Studies on the kinetics of tripropylammonium fluorochromate oxidation of some aromatic alcohols in non-aqueous media

Article abstract of DOI:10.1016/j.molliq.2010.05.012

The oxidation of benzyl alcohol (BnOH) and a few para-substituted benzyl alcohols by tripropylammonium fluorochromate (TriPAFC) in dimethylsulphoxide (DMSO) leads to the formation of corresponding aldehydes. The reaction is first order each in TriPAFC and the alcohols. The reaction is catalysed by hydrogen ions. The hydrogen ion dependence has the form: k_obs = a + b[H ⁺]. The oxidation of α α′-dideuterio benzyl alcohol exhibited a substantial primary kinetic isotope effect (k_H/k _D = 5.45 at 303 K). Oxidation of benzyl alcohol was studied in 19 different organic solvents. The solvent effect has been analysed using Kamlet's and Swain's multiparametric equation. A mechanism involving a hydride ion transfer via chromate ester is proposed.

Full text of DOI:10.1016/j.molliq.2010.05.012

Journal of Molecular Liquids 155 (2010) 85–90
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Studies on the kinetics of tripropylammonium fluorochromate oxidation of some
aromatic alcohols in non-aqueous media
b
S. Sheik Mansoor a, , S. Syed Shafi
⁎
a
Post Graduate and Research Department of Chemistry, C. Abdul Hakeem College, Melvisharam—632 509, Tamil Nadu, India
Post Graduate and Research Department of Chemistry, Islamiah College, Vaniyambadi—635752, Tamil Nadu, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The oxidation of benzyl alcohol (BnOH) and a few para-substituted benzyl alcohols by tripropylammonium
fluorochromate (TriPAFC) in dimethylsulphoxide (DMSO) leads to the formation of corresponding
aldehydes. The reaction is first order each in TriPAFC and the alcohols. The reaction is catalysed by
hydrogen ions. The hydrogen ion dependence has the form: kobs =a+b[H+]. The oxidation of α α′-dideuterio
benzyl alcohol exhibited a substantial primary kinetic isotope effect (kH/kD =5.45 at 303 K). Oxidation of
benzyl alcohol was studied in 19 different organic solvents. The solvent effect has been analysed using Kamlet's
and Swain's multiparametric equation. A mechanism involving a hydride ion transfer via chromate ester is
proposed.
Received 5 February 2010
Received in revised form 4 May 2010
Accepted 13 May 2010
Available online 21 May 2010
Keywords:
Tripropylammonium fluorochromate
Aromatic alcohols
Kinetics
© 2010 Elsevier B.V. All rights reserved.
Solvent effect
1. Introduction
2. Experimental
Specific and selective oxidation of organic compounds under non-
aqueous conditions is an important reaction in synthetic organic
chemistry. For this, a number of different chromium (VI) derivatives
like pyridinium chlorochromate [1,2], tributylammonium chlorochro-
mate [3], pyridinium fluorochromate [4], imidazolium fluorochro-
mate [5], quinolinium chlorochromate [6], quinolinium flurochromate
[7], tetraethylammonium chlorochromate [8], isoquinilinium bromo-
chromate [9], benzimidazolum fluorochromate [10] and benzyltri-
methylammonium fluorochromate [11] have been reported.
Tripropylammonium fluorochromate (TriPAFC) is one such com-
pound used for the oxidation of primary alcohols [12]. Kinetics of
oxidation of alcohols by various oxidizing agents have been well
studied [13–19].
Substituted benzyl alcohols are suitable for studying the structure–
rate correlations, as a large number of monosubstituted benzyl
alcohols are either commercially available or can be easily prepared.
There seems to be no report on the oxidation of benzyl alcohols by
TriPAFC. Therefore, we studied the kinetics of oxidation of benzyl
alcohol and several para-substituted benzyl alcohols by TriPAFC in
DMSO. The major objective of this investigation is to study the solvent
effect for the substrates undergoing oxidation.
2.1. Materials
Tripropylamie and chromium trioxide were obtained from Fluka
(Buchs, Switzerland). The benzyl alcohols used were with substitu-
ents H, p-NMe2, p-OMe, p-OEt, p-Me, p-F, p-Cl, p-Br, p-COOEt, p-CN
and p-NO2. The procedure used for the purification of alcohols has
been described earlier [20]. Due to non-aqueous nature of the solvent,
p-toluenesulfonic acid was used as a source of hydrogen ions. Solvents
were purified by the usual methods [21].
2.2. Preparation of tripropylammonium fluorochromate
Tripropylammonium fluorochromate is easily prepared as follows:
chromium (VI) oxide (15.0 g, 0.150 mol) was dissolved in water in a
polyethylene beaker and 40% hydrofluoric acid (11.3 mL, 0.225 mol)
was added with stirring at 0 °C. To the resultant orange solution,
tripropylammine (28.3 mL, 0.150 mol) was added drop wise with
stirring to this solution over a period of 0.5 h and stirring was continued
for 0.5 h at 0 °C. The precipitated orange solid was isolated by filtration,
washed withpetroleum ether (3×60 mL) and dried in vacuum for 2 h at
room temperature [12]. Yield 37.5 g (95%); mp 142 °C.
CrO3 =40%HF
→
NðC3H7Þ3
ðC3H7Þ NHþ½CrO3Fꢀ−
ð1Þ
3
0BC
Tripropyl amine
Tripropylammonium fluorochromate
The bright orange crystalline reagent can be stored in polyethylene
containers for long periods without decomposition. The chromium
(VI) content may be easily determined iodometrically.
⁎
Corresponding author. Tel.: +91 4172 266187.
E-mail address: smansoors2000@yahoo.co.in (S.S. Mansoor).
0167-7322/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molliq.2010.05.012

86
S.S. Mansoor, S.S. Shafi / Journal of Molecular Liquids 155 (2010) 85–90
Table 1
2.3. Product analysis
Effect of varying the concentration of [BnOH], [TriPAFC] and [H+] on the rate of reaction
at 303 K.
Product analysis was carried under kinetic conditions. In a typical
experiment, benzyl alcohol (5.4 g, 0.05 mol) and tripropylammonium
fluorochromate (2.63 g, 0.01 mol) were made up to 50 cm3 in DMSO
and kept in the dark for 24 h to ensure completion of the reaction. The
solution was then treated with an excess (150 cm3) of a saturated
solution of 2, 4-dinitro phenyl hydrazine in 2 mol dm−3 HCl and kept
overnight in a refrigerator. The precipitated 2, 4-dinitro phenyl
hydrazone (DNP) was filtered off, dried, weighed, recrystallised from
ethanol, and weighed again. The DNP was found identical (m.pt and
mixed m.pt.) with DNP of benzaldehyde. (m.pt 234–236 °C, lit
237 °C). Similar experiments were performed with other alcohols
also.
103[TriPAFC]
mol dm−3
102 [BnOH]
mol dm−3
[TsOH]
105 k1
mol dm−3
s−1
0.5
1.0
1.5
2.0
2.5
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
2.0
2.0
2.0
2.0
2.0
4.0
6.0
8.0
10.0
2.0
2.0
2.0
2.0
2.0
2.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.1
0.2
0.4
0.6
0.8
0.0
15.08
15.26
15.12
15.04
15.06
30.10
45.40
60.74
76.10
16.50
19.80
23.10
27.10
31.60
14.98a
2.4. Kinetic measurements
The pseudo-first-order conditions were attained by maintaining a
large excess (×15 or more) of alcohol over TriPAFC. The solvent was
DMSO, unless specified otherwise. The reactions were followed, at
constant temperatures ( 0.01 K), by monitoring the decrease in
[TriPAFC] spectrophotometrically at 368 nm. No other reactant or
product has any significant absorption at this wavelength. The
pseudo-first-order rate constant. kobs, was evaluated from the linear
(r=0.990 to 0.999) plots of log [TriPAFC] against time for up to 80%
reaction. Duplicate kinetic runs showed that the rate constants were
reproducible to within 3%. The second order rate constant k2, was
obtained from the relation k2 =kobs /[alcohol]. All experiments, other
than those for studying the effect of hydrogen ions, were carried out in
the absence of TsOH.
Contained 0.001 mol dm−3 acrylonitrile.
a
and b, for benzyl alcohol at 303 K are 14.87 0.04×10−5 s−1 and
20.74 0.09×10−5 mol−1 dm3 s−1 respectively (r2 =0.996).
3.5. Kinetic isotope effect
To ascertain the importance of the cleavage of the α-C–H bond in
the rate-determining step, oxidation of α,α′-dideuterio benzyl
alcohol was studied. Results showed the presence of a substantial
primary kinetic isotope effect (Table 2).
3. Results and discussion
3.6. Effect of solvent
The rate and other experimental data were obtained for all the
alcohols. Since the results are similar, only representative data are
reproduced here.
The rate of oxidation of benzyl alcohols were determined in 19
different organic solvents. The choice of the solvents was limited by
the solubility of TriPAFC and its reactivity with primary and secondary
alcohols. There was no noticeable reaction with the solvents chosen.
The kinetics were similar in all the solvents. The values of k2 at 303 K
are recorded in Table 3.
3.1. Stoichiometric studies
The stoichiometry of the reaction was determined by carrying out
several sets of experiments with varying amounts of TriPAFC largely
in excess over benzyl alcohol. The estimation of unreacted TriPAFC
showed that 1 mol of TriPAFC reacts with 1 mol of benzyl alcohol.
3.7. Effect of substituents
In order to study the effect of structure on reactivity, some para-
substituted benzyl alcohols were subjected to oxidation kinetics by
TriPAFC at four different temperatures viz., 298, 303, 308 and 313 K in
DMSO. The activation parameters were calculated from k2 at 298, 303,
308 and 313 K using the Eyring relationship by the method of least
square and presented in Table 3. The least square method gives the
values and standard errors of enthalpy and entropy of activation
respectively. Statistical analysis of the Eyring equation clearly
confirms that the standard errors of ΔH# and ΔS# correlate [22]. The
negative entropy of activation in conjunction with other experimental
data supports the mechanism outlined in (Scheme 1).
C6H5CH2OH þ CrO2FOTriPNH→C6H5CHO þ CrOFOTriPNH þ H2O ð2Þ
3.2. Rate law
The reactions were found to be first order with respect to TriPAFC.
The individual kinetic runs were strictly first order to TriPAFC. Further,
the pseudo-first-order rate constants do not depend on the initial
concentration of TriPAFC. The reaction rate increases with an increase
in the concentration of benzyl alcohols (Table 1).
3.3. Induced polymerization of acrylonitrile
The oxidation of benzyl alcohol by TriPAFC, in an atmosphere of
nitrogen failed to induce the polymerization of acrylonitrile. Further,
an addition of a radical scavenger, acrylonitrile, has no effect on the
rate (Table 1).
Table 2
Kinetic isotope effect on the oxidation of benzyl alcohol by TriPAFC.
Substrate
105 ×k1, s−1
298 K
303 K
308 K
313 K
3.4. Effect of acidity
H
9.76
1.91
5.11
15.26
2.80
5.45
23.64
4.19
5.64
36.64
6.16
5.95
α,α′-BnOH
kH/kD
The reaction is catalysed by hydrogen ions (Table 1). Hydrogen ion
dependence has the following form kobs =a+b[H+]. The values of a
[BnOH]=2.0×10−2 M, [TriPAFC]=1.0×10−3 M.

S.S. Mansoor, S.S. Shafi / Journal of Molecular Liquids 155 (2010) 85–90
87
Table 3
Activation parameters and second order rate constants for the oxidation of para-substituted benzyl alcohols by TriPAFC.
Substrate
103×k2, dm3mol−1 s−1
ΔH#
−ΔS#
ΔG#
(303 K)
298 K
303 K
308 K
313 K
H
4.88
18.86
9.20
3.98
2.96
0.51
0.28
102.40
17.92
2.42
7.63
30.42
14.62
6.23
4.56
0.77
11.82
46.84
22.22
9.96
7.30
1.22
18.32
73.00
34.42
15.92
11.54
1.96
65.78 0.5
67.13 0.6
60.17 0.6
69.20 1.1
68.07 1.2
67.16 1.8
74.55 2.0
66.99 3.5
67.37 1.8
68.77 1.6
70.95 2.0
68.50 1.8
52.65 1.8
64.66 2.1
58.80 3.5
65.26 3.9
82.79 5.8
82.35 7.0
56.63 10.5
43.22 5.6
28.01 5.0
30.65 6.8
86.54 1.0
83.08 1.1
79.76 1.3
87.02 2.1
87.84 2.2
92.24 3.4
99.50 4.0
84.15 6.6
80.46 3.5
77.25 3.0
80.23 4.0
p-OMe
p-Me
p-F
p-Cl
p-CN
p-NO2
p-NMe2
p-OEt
p-Br
0.42
0.68
1.10
166.96
26.85
3.66
236.20
43.92
5.96
407.14
68.40
9.34
p-COOEt
0.88
1.54
2.44
3.66
ΔH# in kJ mol−1; ΔS# in J K−1 mol−1; ΔG# in kJ mol−1
.
[BnOH]=2.0×10−2 M, [TriPAFC]=1.0×10−3 M.
3.9. Hammett plot
3.8. Isokinetic relationship
A Hammett's plot for the oxidation of benzyl alcohols by TriPAFC at
various temperatures was found to be linear (Table 4). The value of
the slope of the Hammett plot is known as reaction constant (ρ).
Reaction constant values at different temperatures are given in
Table 5. According to Hammett, the reaction with positive ρ values are
accelerated by electron withdrawal from benzene ring, whereas those
with negative ρ values are retarded by electron withdrawal from
benzene ring [28]. In this oxidation reactions, the electron withdraw-
ing groups decrease the rate and the electron donating groups
increase the rate. These observations support the negative ρ values
obtained from the Hammett plot.
The reaction is neither isoenthalpic nor isoentropic but complies
with the compensation law also known as the isokinetic relationship.
ΔH# = ΔHo + βΔS#
ð3Þ
The isokinetic temperature β is the temperature at which all the
compounds of the series react equally fast. Also, at the isokinetic
temperature, the variation of substituent has no influence on the free
energy of activation. In an isoentropic reaction, the isokinetic
temperature lies at infinite and only enthalpy of activation determines
the reactivity. The isokinetic temperature is zero for an isoenthalpic
series, and the reactivity is determined by the entropy of activation
[23]. The isokinetic relationship is tested by plotting the logarithms of
rate constants at two different temperatures (T2 NT1) against each
other according to Eq. (6).
3.10. The Kamlet–Taft Method for the examination of solvent effect
The rate constants of oxidation, k2, in 18 solvents (CS2 was not
considered, as the complete range of solvent parameters was not
available) were correlated in terms of the linear salvation energy
relationship in Eq. (5) of Kamlet et al. [29]
log kðat T2Þ = a + b log kðat T1Þ:
ð4Þ
log k2 = Ao + pπ* + bβ + aα
ð5Þ
The linear relationship in Exner plots [24,25] at 4+log k2 (298 K)
and 4+log k2 (303 K) observed in the present study implies the
validity of the isokinetic relationship. Isokinetic temperature obtained
is 484 K. The operation of isokinetic relationship reveals that all the
substituted benzyl alcohols examined are oxidized through a common
mechanism [26].
In this equation, π* represents the solvent polarity, β the hydrogen
bond acceptor basicities and α is the hydrogen bond donor acidity. Ao
is the intercept term. It may be mentioned here that out of the 18
solvents, 12 have a value of zero for α.
Scheme 1. Acid-independent path.

88
S.S. Mansoor, S.S. Shafi / Journal of Molecular Liquids 155 (2010) 85–90
Table 4
Table 6
Second-order rate constants for the oxidation of benzyl alcohol by TriPAFC at 303 K in
various solvents.
Solvent parameters [28].
Solvent
π*
α
β
Solvents
k2104
Chloroform
1,2-Dichloroethane
Dichloromethane
DMSO
Acetone
DMF
Butanone
Nitrobenzene
Benzene
Cyclohexane
Toluene
Acetophenone
THF
0.58
0.81
0.82
1.00
0.71
0.88
0.67
1.01
0.59
0.00
0.54
0.90
0.58
0.41
0.55
0.53
–
0.44
0.00
0.30
0.00
0.08
0.00
0.06
0.00
0.00
0.00
0.00
–
0.00
0.00
0.00
0.76
0.48
0.69
0.48
0.39
0.10
0.00
0.11
0.49
0.55
1.01
0.37
0.41
–
dm−3 mol−1 s−1
Chloroform
1,2-Dichloroethane
Dichloromethane
DMSO
Acetone
DMF
Butanone
Nitrobenzene
Benzene
Cyclohexane
Toluene
Acetophenone
THF
22.3
18.1
25.4
76.3
22.6
37.0
17.5
32.5
8.10
0.80
6.90
34.5
11.0
9.40
13.1
6.10
3.20
4.40
8.60
0.00
0.68
0.00
0.00
–
tert-butyl alcohol
1,4-Dioxane
1,2-Dimethoxyethane
Carbon disulfide
Acetic acid
tert-butyl alcohol
1,4-Dioxane
1,2-Dimethoxyethane
Carbon disulfide
Acetic acid
0.64
0.55
1.12
0.00
–
Ethyl acetate
0.45
Ethyl acetate
logk2 = –4:00 + ð1:70 ꢁ 0:16Þπ* + ð0:12 ꢁ 0:14Þβ
R2 = 0:8624; SD = 0:12; n = 18; ψ = 0:22
ð7Þ
Kamlet et al. [30] established that the effect of a solvent on the
reaction rate should be given in terms of the following properties: i)
the behaviour of the solvent as a dielectric, facilitating the separation
of opposite charges in the transition state, ii) the ability of the solvent
to donate a proton in a solvent-to-solute hydrogen bond and thus
stabilize the anion in transition state and iii) the ability of the solvent
to donate an electron pair and therefore stabilize the initial benzyl
alcohol, by way of a hydrogen bond between the alcoholic proton and
the solvent electron pair. The parameter π* is an appropriate measure
of the first property, while the second and third properties are
governed by the parameters α and β respectively. The solvent
parameters (π*, α and β) are taken from the literature [29] and are
given in Table 6. The linear dependence(LSER) on the solvent
properties were used to correlate and predict a wide variety of
solvent effect.
In order to explain the kinetic results through solvent polarity and
basicity or acidity, the rate constants were correlated with the
solvatochromic parameters π*, α and β using total solvatochromic
equation, Eq. (5). The correlation of kinetic data was realized by
means of multiple linear regression analysis. It was found that the rate
constants in 19 solvents showed satisfactory correlation with the π*, α
and β solvent parameters. The results of correlation analysis in terms
of Eq. (5), a biparametric equation involving π* and β are given below
in Eqs. (6)–(9).
logk2 = –4:02 + ð1:72 ꢁ 0:17Þπ*
ð8Þ
ð9Þ
r2 = 0:8558; SD = 0:16; n = 18; ψ = 0:29
logk2 = –3:09 + ð0:40 ꢁ 0:34Þβ
r2 = 0:0780; SD = 0:45; n = 18; ψ = 0:92
Here n is the number of data points and ψ is the Exner's statistical
parameter [31].
Kamlet's [29] triparametric equation explains ca. 86% of the effect
of solvent on oxidation. However, by Exner's criterion [31] the
correlation is not even satisfactory (cf. Eq. (9)). The major contribu-
tion is of solvent polarity. It alone accounted for ca. 80% of the data.
Both α and β play relatively minor roles.
3.11. Swain's method
The data on solvent effect were analysed in terms of Swain's
equation [32] of cation- and anion-solvating concept of the solvents
also.
log k2 = aA + bB + C
ð10Þ
Here A represents the anion-solvating power of the solvent and B
the cation-solvating power. C is the intercept term. (A+B) is
postulated to represent the solvent polarity. The rates in different
solvents were analysed in terms of Eq. (10), separately with A and B
and with (A+B).
logk2 = –3:95 + ð1:65 ꢁ 0:20Þπ* +ð0:15 ꢁ 0:15Þβ – ð0:11 ꢁ 0:14Þα
R2 = 0:8670; SD = 0:14; n = 18; ψ = 0:29
ð6Þ
Table 5
Reaction constant values for the oxidation of benzyl alcohol by TriPAFC at different
logk2 = ð0:66 ꢁ 0:03ÞA + ð1:72 ꢁ 0:02ÞB − 4:22
ð11Þ
temperaturesa.
R2 = 0:9979; SD = 0:02; n = 19; ψ = 0:03
Temperature
(K)
Reaction
Correlation
coefficient
Standard
deviation
constant (ρ)b
logk2 = 0:43ðꢁ0:56ÞA−3:02
ð12Þ
ð13Þ
ð14Þ
r2 = 0:0312; SD = 0:04; n = 19; ψ = 0:98
298
303
308
313
−1.68 0.3
−1.66 0.8
−1.61 0.9
−1.57 0.1
0.994
0.994
0.999
0.993
0.07
0.06
0.05
0.07
logk2 = 1:73ðꢁ0:12ÞB−4:19
r2 = 0:9199; SD = 0:12; n = 19; ψ = 0:18
[BnOH]=2.0×10−2 M, [TriPAFC]=1.0×10−3 M.
a
σ
p values were taken from reported works [27].
The values were obtained by correlating log k2 with σp for the reactions of
oxidations.
b
logk2 = 1:36 ꢁ 0:14ðA + BÞ−4:19
r2 = 0:8576; SD = 0:18; n = 19; ψ = 0:25

S.S. Mansoor, S.S. Shafi / Journal of Molecular Liquids 155 (2010) 85–90
89
Scheme 2. Acid-dependent path.
The rates of oxidation of benzyl alcohol in different solvents
showed an excellent correlation in Swain's equation with the cation-
solvating power playing the major role. In fact, the cation solvation
alone accounts for ca. 99% of the data. The correlation with anion-
solvating power was very poor. The solvent polarity, represented by
(A+B), also accounted for ca. 86% of the data.
The mechanism depicted in Scheme 1 leads to the following
equation:
Rate = kK½BnOHꢀ½TriPAFCꢀ
ð19Þ
ð20Þ
ð21Þ
It can be shown that
3.12. Mechanism of oxidation
½TriPAFCꢀ = ½TriPAFCꢀT = ð 1 + K½BnOHꢀÞ
Therefore,
A hydrogen abstraction mechanism leading to the formation of the
free radicals is unlikely in view of the failure to induce polymerization
of acrylonitrile and no effect of the radical scavenger on the reaction
rate. Therefore, a hydride ion transfer in the rate-determining step is
suggested (Scheme 1). The hydride ion transfer may take place either
by a cyclic process via an ester intermediate or by an acyclic one-step
bimolecular process. Negative reaction constants are traditionally
associated with an electron deficient centre in transition states: a
convention originally developed from the analysis of substituent
effects in nucleophilic displacement reactions. Negative reaction
constants have been used by Banerji [33–35] as supporting evidence
for oxidation mechanisms involving a hydride ion transfer in the rate-
determining step.
−d½TriPAFCꢀ= dt = kK½BnOHꢀ½TriPAFCꢀT = ð1 + K½BnOHꢀÞ:
Since the reaction is the first order with respect to each the alcohol
and TriPAFC, one can assume that 1NNK[BnOH]. The assumption is
made because the concentration of alcohol is in the order of 0.001 to
0.002 mol dm−3 and which is very small compared with 1. The
Eq. (22) can therefore, be written as
−d½TriPAFCꢀ= dt = kK½BnOHꢀ½TriPAFCꢀ:
ð22Þ
This rate equation is in accord with the experimental results.
Kwart and Nickel [36] have showed that a dependence of kinetic
isotope effect on temperature can be gainfully employed to determine
whether the loss of hydrogen proceeds through a concerted cyclic
process or by an acylic one. The data for protio- and deuterio-benzyl
The observed dependence on the hydrogen ion concentration in
the reaction shows that there is an additional acid-catalysed pathway.
The acid-dependent pathway is given in Scheme 2.
⁎
alcohols, fitted to the expression: kH/kD =AH /AD (−ΔH /RT) [37,38]
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the entropies of activation of the respective reactions are almost
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