Solid - Liquid phase-transfer-catalysed methoxylation of chloronitrobenzenes to nitroanisoles

Article abstract of DOI:10.1021/op9800562

Nitroanisoles are important precursors in the organic process industry and are made from chloronitrobenzenes by using sodium hydroxide and methanol under liquid-liquid phase-transfer catalysis, which requires higher temperatures, leading to loss in selectivity. Synthesis of p-nitroanisole was carried out fromp-chloronitrobenzene (PCNB) and solid sodium methoxide by using tetrabutylammonium bromide as a catalyst under solid-liquid phase-transfer catalysis (S-L PTC) as opposed to the standard liquid-liquid (L-L) PTC system. The advantages of the S-L PTC^re that the reaction is conducted at room temperature with 100% conversion and 100% selectivity. The kinetics and mechanism are also established for this reaction. The methoxylation of o-chloronitrobenzene (OCNB) was also studied. The rate of methoxylation of PCNB was much greater than that of OCNB at 30 °C. m-Chloronitrobenzene does not react with sodium methoxide under otherwise similar conditions. For the reaction of PCNB with sodium methoxide, the diffusional resistance was found to be unimportant. The rate of the reaction was found to be of first order with respect to both the concentration of catalyst and the substrate.

Full text of DOI:10.1021/op9800562

Organic Process Research & Development 1999, 3, 83−91
Articles
Solid-Liquid Phase-Transfer-Catalysed Methoxylation of Chloronitrobenzenes
to Nitroanisoles
G. D. Yadav* and Suguna S. Naik
Chemical Engineering DiVision, UniVersity Department of Chemical Technology (UDCT), UniVersity of Mumbai,
Matunga, Mumbai-400 019, India
Abstract:
anisidine and o-dianisidine, both of which are important dye
intermediates for making the dyes Fast scarlet base and basic
5-nitro-o-anisidine (Fast red B base). The preparation of
p-nitroanisole starting from p-chloronitrobenzene (PCNB)
can be achieved under liquid-liquid (L-L) PTC by using
aqueous sodium hydroxide, methanol, and a phase-transfer
catalyst at reflux temperatures, which also gives p-nitrophe-
nol as a side product. The conversion of a liquid-liquid
system into a solid-liquid (S-L) system can alter the rates
of reactions as well as selectivities.
Nitroanisoles are important precursors in the organic process
industry and are made from chloronitrobenzenes by using
sodium hydroxide and methanol under liquid-liquid phase-
transfer catalysis, which requires higher temperatures, leading
to loss in selectivity. Synthesis of p-nitroanisole was carried out
from p-chloronitrobenzene (PCNB) and solid sodium methoxide
by using tetrabutylammonium bromide as a catalyst under
solid-liquid phase-transfer catalysis (S-L PTC) as opposed to
the standard liquid-liquid (L-L) PTC system. The advantages
of the S-L PTC are that the reaction is conducted at room
temperature with 100% conversion and 100% selectivity. The
kinetics and mechanism are also established for this reaction.
The methoxylation of o-chloronitrobenzene (OCNB) was also
studied. The rate of methoxylation of PCNB was much greater
than that of OCNB at 30 °C. m-Chloronitrobenzene does not
react with sodium methoxide under otherwise similar condi-
tions. For the reaction of PCNB with sodium methoxide, the
diffusional resistance was found to be unimportant. The rate
of the reaction was found to be of first order with respect to
both the concentration of catalyst and the substrate.
Solid-liquid PTC has been employed as a useful vehicle
for conducting a wide variety of organic transformations
using quaternary salts and trialkylamines as well as cyclic
and linear polyethers. While liquid-liquid PTC involves het-
erogeneous reaction processes between two liquid phases
(usually an aqueous phase and an organic phase), solid-
liquid PTC usually involves reaction of an anionic reagent
in a solid phase (usually a salt) with a reactant located in a
continuous organic phase. Several reports have appeared in
which liquid-liquid (L-L) PTC fails while solid-liquid
(S-L) PTC is successful. In addition, the elimination of
water of hydration often increases the activity of the anion
species in nonpolar, aprotic organic solvents. In some reac-
tions, addition of a little water enhances the reaction, espe-
cially in cases where ammonium salts are used in S-L PTC.
Arrad and Sasson4 have reported the synthesis of p-
nitroanisole from PCNB and sodium methoxide using
Amberlyst (A 27) as a catalyst to give 70% yield and 98%
purity. De la Zerda et al.5 synthesized p-nitroanisole using
PCNB, sodium hydroxide, methanol, and tetrabutylammo-
nium bromide (TBAB) at 80 °C to give 99% conversion,
whereas Yu and Liu6 used polystyrene-supported PEG-600
at 60-70 °C for 7 h to get 96.7% p-nitroanisole. Naik has
reviewed some aspects of S-L PTC.7 Recently, Yadav and
Sharma8 proposed the first model of S-L PTC for the
reaction of benzyl chloride with solid sodium acetate and
Introduction
Phase-transfer catalysis (PTC) has been applied to over
600 conventional processes in agrochemical, dyestuff, phar-
maceutical, polymer, perfumes, flavour, and other fine
chemical industries.1-3 The sulfide and bisulfide reduction
of nitroaromatics with PTC is one class of reactions.
Etherification and alkoxylation reactions are another impor-
tant class of reaction involving the use of PTC. Nitroanisoles
are important precursors in the organic process industry. The
reduction of p-nitroanisole leads to p-anisidine, which is a
dye intermediate, used in the preparation of the dye Fast
Bordeaux GP base. o-Nitroanisole can be reduced to o-
* To whom correspondence should be addressed. Fax: 91-22-4145614. Tel.:
91-22-4145616. E-mail: gdy@udct.ernet.in.
(1) Starks, C. M.; Liotta, C.; Halpern, M. Phase Transfer Catalysis: Funda-
mentals, Applications, and Industrial PerspectiVes; Chapman and Hall
Publications: New York, 1994.
(4) Arrad, O.; Sasson, Y. J. Org Chem. 1989, 54, 4993.
(5) De la Zerda, X.; Cohen, S.; Sasson, Y. J. Chem. Soc., Perkin Trans. 1990,
2, 1.
(6) Yu, S.; Liu, W. Chem. Abstr. 1994, 121, 255335x.
(7) Naik, S. S. Insights in catalysis: Novelties of multiphase transfer catalysis.
Ph.D. Thesis, Mumbai University, 1998.
(2) Sasson, Y., Neumann, R., Eds. Handbook of Phase Transfer Catalysis;
Blackie Academic and Professional: London, 1997.
(3) Dehmlow, E. V.; Dehmlow, S. S. Phase Transfer Catalysis, 3rd ed.; VCH:
New York, 1993.
(8) Yadav, G. D.; Sharma, M. M. Ind. Eng. Chem. Process Des. DeV. 1981,
20, 2.
10.1021/op9800562 CCC: $18.00 © 1999 American Chemical Society and Royal Society of Chemistry
Published on Web 01/08/1999
Vol. 3, No. 2, 1999 / Organic Process Research & Development
•
83

benzoate to produce the commercially important benzyl
esters. Naik and Doraiswamy9 have proposed different
models of S-L PTC. There are other mechanistic aspects
of S-L PTC discussed in some detail.10,11 However, there
is no information on the process development and kinetics
of the preparation of nitroanisoles under S-L PTC. This
paper deals with these aspects. A systematic investigation
was done with PCNB as the model compound, and the results
were extended to the methoxylation of o-chloronitrobenzene
(OCNB) in comparison with the usual L-L PTC.
rates of reaction under milder conditions, it was thought
worthwhile to study the mechanism and kinetics of this
reaction. The effect of various parameters on the rates of
reaction was also studied.
Mechanism. The usual mechanistic descriptions of S-L
PTC found in the literature are not as elaborate as those
forL-L PTC. In S-L PTC, the first step involves the
transport of a reactant anion from the solid phase to the
organic phase by a phase-transfer cation. This could be an
organophilic quaternary cation or an organophilic cation
derived from the complexation of a metal cation with a
multidentate ligand (crown ether, cryptand, poly(ethylene
oxide), etc.). The second step involves the reaction of the
transferred anion with the reactant located in the organic
phase. The reactant anion must be in an active form. Finally,
the third step involves the transport of the product anion by
the phase-transfer cation to the solid phase and the transport
of another reactant anion into the organic phase. The S-L
Chemicals
PCNB was obtained from M/S Aarti Industries Ltd.,
Mumbai, India. Sodium methoxide (pure grade), tetrabutyl-
ammonium bromide (TBAB), Aliquat-336, tetrabutylammo-
nium hydrogen sulfate (TBAHS), and cetyltrimethylammo-
nium bromide (CTAB), all of pure grade, were obtained from
Loba Chemie, Mumbai, India. Toluene of LR grade was
obtained from s.d. Fine Chemicals, Mumbai, India.
PTC
mech-
anistic description suggests that most of the reaction takes
place in anhydrous conditions and that both solid and liquid
phases are dry.8 However, if the reagents are not properly
dried or freed of moisture, water is usually associated with
inorganic as well as organic salts, which is called the ω-
phase3. Small amounts of water can dramatically affect the
overall rate of the reaction. Very recently, Naik and Do-
raiswamy9 have reviewed the S-L PTC mechanism, ac-
cording to which there is both homogeneous and heteroge-
neous solubilisation (Figure 1).
Experimental Procedure
The reactions were studied in a 5-cm-i.d. fully baffled,
mechanically agitated contactor of 250 cm3 total capacity,
equipped with a six-blade turbine impeller and a reflux
condenser. Typical experiments were conducted by taking
0.025 gmol of PCNB, 0.1 gmol of sodium methoxide, the
required amount of catalyst, and toluene as solvent at 30
°C. Typically, toluene was added initially to the reactor,
followed by sodium methoxide addition. The mixture was
stirred, and then PCNB was added to it along with the
catalyst. The reaction chemistry is given below.
The schematics of models of the homogeneous solubili-
sation are presented in Figure 2, where model A is the one
developed by Yadav and Sharma.8 Figure 3 shows the
modeling approach to heterogeneous solubilisation.9 Model
A considers only the organic reaction to be controlling, with
ion exchange in equilibrium and the solid dissolution and
mass-transfer steps very fast in comparison to the organic
reaction. Model B accounts for the ion-exchange reaction
in addition to the organic reaction as contributing to the
overall rate of the overall PTC cycle, with solid dissolution
and mass transfer still fast. Both the ion-exchange and
organic reactions are under kinetic control. Model C consid-
ers solid dissolution to contribute to the overall reaction rate
in addition to the ion-exchange and organic reactions. In
model D, the ion-exchange reaction occurs in the film, and
then it incorporates the effects of transport of Q+Y- between
the film and the organic bulk phase. Here, the reaction can
occur in three different ways:12 (i) in the bulk organic phase
between QY with RX in such a way that [QY] would be
zero beyond the film thickness δ from the solid surface; (ii)
in the film, where diffusion of QY and reaction with RX
are parallel steps and no free QY exists in the bulk phase;
and (iii) M+Y- ion exchange with Q+X- at the solid surface
to form a Q+Y- ion pair in such a way that Q+Y- diffuses
from the solid surface a small distance λ in the liquid film
(λ < δ) in this region and then reacts with RX at that plane
λ. No Q+Y- diffuses into the bulk organic phase beyond
that plane within the film, and no RX diffuses beyond the
Method of Analysis
Samples were withdrawn at particular time intervals and
analyzed using gas-liquid chromatography on a Chemito
model 8510 instrument. Quantitative results were obtained
through calibration using synthetic mixtures of pure com-
ponents. A 2.0-m × 3.2-mm-i.d. stainless steel column
packed with Chromosorb WHP, which was impregnated with
10% SE-30, was used for analysis. Upon completion of the
reaction, the mixture was filtered to remove the solids, and
the organic phase was washed with water. The product was
separated by distillation, and GC-MS analysis was per-
formed.
Results and Discussion
Preliminary experiments established that the reaction was
quite facile at room temperature. To understand the enhanced
(9) Naik, S. D.; Doraiswamy, L. K. Chem. Eng. Sci. 1997, 52, 4533.
(10) Esikova, I. A.; Yufit, S. S. J. Phys. Org. Chem. 1991, 4, 149, 341, 336.
(11) Halpern, M., Ed. Phase transfer Catalysis: Mechanism and Synthesis; ACS
Symposium Series 659; American Chemical Society: Washington, DC,
1996.
(12) Yadav, G. D.; Haldavanekar, B. V. J. Phy. Chem. 1997, 101, 36.
84
•
Vol. 3, No. 2, 1999 / Organic Process Research & Development

Figure 1. Schematic of S-L PTC mechanism.
same plane toward the solid surface. Both have zero
concentration at this plane λ.
The equilibrium constant for the reaction 3 is given
[Q+Y-][M+X-]
by
Model A for S-L PTC, which was proposed by Yadav
and Sharma,8 was found to be appropriate for this system
and is illustrated by the SN2 type of reaction of the
substrateRX with the nucleophile Y- of the solid reactant
MY, as given below. M+ is the inorganic metallic counterion
of the nucleophile Y-. The overall reaction is
Ke )
(6)
[Q+Y-][M+Y-]
The rate of reaction for eq 5 is given by
d[RX]
-
) k2[RX][Q+Y-]
(7)
(8)
dt
kobsd
RX(org) + MY(s) + [Q+X-](org)
8
[M+Y-]
[M+X-]
) k2Ke
[RX][Q+X-]
RY(org) + MX(s) + [Q+X-](org) (1)
where Q+ is the quaternary cation of the catalyst. Q+X- is
a loosely bound ion pair. Brackets are used to show ion pairs.
The solid reactant is in equilibrium with its dissolved part
in the organic phase.
[M+Y-]/[M+X-] ) r can be assumed to be a constant due
to the limited solubility of both the nucleophile and the
leaving group in the organic phase. Then over a small range,
for quaternary salt catalysis, eq 7 becomes
MY(s) a [M+Y-](org)
(2)
d[RX]
dt
-
) k1[RX]
(9)
The quaternary salt (Q+X-) reacts with the dissolved part
in the organic phase.
where
Ke
\
k1 ) k2Ker[Q+X-]
(10)
[Q+X-] + [M+Y-] {
} [Q+Y-] + [M+X-]
(3)
(4)
[M+X-](org) a MX(s)
Equation 9 can be integrated for fixed values of r, or eq 7
can be numerically solved.
The substrate RX reacts with [Q+Y-] according to
Integration of eq 9 yields the following equation:
k2
RX + [Q+Y-]
8 RY + [Q+X-]
(5)
ln([RX]/[RX]0) ) -k1t
with the boundary condition
(11)
(product)
The quaternary salt is thus generated repeatedly to catalyse
the reaction. However, there is no transfer of the catalyst
across the interface as is normally observed in the case of
liquid-liquid phase-transfer process.
at t ) 0: [RX] ) [RX]0
(12)
or
Vol. 3, No. 2, 1999 / Organic Process Research & Development
•
85

Figure 2. Schematic models for homogeneous solubilisation.
-ln(1 - XA) ) k1t
Effect of Speed of Agitation. Figure 4 shows that the
speed of agitation had no effect on conversion in the range
of 750-1200 rpm. The conversion was slightly lower at 500
rpm than that in the range 750-1200 rpm. Therefore, the
speed of agitation was kept at around 1000 rpm to eliminate
the external mass-transfer resistance and for assessing the
effect of other variables on the rate of the reaction. This
suggests that the catalyst concentration [Q+Y-]org, due to
anion-exchange solubilisation in organic phase, is constant,
where Y- ) CH3O-.
where
XA (fractional conversion of RX) ) ([RX]0 - [RX])/[RX]0)
(13)
Here, k1 is a constant for a fixed catalyst concentration [Q0]
added to the system. Thus a plot of -ln(1 - XA) against t
will give a slope of k1, the rate constant. The value of this
slope can be changed by varying the catalyst concentration.
A plot of k1 against concentration of catalyst [Qo] will give
a slope of k2′ ) k2Ker.
The above model was verified by conducting several
experiments, during which the effects of different parameters
on the rate of reaction were studied.
The rate of mass transfer is given by
RMY ) kSLap ([MY]*org - [MY]org
)
(14)
where [MY]* is the saturation solubility of the solid reactant
in the organic phase. This rate of mass transfer should always
86
•
Vol. 3, No. 2, 1999 / Organic Process Research & Development

Figure 3. Modeling approach to heterogeneous solubilisation.
Figure 5. Effect of sodium methoxide loading. Conditions:
PCNB, 0.025 gmol; temperature, 30 °C; TBAB, 7.5 × 10-4
gmol; solvent, toluene; speed of agitation, 1000 rpm.
Figure 4. Effect of speed of agitation. Conditions: PCNB,
0.025 gmol; NaOCH3, 0.1 gmol; temperature, 30 °C; TBAB,
7.5 × 10-4 gmol; toluene, 45 cm3; speed of agitation, 1000 rpm.
PCNB under otherwise similar conditions, where the organic
phase volume was kept constant. Figure 5 shows that the
reaction was independent of the loading of the solid beyond
0.05 gmol, which was twice the limiting reactant. The rate
was dependent on the loading of the solid sodium methoxide
be greater than the rate of reaction in order to eliminate mass-
transfer effects. kSL represents the solid-liquid mass-transfer
coefficient, and ap is the solid surface area per unit liquid
volume. ap is dependent on solid loading, and hence it was
necessary to assess the effect of solid loading on the rate of
reaction.
Effect of Sodium Methoxide Loading. Sodium meth-
oxide loading was varied from 0.025 to 0.125 gmol, i.e.,
from a mole ratio of 1:1 to 1:5 times the limiting reactant
up to 0.05 gmol; until then [Q+OCH3 ]org ()[M+Y-]org) was
-
not equal to the saturation concentration [MY]*org, and some
catalyst was still in the form of [Q+X-]. Therefore, in further
experiments, the solid reactant was taken four times in excess
over that theoretically required for complete conversion of
Vol. 3, No. 2, 1999 / Organic Process Research & Development
•
87

Table 1. S-L PTC reaction of PCNB with NaOCH3 at 30
°C
TBAB concn,
pseudo-first-order rate
k2′, cm3
[Q0] × 105 gmol/cm3
constant, k1 x 104 s-1
gmol-1 s-1
0.55
1.66
3.30
11.1
0.53
4.96
7.46
14.8
9.63
29.87
22.60
13.33
Table 2. Efficacy of different catalysts in methoxylation of
chloronitrobenzenes at 30 °C
k2′, cm3 relative
[Q0] × 105 k1 × 104 gmol-1 rate wrt
compound
PCNB
catalyst
gmol/cm3
s-1
s-1
TBAB
Aliquat-336
TBAB
TBAHS
TBAB
1.66
1.66
1.66
1.66
1.66
1.25
4.9
5.6
7.5
29.5
33.7
12.5
0.25
1
1.142
1
0
OCNB
MCNB TBAB
2.01
Figure 6. Effect of catalyst loading. Conditions: PCNB, 0.025
gmol; NaOCH3, 0.1 gmol; temperature, 30 °C; toluene, 45 cm3;
speed of agitation, 1000 rpm.
PCNB. In fact, in all of these experiments, the amount of
sodium methoxide was kept constant in order to have the
same surface area under the dispersion. The surface area is
determined on the basis of the number of particles in a given
solid loading and is ap ) 6w/(Fpdp), where w is the solid
loading in g/cm3, Fp is particle density in g/cm3, and dp is
particle diameter in cm. Thus, the particle surface area ap in
cm2/cm3 of liquid phase is the same in all cases. The particle
surface area (ap) at a loading of 5.04 g (0.1 gmol) was around
200 and 100 cm2/cm3 of the organic phase at the beginning
and the end of the reaction, respectively (Fp ) 1.2 g/cm3).
The value of the solid-liquid mass-transfer coefficient, kSL,
assuming the limiting Sherwood number to be equal to 2, is
0.15 cm/s. The actual Sherwood number would be much
greater than the limiting value of 2 when the system is under
agitation. However, to demonstrate that mass-transfer effects
were, indeed, absent, the lowest value was chosen. The bulk
liquid-phase diffusivity was calculated as 3.5 × 10-5 cm2/s
at 30 °C. Thus, the rate of solid dissolution, represented by
kSLap (30 s-1), was very much greater than the apparent rate
of reaction (k1) under otherwise similar conditions. (Tables
1 and 2). Thus, it is clear that the diffusional resistance was
unimportant.
Effect of Catalyst Loading. The concentration of catalyst
(TBAB) was varied from 5.5 × 10-6 to 1.1 × 10-4 gmol/
cm3. The conversion is plotted against time for different
concentrations of the catalyst under otherwise similar condi-
tions (Figure 6). As the concentration of the catalyst is
increased, there is an increase in conversion. The pseudo-
first-order kinetic plots are shown in Figure 7 at different
catalyst concentrations, from which the pseudo-first-order
constant k1 values are obtained. Figure 8 shows that k1 varies
fairly linearly with the catalyst concentration in the range
from 5.5 × 10-6 to 1.1 × 10-4 gmol/cm3. Thus, the
solubilisation of the reactant MY is linear in catalyst
concentration in the organic phase. In the absence of the
catalyst, no reaction was found to take place.
Figure 7. Kinetic plot for the effect of catalyst concentration.
Conditions: PCNB, 0.025 gmol; NaOCH3, 0.1 gmol; tempera-
ture, 30 °C; toluene, 45 cm3; speed of agitation, 1000 rpm.
Effect of Concentration of PCNB. Effect of concentra-
tion of PCNB was studied in the range from 2.78 × 10-4 to
8.33 × 10-4 gmol/cm3, under otherwise similar experimental
conditions. Figure 9 shows that the initial rate of the reaction
was proportional to the initial concentration of PCNB. This
is in accordance with eq 9.
Effect of Structure of PTC. The different catalytic
activities are mainly due to the different solubilities of the
ion pair [M+Y-] in the presence of catalyst in the organic
phase, which in turn can be attributed to the nature and
bulkiness of Q+ and the property of the medium. Efficiency
increases if the molecule is symmetrical. It seems that
symmetrical molecules with the same number of C atoms
or bulkiness distribute the charges more evenly and are,
therefore, more active than the unsymmetrical molecules.1-3
88
•
Vol. 3, No. 2, 1999 / Organic Process Research & Development

Figure 8. Effect of catalyst concentration on pseudo-first-order
rate constant. Conditions: PCNB, 0.025 gmol; NaOCH3, 0.1
gmol; temperature, 30 °C; solvent, toluene; speed of agitation,
1000 rpm.
Figure 10. Effect of structure of PTC. Conditions: PCNB,
0.025 gmol; NaOCH3, 0.1 gmol; temperature, 30 °C; toluene,
45 cm3; catalyst, 7.5 × 10-4 gmol; speed of agitation, 1000 rpm.
Figure 9. Effect of [PCNB]0 on initial rate. Conditions:
NaOCH3, 0.1 gmol; TBAB, 7.5 × 10-4 gmol; temperature, 30
°C; toluene, 45 cm3; speed of agitation, 1000 rpm.
Figure 11. Kinetic plot for the effect of different catalysts.
Conditions: PCNB, 0.025 gmol; NaOCH3, 0.1 gmol; tempera-
ture, 30 °C; catalyst, 7.5 × 10-4 gmol; toluene, 45 cm3; speed
of agitation, 1000 rpm.
Tetrabutylammonium bromide (TBAB), tricaprylmethyl-
ammonium chloride (Aliquat-336), cetyltrimethylammonium
bromide (CTMAB), and tetrabutylammonium hydrogen
sulfate (TBAHS) were used as the catalysts. There was not
much difference in the efficacies due to the fact that the
formation of Bu4NOCH3 was much faster. The different types
of anion X- with the same symmetrical tetrabutylammonium
ion do not matter. The bulkier catalyst Aliquat-336 gave
Effect of Solvent. Toluene, benzene, and carbon tetra-
chloride were used as the solvent (Figure 12). Thus, the final
conversions were found to be in the ascending order: CCl4
(least) , benzene < toluene (maximum). During the initial
period, the increase is marginal, but as time progresses, the
generation of MX salt leads to a fairly constant value of the
ratio r ()[MY]/[MX]). This is again due to the increase in
the concentration of dissolved nucleophile Y- in these
solvents, or correspondingly increased r values in the same
order.
Effect of Temperature. The reactions were studied at
different temperatures under otherwise similar conditions.
The conversion was found to increase with temperature.
However, at 72 °C, the yield was found to decrease due to
decomposition of sodium methoxide. At 100 °C, the conver-
-
lower conversion because the formation of (Q+OCH3 ) in
this case was relatively slow. CTMAB did not yield any
results due to nonsolubilisation of the particle. In addition,
the catalyst formed a separate phase. CTMAB, being a
surface-active agent, was found to be strongly bound to the
solid particle (Figures 10 and 11). Table 1 gives the relevant
data.
Vol. 3, No. 2, 1999 / Organic Process Research & Development
•
89

Figure 14. Arrhenius plot. Conditions: PCNB, 0.025 gmol;
NaOCH3, 0.1 gmol; TBAB, 7.5 × 10-4 gmol; toluene, 45 cm3;
speed of agitation, 1000 rpm.
Figure 12. Effect of different solvents. Conditions: PCNB,
0.025 gmol; NaOCH3, 0.1 gmol; temperature, 30 °C; toluene,
45 cm3; catalyst, 7.5 × 10-4 gmol; speed of agitation, 1000 rpm.
Figure 13. Effect of temperature. Conditions: PCNB, 0.025
gmol; NaOCH3, 0.1 gmol; TBAB, 7.5 × 10-4 gmol; toluene, 45
cm3; speed of agitation, 1000 rpm.
Figure 15. Effect of isomer. Conditions: PCNB, 0.025 gmol;
NaOCH3, 0.1 gmol; temperature, 30 °C; TBAB, 7.5 × 10-4
gmol; toluene, 45 cm3; speed of agitation, 1000 rpm.
sion suffers due to extensive decomposotion of methoxide
(Figure 13). Figure 14 gives the Arrhenius plot for the
reaction. The energy of the activation for the reaction is 14.4
kcal/gmol. Thus, the reaction is kinetically controlled.
Alkoxylation of OCNB and MCNB. When under
otherwise similar conditions the methoxylation was carried
out with o-chloronitrobenzene (OCNB), the conversion was
less than that with PCNB (Figure 15). The rate constant for
the methoxylation of OCNB at 30 °C was found to be 12.5
cm3 gmol-1 s-1. m-Chloronitrobenzene (MCNB) did not react
at 30 °C. When a mixture of PCNB and OCNB (1:1 mole
ratio) was reacted with 1 mol of sodium methoxide, both
were found to react, with the product distribution being in
the ratio of the rate constants for the respective species. Table
2 contains the pertinent data.
Conclusions
Preparation of p-nitroanisole from PCNB and solid
sodium methoxide was achieved successfully at room tem-
perature with 100% selectivity to the alkoxylated product
under solid-liquid PTC. The reaction was found to be
kinetically controlled. The rate of the reaction was found to
be first order with respect to both the concentrations of
catalyst and the substrate. Toluene was the best solvent, and
tetrabutylammonium salts were better catalysts. The type of
anion in the ion pair Q+X- did not affect the catalyst efficacy.
The methoxylation of OCNB was much slower than that of
PCNB. MCNB does not react with sodium methoxide at
room temperature.
90
•
Vol. 3, No. 2, 1999 / Organic Process Research & Development

QY
quaternary ammonium salt of nucleophile Y
Acknowledgment
r ) [M+Y-]/
ratio of concentrations of dissolved salts of nu-
cleophile and leaving group
S.S.N. would like to acknowledge AICTE for the Senior
Research Fellowship. G.D.Y. thanks AICTE and CSIR for
research funding. G.D.Y. acknowledges funding by Darbari
Seth Chair Endowment.
[M+X-]
R
alkyl or aralkyl group of substrate RX
RX
RY
X
substrate
product
NOMENCLATURE
ap
D
particle surface area, cm2/cm3 organic phase
leaving group
nucleophile
Y
diffusivity of dissolved solid reactant in organic
phase, cm2/s
Greek Letters
k1 ) k2Ker[Q+X-] pseudo-first-order rate constant, s-1
δ
λ
liquid film thickness, cm
k2
second-order rate constant for reaction, cm3 gmol-1
s-1
distance from the solid surface in the liquid film
surrounding the particle, cm
kobsd
observed second-order rate constant of the overall
reaction 1, cm3 gmol-1 s-1
Fp
density of solid reactant, MY, g/cm3
AbbreViations
Aliquat-336
CTMAB
TBAB
kSL
KA
solid-liquid mass-transfer coefficient, cm/s
tricaprylmethylammonium chloride
cetyltrimethylammonium bromide
tetrabutylammonium bromide
tetrabutylammonium hydrogen sulfate
p-chloronitrobenzene
anion partitioning equilibrium coefficient, dimen-
sionless
Ke
equilibrium constant, dimensionless
TBAHS
PCNB
M
metallic (inorganic) counterpart of leaving group
X as well as nucleophile Y
OCNB
o-chloronitrobenzene
MX
MY
(MY)*
RMY
Q
inorganic salt of leaving group X
MCNB
m-chloronitrobenzene
inorganic salt of nucleophile Y
saturated solubility of nucleophile in organic phase
rate of mass transfer of MY into the organic phase
Received for review June 29, 1998.
OP9800562
quaternary cationic part of leaving group X as well
as nucleophile Y
QX
quaternary ammonium salt of leaving group X
Vol. 3, No. 2, 1999 / Organic Process Research & Development
•
91