Mechanistic and kinetic investigation of liquid-liquid phase transfer catalyzed oxidation of benzyl chloride to benzaldehyde

Article abstract of DOI:10.1021/jp961678x

The kinetics and mechanism of liquid-liquid phase transfer catalyzed oxidation of benzyl chloride with chromate salts is investigated in great detail in a systematic way to throw light on the selectivity of the reaction to benzaldehyde. This system presen

Full text of DOI:10.1021/jp961678x

36
J. Phys. Chem. A 1997, 101, 36-48
Mechanistic and Kinetic Investigation of Liquid-Liquid Phase Transfer Catalyzed
Oxidation of Benzyl Chloride to Benzaldehyde
Ganapati D. Yadav* and Bhagyashree V. Haldavanekar
Chemical Engineering DiVision, UniVersity Department of Chemical Technology, UniVersity of Bombay,
Matunga, Bombay/Mumbai 400 019, India
ReceiVed: June 6, 1996X
The kinetics and mechanism of liquid-liquid phase transfer catalyzed oxidation of benzyl chloride with
chromate salts is investigated in great detail in a systematic way to throw light on the selectivity of the
reaction to benzaldehyde. This system presents a complex case of consecutive and parallel reactions in which
benzyl alcohol is formed as an intermediate which is further oxidized to benzaldehyde. The results are novel,
and the experimental data fit the theoretical model very well.
1. Introduction
chromic acid in which the reaction was found to be first order
in the concentration of each of benzyl chloride, oxidant, and
H+. The mechanism involves rapid formation of a chromate
ester which subsequently decomposes to afford benzaldehyde
and Cr(IV) species. The reaction was overall third order.
Pletcher and Tait16 have studied the oxidation of alcohols with
stoichiometric quantity of dichromate in 3 M aqueous sulfuric
acid using tetrabutylammonium bisulfate as the phase transfer
catalyst (PTC). They have suggested that the reaction proceeds
via disproportionation of chromate ester in which the proton
catalyzed disproportionation of Cr(V) or Cr(IV) to Cr(VI) is
important. Cr(VI) in aqueous solution exists as a pH-dependent
mixture of several species as given below:
Phase transfer (PT) catalysis has become a pervasive and
widely accepted synthetic tool, notwithstanding several limita-
tions on its commercial exploitation. However, there is very
limited information on the kinetics of multiphase PT catalysis
reactions, in liquid-liquid, liquid-solid, liquid-liquid-solid
(triphase) systems, particularly those involving complex mech-
anisms and series and/or parallel steps; for instance, the
oxidation of benzyl chloride with aqueous chromate salts is such
a case.
On laboratory scale, inorganic oxidizing agents have been
reported for the oxidation of benzyl chloride or benzyl alcohol
using solid-liquid (S-L) or liquid-liquid (L-L) systems, and
some of these include hypochlorite, chromic acid, chromates,
dichromates, hydrogen peroxide, etc. There is a dearth of
accounts on the mechanism and the kinetics of oxidation of alkyl
or aralkyl halides with chromate and dichromate ions under
phase transfer catalysis. This study was therefore undertaken
to investigate the liquid-liquid phase transfer catalyzed oxida-
tion of benzyl chloride with aqueous chromate/dichromate salts.
H
H
2-
-
CrO4
+8 HCrO4
+8 H2CrO4
(2)
HCrO4- + HCrO4 -H O8 Cr2O7
+8 HCr2O7 (3)
2
+H
-
2-
-
All of the above species are capable of forming ion pairs
with the PTC cation which are partitioned into the organic phase.
However, the different Cr(VI) species may not be equally active
for alcohol oxidation. Indeed there is an evidence16 that the
first step of oxidation of alcohol is
2. Literature Review and Suggested Mechanisms
Various phase transfer catalysts, such as quaternary onium
salts, crown ethers, and poly(ethylene glycol)s, have been used
for oxidation of alkyl and aralkyl halides and also of alcohols.
Haldavanekar1 has reviewed the literature2-17 on the preparation
of benzaldehyde under phase transfer catalysis. A new tech-
nique of capsule membrane phase transfer catalyzed oxidation
of benzaldehyde has emanated from this laboratory.18-19
Cardillo et al.14 have reported the usefulness of chromate ion
as a nucleophile for the crown ether catalyzed oxidation of alkyl
halide to aldehyde in hexamethylphosphoric triamide (HMPT),
where the reaction proceeds via SN2 displacement of the halogen
atom by chromate ion to form a chromate ester which further
decomposes to the corresponding carbonyl compound.
HCrO4- + ROH f ROCrO3- + H2O
(4)
Chromate ester has a greater solubility in organic solvents
than water. Hence, Pletcher and Tait16 have suggested the
following mechanism:
-
HCrO4- + Q+ f Q+HCrO4 (in aqueous phase) (5)
org. solvent
-
-
[Q+HCrO4 ]aq
8 [Q+HCrO4 ]org
(6)
-
[Q+HCrO4 ]org + RCH2OH f
HMPT
-
[RCH2OCrO3 Q+]org + H2O (7)
R-CH2X + K2CrO4 crown ether8 RCH2CrO4K f RCHO
-
[RCH2OCrO3 Q+]org f RCHO + Cr(IV) + Q+ + H+ (8)
(1)
Gopalan and Subbarayan15 have, on the basis of kinetic study,
proposed a mechanism for the oxidation of benzyl chloride by
3Cr(IV) f Cr(VI) + 2Cr(III)
(9)
where the bracketed quantities denote ion pairs.
* To whom correspondence should be addressed. FAX: (91-22) 414
5614. E-mail: gdy@udct.ernet.in.
Dey and Mahanti17 have studied the kinetics of oxidation of
substituted benzyl alcohols by quinolinium dichromate in DMF
X Abstract published in AdVance ACS Abstracts, November 1, 1996.
S1089-5639(96)01678-7 CCC: $14.00 © 1997 American Chemical Society

Oxidation of Benzyl Chloride to Benzaldehyde
J. Phys. Chem. A, Vol. 101, No. 1, 1997 37
in the presence of an acid and found that no further oxidation
of benzaldehyde occurred under their experimental conditions.
these reactions into four regimes. The theory is presented here
in brief in the context of L-L PT catalysis on the basis of a
two-film model (Figure 1).
3PhCH2OH + 2Cr(VI) f 3PhCHO + 2Cr(III) + 6H+ (10)
Regime 1: Very Slow Reaction (Figure 1a). The rate of mass
transfer of the ion pair containing the nucleophile [Q+Y-] into
the reaction phase is far greater than the rate of its reaction
with the substrate RX. The concentration of the ion pair will
be uniform throughout the reaction (organic phase), and the
overall rate is governed by the kinetics of the homogeneous
chemical reaction. Here the speed of agitation will have no
influence on the rate of chemical reaction.
The literature reveals that most of the published information
is patented and very few researchers have reported details of
kinetics of the oxidation of benzyl chloride under L-L PT
catalysis.
3. Experimental Section
ReactiVe 2: Slow Reaction (Figure 1b). In this regime, the
rate of reaction is much greater than the rate of transfer of the
ion pair [Q+Y-] from the interface through the liquid (organic)
film to the bulk liquid face. The overall rate is governed by
the rate of mass transfer. There is a concentration gradient for
the ion pair, and its concentration in the bulk reaction (organic
phase) is zero. However, no reaction occurs in the diffusion
film next to the L-L interface in the reaction phase. Here speed
of agitation has a pronounced effect on the overall rate because
the mass transfer coefficient and the liquid-liquid interfacial
areas are dependent on the speed of agitation.
Regime 3: Fast Reaction (Figure 1c). Under certain condi-
tions, the reaction of ion pair [Q+Y-]org and substrate RX occurs
while the ion pair is diffusing through the liquid (organic) film
from the interface. The chemical reaction and diffusion become
steps in parallel. There is an enhancement in the rate of
diffusion due to reaction.
Regime 4: Instantaneous Reaction (Figure 1d). In this case,
the reaction is so fast that the ion pair [Q+Y-]org and substrate
RX cannot coexist. At a certain distance λ from the L-L
interface in the liquid (organic) film, a reaction plane is formed
at which both the ion pair and substitute RX are consumed by
the reaction. The rate of mass transfer here is controlled by
the rate at which [Q+Y-]org and RX diffuse to the reaction plane.
There are also cases of overlaps between regimes 1 and 2,
regimes 1, 2, and 3, and regimes 3 and 4 depending on the
reaction conditions and mass transfer effects (see Doraiswamy
and Sharma20).
3.1. Chemicals and Catalysts. Benzyl chloride (BnCl),
toluene, potassium dichromate, and sodium carbonate of AR
grade were procured from M/s s.d. Fine Chemicals Pvt. Ltd.,
India. Tetrabutylammonium bromide (TBAB) and cetyltrim-
ethylammonium bromide (CTMAB) were obtained from Spec-
trochem, India. Aliquat-336 was obtained from Aldrich.
3.2. Experimental Setup and Procedure. Reactions were
studied in a 5 cm i.d. fully baffled mechanically agitated
contactor of 250 mL capacity equipped with a six-bladed
pitched-turbine impeller and a condenser. The reactor was kept
in a thermostatic bath whose temperature was maintained at
the desired value. Initially, the required amounts of BnCl and
toluene were fed to the reactor and the mixture was heated to
the desired temperature. A known amount of the catalyst was
then added to the reactor. An aqueous solution of potassium
dichromate and sodium carbonate which had already attained
the reaction temperature was added to the reactor at the same
temperature, and the moment of its addition was considered as
the zero time. Samples were withdrawn at a definite interval
of time, and the organic phase was analyzed by gas chroma-
tography.
Preliminary experiments were conducted with 34.1 g (0.3705
mol) of toluene, 11.803 g (0.0932 mol) of BnCl, 1.5030 g of
TBAB (5% mol/mol of benzyl chloride), 50 mL of 0.93 M
potassium dichromate solution, and 2.4 g (0.0222 mol) of
sodium bicarbonate at 100 °C for 4 h. The pH of the aqueous
phase was measured after addition of sodium bicarbonate and
was found to be in the range 6.65-6.75. All other experiments
were done at a temperature of 100 °C with TBAB as a catalyst
except when the effect of temperature on the rate of reaction
was studied.
The theory suggests that the mass transfer rates will be
important in regimes 2 and 4, and here the speed of agitation
will play a dominant role.
3.3. Method of Analysis. The progress of the reaction was
studied by analyzing the samples with a gas chromatograph
(Perkin-Elmer 8500 model) equipped with a flame ionization
detector (column: 3 mm × 4 m, stainless steel; stationary
phase: 5% OV-17 on chromosorb WHP). Synthetic mixtures
were prepared and used for quantifying the reactant and
products.
4.2. Effect of Speed of Agitation. The speed of agitation
was varied under otherwise similar conditions from 500 to 1500
rpm (Figure 2). The conversion of BnCl remained unchanged
at a particular time beyond 1000 rpm, thereby indicating absence
of mass transfer resistance to the transfer of ion pairs across
the interface. It is possible to calculate the rate of mass transfer
at 1000 rpm from established correlations for L-L agitated
systems as given by Doraiswamy and Sharma.20 Thus for the
concentration of PTC used in standard experiments it would be
most appropriate to compare the relative rates of mass transfer
and chemical reaction. A typical calculation is given here. The
estimated value of volumetric mass transfer coefficient for the
ion pair at 1000 rpm is 0.3 s-1 and the rate of mass transfer for
the PTC concentration of 9.32 × 10-5 mol/cm3 is 2.796 × 10-5
(mol/cm3 of organic phase)/s, whereas the measured value of
the rate of reaction is 3.13 × 10-7 (mol/cm3 of organic phase)/
s, which is almost 2 orders of magnitude lower. Thus, it is
clear that the rate of organic phase reaction is very slow in
comparison with the rate of mass transfer of the ion pair from
the interface. The reaction belongs to the so-called regime 1
of very slow reaction free of any mass transfer effects. To be
on the safe side, the speeds of reaction were always maintained
at or beyond 1000 rpm in all other experiments.
4. Results and Discussion
4.1. Mass Transfer Accompanied by Chemical Reaction
and Locale of Reaction. L-L PT catalyzed reactions involve
diffusion of the ion pairs back and forth between the aqueous
and organic phases and the reaction between the substrate and
the ion pair incorporating the nucleophile. Most of the L-L
PT catalyzed reactions are known to occur in the organic phase
wherein the concentration of the desired ion pair must be
maximum for achieving higher reaction rates and conversions.
It is thus necessary to ascertain the values of the relative rates
of mass transfer and chemical reaction to deduce mechanisms
and kinetics. The theory of mass transfer accompanied by
chemical reaction in G-L, L-L, S-L, G-L-S (reactant), and
G-L-S (catalyst) multiphasic systems is well documented, for
instance, by Doraiswamy and Sharma,20 who have classified

38 J. Phys. Chem. A, Vol. 101, No. 1, 1997
Yadav and Haldavanekar
Figure 1. Theory of mass transfer accompanied by chemical reaction in L-L PT calatysis: Typical concentration profiles according to the four
regimes of operation. [Q+Y-]aq ) ion pair of nucleophile in aqueous phase. [Q+Y-]org ) ion pair of nucleophile in organic phase. RX ) substrate.
δa ) thickness of aqueous phase film next to interface I. δo ) thickness of organic phase film next to interface I. (a) Regime 1: very slow reaction.
The reaction occurs in the bulk organic phase as a homogeneous reaction. Mass transfer effects are unimportant. (b) Regime 2: slow reaction. The
reaction occurs in the bulk organic phase. No free concentration of ion pairs exists in the bulk organic phase. (c) Regime 3: fast reaction. Diffusion
of [Q+Y-]org and its reaction with RX are steps in parallel, and the reaction occurs in the organic liquid film next to the L-L interface. (d) Regime
4: instantaneous reaction. The reaction between [Q+Y-]org and RX is so fast that they cannot coexist in the organic phase. [Q+Y-]org diffuses from
the L-L interface, and RX diffuses from the bulk organic phase into the organic liquid film and react at a plane λ from the interface. No free
[Q+Y-]org exists beyond distance λ, and no free RX exists beyond distance (δo - λ) toward the interface. Note that for simiplicity other ion pairs
are not shown.
-
4.3. Mechanism and Kinetics of Formation of Benzalde-
hyde. 4.3.1. Formation of Chromate Ester. The various ionic
reactions of potassium dichromate and water are given below.
only taken into account in this work. The species HCr2O7 is
-
formed in strong acid. The species HCrO4 is the only
predominant species in the pH range 6-8, which was used in
the current work.
K2Cr2O7 a Cr2O72- + 2K+
(11)
(12)
4.3.2. Reaction Paths. Since several species are involved in
the reaction mixture which can possibly follow different series
and parallel reactions resulting in the formation of benzyl alcohol
(BnOH), benzaldehyde (PhCHO), dibenzyl ether (BnOBn),
benzoic acid (BnCOOH), and benzyl benzoate (BnCOOBn), it
is essential to conduct the experiments under controlled condi-
tions limiting the byproduct formation. Some experiments were
designed to trace the path of the reaction of benzyl chloride,
leading to the formation of benzaldehyde, the main product.
The selectivity S of benzaldehyde, in general, is defined as
Cr2O72- + H2O a 2HCrO4
-
Thus, in this case along with HCrO4-, an appreciable amount
2-
of Cr2O7 is formed. Starks and Liotta21 have reported that
the transfer of monovalent ion from aqueous phase to organic
2-
phase is easier than that of the divalent ion. To convert Cr2O7
into HCrO4-, it was thought worthwhile to add Na2CO3 to the
reaction mixture.
[PhCHO]
Na2CO3 + Cr2O72- a 2 Na+ + 2CrO42- + CO2 (13)
S )
[BnOH] + [BnOBn] + [BnCOOH] + [BnCOOBn]
CrO42- + H+ a HCrO4
(14)
-
(15)
Starks and Liotta21 have given an account of chromate
The preliminary L-L PT catalysis experiments on the
chromate oxidation of benzyl chloride (BnCl) with tetrabuty-
lammonium bromide (TBAB) as a phase transfer catalyst at 100
°C showed that the reaction led to the formation of BnOH and
PhCHO only. There was no oxidation of the solvent toluene
at all which was independently confirmed under otherwise
similar conditions except the addition of benzyl chloride. There
was no formation of dibenzyl ether or benzyl benzoate. Further,
since PhCHO could be overoxidized to benzoic acid (BnCOOH),
both the aqueous and organic phases were further analyzed. The
2-
extraction, particularly HCrO4-, CrO42-, HCr2O7-, and Cr2O7
-
into organic solvents by using the PTC. It is shown that HCrO4
and HCr2O7- are readily phase transferred provided the aqueous
phase is acidic, but almost no transfer of chromate anions occurs
2-
from alkaline aqueous solutions. The dichromate anion Cr2O7
,
in contrast to HCrO4-, is reported to be difficult to transfer into
an organic solution, as is typical of divalent anions.
In the current studies, the pH of the solution was such that it
was always slightly acidic. Thus, formation of [Q+HCrO4-] is

Oxidation of Benzyl Chloride to Benzaldehyde
J. Phys. Chem. A, Vol. 101, No. 1, 1997 39
Figure 3. Triangular paths of reaction. The PT catalysis path is due
-
to the formation of [Q+HCrO4
] .
org
(i) Path a: in the presence of PTC (rate constant ka). This
path is possibly due to the formation of an intermediate chromate
ester [RCH2OCrO3-H+], which decomposes to BnOH and CrO3,
which is immediately transformed to aqueous phase to regener-
Figure 2. Effect of speed of agitation on conversion of benzyl chloride.
Temperature ) 100 °C. [BnCl] ) 1.86 × 10-3 mol/cm3of organic phase.
[K2CrCr2O7] ) 9.33 × 10-4 mol/cm3 of aqueous phase. Volorg ) Volaq
) 50 mL. [Na2CO3] ) 4.44 × 10-4 mol/cm3 of aqueous phase.
[TBAB]org ) 9.32 × 10-5 mol/cm3.
-
ate HCrO4 ions.
(ii) Path b: without a PTC (rate constant kb). Since trace
hydrolysis of BnCl is known to occur in water, it is essential to
compare the relative rates of paths a and b leading to the
formation of BnOH and whether the contribution due to the
noncatalytic path b could be neglected.
TABLE 1: Typical Selectivity Profile of Benzaldehyde in
Standard Experimentsa
selectivity ratio
selectivity ratio
2. Path c: Oxidation of benzyl alcohol, formed in situ, with
aqueous HCrO4- to benzaldehyde under PT catalysis conditions.
This reaction is possible because of the formation an intermedi-
ate chromate ester [PhCH2OCrO3-Q+] of BnOH which further
decomposes to PhCHO. The possibility of formation of PhCHO
by a noncatalytic oxidation of BnOH ought to be also verified.
This is not shown in Figure 3 for reasons put forward later.
3. Path d: Oxidation of benzyl chloride directly to benzal-
time (min) [PhCHO]/[BnOH] time (min) [PhCHO]/[BnOH]
0
15
30
45
60
0
90
120
180
240
0.85
0.91
1.28
2.00
0.21
0.24
0.38
0.45
a Temperature ) 100 °C. [BnCl] ) 1.86 × 10-3 mol/cm3of organic
phase. [K2CrCr2O7] ) 9.33 × 10-4 mol/cm3 of aqueous phase. Volorg
) Volaq ) 50 mL. [Na2CO3] ) 4.44 × 10-4 mol/cm3 of aqueous phase.
[TBAB]org ) 9.32 × 10-5 mol/cm3.
-
dehyde with aqueous HCrO4 in the presence of a PTC.
4.4. Testing of the Hypothesis on Reaction Paths. The
non-PT catalysis reactions were conducted under otherwise
similar conditions to find out the contributions of these reactions
to overall conversion in the absence of any phase transfer
catalyst.
pH-metric titration of the aqueous phase as well as extraction
of the organic phase with aqueous NaOH did not detect any
benzoic acid.
4.4.1. Path b: Hydrolysis of Benzyl Chloride(BnCl) with
-
Thus,
Aqueous HCrO4 without Phase Transfer Catalyst. The hy-
drolysis of BnCl was carried out at 100 °C with aqueous
chromate solution but without the addition of a phase transfer
catalyst. It was interesting to note that the hydrolysis of BnCl
took place even in the absence of phase transfer catalyst and
BnOH was the sole product. However, the formation of
benzaldehyde was detected only in the presence of a phase
transfer catalyst. The rate of reaction can be described by a
first-order kinetics.
[PhCHO]
S )
(16)
[BnOH]
Table 1 shows a typical selectivity profile of benzaldehyde,
which increases with time, thereby indicating that BnOH was
further oxidized to PhCHO. Thus, triangular reaction paths
starting from BnCl leading to PhCHO, as depicted in Figure 3,
were formulated and the hypothesis was tested by conducting
different isolated experiments under otherwise similar conditions
at 100 °C and in the absence of any mass transfer limitations.
d[BnCl]
-
) kb[BnCl]
(17)
dt
Upon integration eq 17 gives
The type of experiments required per these paths are as
follows (see Figure 3):
-ln(1 - XA) ) kbt
(18)
-
1. Hydrolysis of benzyl chloride with aqueous HCrO4 to
benzyl alcohol by two paths a and b:
where XA is the fractional conversion of benzyl chloride.

40 J. Phys. Chem. A, Vol. 101, No. 1, 1997
Yadav and Haldavanekar
Figure 5. Oxidation of benzyl alcohol with HCrO4- under PT catalysis
conditions. Speed ) 1000 rpm. Temperature ) 100 °C. [BnOH] )
1.09 × 10-3 mol/cm3. [K2CrCr2O7] ) 9.33 × 10-4 mol/cm3. Volorg
)
Volaq ) 50 mL. [Na2CO3] ) 4.44 × 10-4 mol/cm3. [TBAB]org ) 9.32
× 10-5 mol/cm3.
Figure 4. First-order kinetics plot for the hydrolysis of benzyl chloride
in the absence of phase transfer catalyst. [K2CrCr2O7] ) 9.33 × 10-4
mol/cm3. Volorg ) Volaq ) 50 mL. [Na2CO3] ) 4.44 × 10-4 mol/cm3.
and PhCHO given in Figure 5 shows a pseudo-first-order
behavior. Further, there was no benzoic acid formed because
the analysis of aqueous phase did not show any detectable
benzoic acid. The kinetics of this reaction were established from
the data up to 2 h.
The kinetic plot of -ln(1 - XA) vs time is shown in Figure
4. It confirms that the reaction is first order with respect to
benzyl chloride. The rate constant kb was found to be 3.47 ×
10-5 s-1 at 100 °C.
The rate of reaction of benzyl alcohol in presence of a PTC
is given by
The above experiment also demonstrated that there was no
simultaneous formation of PhCHO directly from BnCl by a
noncatalytic oxidation route.
d[BnOH]
-
) kc1[BnOH]
(19)
(20)
dt
where kc1 ) kc[PTC]
To elucidate whether the presence of chromate is essential
or not, another set of experiments was conducted at a pH of
6.6 between BnCl and water, both in the presence and absence
of phase transfer catalyst at 100 °C. However, a very negligible
amount of BnOH was noticed even after 4 h of reaction. This
suggested that the species HCrO4- was necessary for hydrolysis
of BnCl.
where kc1 is a pseudo-first-order rate constant.
Equation 19 can be integrated, to get
-ln(1 - XB) ) kc1t
(21)
-
4.4.2. Oxidation of Benzyl Alcohol (BnOH) with HCrO4
without Phase Transfer Catalyst. To verify whether the
oxidation of BnOH requires a phase transfer catalyst or not,
typical experiments were conducted with BnOH and potassium
dichromate without the phase transfer catalyst at 100 °C. No
formation of benzaldehyde was observed even after 4 h, which
suggested that the presence of phase transfer catalyst was needed
for the oxidation of BnOH. Thus, further experiments were
conducted in the presence of a PTC.
where XB is the fractional conversion of benzyl alcohol.
A plot of -ln(1 - XB) against time t is given in Figure 6,
from which the value of kc1 is obtained as 1.13 × 10-4 s-1 for
the TBAB concentration of 9.32 × 10-5 mol/cm3 (org). The
value of kc was calculated as 1.2123 cm3/(mol s). It is thus
obvious that only the phase transfer catalyzed oxidation of
benzyl alcohol leads to the formation of benzaldehyde (path
c).
-
4.4.3. Path c: Oxidation of Benzyl Alcohol with HCrO4
-
4.4.4. Reaction of Benzyl Chloride with Aqueous HCrO4
with Phase Transfer Catalyst. To examine the role of phase
transfer catalyst in the oxidation of BnOH, some experiments
were conducted, under otherwise similar conditions, with
tetrabutylammonium bromide (TBAB) as the phase transfer
catalyst at 100 °C.
in the Presence of PTC: Path a or Path d or Both? It is rather
tricky to verify the formation of PhCHO directly from BnCl
by path d because of its parallel reaction by path a followed by
path c. One of the ways is to tag some molecules of BnOH
and add them in the reaction mixture. However, a simple way
is to carefully analyze the progress of the reaction vis-a`-vis the
concentration profiles of BnCl, BnOH, and PhCHO.
To establish the correct paths for the formation of benzal-
dehyde, several experiments were conducted with stoichiometric
amounts of benzyl chloride and HClO4- under otherwise similar
conditions, with TBAB as the PTC. Figure 7 depicts the typical
There was a suspicion that dibenzyl ether might be formed
from BnOH. However, the earlier literature on oxidation of
alcohol to benzaldehyde under PT catalysis conditions men-
tioned no formation of dibenzyl ether. When samples drawn
at different times were analyzed, no detectable dibenzyl ether
was noticed up to 3 h or so. The concentration profile of BnOH

Oxidation of Benzyl Chloride to Benzaldehyde
J. Phys. Chem. A, Vol. 101, No. 1, 1997 41
throughout in region II. The concentration of benzaldehyde is
less than that of benzyl alcohol in region I, and it becomes
greater in region II.
The concentration of benzyl chloride decreases continuously
in both the regions. However, an interesting trend is seen in
region II. The BnCl concentration drop is linear in time in that
region, and simultaneously, the increase in concentration of
PhCHO is also linear in time. This suggests that the rate of
reaction of BnCl in region II is independent of its concentration
and that of formation of PhCHO also independent of benzyl
chloride concentration. Further, the slope of concentration of
BnCl with time in region II is identical and opposite in sign
with the slope of concentration of PhCHO in that region.
Since the concentration of BnOH is constant in region II, its
net rate is zero, that is, the rate of formation of benzyl alcohol
(by path a) is equal to its consumption by path c to produce
benzaldehyde. It is obvious that the rate of reaction of benzyl
chloride is equal to the rate of formation benzaldehyde in region
II.
If both the concentrations of benzyl alcohol and benzaldehyde
had increased with time, then it would have suggested that there
was no oxidation of benzyl alcohol and that benzaldehyde was
formed directly from benzyl chloride by path d. Under such a
case, benzyl chloride would disappear by two parallel reactions,
namely, one leading to the formation of benzyl alcohol and
another leading to the formation of benzaldehyde. However,
this supposition is contrary to the facts.
-
Figure 6. Kinetics of plot of oxidation of benzyl alcohol with HCrO4
The material balance for region II of Figure 7 is as follows:
under PT catalysis conditions. Speed ) 1000 rpm. Temperature ) 100
°C. [BnOH] ) 1.09 × 10-3 mol/cm3. [K2CrCr2O7] ) 9.33 × 10-4 mol/
cm3. Volorg ) Volaq ) 50 mL. [Na2CO3] ) 4.44 × 10-4 mol/cm3.
[TBAB]org ) 9.32 × 10-5 mol/cm3.
Rate of reaction of benzyl chloride
-d[BnCl]
) kd[PTC][BnCl] + ka[PTC][BnCl] (22)
dt
Net rate of formation of benzaldehyde
d[BnOH]
) ka[PTC][BnCl] - kc[PTC][BnOH] (23)
dt
) 0 in region II
Net rate of formation of benzaldehyde
d[PhCHO]
) kd[PTC][BnCl] + kc[PTC][BnOH] (24)
dt
In eqs 21-23,
-
[PTC] ) [Q+HCrO4 ]org
(25)
From eq 23, ka[BnCl] ) kc[BnOH] in region II. Since both ka
and kc are finite and BnOH is not overoxidized to benzoic acid,
it follows that for region II
-d[BnCl] d[PhCHO]
)
-
dt
dt
Figure 7. Reaction of benzyl chloride with aqueous HCrO4 in the
presence of PTC. Speed ) 1000 rpm. Temperature ) 100 °C. [BnCl]
) 1.86 × 10-3 mol/cm3. [K2CrCr2O7] ) 9.33 × 10-4 mol/cm3. Volorg
) ka[PTC][BnCl] ) rate of formation of
) Volaq ) 50 mL. [Na2CO3] ) 4.44 × 10-4 mol/cm3. [TBAB]org
9.32 × 10-5 mol/cm3.
)
BnOH from BnCl
) kc[PTC][BnOH] ) rate of reaction of BnOH
concentration profiles of the reactants and products, and it is
divided in two regions, I (up to 2 h) and II (beyond 2 h).
It is seen that the concentration of benzyl alcohol increases
in region I up to 2 h, and thereafter, it remains constant
generated in-situ to yield PhCHO (26)
Therefore, kd ) 0

42 J. Phys. Chem. A, Vol. 101, No. 1, 1997
Yadav and Haldavanekar
Transfer of HCrO4- from Aqueous Phase to Organic Phase
-
-
(Q+X-)aq + HCrO4 aq a (Q+HCrO4 )aq + X- (27)
aq
(The parentheses show the ion pair.)
Extraction of Ion Pairs into Organic Phase
-
-
(Q+HCrO4 )aq a (Q+HCrO4 )org
(28)
Reactions in the Organic Phase
-
PhCH2Cl + (Q+HCrO4 )org a
PhCH2OCrO3H + (Q+Cl-)org (29)
PhCH2OCrO3H a PhCH2OH + CrO3 (path a) (30)
Chromium trioxide dissolves in water with accompanying
depolymerization, as given below:
Figure 8. Concentration profiles for reaction of benzyl chloride with
-
aqueous HCrO4 in the presence of PTC (at higher concentration).
(CrO3)n + nH2O f nH2CrO4(aq)
(31a)
(31b)
Speed ) 1000 rpm. Temperature ) 100 °C. [BnCl] ) 1.86 × 10-3
mol/cm3. [K2CrCr2O7] ) 9.33 × 10-4 mol/cm3. Volorg ) Volaq ) 50
mL. [Na2CO3] ) 4.44 × 10-4 mol/cm3. [TBAB]org ) 1.86 × 10-4 mol/
cm3.
-
H2CrO4 f H+ + HCrO4 (aq)
-
which helps in regeneration of HCrO4 (refer to Cainelli and
Further proof for the existence of path a through path c for
the formation of the benzaldehyde is provided in what follows.
The independent experiments on the oxidation of BnOH (Figure
5 and Figure 6) show that the PT catalyzed oxidation of BnOH
is first order in BnOH concentration. In region I of Figure 7,
the concentration of BnOH is always higher than that of PhCHO,
thereby indicating that the net rate of formation of BnOH (by
path a) is higher than the net rate of formation of PhCHO by
both paths c and d, up to 2 h. The initial rate of formation of
BnOH and the initial rate of reaction of BnCl, as calculated
from the concentration profiles in region I of Figure 7 indicate
that they are practically the same and hence contribution by
the path d to the formation of PhCHO could be neglected.
Indeed, all experiments conducted thereafter confirmed the same.
Table 1 also shows that the selectivity of benzaldeyde increases
with time at the cost of benzyl alcohol.
Cardillo22). Equation 31 represents a hydrolysis reaction and
not oxidation.
-
Now benzyl alcohol generated in situ reacts with (Q+HCrO4
)
in the organic phase.
-
PhCH2OH + (Q+HCrO4 )org a
-
(PhCH2OCrO3 Q+)org + H2O (32)
-
(PhCH2OCrO3 Q+)org a PhCHO + CrO2 + Q+ + H+
(33)
Another mechanistic interpretation is based on the relative
HCrO4- + H+ a H2CrO4
(Q+Cl-)org a (Q+Cl-)aq
(34)
(35)
-
rates of formation of the chromate esters [PhCH2OCrO3 H+]
and [PhCH2OCrO3-Q+] from BnCl and BnOH, respectively,
in the organic phase where the ion pair [Q+HCrO4-] is
preferentially partitioned. [PhCH2OCrO3-H+] immediately
decomposes to BnOH with CrO3 as byproduct, which goes back
into the aqueous phase and is regenerated as HCrO4-, thereby
indicating that it is a hydrolysis reaction (path a) and not
oxidation. The decomposition of PhCH2OCrO3-H+ directly to
PhCHO is a dehydration reaction which is not favored in the
organic phase. To supplement these arguments, higher con-
centrations of catalyst and longer reaction times were chosen
as shown in Figure 8, which depicts a continuous decline in
[BnCl], [BnOH] goes through a maximum and declines slowly,
and [PhCHO] increases with time accordingly.
Let Qo be the amount of quaternary salt added to the organic
phase at time t ) 0. The material balance on Qo is as follows:
-
-
Qo ) [Q+HCrO4 ]aq + [Q+HCrO4 ]org + [Q+X-]aq
+
[Q+X-]org (36)
As the distribution coefficient of quaternary salts is generally
high in organic phase than in aqueous phase,
Thus, the reaction path for the formation of benzaldehyde
under L-L PT catalysis from benzyl chloride is via the
formation of benzyl alcohol (path a) followed by its subsequent
oxidation (path c).
-
Qo ) [Q+HCrO4 ]org + [Q+X-]org
(37)
4.5. Measurement of Kinetics. The L-L PT catalysis
mechanism for formation of benzaldehyde from benzyl chloride
can now be described:
-
if [Q+HCrO4 ]aq ) [Q+X-]aq ) 0
(37a)

Oxidation of Benzyl Chloride to Benzaldehyde
J. Phys. Chem. A, Vol. 101, No. 1, 1997 43
d[PhCHO]
The anion exchange equilibrium across the interface is
) kc[BnOH]{Kcat} ) kc1[BnOH] (51)
dt
K
-
-
(Q+X-)org + (HCrO4 )aq
{
} (Q+HCrO4 )org + X-
(38)
(39)
aq
The above eq 51 can be analytically solved to get the
following (see for instance, Levenspiel23), provided Kcat is
constant.
-
[Q+HCrO4 ]org[X-]aq
K )
-
[Q+X-]org[HCrO4 ]aq
a1t
[BnCl] ) [BnCl]o e-k
(52)
-
But, [Q+HCrO4 ]org[X-]aq
)
a1t
[BnOH] ) [BnCl]o{ka1/(kc1 - ka1)}(e-k t - e-k
)
(53)
(54)
a1
-
-
K{Qo - [Q+HCrO4 ]org}[HCrO4 ]aq (40)
-
[PhCHO] ) [BnCl]o - [BnCl] - [BnOH]
KQo[ HCrO4 ]aq
[X-]aq + K[HCrO4 ]aq
-
Hence, [Q+HCrO4 ]org
)
(41)
-
where the subscript represents initial concentration condition.
The time, tmax, when [BnOH] attains a maximum is obtained
by setting the derivative of [BnOH] with respect to time as zero.
Now, the net rates of reaction of BnCl and of formation
BnOH and PhCHO can be written as
ln(ka1/kc1)
-
KQo[HCrO4 ]aq
tmax
)
(55)
-d[BnCl]
dt
(kc1 - ka1)
) ka[BnCl]
(42)
-
-
{
}
[X ]aq + K[HCrO4 ]aq
From eq 52,
d[BnOH]
) ka{[BnCl] -
)
dt
[BnOH]max ) [BnCl]o{ka1/ kc1}ka1/(ka1-kc1
(56)
(57)
-
KQo[HCrO4 ]aq
kc[BnOH]}
(43)
(44)
[BnCl]
-
-
{
}
[X ]aq + K[HCrO4 ]aq
ln
) -ln(1 - XA) ) ka1t
[BnCl]o
-
KQo[HCrO4 ]aq
d[PhCHO]
-
KQo[HCrO4 ]aq
) kc[BnOH]
-
-
{
}
dt
) ka
t (58)
[X ]aq + K[HCrO4 ]aq
-
-
{
}
[X ]aq + K[HCrO4 ]aq
The instantaneous selectivity S of PhCHO over BnOH is given
If K[HCrO4-] > [X-]aq, then eq 58 can be equated to ka1Qo,
from the slope of which ka1 can be calculated.
by the ratio of eq 44 to 43:
kc[BnOH]
From the independent oxidation of BnOH, the value of kc1
can be similarly found (see eqs 19 and 21). The slope will be
equal to kcQo.
d[PhCHO]
)
(45)
d[BnOH]
ka[BnCl] - kc[BnOH]
Since ka1 is known and kc1 is also known independently as
given in section 4.4.3 it is possible to calculate tmax, the time at
which the concentration of BnOH goes to a maximum value,
from eq 55 at a particular concentration Qo of a PTC. This
value can now be verified with that obtained from the plot of
concentration vs time for the PTC catalyzed oxidation of BnCl.
Upon the basis of the above mechanistic and kinetic analysis,
the effects of the various pertinent variables which appear in
the final rate equation were studied to ascertain the validity of
the theory.
4.6. Effect of Catalyst Concentration. As given by eq 48,
the rate of reaction of benzyl chloride should be directly
proportional to Kcat, which is a function of the effective catalyst
concentration in the form of [Q+HCrO4-] ion pair in the organic
phase. In other words, it is a function of the quaternary species
Q+ and its distribution between the organic phase and aqueous
The left-hand side of eq 45 is the difference in concentrations
in time increments ti and ti+1 and is given by
kc[BnOH]
∆[PhCHO]
∆[BnOH]
)
(46)
ka [BnCl] - kc [BnOH]
Thus, the selectivity is independent of catalyst concentration
or its type.
-
KQo[HCrO4 ]aq
Let
) constant ) Kcat (47)
-
[X-]aq + K[HCrO4 ]aq
Equation 47 is the same as eq 41, and Kcat is the concentration
of [Q+HCrO4-] in the organic phase.
Equations 42 and 43 can be rewritten as
phase. Although it is most desirable to have the distribution
-
coefficient ([Q+HCrO4
]
org/[Q+Cl-]aq) greater than 100, a
-d[BnCl]
) kaKcat[BnCl] ) ka1[BnCl]
(48)
conservative estimate shows that a value of above 10 or so
makes the PT catalysis process very effective and the rate of
reaction will be directly proportional to the catalyst concentration
added initially (Qo) (see Dehmlow and Dehmlow24).
The effect of catalyst concentration was studied for TBAB
in the range 9.322 × 10-5 to 3.72 × 10-4 mol/cm3 of organic
phase, by keeping all other variables constant. The results are
plotted in Figure 9, from which the initial rates of benzyl
chloride were calculated. Figure 10 shows the plot of initial
rate of reaction against Qo. During the initial period, the
dt
d[BnOH]
) {ka[BnCl] - kc[BnOH]}{Kcat} (49)
) ka1[BnCl] - kc1[BnOH] (50)
dt
The value of kc was obtained independently from the PT
catalysis oxidation of BnOH to PhCHO from the slope of
-ln(1 - XB) vs t (see section 4.4.3, Figure 6).

44 J. Phys. Chem. A, Vol. 101, No. 1, 1997
Yadav and Haldavanekar
Figure 9. Effect of catalyst concentration on conversion of benzyl
chloride. Speed ) 1000 rpm. Temperature ) 100 °C. [BnCl] ) 1.86
× 10-3 mol/cm3. [K2CrCr2O7] ) 9.33 × 10-4 mol/cm3. Volorg ) Volaq
) 50 mL. [Na2CO3] ) 4.44 × 10-4 mol/cm3. [TBAB]org ) 9.32 ×
10-5 mol/cm3.
Figure 11. Effect of concentration of benzyl chloride on conversion.
Speed ) 1000 rpm, Temperature ) 100 °C. [K2CrCr2O7] ) 9.33 ×
10-4 mol/cm3. Volorg ) Volaq ) 50 mL. [Na2CO3] ) 4.44 × 10-4 mol/
cm3. [TBAB]org ) 9.32 × 10-5 mol/cm3.
TABLE 2: Effect of Catalyst Concentration on Selectivity of
Benzaldehyde after 4 h
catalyst
concn
catalyst
concn
selectivity
[PhCHO]/[BnOH]
selectivity
[PhCHO]/[BnOH]
(mol/cm3)
(mol/cm3)
1.864 × 10-5
0.56
4.17
2.793 × 10-4
8.63
11.70
1.864 × 10-4
3.729 × 10-4
taken equal to the amount of catalyst added initially divided by
the volume of the organic phase.
Table 2 demonstrates the effect of concentration of catalyst
on the selectivity of benzaldehyde (after 4 h). It can be seen
that the selectivity improves with an increase in the concentra-
tion of catalyst. This is due to the fact that the pseudo rate
constant kc1 increases, leading to higher rates of oxidation of
BnOH formed in situ.
4.7. Effect of Concentration of Benzyl Chloride and
Benzyl Alcohol. Figure 11 shows typical plots of benzyl
chloride conversion against time under otherwise similar condi-
tions, from which the initial rates were calculated. A plot of
initial rate of reaction of BnCl against its initial concentration
is shown in Figure 12, which demonstrates a linear relationship.
The reaction is first order in BnCl concentration.
To study the effect of concentration of benzyl chloride on
selectivity, the selectivity of benzaldehyde after 4 h was
calculated for each run (Table 3). The selectivity of benzal-
dehyde was found to increase with an increase in the concentra-
Figure 10. Plot of initial rate of reaction of benzyl chloride against
concentration catalyst. Speed ) 1000 rpm. Temperature ) 100 °C.
[BnCl] ) 1.86 × 10-3 mol/cm3. [K2CrCr2O7] ) 9.33 × 10-4 mol/cm3.
Volorg ) Volaq ) 50 mL. [Na2CO3] ) 4.44 × 10-4 mol/cm3. [TBAB]org
) 9.32 × 10-5 mol/cm3.
formation of BnOH is predominant and 1 mol of BnCl requires
1 mol of [Q+HCrO4-].
tion of benzyl chloride.
4.8. Effect of Concentration of Nucleophile, HCrO4
-
.
-
The best fit shows that the rate of reaction BnCl, expressed
as (mol/cm3 of organic phase)/min, is linearly proportional to
the catalyst concentration Qo. This also suggests that the
equilibrium constant K is quiet high and most of the catalyst is
in the organic phase as [Q+HCrO4-]. The reasons for plotting
initial rates is that the contribution of X-aq in the denominator
of the eq 47 for Kcat is very small because the leaving species
Cl- has less concentration in the aqueous phase.
Since the pH was maintained at 6.6-6.7, only HCrO4 was
the predominant oxidizing species present. The amount of K2-
CrCr2O7 added to the aqueous phase was varied from 4.66 ×
10-4 to 1.87 × 10-3 mol/cm3 of the aqueous phase by keeping
Na2CO3 constant at 4.44 × 10-4 mol/cm3 under otherwise
similar conditions. Figure 13 shows the conversion of BnCl
against time. The conversions are practically the same because
-
the concentration of the species Q+CrO4 is constant in the
The linear dependence of the rate of reaction on the catalyst
concentration is truely characteristic of PT catalysis. Thus, the
oxidation of BnCl to BnOH and PhCHO is a PT catalysis
reaction. The catalyst concentration in the organic phase is
organic phase. This further lends credence to the argument
proposed in section 4.4 that Kcat is constant for a given Qo and
thus, from eq 47, Qo ) [Q+HCrO4-], for the K2CrCr2O7
concentration used.

Oxidation of Benzyl Chloride to Benzaldehyde
J. Phys. Chem. A, Vol. 101, No. 1, 1997 45
TABLE 3: Effect of BnCl Concentration on Selectivity of
Benzaldehyde after 4 h
TABLE 4: Effect of the Concentration of K2Cr2O7 on the
Selectivity of Benzaldehyde after 4 h
BnCl
concn
BnCl
concn
K2CrCr2O7
concn
K2CrCr2O7
concn
selectivity
[PhCHO]/[BnOH]
selectivity
[PhCHO]/[BnOH]
selectivity
selectivity
(mol/cm3)
(mol/cm3)
(mol/cm3)
[PhCHO]/[BnOH] (mol/cm3) [PhCHO]/[BnOH]
0.932 × 10-3
1.44
1.83
3.732 × 10-3
6.50
8.38
0.466 × 10-3
1.96
2.00
1.44 × 10-3
2.18
2.22
1.866 × 10-3
5.596 × 10-3
0.933 × 10-3
1.87 × 10-3
Figure 12. Plot of initial rate of reaction against conversion of BnCl.
Speed ) 1000 rpm. Temperature ) 100 °C. [K2CrCr2O7] ) 9.33 ×
10-4 mol/cm3. Volorg ) Volaq ) 50 mL. [Na2CO3] ) 4.44 × 10-4 mol/
cm3. [TBAB]org ) 9.32 × 10-5 mol/cm3.
Figure 14. Effect of concentration of nucleophile on the rate of
formation of benzaldehyde. Speed ) 1000 rpm. Temperature ) 100
°C. [BnCl] ) 1.86 × 10-3 mol/cm3. Volorg ) Volaq ) 50 mL. [Na2-
CO3] ) 4.44 × 10-4 mol/cm3. [TBAB]org ) 9.32 × 10-5 mol/cm3.
Figure 13. Effect of concentration of nucleophile HCrO4- on rate of
reaction of benzyl chloride. Speed ) 1000 rpm. Temperature ) 100
°C. [BnCl] ) 1.86 × 10-3 mol/cm3. Volorg ) Volaq ) 50 mL. [Na2-
CO3] ) 4.44 × 10-4 mol/cm3. [TBAB]org ) 9.32 × 10-5 mol/cm3.
The effect of concentration of nucleophile on the initial rate
of reaction is presented in Figure 14. Table 4 shows that the
selectivity is practically the same.
4.9. Effect of Sodium Carbonate, Na2CO3. Since sodium
carbonate was added to the aqueous solution of potassium
dichromate to maintain a pH of 6.6, the effect of sodium
carbonate on the rate of consumption of BnCl was found by
carrying out the reaction with different amounts of sodium
carbonate under otherwise identical reaction conditions, includ-
Figure 15. Effect of addition of sodium carbonate on conversion of
benzyl chloride. Speed ) 1000 rpm. Temperature ) 100 °C. [BnCl]
) 1.86 × 10-3 mol/cm3. [K2CrCr2O7] ) 9.33 × 10-4 mol/cm3. Volorg
) Volaq ) 50 mL. [TBAB]org ) 9.32 × 10-5 mol/cm3.
ing an experiment without sodium carbonate. The pH deter-
mines the existence of the oxidizing species in a particular form.
Figure 15 demonstrates the effect of the amount of sodium
carbonate on the conversion of BnCl.
The quantity of Na2CO3 added to vary the pH resulted in the
four experiments resulted in pHs of 2.92, 6.6, 7.04, and 9. When
no Na2CO3 was added, the pH was 2.92, at which the species
-
Cr2O7 predominantly exists and is difficult to extract as Q2-
Cr2O7 in organic phase (Starks and Liotta21), and therefore, rates

46 J. Phys. Chem. A, Vol. 101, No. 1, 1997
Yadav and Haldavanekar
Figure 17. Effect of type of catalyst. Speed ) 1000 rpm. Temperature
) 100 °C. [BnCl] ) 1.86 × 10-3 mol/cm3. [K2CrCr2O7] ) 9.33 ×
10-4 mol/cm3. Volorg ) Volaq ) 50 mL. [Na2CO3] ) 4.44 × 10-4 mol/
cm3. [TBAB]org ) 9.32 × 10-5 mol/cm3.
TABLE 6: Effect of the Quantity of Water on the
Selectivity of Benzaldehyde after 4 h
Figure 16. Effect of amount of water on conversion of benzyl chloride.
Speed ) 1000 rpm. Temperature ) 100 °C. [BnCl] ) 1.86 × 10-3
mol/cm3. [K2CrCr2O7] ) 9.33 × 10-4 mol/cm3. Volorg ) 50 mL. [Na2-
CO3] ) 4.44 × 10-4 mol/cm3. [TBAB]org ) 9.32 × 10-5 mol/cm3.
amount of
selectivity
amount of
selectivity
water (cm3) [PhCHO]/[BnOH] water (cm3) [PhCHO]/[BnOH]
0
5
79.32
10.95
50
100
1.82
1.60
TABLE 5: Effect of the Concentration of Na2CO3 on the
Selectivity of Benzaldehyde after 4 h
Na2CO3
concn
Na2CO3
concn
found to enhance until the amount of water was increased to
50 cm3, and then conversion practically remains constant upon
further addition of water up to 100 cm3. The rates were lower
where 150 cm3 of water was used because the distribution
selectivity
[PhCHO]/[BnOH]
selectivity
[PhCHO]/[BnOH]
(mol/cm3)
(mol/cm3)
0
165.66
2.00
2.65
1.865 × 10-3
2.26
1.30
0.453 × 10-3
2.794 × 10-3
-
coefficient of Q+HCrO4 is low and the pH is more than 8.
0.930 × 10-3
The selectivity for benzaldehyde decreases with an increase in
the amount of water as the formation of benzyl alcohol takes
place with water even without catalyst.
were found to be low. The conversion was maximum at a pH
of 6.6 because the species Q+HCrO4- is easily extracted in the
organic phase.
4.11. Effect of Type of Catalyst. Besides TBAB, two other
catalysts were tried. Figure 18 shows plots of conversion against
time. It is seen that practically no change in conversion is
obtained because for all the catalysts the K values are quite
high, rendering the [Q+HCrO4-] species concentration in the
organic phase about the same as Qo, the added quantity. The
selectivity for benzaldehyde was practically the same in
consonance with the theory (see eq 46) that it should be
independent of catalyst type.
4.12. Kinetics of the Reaction. The effect of various
parameters on the rate of reaction established that the oxidation
of benzyl chloride is a true phase transfer catalyzed consecutive
reaction which follows the path a through path c to form
benzaldehyde. The mechanism and mathematics developed
earlier are thus valid (see Figure 18).
Now, as mentioned earlier, the values of ka and kc can be
calculated and are 5 × 10-5 and 3.4 × 10-4 s-1 at 100 °C for
TBAB as catalyst. When back-checked, the value of tmax
obtained by using eq 55 is 6610 s whereas the experimental
Tmax is 7200 s (see Figure 6). There is deviation of 8.2% which
is indeed in good agreement.
4.13. Effect of Temperature. The temperature of the
reaction was varied from 80 to 100 °C in order to elucidate the
effect of temperature on the rate of consumption of BnCl. The
conversion of BnCl was found to increase with an increase in
-
Chromium exists as HCrO4 species in the pH range 6-8,
which being an univalent ion can get extracted easily in the
organic phase, and therefore, the rate of reaction was found to
increase. When the pH is 9, chromium exists as CrO42-, which
being a divalent ion cannot be extracted easily, and hence, the
rate of reaction was found to slightly decrease.
The effect of the amount of sodium carbonate on the
selectivity of PhCHO is given in Table 5. The selectivity of
PhCHO was found to decrease with increase in the amount of
Na2CO3.
When no Na2CO3 was added, the pH was more acidic (i.e.,
2.6) and the oxidizing species was Cr2O72-. Since Q2Cr2O7
ion pairs are difficult to extract, the rates of reaction were low
but the BnOH formed in situ got oxidized to PhCHO and thus
the selectivity was highly favored. Here selectivity was found
to increase at the cost of conversion of BnCl.
4.10. Effect of the Amount of Water. The effect of the
amount of water was studied by varying the amount of water
from 0 to 150 cm3 under otherwise similar conditions. When
no water was added to the reaction mixture, conversion of benzyl
chloride was low because the HCrO4- species was not formed
initially (Figure 16). Chromium exits in other forms which are
not so extractable. The selectivity for benzaldehyde was almost
complete (Table 6). The conversion of benzyl chloride was

Oxidation of Benzyl Chloride to Benzaldehyde
J. Phys. Chem. A, Vol. 101, No. 1, 1997 47
Figure 20. Arrhenius Plots: (1) Hydrolysis of benzyl chloride to benzyl
alcohol (path a, ka1). (2) Oxidation of benzyl alcohol to benzaldehyde
-
(path c, kc1). ka1 and kc1 include [Q+HCrO4
]
org
and are expressed in
min-1
.
Figure 18. First-order kinetics plot. Speed ) 1000 rpm. Temperature
) 100 °C. [BnCl] ) 1.86 × 10-3 mol/cm3. [K2CrCr2O7] ) 9.33 ×
10-4 mol/cm3. Volorg ) Volaq ) 50 mL. [Na2CO3] ) 4.44 × 10-4 mol/
cm3. [TBAB]org ) 9.32 × 10-5 mol/cm3.
TABLE 7: Effect of Temperature on Selectivity of
Benzaldehyde after 4 h
temperature
selectivity
temperature
selectivity
(°C)
[PhCHO]/[BnOH]
(°C)
[PhCHO]/[BnOH]
80
90
2.07
1.95
100
2.00
proportionately with an increase in temperature and the activa-
tion energy values are on the same order of magnitude or about
the same.
Typical Arrhenius plots are shown in Figure 20, from which
the activation energies for the path a and path c are formed as
13.13 and 5.67 kcal/mol, respectively, thereby proving that the
reactions are kinetically controlled.
5. Conclusions
A thorough and systematic analysis of the mechanism and
kinetics of the oxidation of benzyl chloride by dichromate in
L-L PT catalysis has been provided to throw light on the course
of reaction.
The rate of reaction depends on the pH of the aqueous phase
in view of the fact that chromium exists as dichromate,
perchromate, or chromate depending on the pH. It was observed
-
that Q+HCrO4 was the active species for oxidation. The
reaction proceeds via the formation of benzyl alcohol, which is
oxidized to benzaldehyde.
Acknowledgment. G.D.Y. gratefully acknowledge the award
of the Herdillia Chemicals UDCT Diamond Jubilee Distin-
guished Fellowship, which allowed him to write this paper.
Figure 19. Effect of temperature. Speed ) 1000 rpm. [BnCl] ) 1.86
× 10-3 mol/cm3. [K2CrCr2O7] ) 9.33 × 10-4 mol/cm3. Volorg ) Volaq
) 50 mL. [Na2CO3] ) 4.44 × 10-4 mol/cm3. [TBAB]org ) 9.32 ×
10-5 mol/cm3.
References and Notes
(1) Haldavanekar, B. V. Supported Catalysis: Phase Transfer and Acid
Catalysis. Ph.D. (Science) Thesis, University of Bombay, Mumbai,
September 1993.
(2) Dehmlow, E.; Dehmlow Fisher, T.; Dowd, W. U.S. Patent 4,174,-
352, 1979; Chem. Abstr. 1980, 92, 41600p.
(3) Hanessian, S.; Wong, D. H.; Therian, M. Synth. Commun. 1981,
394.
the temperature (Figure 19). The effect of temperature on
selectivity of PhCHO (after 4 h) is presented in Table 7. It can
be seen that the selectivity of PhCHO almost remains constant
even if the temperature is increased from 80 to 100 °C. This
proves that the rate constants for both of the reactions increase
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