A highly efficient synthetic method for the preparation of 3,4-dimethylbenzaldehyde from o-xylene in aqueous media

Article abstract of DOI:10.1007/BF03246036

In this study, 3,4-dimethylbenzaldehyde was obtained in 82.3% total yield by oxidation of 3,4-dimethylbenzyl chloride with NaNO₃/AcOH catalyzed by PEG-600 in aqueous media. The starting material 3,4-dimethylbenzyl chloride was prepared by chloromethylation of o-xylene in CTAB micellar catalytic system. Compared with other synthetic methods, this method not only enhanced the yield, but also afforded an efficient work-up procedure. The structures of the products were confirmed by Elemental analysis,¹H NMR and ¹³C NMR or compared with authentic samples.

Full text of DOI:10.1007/BF03246036

J. Iran. Chem. Soc., Vol. 7, No. 2, June 2010, pp. 487-494.
JOURNAL OF THE
Iranian
Chemical Society
A Highly Efficient Synthetic Method for the Preparation of 3,4-Dimethylbenzaldehyde
from o-Xylene in Aqueous Media
Y.L. Hu, M. Lu*, Q.F. Liu, W. Wei and X. Liu
Chemical Engineering College, Nanjing University of Science and Technology, Nanjing 210094, PR China
(Received 4 July 2009, Accepted 10 August 2009)
In this study, 3,4-dimethylbenzaldehyde was obtained in 82.3% total yield by oxidation of 3,4-dimethylbenzyl chloride with
NaNO /AcOH catalyzed by PEG-600 in aqueous media. The starting material 3,4-dimethylbenzyl chloride was prepared by
3
chloromethylation of o-xylene in CTAB micellar catalytic system. Compared with other synthetic methods, this method not only
enhanced the yield, but also afforded an efficient work-up procedure. The structures of the products were confirmed by Elemental
1
13
analysis, H NMR and C NMR or compared with authentic samples.
Keywords: 3,4-Dimethylbenzaldehyde, Micellar catalysis, Phase transfer catalysis, Chloromethylation, Oxidation
INTRODUCTION
special apparatus and drastic reaction conditions. Another
procedure starting from 1,2,4-trimethylbenzene via an indirect
selective electro-oxidation with metal complexes (e.g.
3
,4-Dimethylbenzaldehyde (3,4-DBAL) is an important
4+
3+
4+
3+
intermediate in the synthesis of a variety of fine or special
chemicals such as pharmaceuticals, agrochemicals, dyes,
flavors, fragrances, polymers, etc. [1-3]. Moreover, it is known
Mn /Mn , Ce /Ce ) as redox mediator system has also been
reported [18-20], which, however, requires high reaction
temperature, special equipment and troublesome work-up
procedures. Consequently, there is a great need for an efficient
and improved procedure for the synthesis of 3,4-
dimethylbenzaldehyde.
as
a key intermediate in the synthesis of bis-3,4-
dimethyldibenzylidene sorbitol (3,4-DMDBS), which is a
well-known nucleation transparent agent of polyolefin and the
primary clarifying agent throughout the polylefin market [4-7].
Generally, 3,4-dimethylbenzaldehyde is prepared by the
Gatterman-Koch reaction via a direct carbonylation of
o-xylene by the use of carbon monoxide and hydrogen
chloride in the presence of AlCl and Cu Cl [8]. The reaction
Micellar catalysis is an effective means to accelerate
organic reactions between oil phase and water phase reactants.
In micellar catalysis system, lipophilic reactants are
solubilized in the surfactant micelles, and the swelling
micelles disperse in water phase containing hydrophilic
reactants, so that the reaction interface area between oil phase
reactants and water phase reactants is enlarged greatly. The
interface magnifying effect as well as electrostatic interaction
and concentrating effect result in dramatic increases of
reaction rates [21,22]. In addition, micellar catalysis can make
reaction conditions gentle, can effectively inhibit side
reactions to occur, and can enhance the efficiency of organic
3
2
2
was subsequently expanded to include other catalysts such as
HF-BF3 [9-11], SbF -HF [12-14], HFO S-NbF5 [15],
5
3
CF
3
SO
3
H-NbBr [16], etc. [17]. However, this procedure is
5
invariably associated with certain limitations such as low yield,
poor selectivity, long reaction time, environmental hazards,
*
Corresponding author. E-mail: luming1963@163.com

Hu et al.
Scheme 1. Two-step synthesis of 3,4-dimethylbenzaldehyde
synthesis. In our previous paper, we successfully applied
(HPLC) experiments were performed on
a
liquid
surfactant micelles for the chloromethylation of 2-
bromoethylbenzene and found that cetyltrimethylammonium
bromide (CTAB) was the most active surfactant [23]. Phase
transfer catalysis (PTC) is another versatile synthetic
technique that has been widely applied to intensify otherwise
slow heterogeneous reactions involving an organic substrate
and an ionic reactant, either dissolved in water (liquid-liquid)
or present in solid state (liquid-solid) [24]. Because PTC can
decrease the reaction activation energy, accelerate reaction
speed, make conditions convenient and inhibit side reaction to
occur, it has been applied in various organic syntheses [25-27].
The objectives of the present work are to report an
improved and economic procedure for the synthesis of
chromatograph (Dionex Softron GmbH, USA), consisting of a
pump (P680) and an ultraviolet-visible light detector (UVD)
system (170U). The experiments were performed on
Diacovery C18 column, ø 4.6×150 mm. Elemental analyses
were performed on a Vario EL III instrument (Elmentar
Anlalysensy Teme GmbH, Germany).
Preparation of 3,4-Dimethylbenzyl Chloride (2)
A mixture of o-xylene (1, 10.6 g, 0.1 mol), CTAB (0.8 g,
2.3 mmol), 20% H SO (60 ml) and AcOH (30 ml) was stirred
2
4
in 250 ml round flask for 2 h at room temperature in order to
solubilize o-xylene fully in the surfactant micelle solution.
Then paraformaldehyde (3.15 g, 0.105 mol) was added and
anhydrous hydrogen chloride gas was bubbled into the flask at
3
,4-dimethylbenzaldehyde by oxidation of 3,4-dimethylbenzyl
-
1
chloride with NaNO /AcOH catalyzed by PEG-600 in aqueous
the flow rate of 60 ml min . The mixture was stirred for 4
additional hours at 45 °C and then cooled to room temperature.
The reaction progress was monitored by TLC and HPLC. The
residue obtained was extracted with methylene chloride (3 ×20
ml). The combined organic phase was washed to neutral with
3
media under PTC conditions. The starting material 3,4-
dimethylbenzyl chloride was prepared by chloromethylation of
o-xylene in CTAB micellar catalytic system (Scheme 1).
EXPERIMENTAL
20% NaHCO
3
solution (3×20 ml) and water (3×20 ml), then
dried over anhydrous Na SO . The solvent was evaporated and
2
4
Materials and Apparatus
Cetyltrimethylammonium bromide (CTAB), tetraethyl-
ammonium bromide (TEAB), tetrabutylammonium bromide
then purified by column chromatography over silica gel
(eluent:hexane/methylene chloride = 4.5/1) to give pure
3,4-dimethylbenzyl chloride (2, a colorless liquid, 13.8 g,
1
(
TBAB), tetradecyltrimethylammonium bromide (TTAB),
yield 89.7%). H NMR (400 MHz, CDCl ): 2.29 (s, 3H, CH ),
3
3
dodecyltrimethylam monium bromide (DTAB), nonylphenol
polyoxyethylene ether (NP-10), sodium dodecyl sulfonates
2.30 (s, 3H, CH ), 4.56 (s, 2H, CH ), 7.04-7.13 (m, 3H, Ar-H).
3
2
13
C NMR (100 MHz, CDCl ): 21.9, 22.5, 46.8, 126.1, 129.7,
3
(SDS) and (polyethylene glycol 600) PEG-600 all purchased
130.5, 135.8, 137.1, 137.5. Anal. Calcd. For C H Cl: C,
9
11
from Aldrich Chemical Co., were of analytical grade and used
without further purification. Other reagents purchased from
Chinese companies were all of analytical or chemical grades.
Distilled water was used for all the reactions. NMR spectra
were recorded on a Bruker 400-MHz spectrometer using
69.86%; H, 7.18%; Cl, 22.96% . Found: C, 69.90%; H, 7.17%;
Cl, 22.93%.
Preparation of 3,4-Dimethylbenzaldehyde (3)
A mixture of 3,4-dimethylbenzyl chloride (15.4 g, 0.1 mol),
PEG-600 (0.73 g, 1.2 mmol) and AcOH 10 ml was stirred in
CDCl as the solvent with tetramethylsilane (TMS) as an
3
internal standard. High performance liquid chromatography
250 ml round flask. Then a solution of NaNO (9.35 g, 0.11
3
4
88

A Highly Efficient Method for the Preparation of 3,4-Dimethylbenzaldehyde
100
mol) in H O (10 ml) was added dropwise while the mixture
2
was being heated at reflux. The reaction progress was
monitored by TLC and HPLC. The reaction was completed in
80
6
4
0
0
3
h, and then cooled to room temperature. The organic product
was extracted with methylene chloride (3×30 ml). The
combined organic phase was washed to neutral with 10%
20
0
NaHCO solution (3×20 ml) and water (3×20 ml), then dried
3
over anhydrous Na SO . The solvent was removed and the
2
4
0
2
4
6
8
residue was distilled under vacuum to give pure 3,4-
Time (h)
dimethylbenzaldehyde (3, a colorless liquid, 12.3 g, 92%). b.p.:
1
8
3 °C/5 mmHg (lit. [20] b.p.: 233-235 °C). H NMR (400
Fig. 1. Plot of the chloromethylation degree of o-xylene vs.
time in presence and in absence of CTAB: (ꢀ) yield,
no CTAB; ( ) yield, CTAB; (ꢀ) conversion, no
CTAB; (=) conversion, CTAB.
MHz, CDCl ): 2.32 (s, 3H, CH ), 2.34 (s, 3H, CH ), 7.38-7.53
3
3
3
1
3
(
m, 3H, Ar-H), 9.91 (s, H, -CHO). C NMR (100 MHz, CDCl
3): 17.9, 18.7, 127.2, 129.7, 130.2, 134.1, 136.9, 143.5, 194.2.
Anal. Calcd. For C H O: C, 80.54%, H, 7.51%, O, 11.94%
9
10
Found: C, 80.56%, H, 7.51%, O, 11.92 %.
1
00
RESULTS AND DISCUSSION
8
0
In a preliminary study, the chloromethylation was carried
out in oil-water biphasic system in the presence and absence of
CTAB. As shown in Fig. 1, in the absence of surfactant
60
4
2
0
0
0
(CTAB), the chloromethylation reaction proceeded very
CM
18CMC
-0.5
slowly, the yield was less than 20% after 7 h, and the
conversion was only 73%. Reaction performed with CTAB (its
critical micelle concentration, CMC, in pure water at 25 °C is
-4.5
-2.5
-1
CTAB concentration (log[c (M) ])
-
4
-2
9
.20 × 10 M [28]) at a concentration of 2.57 × 10 M (18
CMC) proceeded very rapidly, the yield reached 89.7% in a
shorter time (4 h), and the conversion increased to 94%. After
that, the conversion and the yield leveled off.
Fig. 2. Influences of concentration of CTAB on the
chloromethylation: (ꢁ) conversion, (ꢀ) yield.
Figure 2 shows the effects of CTAB concentration on the
chloromethylation. When CTAB concentration was below
CMC, the yield after 4 h was only 25.4%, the conversion was
about 78%, and hardly varied with surfactant concentration.
However, the yield and the conversion increased with an
increase in the surfactant concentration higher than CMC, and
leveled off after the surfactant concentration reached 18CMC.
The experimental facts distinctly display the high efficiency of
micellar catalysis system. When no surfactant is used or
surfactant concentration is below CMC, the reaction system is
a suspension (under stirring) with two phases, and th the
reaction rate is very slow. However, when surfactant e
interface between oil phase and water phase is very small, so
micelles were formed, o-xylene was solubilized into the
micelles, the interface area of oil phase/water phase was
magnified suddenly and the rate of chloromethylation reaction
occurring at the interface was accelerated abruptly, so the
conversion showed a break point at CMC. Above CMC, the
number of micelles increased with the increasing surfactant
concentration, so the rate of chloromethylation reaction
speeded up and a higher conversion was obtained. Further
increase of the surfactant concentration induced micelles to
expand, which in turn caused slow increase of oil/water
interfacial area. Therefore, at high CTAB concentration, the
rate of increase gradually slowed down and the conversion of
4
89

Hu et al.
1
00
100
8
6
4
2
0
0
0
0
0
8
0
60
4
0
0
1
2
3
4
20
0
Time (h)
0
10
20
30
40
Fig. 3. Influences of different types of surfactants on the
chloromethylation: (=) no surfactant, (ꢀ) SDS, (ꢀ)
NP-10, (ꢀ) TTAB, ( ) CTAB.
Sulfuric acid concentration (wt-%)
Fig. 5. Influences of concentration of sulfuric acid on the
chloromethylation: (ꢁ) conversion, (ꢀ) yield.
1
00
8
6
4
2
0
0
0
0
0
100
80
6
4
2
0
0
0
0
1
0
30
50
70
0
0.2
0.4
0.6
0.8
Te m pe ra t u re ( °C )
-1
Ratio [(ml AcOH) (ml H SO ) ]
2
4
Fig. 4. Influences of temperature on the chloromethylation:
ꢁ) conversion, (ꢀ) yield.
(
Fig. 6. Influences of volume ratio of acetic acid and sulfuric
acid on the chloromethylation: (ꢁ) conversion, (ꢀ)
yield.
o-xylene did not change significantly. Besides CTAB, we also
tried to use other types of surfactants such as SDS, NP-10 and
TTAB as catalysts in the reaction. As shown in Fig. 3, it was
observed that the yield was highest for the CTAB system,
higher for the TTAB, lower for the NP-10 and much lower for
the SDS. The different catalytic abilities of surfactants could
be attributed to their different solubilization abilities. CTAB,
TTAB, NP-10 and SDS have various CMC, the lower CMC
leads to more micelles at the same concentration causing more
o-xylene to be solubilized into micelles and greater encounter
probability between o-xylene and reactive species. Thus, the
observed rate and yield of the reaction is CTAB > TTAB >
NP-10 > SDS.
temperature to 45 °C; however, the yield decreased with
further increase of the temperature. The yield reached
maximum at 45 °C under which the conversion could be kept
at the highest. These findings show that a moderate
temperature such as 45 °C enhances the chloromethylation.
The effects of the concentration of sulfuric acid at 45 °C
are shown in Fig. 5. No reaction occurred in the absence of
sulfuric acid, and an increase in its concentration enhanced the
catalysis. The yield reached maximum at the 20% sulfuric acid;
however, it decreased with further increase of its concentration.
Fig. 6 shows the effects of the volume ratio of AcOH to 20%
Figure 4 shows the effects of reaction temperature on the
chloromethylation. The catalytic activity increased with the
H SO4 at 45 °C. In the absence of acetic acid, the
chloromethylation reaction proceeded poorly, only 67.7%
2
4
90

A Highly Efficient Method for the Preparation of 3,4-Dimethylbenzaldehyde
Table 1. PEG-600 Catalyzed Oxidation of 3,4-Dimethylbenzyl Chloride with NaNO
in Aqueous Mediaa
3
NaNO3
Entry
PEG-600 (mmol)
AcOH (ml)
Time (h)
Yield (%)b
(
mol)
1
2
3
4
5
6
7
8
0.11ꢀ
0.11
0.11
0.11
0.11
0.11
0.11
0.11
-
-
-
-
10
10
10
12
15
10
10
10
3
3
3
17
53
53
92
92
67
92
92
1.2
1.3
1.2
1.3
-
1.2
1.2
3
3
a
Reaction conditions: 3,4-dimethylbenzyl chloride (0.1 mol), NaNO
Isolated yield.
3
(0.11 mol), H O (10 ml), reflux.
2
b
yield was obtained and the conversion decreased to 87%. An
increase in the amount of acetic acid (i.e. increase in volume
ratio) enhanced the catalysis, the yield reached maximum at
colorless to yellow and a red-brown gas was observed as the
reaction proceeded. After the addition of AcOH 10 ml to the
catalytic system, the yield increased to 92% (Table 1, entry 4)
within 3 h, which shows that AcOH can promote the oxidation
reaction to some extent. Further addition of the amount of
AcOH, under the same conditions, did not enhance the yield
significantly (Table 1, entries 7 and 8). The results are shown
in Table 1.
0.5 of the ratio. However, the yield decreased slowly with
further increase in the amount of acetic acid although the
conversion seemed to be stable. Such a pattern of behavior
shown by acetic acid may be due to the enhancement of
solubility to prompt the contact of paraformaldehyde with
o-xylene via solvation. Nevertheless, further studies are
required for an adequate clarification of the mechanism.
Phase-transfer catalyst and AcOH must coexist in this
oxidation system (Table 1, entries 3-8). The yield was raised to
92% in a shorter period (3 h) if both PEG-600 and AcOH were
present in the reaction system (Table 1, entry 4). The PEG-600
was also crucial for achieving high yields because the yield
decreased to 67% in the absence of PEG-600 (Table 1, entry 6)
but increased to 90% in its presence. Using 1.3 mmol of
PEG-600, under the same conditions, did not enhance the
yield significantly (Table 1, entries 3 and 5). Therefore, the
optimal reaction conditions were observed in entry 4 in Table
1. Besides PEG-600, we also used cetyltrimethylammonium
bromide (CTAB), tetraethylammonium bromide (TEAB),
tetrabutylammonium bromide (TBAB), dodecyltrimethylam
monium bromide (DTAB) and sodium dodecyl sulfonates
(SDS) as phase-transfer catalysts and the yields were merely
82%, 70%, 79%, 73% and 77%, respectively (Table 2).
Therefore, the best phase transfer catalyst was PEG-600.
Figure 8 shows the effects of the amount of sodium nitrate
on the oxidation. No reaction occurred in the absence of
NaNO , and the increase in the amount of NaNO (i.e. the
3,4-Dimethylbenzyl chloride was initially oxidized using
sodium nitrate (NaNO ) as oxidant in the presence of
3
PEG-600 and AcOH at reflux and the yield was 92% (Table 1,
entry 4) after 3 h. Other nitrates such as potassium nitrate
(
(
KNO ), calcium nitrate (Ca(NO ) ) and sodium nitrite
3
3 2
NaNO ), are also excellent oxidants for the oxidation of
2
3
,4-dimethylbenzyl chloride with yields over 85% under the
same reaction conditions. However, the yield was decreased in
the order: NaNO > KNO > NaNO > Ca(NO ) . Thus, based
3
3
2
3 2
on the results obtained, the best oxidant was NaNO . Typical
3
results are shown in Fig. 7.
The oxidation of 3,4-dimethylbenzyl chloride (2) was then
carried out by vigorously stirring the two-phase system (2 and
1
0 ml H O) at reflux temperature with NaNO (0.11 mol). The
2
3
yield was only 17% (Table 1, entry 1) after 10 h. Using
PEG-600 (1.2 mmol) as phase-transfer catalyst, under the
same conditions, 3 was obtained in a higher yield 53% (Table
1
, entry 2). The colors of the reaction mixture varied from
3
3
4
91

Hu et al.
9
8
8
2
8
4
1
2
3
4
Ca(NO
3
NaNO
2
KNO
3
NaNO
Oxidant
Fig. 7. Influences of type of nitrates on the oxidation.
Table 2. PTC Catalyzed Oxidation of 3,4-Dimethylbenzyl
100
80
a
Chloride with NaNO /AcOH in Aqueous Media
3
6
0
40
Phase transfer catalyst
Time (h)
Yield (%)b
2
0
0
PEG-600
CTAB
TEAB
TBAB
DTAB
SDS
3
5
6
4
7
3
92
82
70
79
73
77
0
0.5
1
1.5
-1
3
Sodium nitrate [(mol NaNO ) (mol 2) ]
Fig. 8. Influences of amount of sodium nitrate on the
oxidation.
a
Reaction conditions: 3,4-dimethylbenzyl chloride (0.1 mol),
NaNO (0.11 mol), PTC (1.2 mmol), AcOH (10 ml), H O (10
3
2
b
ml), reflux. Isolated yield.
3
,4-dimethylbenzaldehyde was developed in this research, by
CTAB micellar-catalyzed chloromethylation of o-xylene as the
starting material and the subsequent oxidation with
ratio of NaNO to 2) enhanced the yield of the reaction and the
yield reached maximum at 1.1 of the ratio. However, further
3
NaNO /AcOH catalyzed by PEG-600 in aqueous media.
3
One of the notable aspects of our work was that the CTAB
micellar-catalyzed chloromethylation of o-xylene was carried
out successfully. The conversion for 2 in the micellar solutions
was remarkably improved to 94% which provided a striking
contrast to other procedures catalyzed by lewis acids, ionic
liquids or rare-earth metal triflates whose conversion for 2 was
usually 72~80% [29-34], which demonstrated a higher yield
addition of NaNO resulted in the decrease of the yield, which
is due to the formation of 3,4-dimethylbenzoic acid, which
was confirmed by HPLC.
3
CONCLUSIONS
A facile and efficient procedure for the synthesis of
4
92

A Highly Efficient Method for the Preparation of 3,4-Dimethylbenzaldehyde
for the chloromethylation of o-xylene. Another noteworthy
(1985) 1483.
point in our study was that we managed to develop a novel and
efficient procedure for the preparation of 3 from 2 via an
oxidation with sodium nitrate and acetic acid catalyzed by
PEG-600 in aqueous media under PTC conditions. Compared
with other oxidation procedures [35-42], this procedure not
only enhanced the yield, but also had the merits of being
economical, convenient and eco-safe. The good yields
obtained in all cases, makes this oxidation procedure very
attractive.
[13] Toshiaki, Kobayashi, Nara, K. Kujitani, U.S. Patent
4902807, 1990.
[14] M. Tanaka, J. Iyoda, Y. Souma, J. Org. Chem. 57 (1992)
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In conclusion, we have developed an excellent procedure
for the preparation of 3,4-dimethylbenzaldehyde in two steps
in an 82.5% total yield. Judging from the conditions employed,
this method has great prospects for industrial applications.
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(
2007) 113.
We thank the Ministry of Education of the Republic of
China and Natural Science Foundation of Jiangsu Province for
their support of this research.
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