Oxidation of aromatic aldehydes with potassium bromate-bromide reagent and an acidic catalyst

Article abstract of DOI:10.1007/s11164-012-0836-y

We report herein an easy oxidation procedure for converting aromatic aldehydes to aromatic carboxylic acids by use of a combination of commercially and readily available potassium bromate with potassium bromide in the presence of hydrochloric acid catalyst.

Full text of DOI:10.1007/s11164-012-0836-y

Res Chem Intermed
DOI 10.1007/s11164-012-0836-y
Oxidation of aromatic aldehydes with potassium
bromate–bromide reagent and an acidic catalyst
•
G. V. R. Sharma Alice R. Robert
Received: 21 May 2012 / Accepted: 25 September 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract We report herein an easy oxidation procedure for converting aromatic
aldehydes to aromatic carboxylic acids by use of a combination of commercially
and readily available potassium bromate with potassium bromide in the presence of
hydrochloric acid catalyst.
Keywords Oxidation ꢀ Aromatic aldehydes ꢀ Aromatic carboxylic acids ꢀ
Oxidizing agents
Introduction
Many oxidizing agents are known for oxidation of aromatic aldehydes to aromatic
carboxylic acids. Some of the best known oxidizing agents are chromium(VI) agents
[1, 2], silver oxide (Ag2O) [3], potassium permanganate (either in acidic or basic
media), various chromium reagents, for example chromium oxide, PCC, PDC,
Collin’s reagent, ozone [5], molecular oxygen [6], peroxy acids [4], etc., the details
of which are well covered in Refs. [1–15]. However, these methods suffer from
several disadvantages, for example cost, availability of reagents, and tolerance of
the reagents to reaction conditions, etc.
Potassium bromate and potassium bromide are common reagents in any basic
chemistry research laboratory and are not expensive. Hydrochloric acid, also, is readily
available in the chemistry laboratory. Herein, we report a simple, rapid method, using
these reagents, for oxidation of aromatic aldehydes into their corresponding carboxylic
acids. We encountered this interesting system while pursuing our interest in the use of
G. V. R. Sharma (&) ꢀ A. R. Robert
Department of Chemistry, GITAM Institute of Technology, GITAM University,
Rushikonda, Visakhapatnam 530045, AP, India
e-mail: sharmagvr@yahoo.co.uk; gvrs276@gmail.com
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G. V. R. Sharma, A. R. Robert
aromatic aldehydes for some of our studies. The reaction is conducted at room
temperature after addition of the reagents to the aldehydes.
Results and discussion
Aromatic carboxylic acids are very important intermediates in organic synthesis. As
mentioned in the Introduction, when we were pursuing the use of aromatic
aldehydes, we encountered an interesting oxidation system that can oxidize
aromatic aldehydes to aromatic carboxylic acids effectively. Although several
methods are available in the literature for similar oxidation, this method is an
alternative route making use of readily available and not so expensive reagents. The
reaction involves treating an aromatic aldehyde with potassium bromate, potassium
bromide, and catalytic quantity of hydrochloric acid in aqueous medium. Usually
the reaction is carried out at room temperature. The reaction conditions were
optimized to furnish good yields in each reaction. The time taken for the completion
of the reaction is approximately 3 h. Initial experiments were conducted by adding
one equivalent each of bromide and bromate. However, after confirmation of the
formation of carboxylic acids, experiments were conducted with five equivalents of
bromide and one equivalent of bromate, which improved the yields. Our attempts to
increase yields by increasing the duration of the reaction had a marginal effect only
on the yields of the final products. The identities of the aromatic carboxylic acids
obtained were confirmed by comparison of melting points and spectral data, for
example 1H NMR, IR, and mass spectra, with reported data. Table 1 shows a variety
of aromatic carboxylic acids obtained from the respective aromatic aldehydes, with
duration and yield. Initially we conducted the reaction on selected aldehydes; the
method has, however, been extended to other aromatic aldehydes also. We are also
currently working on application of this method to other aldehydes.
COOH
R
CHO
R
KBrO3, KBr, HCl, Water
R = H, Cl, Br, OMe, substituted amino, etc. Besides these aldehydes,
cinnamaldehyde has also been used as the starting aldehyde.
The oxidation may be proceeding by the following mechanism. Initial in-situ
reaction of bromide with bromate in the presence of hydrochloric acid liberates free
bromine which oxidizes the aldehyde to the carboxylic acid.
6Hþ þ 5Brꢁ þ BrO3ꢁ ! 3Br2 þ 3H2O
We also attempted to perform the oxidation in the absence of potassium bromide
and observed that the reaction proceeds very slowly. Hence the presence of
potassium bromide is necessary for the accelerated oxidation.
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Oxidation of aromatic aldehydes with an acidic catalyst
Table 1 Oxidation of aromatic aldehydes to aromatic carboxylic acids
No. Reactant
Product
Duration
(h)
Yield
(%)
1
2
3
4
5
6
7
8
Benzaldehyde
Benzoic acid, mp 120 8C
3
12
3
50
62
50
55
55
58
45
53
50
55
50
52
50
53
51
55
Cinnamaldehyde
Cinnamic acid, mp 130 8C
2-Chlorobenzoic acid, mp 140 8C
12
3
2-Chlorobenzaldehyde
12
3
4-N,N-
4-N,N-Dimethylaminobenzoic acid, mp
237 8C
Dimethylaminobenzaldehyde
12
3
4-Chlorobenzaldehyde
4-Chlorobenzoic acid, mp 240 8C
12
3
2,4,6-Trihydroxybenzaldehyde 2,4,6-Trihydroxybenzoic acid, mp
205 8C
12
3
3,4,5-Trihydroxybenzaldehyde 3,4,5-Trihydroxybenzoic acid, mp
245 8C
12
3
3,4-Dihydroxybenzaldehyde
3,4-Dihydroxybenzoic acid, mp 192 8C
12
General experimental procedure
Aromatic aldehyde (9 mmol) was added to a mixture of potassium bromate (2 g,
12 mmol), potassium bromide (2.38 g, 20 mmol), and hydrochloric acid (37 %,
30 ml). The reaction mixture was stirred for the time given in Table 1, then left to
cool. The reaction was monitored by TLC. The product settled as a precipitate
which was isolated by filtration and dried. The melting point was determined and
1
compared with the literature melting point. The products were analysed by IR, H
NMR, and mass spectrometry.
1H NMR of 4-chlorobenzoic acid in CDCl3: 7.0–8.0 (2d, 4H), 11.0 (s, 1H,
COOH).
Conclusion
Herein, we report, as a preliminary communication, a brief oxidation procedure for
oxidation of aromatic aldehydes. The products are obtained in good yields and are
characterized well by comparison with standard compounds. Further studies are in
progress to improve the yields and utility of the reaction. We strongly believe that
this reaction will find utility in organic synthesis.
Acknowledgments We thank the management of GITAM University for inspiration and support. One
of the authors, Alice R. Robert, acknowledges a minor research grant received from GITAM University
for carrying out her research work for a Ph.D. degree.
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