100 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
1998, 100±101
Oxidation of Benzylic and Secondary Alcohols to
Carbonyl Compounds by NaBrO3^NH4Cl Reagent in
Aqueous Acetonitrile
Ahmad Shaabani* and Majid Ameri
Chemistry Department, Shahid Beheshti University, P.O. Box19396-4716,Tehran, Iran
NaBrO3 combined with NH4Cl is found to be an efficient reagent for the conversion, in aqueous acetonitrile and under mild
conditions, of benzylic and secondary alcohols into aldehydes and ketones, respectively.
The oxidation of alcohols to carbonyl compounds is a
fundamental transformation of organic chemistry which
is attracting much current interest.1±4 A great number of
oxidizing agents can eect the conversion of alcohols into
carbonyl compounds, and synthetic chemists are faced with
an wide choice of methods for this reaction. However, the
susceptibility of aldehydes to further oxidation narrows the
choice of reagents for the oxidation of primary alcohols to
aldehydes in good yield, and if the alcohol group is part
of a complex molecule that is sensitive to acidic or basic
reagents then the choice of eective oxidants is narrowed
still further. The discovery of new oxidants for the trans-
formation of alcohols to carbonyl compounds under mild
conditions with a variety of alcohols is of prime importance
in synthetic organic chemistry. Oxidations of alcohols by
NaBrO3 in the presence of cerium(IV) ammonium nitrate
solutions in the absence of chloride ion. Thus we repeated
the oxidation of cyclohexanol in HOAc±NaOAc (7 ml
CH3CN + 3 ml buer solution with pH = 4.62), potassium
hydrogen phthalate (7 ml CH3CN + 3 ml buer solution
with pH = 3.99) and NaH2PO4 (7 ml CH3CN + 3 ml buer
solution with pH = 3.86). No reaction occurred in any of
these experiments after 3 h at 80 8C. These experiments
showed that the reaction is not only pH-dependent, but also
requires the de®nite presence of Cl from NH4Cl in order to
proceed. The chloride ion mentioned above is suggested to
generate bromine and chlorine via the following reaction:12
2BrO3 2X 12H 4Br2 X2 6H2O
X Cl; Br
and can in turn oxidize alcohols.13
We have found that Br2 is generated after ca. 2.5 h when
NaBrO3 (5 mmol) is added to a solution of NH4Cl (7 mmol,
in 7 ml CH3CN + 3 ml H2O) at room temperature.
Also, a of NH4Br±NaBrO3 mixture was observed to
release bromine and we suggest that this system could be a
good candidate for the oxidation of alcohols or as a bromi-
nating agent of alkenes.
10
(CAN),5 bromine,6 NaHSO3,7 HBr,8 H2SO4,9 HClO4 and
HOAc,11 have been reported, most of the reactions having
occurred in relatively strong acidic solutions. We report here
the oxidation of benzylic and secondary alcohols with
NaBrO3±NH4Cl into the corresponding aldehydes and
ketones.
We have found that this method of oxidation is very con-
venient for the conversion of alcohols into carbonyl com-
pounds because of its simplicity and use of mild reaction
conditions. Furthermore, NH4Cl and NaBrO3 are both
cheap and easily available compared to most other oxidizing
agents that have so far been employed. As shown in Table 1,
a wide variety of secondary alcohols and some benzylic
alcohols could be easily oxidized to the corresponding
carbonyl compounds. However, other primary alcohols
(Table 1, entries 3±5) were recovered practically unchanged.
In order to obtain some information about the reaction
pathway, cyclohexanol was allowed to react with (a)
NaBrO3±NH4OAc [5 mmol NaBrO3 and 7 mmol NH4OAc
in 10 ml solvent mixture (acetonitrile±water 7:3)] and (b)
NaBrO3 (5 mmol NaBrO3 in 10 ml same solvent mixture)
in the absence of NH4Cl. Neither NaBrO3±NH4OAc nor
bromate ion alone was capable of oxidizing cyclohexanol.
This fact excludes the possibility of the alcohols being
oxidized with just BrO3 ion and also when bromate ion
exists in the presence of NH+4 ion in a solution which does
not have any acidic property (the pH of 5 mmol NaBrO3
and 7 mmol NH4OAc in 3 ml of H2O is 7.20). However, in a
solution in which both bromate and NH+4 ions co-exist,
NH+4 ion hydrolysis gives an acidic solution (the pH of
5 mmol NaBrO3 and 7 mmol NH4Cl in 3 ml of H2O is 4.00)
while the BrO3 ions are capable of oxidizing the alcohols.
In order to illustrate the role of NH4Cl in providing an
acidic solution, we performed experiments in various buer
In conclusion, NaBrO3±NH4Cl is an excellent oxidizing
agent which promises to be economical with high yields,
employs simple and mild reaction conditions, and is thus a
convenient reagent for selective oxidation of secondary and
benzylic alcohols.
Experimental
All products are known compounds and were identi®ed by com-
parison of their physical and spectral data with those of authentic
samples. Melting points were determined in open capillaries using
an oil-bath and are uncorrected. IR spectra were recorded as neat
®lms or as KBr pellets on a Shimadzu 470 spectrometer. 1H NMR
spectra were recorded at 90 MHz on a JEOL EX-90 instrument
with CDCl3 as solvent and Me4Si as an internal standard. pH
Measurements were carried out with a Schott CG model 825 pH
meter equipped with a combined glass±calomel electrode; all the
pH measurements were performed in aqueous solution. All alcohols
are commercial materials and were purchased from Fluka, Aldrich
or Merck. Reagent-quality solvents were used without further
puri®cation. Yields reported refer to isolated products or 2,4-di-
nitrophenylhydrazone derivatives (2,4-DNP)14,15 of the carbonyl
compounds.
General Procedure.ÐAlcohol (5 mmol) was added to a mixture
of NaBrO3 (0.755 g, 5 mmol) and NH4Cl (0.4 g, 7.5 mmol) in
aqueous acetonitrile (CH3CN±H2O = 7/3 v/v; 10 ml). The mixture
was stirred at 80 8C for 1±3 h. When the reaction was complete,
the resulting solution was extracted with methylene dichloride
(20 ml 2). The combined organic layers were washed with a satu-
rated aqueous solution of NaHCO3 and dried over MgSO4. After
®ltration, the solution was concentrated to aord the crude carbo-
nyl compound, which was frequently of good purity without further
treatment, although, if necessary, it could be puri®ed by distillation,
crystallization or chromatography, as appropriate.
To receive any correspondence.
This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
Financial support by the Research Council of Shahid
Beheshti University is gratefully acknowledged.