Oxidation of dibenzyl ethers by dilute nitric acid

Article abstract of DOI:10.1021/op000085o

A simple process for the manufacture of benzaldehyde (BzH) by oxidation of dibenzyl ether (DBE) has been reported in this paper. Two-phase reaction between dibenzyl ether and aqueous dilute nitric acid in the presence of catalytic amount of sodium nitrite has been investigated. Effect of stirring speed, concentration of nitric acid in the aqueous phase, mole ratio of reactants, sodium nitrite concentration, and reaction temperature on the conversion of DBE and yield of BzH has been investigated. 80% yield of BzH with 95% conversion of DBE has been reported. Oxidation of bis(chlorobenzyl) ethers to corresponding aldehydes is difficult compared to oxidation of DBE under otherwise identical conditions. Initial concentration of sodium nitrite does not affect the overall progress of the reaction.

Full text of DOI:10.1021/op000085o

Organic Process Research & Development 2001, 5, 152−157
Oxidation of Dibenzyl Ethers by Dilute Nitric Acid
S. R. Joshi, S. B. Sawant,* and J. B. Joshi
Department of Chemical Technology, UniVersity of Mumbai, Matunga, Mumbai - 400 019, India
Abstract:
two-phase reaction between dibenzyl ether and aqueous dilute
A simple process for the manufacture of benzaldehyde (BzH)
by oxidation of dibenzyl ether (DBE) has been reported in this
paper. Two-phase reaction between dibenzyl ether and aqueous
dilute nitric acid in the presence of catalytic amount of sodium
nitrite has been investigated. Effect of stirring speed, concentra-
tion of nitric acid in the aqueous phase, mole ratio of reactants,
sodium nitrite concentration, and reaction temperature on the
conversion of DBE and yield of BzH has been investigated. 80%
yield of BzH with 95% conversion of DBE has been reported.
Oxidation of bis(chlorobenzyl) ethers to corresponding alde-
hydes is difficult compared to oxidation of DBE under otherwise
identical conditions. Initial concentration of sodium nitrite does
not affect the overall progress of the reaction.
nitric acid in the presence of catalytic amount of sodium
nitrite was taken up for investigation. All of the experiments
were performed in the absence of any organic solvent. This
increases output per batch and reduces a step of separation
of the solvent from the product. Effect of stirring speed,
concentration of nitric acid in aqueous phase, mole ratio of
reactants, sodium nitrite concentration, and temperature on
the conversion of DBE and yield of BzH was investigated
in the present work. The main focus of the study was to
develop a simple and commercially viable process for the
manufacture of benzaldehyde from inexpensive raw material.
Oxidation of chloro-substituted dibenzyl ethers was also
investigated.
2. Experimental Section
1. Introduction
2.1. Method. All the experiments were carried out in a
batch manner. A borosilicate glass reactor of 75 mm i.d.,
400 mL capacity, provided with a pitch-blade turbine down
flow impeller (25 mm diameter), and baffles were used.
Reaction temperature was maintained by using a constant-
temperature bath.
Benzyl alcohol (BnOH) is manufactured industrially by
hydrolysis of benzyl chloride (BnCl) with sodium carbonate.
This process gives 10-12% dibenzyl ether (DBE) as an
unavoidable byproduct.1 Thus substantial quantity of dibenzyl
ether is available as the byproduct. It is possible to convert
this DBE to benzaldehyde (BzH) which is a value added
product. When 1 mmol DBE was stirred with chromato-
graphic silica gel in CCl4 under refluxing conditions, in an
atmosphere of NO2, 1.94 mmol BzH was obtained.2 Sodium
nitrate in the presence of sodium nitrite seed is reported to
be efficient catalyst for oxidation of dibenzyl ether by oxygen
in aqueous perchloric acid solution to give benzaldehyde.3
DBE when passed over Al2O3 at 340 °C and a space velocity
of 0.23 hr-1 gave 35% BzH.4 DBE (100 g) when treated
with HNO3 (390 g, 15%) under refluxing conditions, gave
BzH (46.5-57 g) after final workup.5 Dilute nitric acid, is
also an industrial byproduct and inexpensive oxidizing agent
which can be used for oxidation of DBE. Oxidation of DBE
to BzH with dilute nitric acid becomes economically viable
route for manufacture of BzH as both the raw materials are
industrial byproducts. Detailed information regarding yields
of BzH, and other possible byproducts viz. BnOH and
benzoic acid (BzOH) with respect to different operating
parameters is not available in the published literature. Hence
A predetermined quantity of dibenzyl ether was added to
the reactor placed in the constant-temperature bath. A
measured quantity of sodium nitrite was then added, followed
by addition of the aqueous nitric acid preheated to the
required temperature and then the run was started. Small
quantities of samples were withdrawn at predetermined time
intervals. The sample was divided into two parts. The first
part was analysed by gas chromatography to estimate the
amount of benzaldehyde and benzyl alcohol formed and
dibenzyl ether consumed. The second part of the sample was
analysed by TLC using a Dessagga applicator and scanner
to determine the amount of benzoic acid (BzOH) formed.
2.2. Analysis. The gas chromatographic analysis was done
using a 2 m 10% OV-17 column. Nitrogen was the carrier
gas and the detector was FID. The other parameters were as
follows:
injection temperature: 350 °C
detector temperature: 350 °C
nitrogen flow rate: 30 mL/min
temperature programming: 110 °C for 5 min, 110 to 265
°C at the rate of 25 °C/min, temperature 265 °C for 2 min.
For TLC analysis precoated silica gel plates (Merck) were
used. 0.2 mL portion of the sample was diluted to 10 mL
with methanol. Known quantities of diluted sample and
authentic sample of BzOH was applied in form of 5 mm
bands by using applicator (Dessagga). The plates were
developed in a solvent system consisting of benzene and
* To whom correspondence should be addressed. E-mail: sbs@udct.ernet.in.
Fax: 91-22-414 5614. Telephone: 91-22-414 5616.
(1) Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; VCH Verlags-
gesellschaft mbH: Weinheim, 1985; Vol. A4.
(2) Nishiguchi, T.; Hiroaki O. J. Chem. Soc., Chem. Commun. 1990, 22, 1607.
(3) Levina, A. B.; Trusov, S. R. LatV. Kim. Z. 1991, 6, 686. (Russ.) [Chem.
Abstr. 1992, 116, 193511n].
(4) Alfred R.; Helmut S.; Gisela R. J. Prakt. Chem. 1962, 15, 139.
(5) Bagdonov, K. A.; Antonova, V. A. Masloboino-ZhiroVaya Prom. 1961,
27(2), 32 [Chem. Abstr. 1961, 55, 15395f].
152
•
Vol. 5, No. 2, 2001 / Organic Process Research & Development
10.1021/op000085o CCC: $20.00 © 2001 American Chemical Society and The Royal Society of Chemistry
Published on Web 01/27/2001

Figure 2. Effect of HNO3 concentration on the conversion of
DBE at constant mole ratio of the reactants. DBE ) 0.15 mol;
HNO3 ) 0.1875 mol; temperature ) 90 °C; NaNO2 ) 1 g; speed
of stirring ) 2500 rpm.
Figure 1. Effect of speed of stirring on the progress of reaction
at 90 °C. [: DBE ) 0.15 mol; HNO3 ) 3.18 M, 59 mL; NaNO2
) 1 g; speed of stirring ) 2500 rpm. 9: DBE ) 0.15 mol; HNO3
) 3.18 M, 59 mL; NaNO2 ) 1 g; speed of stirring )1500 rpm.
0: DBE ) 1.05 mol; HNO3 ) 3.18 M, 413 mL; NaNO2 ) 7 g;
speed of stirring ) 1500 rpm.
1500 rpm. All of the further experiments were performed at
2500 rpm to ensure that the effects of various operating
parameters are evaluated when the reaction is kinetically
controlled. If the experiments are performed under mass
transfer controlled conditions, then the change in the reaction
rate cannot be attributed to the variable under investigation
alone.
Also to know if the method is scaleable we reacted 1.05
mol DBE with 413 mL, 3.18 M aqueous HNO3 and 7 g of
NaNO2 at 90 °C. Figure 1 also shows the conversion profile
of this experiment, which matches, well with reaction profiles
of experiments at 2500 and 1500 rpm. The yields of
benzaldehyde, benzyl alcohol, and benzoic acid were found
to be same for all the three experiments given in the Figure
1.
methanol (5:1 by volume). The developed plates were
analysed by using densitometer (Dessagga).
3. Results and Discussion
The conversion of DBE (%) and yield of BzH (%) were
determined by using following formulae:
moles of DBE consumed
moles of DBE charged
conversion of DBE (%) )
yield of BzH (%) )
× 100
moles of DBE consumed for the formation of BzH
moles of DBE consumed
× 100
3.2. Effect of Nitric Acid Concentration. To study the
effect of concentration of nitric acid, 0.15 mol DBE was
reacted with different concentrations of HNO3 viz. 1.59 M,
3.18 M and 4.77 M. The amount of HNO3 taken in each
experiment was kept constant at 0.1875 mol by appropriately
reducing the volume of aqueous phase as the concentration
of HNO3 increased. The experiments were performed at 90
°C and 2500 rpm; 1 g of NaNO2 was added to the reaction
mixture in each case. Figure 2 shows conversion of DBE
versus time for each case. As can be seen from the figure,
the rate of reaction increases as the concentration of the
aqueous phase increases. Similar result has been reported
by Levina et al.6 However, the increase in the rate of reaction
is not pronounced as concentration increases from 3.18 to
4.77 M. Figures 3-5 show variation of yield of BzH, BnOH,
and BzOH with respect to conversion of DBE. The yield of
BzH almost remains constant. The yield of BnOH goes on
decreasing and that of BzOH increases with increasing
conversion of DBE. During the oxidation of DBE, BnOH is
formed and oxidised to BzH.7 Benzoic acid is formed by
oxidation of benzaldehyde. Therefore, with the progress of
The effect of the following variables on the conversion
of substrate was studied: (1) speed of stirring, (2) HNO3
concentration, (3) mole ratio of reactants, (4) NaNO2 loading,
(5) temperature, (6) different substrates.
3.1. Effect of Speed of Stirring. A chemical reaction
involving two phases is regarded as mass transfer-controlled
when the rate of mass transfer across the interface is slower
compared to the rate of chemical reaction, while the same
is kinetically controlled when the rate of mass transfer across
the interface is fast but the rate of chemical reaction is slow.
At the low stirring speed, the requirement for sufficiently
rapid mass transfer of the reacting species is not met, and
mass transfer controlled kinetics is observed. As we increase
the speed of stirring, the rate of mass transfer across the
interface increases and becomes faster than the chemical
reaction rate. Thus, at higher stirring speeds, the reaction
goes from being mass transfer-controlled to kinetically
controlled.
To investigate the effect of the speed of stirring, at 90
°C, 0.15 mol DBE was treated with 59 mL, 3.18 M aqueous
nitric acid in the presence of 1 g of NaNO2. The reaction
was performed at stirring speeds of 1500 and 2500 rpm.
Figure 1 shows the conversion of DBE with time. The results
show that the reaction is not affected by stirring speed beyond
1500 rpm, and the reaction is kinetically controlled beyond
(6) Levina, A. B.; Trusov, S. R. LatV. Kim. Z. 1991, 6, 695. (Russ.) [Chem.
Abstr. 1992, 116, 193512p].
(7) Levina, A. B.; Trusov, S. R. Kinet. Ketal. 1991, 32(6), 1343 (Russ) [Chem.
Abstr. 1992, 116, 83051e].
Vol. 5, No. 2, 2001 / Organic Process Research & Development
•
153

Table 1. Material balance and consumption of HNO3 for different parameters (DBE ) 0.15mol, NaNO2 ) 1 g, temp ) 90 °C)
DBE left in
reaction mix. (mol)
BzH in
reaction mix. (mol)
BnOH in
reaction mix. (mol)
BzOH in
reaction mix. (mol)
O2 required
(mol)
HNO3 consumed
(mol)
expt 1a
expt 2b
expt 3c
expt 4d
0.013
0.021
0.036
0.003
0.219
0.202
0.187
0.233
0.021
0.023
0.022
0.006
0.026
0.021
0.013
0.043
0.141
0.128
0.111
0.162
0.170
0.165
0.132
0.237
a Expt 1: HNO3 ) 4.77 M, 39 mL (0.1875 mol); batch time ) 140 min. b Expt 2: HNO3 ) 3.18 M, 59 mL (0.1875 mol); batch time ) 150 min. c Expt 3: HNO3
) 1.59 M, 118 mL (0.1875 mol); batch time ) 210 min. d Expt 4: HNO3 ) 4.77 M, 79 mL (0.377 mol); batch time ) 85 min.
Figure 5. Effect of HNO3 concentration on the yield of BzOH
at constant mole ratio of the reactants. DBE ) 0.15 mol; HNO3
) 0.1875 mol; temperature ) 90 °C; NaNO2 ) 1 g; speed of
stirring ) 2500 rpm.
Figure 3. Effect of HNO3 concentration on the yield of BzH
at constant mole ratio of the reactants. DBE ) 0.15 mol; HNO3
) 0.1875 mol; temperature ) 90 °C; NaNO2 ) 1 g; speed of
stirring ) 2500 rpm.
Figure 4. Effect of HNO3 concentration on the yield of BnOH
at constant mole ratio of the reactants. DBE ) 0.15 mol; HNO3
) 0.1875 mol; temperature ) 90 °C; NaNO2 ) 1 g; speed of
stirring ) 2500 rpm.
Figure 6. Effect of mole ratio of reactants on the conversion
of DBE. DBE ) 0.15 mol; HNO3 ) 4.77 M; NaNO2 ) 1 g;
temperature ) 90 °C; speed of stirring ) 2500 rpm.
the presence of 1 g of NaNO2. The speed of stirring was
2500 rpm in each case. The volume of aqueous phase varied
(viz. 39.33 mL, 79 mL, 118 mL) with change in the mole
ratio of the reactants. Figure 6 shows conversion versus time
plots for this set of the experiments. As DBE to HNO3 mole
ratio changes from 1:1.25 to 1:2.5 there is increase in the
rate of consumption of DBE, but when mole ratio changes
from 1:2.5 to 1:3.75, there is marginal increase in the rate
of the reaction. At higher mole ratios, concentration of
unreacted HNO3 in the aqueous phase is higher, and as seen
from Figure 6, this higher concentration increases the
conversion of DBE under otherwise identical conditions.
the reaction the yield of BnOH decreases and that of BzOH
increases. Presence of BnOH in the final product may not
be a serious problem as it can be recovered from the reaction
mixture by fractional distillation and is a value-added
product.
Material balance and consumption of nitric acid for
different parameters has been presented in Table 1. Nitric
acid concentration in case of experiment 3 is 0.5 M which
indicates even at low nitric acid concentrations the reaction
takes place.
3.3. Effect of Mole Ratio of the Reactants. DBE (0.15
mol, 28.6 mL) was treated with 4.77 M HNO3 at 90 °C in
154
•
Vol. 5, No. 2, 2001 / Organic Process Research & Development

Figure 10. Effect of temperature on the conversion of DBE.
DBE ) 0.15 mL; HNO3 ) 3.18 M, 79 mL; NaNO2 ) 1 g; speed
of stirring ) 2500 rpm.
Figure 7. Effect of mole ratio of reactants on the yield of BzH.
DBE ) 0.15 mol; HNO3 ) 4.77 M; NaNO2 ) 1 g; temperature
) 90 °C; speed of stirring ) 2500 rpm.
Figure 8. Effect of mole ratio of reactants on the yield of
BnOH. DBE ) 0.15 mol; HNO3 ) 4.77 M; NaNO2 ) 1 g;
temperature ) 90 °C; speed of stirring ) 2500 rpm.
Figure 11. Effect of temperature on the yield of BzH. DBE )
0.15 mL; HNO3 ) 3.18 M, 79 mL; NaNO2 ) 1 g; speed of
stirring ) 2500 rpm.
1:2.5 and 1:3.75. Even after 90% conversion of DBE,
there is considerable amount of HNO3 left in the aqueous
phase, which is responsible for further oxidation of BzH to
BzOH.
3.4. Effect of Temperature. As the temperature increases
the rate of consumption of substrate increases. But with
different energies of activations for different reactions, the
change in the magnitude of the rate of reaction is not the
same for all the reactions. The effect of temperature was
mainly investigated to find out whether yield of BzH can be
improved. DBE (0.15 mol) was treated with 79 mL of 3.18
M HNO3 in the presence of 1 g of NaNO2 at 2500 rpm. The
results are presented in Figures 10-13. From Figure 10 it
can be seen that increasing the temperature also increases
the rate of reaction. Figures 12 and 13 also show that the
yields of BnOH and BzOH do not change with temperature.
Figure 11 shows the yield of BzH for different temperatures.
In this set of experiments it is observed that yield of BzH
remains practically constant even after 90% conversion of
DBE. Figures 12 and 13 indicate that at higher conversions
of DBE, the decrease in the yield BnOH corresponds to
increase in the yield of BzOH indicating that rate of
formation of BzH from BnOH almost equals the rate of
oxidation of BzH to BzOH. Further independence of yields
Figure 9. Effect of mole ratio of reactants on the yield of
BzOH. DBE ) 0.15 mol; HNO3 ) 4.77 M; NaNO2 ) 1 g;
temperature ) 90 °C; speed of stirring ) 2500 rpm.
Figures 7-9 show variation of yield of BzH, BnOH, and
BzOH, respectively, with respect to conversion of DBE.
Figure 8 has again demonstrated that the yield of BnOH
decreases as conversion of DBE increases. It is seen from
Figures 7 and 9 that after about 90% conversion of DBE
there is a rather sharp decrease in the yield of BzH and
increase in the yield of BzOH particularly for mole ratios
Vol. 5, No. 2, 2001 / Organic Process Research & Development
•
155

Figure 12. Effect of temperature on the yield of BnOH. DBE
) 0.15 mL; HNO3 ) 3.18 M, 79 mL; NaNO2 ) 1 g; speed of
stirring ) 2500 rpm.
Figure 14. Effect of NaNO2 loading on the conversion of DBE.
DBE ) 0.15 mol; HNO3 ) 1.59 M, 118 mL; temperature ) 90
°C; speed of stirring ) 2500 rpm.
Figure 13. Effect of temperature on the yield of BzOH. DBE
) 0.15 mL; HNO3 ) 3.18 M, 79 mL; NaNO2 ) 1 g; speed of
stirring ) 2500 rpm.
Figure 15. Effect of NaNO2 loading on the yield of BzH. DBE
) 0.15 mol; HNO3 ) 1.59 M, 118 mL; temperature ) 90 °C;
speed of stirring ) 2500 rpm.
of all the products on temperature indicate identical activation
energies for all the reactions.
3.5. Effect of Sodium Nitrite Concentration. When 0.15
mol DBE was treated with 118 mL, 1.59 M HNO3 in the
absence of NaNO2 at 90 °C, the conversion of DBE was
found to be less than 1.5% after 4 h. Previous experiments
have shown that in the presence of sodium nitrite, substantial
conversion (>80%) of DBE can be achieved. We decided
to determine the effect of NaNO2 concentration on the
progress of reaction. 0.15 mol DBE was treated with 118
mL, 1.59 M HNO3 at 90 °C at the stirring speed of 2500
rpm. Figure 14 shows conversion of DBE with time at
sodium nitrite loading of 1, 0.5, and 0.1 g. As can be seen
from the figure, the rate of consumption of DBE is
independent of concentration of NaNO2. Addition of NaNO2
to HNO3 produces nitrous acid, which is the species
necessary for initiation of oxidation reaction.8 Also it has
been reported that nitrous acid formed simply acts an initiator
and rate of oxidation is independent of its concentration.8
The oxidation reaction does not proceed when urea is added
to the reaction mixture which is a scavenger of nitrous acid.8
Figures 15-17 show the effect of NaNO2 concentration on
the yield of BzH, BnOH, and BzOH, respectively. It may
Figure 16. Effect of NaNO2 loading on the yield of BnOH.
DBE ) 0.15 mol; HNO3 ) 1.59 M, 118 mL; temperature ) 90
°C; speed of stirring ) 2500 rpm.
be noted that for low sodium nitrite loading (0.1 g), although
the conversion pattern is identical to that at higher concentra-
tions, the yield of BnOH is higher, while yields of BzH and
BzOH are somewhat lower.
3.6. Effect of Different Substrates. Symmetric chloro-
substituted dibenzyl ethers were synthesized by the reaction
of chlorobenzyl chlorides and corresponding chlorobenzyl
(8) Ogata Y.; Sawaki Y.; Matsunaga F.; Tezuka H. Tetrahedron 1966, 22, 2655.
156
•
Vol. 5, No. 2, 2001 / Organic Process Research & Development

Figure 20. Effect of different substrates on the yield of alco-
hols. Substrate ) 0.15mol; HNO3 ) 4.77 M, 118 mL; NaNO2
) 1 g; temperature ) 90 °C; speed of stirring ) 2500 rpm.
Figure 17. Effect of NaNO2 loading on the yield of BzOH.
DBE ) 0.15 mol; HNO3 ) 1.59 M, 118 mL; temperature ) 90
°C; speed of stirring ) 2500 rpm.
Figure 21. Effect of different substrates on the yield of acid.
Substrate ) 0.15mol; HNO3 ) 4.77 M, 118 mL; NaNO2 ) 1 g;
temperature ) 90 °C; speed of stirring ) 2500 rpm.
Figure 18. Effect of different substrates on the conversion.
Substrate ) 0.15 mol; HNO3 ) 4.77 M, 118 mL; NaNO2 ) 1 g;
temperature ) 90 °C; speed of stirring ) 2500 rpm.
To investigate the progress of reaction with different sub-
strates, 0.15 mol of ethers, were treated with 118 mL, 4.77
M HNO3 at 90 °C in the presence of 1 g of NaNO2. Figure
18 shows conversions of different ethers with time. Figures
19-21 show yield of corresponding aldehydes, alcohols, and
acids, respectively. It can be seen that in the case of PCDBE
and OCDBE yield of corresponding alcohols is high. In case
of DCDBE the overall reactivity towards oxidation is low.
4. Conclusions
(1) Successful synthesis of benzaldehyde from dibenzyl
ether with nitric acid has been demonstrated. The reaction
can be carried out even at low nitric acid concentration.
(2) As dibenzyl ether and dilute nitric acid are both
byproducts, the current strategy is economically viable.
(3) Chloro-substituted dibenzyl ethers are rather difficult
to oxidise to corresponding aldehydes with dilute nitric acid.
(4) Initial concentration of sodium nitrite does not affect
the overall progress of the reaction.
Figure 19. Effect of different substrates on the yield of
aldehyde. Substrate ) 0.15mol; HNO3 ) 4.77 M, 118 mL;
NaNO2 ) 1 g; temperature ) 90 °C; speed of stirring ) 2500
rpm.
alcohols in the presence of concentrated NaOH and trialkyl-
amine as the catalyst.9 The ethers were purified by recrys-
tallization. (4-clorobenzyl)2O, (2-chlorobenzyl)2O, and (2,4-
dichlobenzyl)2O are abbreviated as PCDBE and OCDBE,
and DCDBE, respectively.
(5) As the reaction can be carried out in the absence of
any solvent, it enhances output per batch as well as eliminates
the step of separation of solvent from the product.
Received for review July 31, 2000.
OP000085O
(9) Joshi S. R.; Sawant S. B. Forewarded for publication.
Vol. 5, No. 2, 2001 / Organic Process Research & Development
•
157