Three Reagents in One: Ammonium Permanganate in the Oxidation of Benzyl Alcohol

Article abstract of DOI:10.1515/znb-2001-0816

The oxidation or consecutive ammoxidation reaction of benzyl alcohol with solid ammonium permanganate was studied. The first oxidation step leads to the formation of benzaldehyde, ammonia, and MnO₂. The MnO₂ is present in the system in a colloidal form which facilitates the reaction between aldehyde and ammonia, and this latter reaction then yields benzonitrile. All these products are formed in a heterogeneous system under relatively mild conditions. The yield of benzaldehyde has an optimum at room temperature and increases with increasing reaction time. At higher temperature (e.g. 80°C) benzonitrile is formed together with minor amounts of benzyl benzoate.

Full text of DOI:10.1515/znb-2001-0816

Three Reagents in One: Ammonium Permanganate in the
Oxidation of Benzyl Alcohol
László Kotai3, B éla Kazinczy3, Ágnes Keszler3, Sándor Holly3, István Gács3,
and Kalyan K. Banerjib
a Institute of Chemistry, Chemical Research Center, Hungarian Academy of Sciences,
H-1525 Budapest, P. O. Box 17, Hungary
b Department of Chemistry, J. N. V. University, Jodhpur-342005, India
Reprint requests to L. Kötai. Fax: 36-1-3257554. E-mail: kotail@cric.chemres.hu
Z. Naturforsch. 56b, 823-825 (2001); received December 20, 2000
Ammonium Permanganate, Oxidation
The oxidation or consecutive ammoxidation reaction of benzyl alcohol with solid ammo-
nium permanganate was studied. The first oxidation step leads to the formation of benzalde-
hyde, ammonia, and M n02. The M n02 is present in the system in a colloidal form which
facilitates the reaction between aldehyde and ammonia, and this latter reaction then yields
benzonitrile. All these products are formed in a heterogeneous system under relatively mild
conditions. The yield of benzaldehyde has an optimum at room temperature and increases
with increasing reaction time. At higher temperature (e.g. 80 °C) benzonitrile is formed to-
gether with minor amounts of benzyl benzoate.
Introduction
the reaction of 1 and solid NH4M n04 beside the
inorganic residue. After one hour at 22 °C, only
the formation of benzaldehyde (2) could be ob-
served, and its yield increased with increasing tem-
perature. Benzonitrile (3), however, appeared only
at 60-80 °C after 3 h. Eq.(l) illustrates the forma-
tion of 2.
Benzyl alcohol and its ring-substituted deriva-
ives can be easily oxidized by various permanga-
nates [1-14]. Reaction conditions, such as the na-
jureof the oxidant and the medium, temperature,
lie. have significant influences on the yield, selec-
Jvity, and formation of by-products. Previous in-
vestigations [1-14] have indicated that benzalde-
yde and/or benzoic acid are the main products.
If, however, ammonia is also present beside the
Jxidantthe alcohols or aldehydes are converted to
jitriles [15-19], For this reason, ammonium per-
manganate may behave as a multiple reagent lead-
ngto nitrile formation. At the same time, ammo-
iiumpermanganate is a more stable oxidant than
3
3
Ph-CH2OH + 2 NH4M n04 —
1
Ph-CHO + 2 NH3 + 2 M n02 + 4 HzO
(1)
2
Increasing production of 2 was incident to the
formation of increasing amounts of NH3 and
M n02. The formation of 3 would be attributed to
the reaction between 2 and ammonia produced as
illustrated by eq.(2).
‘h e previously applied permanganates [20], it is
Easily available [21,22], and inexpensive. Thus, its
Application may provide an advantageous new
Method to produce aromatic nitriles.
Accordingly, we studied the benzyl alcohol oxi-
dation with ammonium permanganate with re-
Ph-CHO+NH3
-H->Ph-CN +H 20 (2)
2
3
This mechanism is supported by the observation
that the amount of benzaldehyde decreases with
increasing nitrile production. Changes in the ben-
zyl alcohol/ammonium permanganate molar ratio
did not result in higher nitrile yields. This means
that neither 1 nor NH4M n04 participated in the
ject to reaction products, and the effect of the
tamonium ion.
Results and Discussion
In chlorinated solvents, only benzaldehyde and consecutive reactions of benzaldehyde. (Colloidal
tenzonitrile could be identified as the products of M n02 acts as oxidant in this process.) The process
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24
L. Kötai et al. • Ammonium Permanganate in the Oxidation of Benzyl Alcohol
(
1
) determines the ratio of the final products. Experimental
Since the formation of three equivalents of 2 is
incident to the liberation of only two equivalents has been described previously [21,22]. Benzyl alco­
of ammonia, the whole amount of the aldehyde hol ( ) was used after distillation, while the
could only be consumed (see eq.(
)), if additional solvents were applied without further purification.
The preparation of ammonium permanganate
1
2
The reactions were carried out in a double­
necked flask equipped with a condenser and a
magnetic stirrer. Alcohol 1 was oxidized with dif­
ferent amounts of ammonium permanganate (mo­
ammonia was introduced into the system.
Other reactions [23] may also lead to the forma­
tion of benzonitrile. However, benzoic acid, ben­
zamide and/or benzyl amine could not be detected
among the reaction products or intermediates. The
formation of 3 via the oxidation of benzyl amine
lar ratio alcohol:NH
4
M n0 = 0.4­4): (i) in reflux­
4
ing solvents under atmospheric pressure; (ii) in a
stirred mixture at room temperature. The reaction
time was varied between 1 and 48 h. The conver­
[23,24] could also be excluded.
The reduction product of ammonium permanga­ sion was followed by GC, and the products were
nate was M n0
present in the system in a colloidal identified via their mass and IR spectra. The GC
2
measurements were performed with a Finnigan
MAT GCQ instrument including a medium polar
CP WAX 52CB (ChromPack) column (length: 30
m; i.d.: 0.25 mm; film thickness: 0.25 ium). The lin­
ear flow rate of the helium carrier gas and the in­
jection temperature were set to 35 cm s- 1 and
form. This was confirmed by the UV spectra of
the final solution according to ref. [25], Other Mn­
containing species could not be detected. In com­
parison to microcrystalline MnOz obtained by son­
ification [26] or electrolytically [27], the colloidal
M n0
is a more active oxidant and therefore can
2
2
30 °C, respectively. The column temperature was
behave as an oxidizing agent in the process de­
scribed by equation (2). Increased reaction time
and/or temperature improved the yield of nitrile.
increased from 65 °C to 160 °C with a rate of 5 °C
min­1, then, after 1 min, to 200 °C with a rate of
10 °C min­1. Detection was performed in the 10­
Colloidal M n0
lar redox reaction of the ions of NH
2
can also form by the intramolecu­ 650 amu mass range using the electron impact
M n0
[5], (EI+) technique at 70 eV energy.
4
4
Samples for infrared spectroscopy were pre­
pared as KBr pellets and the spectra were re­
corded in the 400­4000 cm- 1 range with a Nicolet
and the M n02/2 (or benzaldimine) ratio therefore
may increase from the original 2/3 to a higher
value (> ).
1
2
05 FT­IR spectrometer. UV­Vis spectra were re­
At higher temperatures (80 °C) and/or longer
reaction times (e.g., 24­48 h) a small amount
corded in the range of 200­600 nm with a Unicam
UV­4 spectrometer. X­ray powder diffraction data
were obtained by means of a Phillips PW 3710 dif­
fractometer equipped with a Cu monochromator.
(
<
1
%) of benzyl benzoate (
the main products. Since 1 cannot be oxidized to
by solid NH M n0 or colloidal M n02, esterifica­
tion of 4 with 1 does not have to be taken into
account. Benzoate may be derived from other
reactions, e.g.: (i) from the oxidation of hemiacetal
6
) was obtained beside
4
4
4
Conclusion
6
Oxidation of 1 by solid ammonium permanga­
nate leads to the formation of benzaldehyde, am­
(
a­hydroxy­ether) intermediates [16]; (ii) from ox­ monia, and M n02. The M n0
2
present in the sys­
idation of 1 by activated M n0
2
[28]; (iii) from a tem in colloidal form then oxidizes the
a
Cannizaro reaction of 2 (catalyzed by manganese benzaldimine formed from the aldehyde and am­
monia. This latter reaction yields benzonitrile. Dif­
centers, similar to ester formation catalyzed by
ferent molar ratios of the reactants have no influ­
metal centers of enzymes [29]). The molar ratio of
ence on the product selectivity. The yield of 2 has
the reactants has no effects either on the selectiv­
an optimum at room temperature and increases
ity or the amount of the ester found. Therefore,
formation and oxidation of a hemiacetal type in­
termediate as well as oxidation of 1 (by M n02)
could be excluded. The catalytic Cannizaro­type
with increasing reaction time. The formation of 3
is favoured at higher temperature (e.g. 80 °C).
At higher temperatures (80 °C) and longer reac­
tion times (24 h)
reaction or a modified Corey­type reaction [28], via metal­center catalyzed Cannizaro or a cata­
however, are likely pathways.
lysed modified Corey­type reaction of 2.
6
appears as a by­product formed
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L. Kötai et al. • Ammonium Permanganate in the Oxidation of Benzyl Alcohol
825
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