This order is in accordance with the fact that
in the presence of an electron-withdrawing group, elec-
[P2]
[P1]
k2 KB[OH ]
=
(10)
k1 1 + KB[OH ]
Validity of this assumption is also verified by the rel-
atively good agreement between measured and calcu-
latedvalues. ThiscanbestatedinspiteofthefactthatP2
always appeared in lower amount than expected, which
Table III Amounts of Products of Unsaturated Bond
Oxidation (P1) and Those of Alkoxide Group Oxidation
(P2) as a Function of Hydroxide Ion Concentrationa
3
[OH ] (mol dm
)
0.10
0.92
0.25
0.50
1.0
1 Allyl alcohol
P1
P2
P2/P1
0.82
0.71
0.59
0.17
0.290
0.018 0.057 0.11
0.020 0.070 0.155
(P2/P1)
(0.033) (0.086) (0.170) (0.330)
tron or H abstraction is hindered. The Taft
values
7 Propargyl alcohol
for crotyl (5), allyl (1), and propargyl (7) alcohols are
available (0.36, 0.56, and 1.7 [34]). Although the lgk2
P1
P2
P2/P1
(P2/P1)
0.14
0.62
4.4
0.10
0.73
7.3
0.082
0.77
9.4
0.073
0.80
11.0
vs.
plot is not a straight line, the declining trend of
(4.75) (7.71) (9.74) (11.2)
the plot ( < 0) points to an electron-deficient carbon
center in the transition state.
A mechanism based on hydride transfer was gen-
erally suggested for the permanganate oxidation of
a The amounts represent HPLC peak areas related to those of
standard solutions of 1.0 10 3 mol dm 3. The ratios calculated by
Eq. (10) are given in parentheses.

´
JAKY AND SIMON-TROMPLER
566
alcohols both in acidic and alkali media [17–25]. The
same mechanism was proposed for some other oxidants
as well [41–47]. The main argument for H abstraction
is the substantial kinetic isotope effect observed in each
case, which points to the important role of C H bond
cleavage in the rate-determining step. Banerji consid-
ers the H-atom abstraction less probable, as in this case
a higher kinetic isotope effect would be expected than
had been measured [23,24]. According to Lee et al.
[48] there is a possibility of MnO4 addition to the
C H bond, giving an organometallic intermediate. Ho-
molytic cleavage of the C Mn bond would then result
in the formation of free radicals.
Most recently, further possibilities have been raised
based on quantum mechanical computation. According
to the approach of Lee et al. [49], the reaction could be
initiated by HOMO–LUMO interaction. In the opinion
of Strassner et al. [50], concerted 3+2 cycloaddition is
capable of activating the C H bond. Ab initio calcula-
tions of Wiberg and Freeman [51] showed that energet-
ically all four variations (H , H, e , 2e abstraction)
are possible. Considering the above, this problem does
not seem to be fully resolved yet.
In our previous papers we argued against the mech-
anism based on H or H transfer and for direct electron
abstraction from the alkoxide anion [13–16]. Although
the C H bond in the negatively charged alkoxide anion
does not seem to be more loosened than in the neutral
molecule, alcohols are oxidized by orders of magnitude
faster in alkali than in acidic media. Our assumption
was that the interaction between permanganate and the
alcoholic O-atom enhances polarization on the C-atom,
which facilitates the simultaneous nucleophilic attack
of the hydroxide ion. Transition state was suggested
to be
2. The oxidation of ethylene glycol proceeds with
twice as high rate constant in D2O as in H2O. As
is known, DO is more nucleophilic than HO ;
therefore the higher rate constant is in accordance
with the activated complex proposed [14].
3. Meso-tartaric acid was oxidized almost 10 times
as fast as D,L-tartaric acid. In the expected an-
tiparallel open position both hydroxyl groups of
the D,L-form are close to each other, which may
hinder the reaction. This may account for the
lower rate constant. At the same time, the H-
atoms are more accessible than in the meso-form;
therefore H abstraction would be more favor-
able for the D,L-form, which is in contradiction
with experience [16].
In spite of the above considerations, our previous as-
sumption on the mechanism must be revised. In some
cases, for example in the case of mesoxalic acid hy-
drate, only pure electron abstraction is possible. The
fact that the rate constants of alkoxide anions of un-
saturated alcohols are considerably higher than those
of saturated alcohols, however, cannot be explained by
this mechanism, since exactly the opposite could be
expected owing to the negative inductive effect [com-
1
1
pare the values of 87 and 137 dm3 mol
s
for
propanol [this work] and n-butanol [13] with the val-
1
1
ues of 796 and 4028 dm3 mol
s
for allyl alco-
hol (1) and crotyl alcohol (5)]. The following expla-
nation seems plausible. Delocalization of the negative
charge in the mesomer system, comprising the unsatu-
rated bond and electron pairs of the alkoxide O-atom,
promotes hydride ion abstraction. In the case of methal-
lyl alcohol this effect is enhanced by the hyperconju-
gation of the methyl H-atoms. The mesomer system
for acetylenes does not seem to be symmetrical. Delo-
calization of the negative charge is shifted towards the
triple bond, which is less favorable for hydride abstrac-
tion. This might explain the relatively low k2 values of
the acetylenic compounds.
Decomposition of the activated complex may take
place with proton elimination. It should be noted that
proton elimination and two-electron transfer to the ox-
idant manifest themselves as hydride transfer.
In support of this assumption the following facts
were brought up:
Accepting the suggestion made by Lee [49], we pro-
pose the following activated complex:
1. Mesoxalic acid hydrate cannot be oxidized via
H
or H abstraction, as there is no C H bond.
In spite of this, the rate constant is commensu-
rable to that of tartronic, lactic, and D,L-tartaric
acids [15,16,52]. (The somewhat lower value can
rather be ascribed to steric hindrance.)
The lone pairs of electrons of the alcoholic O-atom
might occupy the lowest unoccupied molecular or-
bital of the Mn-atom. This initial interaction may be

PERMANGANATE OXIDATION OF UNSATURATED ALCOHOLS IN ALKALINE MEDIA
567
followed by rate-limiting hydride transfer through in-
ternal cyclic electron transfer. It should be noted that
this assumption also explains the higher rate constant
of meso-tartaric acid than that observed for D,L-tartaric
acid.
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