Indian Journal of Chemistry
Vol. 49A, March 2010, pp. 302-306
Notes
Oxidation of lower oxyacids of phosphorus by
tetraethylammonium chlorochromate: A kinetic
and mechanistic study
Khushboo Vaderaa , D Sharmaa , S Agarwalb &
Pradeep K Sharmaa,*
a
Department of Chemistry, JNV University,
Jodhpur 342 005, India
Email: [email protected]
b
Department of Chemistry, JDB Girls College,
Kota 324 008, India
Received 24 September 2009; revised and accepted 19 February 2010
Oxidation of lower oxyacids of phosphorus by
tetraethylammonium chlorochromate in dimethyl sulphoxide leads
to the formation of corresponding oxyacids with phosphorus in a
higher oxidation state. The reaction exhibits 1:1 stoichiometry.
The reaction is first order each with respect to chlorochromate and
the oxyacids. The reaction does not induce polymerization of
acrylonitrile. The oxidation of deuterated phosphinic and
phosphorous acids exhibits a substantial primary kinetic isotope
effect. The oxidation has been studied in nineteen different
organic solvents. The effect of solvent indicates that the solvent
polarity plays a major role in the process. It has been shown that
the pentacoordinated tautomer of the phosphorus oxyacid is the
reactive reductant and the tricoordinated form of phosphorus
oxyacids does not participate in the oxidation process. A
mechanism involving transfer of a hydride ion in the rate
determining step has been proposed.
Keywords:
Kinetics, Reaction mechanisms,
Halochromates, Phosphorus acids
IPC Code:
Int. Cl.9 CO7B33/00
Oxidations,
Halochromates have been used as mild and
selective oxidizing reagents in synthetic organic
chemistry1,2. Tetraethylammonium chlorochromate
(TEACC) is one such compounds used for the
oxidation of primary aliphatic alcohols3. TEACC
differs from other such compounds in being a
quaternary ammonium salt having four alkyl groups.
Most other halochromates are salts of heterocyclic
bases with chlorochromic acid. We have been
interested in the kinetic and mechanistic studies of the
reactions of chromium(VI) species and have reported
the mechanistic studies by several halochromates,
viz., pyridinium and quinolinium halochromates4-7.
The lower oxyacids are reported to exist in two
tautomeric forms8, and it is of interest to determine
the nature of the oxyacid acid involved in the
oxidation process. There seems to be no report on the
oxidation of oxyacids of phosphorus by TEACC.
Therefore, it was interest to investigate the kinetics of
oxidation of phosphinic (PA), phenylphosphinic
(PPA) and phosphorous (POA) acids by TEACC in
dimethyl sulphoxide (DMSO) as a solvent. A suitable
mechanism has also been proposed.
Experimental
The phosphorus oxyacids were commercial
products from Fluka and were used as supplied.
TEACC was prepared by the reported method3 and its
purity was checked by iodometric method and melting
point determination. Deuterated phosphinic (DPA)
and phosphorus acids (DPOA) were prepared by
literature method involving repeatedly dissolving the
acid in deuterium oxide (BARC, 99.4%) and
evaporating water and the excess deuterium oxide
in vacuo. The isotopic purity of the deuterated PA and
POA, as determined from their NMR spectra, was
93±5% and 94±5% respectively. Due to the nonaqueous nature of the medium, toluene-p-sulphonic
acid (TsOH) was used as a source of hydrogen ions.
TsOH is a strong acid and in non-polar solvents like
DMSO it is likely to be completely ionised. Solvents
were purified by literature methods.
The oxidation of lower oxyacids of phosphorus
leads to the formation of corresponding oxyacids
containing phosphorus in a higher oxidation state. The
products were identified by comparison of the IR
spectra. Reaction mixtures were prepared containing a
known excess (x 5) of phosphinic or phosphorous
acids. On completion of the reaction, the amount of
phosphorous acid formed in the oxidation of
phosphinic acid and the residual reductant in the
oxidation of phosphorous acids were determined by
literature method. To determine the stoichiometry of
the oxidation of PPA, a known excess (x 5) of
TEACC was treated with PPA and the residual
TEACC was determined spectrophotometrically at
365 nm after completion of the reaction. The
oxidation state of chromium in the completely
reduced reaction mixture as determined by iodometric
titration, was 3.90±0.15. The oxidation reaction
NOTES
exhibited 1:1 stoichiometry and the overall reaction
may be written as Eq. (1).
RPH (O) OH + O2CrCl O-N+Et4 →
RP(O)(OH)2 + OCrCl O-N+Et4 … (1)
(R = H, Ph or OH)
TEACC undergoes a two-electron change. This is
in accordance with the earlier observations with
pyridinium fluorochromate (PFC)9, pyridinium
chlorochromate (PCC)10 and 2,2’-bipyridinium
chlorochromate11. It has already been shown that both
PFC9 and PCC10 act as two electron oxidants and are
reduced to chromium (IV) species by determining the
oxidation state of chromium by magnetic susceptibility, ESR and IR studies.
The reactions were studied under pseudo-first order
conditions by keeping an excess (× 15 or greater) of
the [oxyacid] over [TEACC]. The solvent was DMSO,
unless specified otherwise. The reactions were studied
at constant temperature (±0.1 K) and were followed
by monitoring the decrease in the [TEACC] spectrophotometrically at 365 nm for up to 80% completion
of the reaction. Pseudo-first-order rate constants, kobs,
were evaluated from linear plots (r > 0.990) of
log[TEACC] against time. Duplicate kinetic runs
showed that the rates were reproducible to within
±3%. The second order rate constants, k2 were
determined from the relationship: k2 = kobs/ [oxyacid].
Results & discussion
The reactions were found to be first order in
TEACC. The individual kinetic runs were strictly first
order in TEACC. Further, the pseudo-first order rate
constant does not depend on the initial [TEACC]. The
reaction rate increases linearly with an increase in the
[oxyacids] (Table 1). The oxidation of oxyacids, in an
atmosphere of nitrogen, failed to induce the
polymerization of acrylonitrile. Further, the addition
of acrylonitrile had no retarding effect on reaction rate
(Table 1).
The rate of reaction is increased from 31.5 × 10-4 –
79.2 × 10-4 s-1 on addition of hydrogen ions from
0.1 – 1.00 mol dm-3. The hydrogen ion dependence
has the following form kobs = a + b [H+]. The values of
a and b, for PPA, are 12.2±0.48 × 10−4 s−1 and
23.2±0.79 × 10−4 mol−1 dm3 s−1 respectively (r = 0.9976).
The observed hydrogen ion dependence suggests that
the reaction follows two mechanistic pathways; one of
which is acid independent and the other acid
dependent. This may well be attributed to protonation
of TEACC to give a stronger oxidant and electrophile
303
(Eq. 2). Both TEACC and TEACCH+ are reactive
species with the protonated form being more reactive.
+
[O2CrCl O-N+Et4] + H+ [HOCrOCl O-N+Et4]
… (2)
Formation of a protonated Cr(VI) species has
earlier been postulated in the reactions of structurally
similar benzyltriethylammonium chlorochromate
(BTEACC)12 and morpholinium chlorochromate
(MCC)13.
The rates were determined at different temperatures
and the activation parameters have been evaluated
(Table 2). The reactions are characterized by low
enthalpy of reaction and highly unfavourable entropy
factor. Thus, the rates are mainly controlled by the
entropy of activation.
To ascertain importance of the cleavage of the P-H
bond in the rate-determining step, oxidation of
deuteriated PA and POA was studied. Results showed
the presence of a substantial primary kinetic isotope
effect, kH/kD = 5.76 and 5.37 for PA and POA
respectively at 298 K (Table 2). This confirms that a
P-H bond is cleaved in the rate-determining step.
The oxidation of PPA was studied in 19 different
organic solvents. The choice of solvents was limited
due to the solubility of TEACC and its reaction with
primary and secondary alcohols. There was no
reaction with the solvents chosen. The kinetics were
similar in all the solvents. The values k2 at 298 K for
the oxidation of PPA are recorded in Table 3.
The rate constants of the oxidation, k2, in eighteen
solvents (CS2 was not considered, as the complete
Table 1 — Rate constants for the oxidation of oxyacids of
phosphorus by TEACC at 298 K
103[TEACC]
(mol dm-3)
[oxyacid]
(mol dm-3)
1.00
1.00
1.00
1.00
1.00
1.00
2.00
4.00
6.00
8.00
1.00
0.10
0.20
0.40
0.60
0.80
1.00
0.40
0.40
0.40
0.40
0.20
a
PA
105 kobs (s-1)
PPA
POA
11.1
20.5
41.4
62.1
82.8
108
43.2
39.6
41.5
40.5
22.7a
28.1
58.5
117
175
234
288
121
114
118
120
60.3a
2.97
6.30
12.6
19.0
25.5
30.6
13.5
11.7
14.0
12.4
7.20a
contained 0.005 mol dm−3 acrylonitrile.
INDIAN J CHEM, SEC A, MARCH 2010
304
Table 2 — Rate constants and activation parameters of oxidation of phosphorus oxyacids by TEACC
104 k2 ( dm3 mol −1 s−1 )
Acid
∆H*
∆S*
∆G*
288 K
298 K
308 K
318 K
(kJ mol−1)
(J mol1 K−1)
(kJ mol−1)
PA
5.22
10.8
19.8
36.0
46.2±0.5
−147±2
90.0±0.4
PPA
16.2
28.8
47.6
81.0
38.1±0.4
−167±1
87.5±0.3
POA
1.62
3.06
5.76
108
DPA
DPOA
kH/kD
kH/kD
0.88
0.29
5.93
5.59
1.87
0.57
5.76
5.37
3.54
1.10
5.59
5.24
6.77
2.11
5.32 (PA)
5.12 (POA)
45.6±0.6
48.9±0.5
47.8±0.5
−160±2
−153±1
−166±2
93.0±0.5
94.3±0.4
97.2±0.4
Table 3 — Effect of solvents on the oxidation of phenylphosphinic acid by TEACC at 298 K
Solvents
105 k2
(dm mol −1 s−1)
Solvents
3
Chloroform
1,2-Dichloroethane
Dichloromethane
DMSO
Acetone
N,N-Dimethylformamide
Butanone
Nitrobenzene
Benzene
Ethyl acetate
51.3
81.3
66.1
288
70.8
144
45.7
102
22.1
25.1
Acetic acid
Cyclohexane
Toluene
Acetophenone
THF
t-Butyl alcohol
1,4-Dioxane
1,2-Dimethoxyethane
Carbon disulfide
range of solvent parameters was not available) were
correlated in terms of the linear solvation energy
relationship (LESR) of Kamlet and Taft, log k2 = A0 +
pπ* + bβ + aα, where π* represents the solvent
polarity, β the hydrogen bond acceptor basicities and
α is the hydrogen bond donor acidity. A0 is the
intercept term. It may be mentioned here that out of
the 18 solvents, 12 have a value of zero for α. The
results of correlation analyses in terms of the
biparametric equation involving π* and β, and
separately with π* and β are given below as
Eqs. (3) – (6).
log k2 = − 4.28 + 1.89 (±0.24) π* + 0.24 (±0.20) β
+ 0.46 (±0.19) α
… (3)
2
R = 0.8691; sd = 0.22; n = 18; ψ = 0.40
log k2 = − 4.17 + 2.06 (±0.26) π* + 0.09 (±0.22) β
… (4)
R2 = 0.8137; sd = 0.25; n = 18; ψ = 0.46
log k2 = − 4.19 + 2.08 (±0.25) π*
r2 = 0.8117; sd = 0.25; n = 18; ψ = 0.45
105 k2
(dm mol −1 s−1)
3
… (5)
4.27
1.51
17.8
117
33.9
16.2
30.2
12.9
8.13
log k2 = − 3.63 + 0.45(±0.45) β
… (6)
r2 = 0.0582; sd = 0.55; n = 18; ψ = 0.99
Here n is the number of data points and ψ is the
Exner's statistical parameter.
Kamlet's triparametric equation explains ca. 87%
of the effect of solvent on the oxidation. However, by
Exner's criterion the correlation is not even
satisfactory. The major contribution is of solvent
polarity. It alone accounted for ca. 81% of the data.
Both β and α play relatively minor roles.
The data on the solvent effect were analysed in
terms of Swain's equation of cation- and anion-solvating
concept of the solvents also, i. e., log k2 = aA + bB + C.
Here A represents the anion-solvating power of the
solvent and B the cation-solvating power and 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. (7), separately with A and B
and also with (A + B).
log k2 = 0.32 (±0.04) A + 2.15 (±0.03) B − 3.97
… (7)
R2 = 0.9968; sd = 0.03; n = 19; ψ = 0.06
NOTES
305
log k2 = 0.01 (±0.07) A − 2.49
r2 = 0.0011; sd = 0.57; n = 19; ψ = 1.03
log k2 = 2.12 (±0.06) B − 3.86
r2 = 0.9851; sd = 0.07; n = 19; ψ = 0.13
… (8)
Rate = kb Kt [Oxyacid]0 [TEACC]/ (1 + Kt )
… (9)
This rate law (16), can be reduced to (17), again
indicating 1 >> Kt.
log k2 = 1.54 ± 0.24 (A + B) − 3.91
r2 = 0.7163; sd = 0.31; n = 19; ψ = 0.55
… (10)
The rates of oxidation of PPA in different solvents
showed an excellent correlation in Swain's equation
(cf. Eq. 9) with the cation-solvating power playing the
major role. In fact, the cation-solvation alone account
for ca. 99% of the data. The correlation with the
anion-solvating power was very poor. The solvent
polarity, represented by (A + B), also accounted for
ca. 72% of the data. In view of the fact that solvent
polarity is able to account for ca. 72% of the data, an
attempt was made to correlate the rate with the
relative permittivity of the solvent. However, a plot of
log k2 against the inverse of the relative permittivity is
not linear (r2 = 0.5265; sd = 0.39; ψ = 0.71).
Lower oxyacids of phosphorus are reported to exist
in two tautomeric forms8. The predominant species is
the pentacoordinated form (A). The value14 of the
equilibrium constant, Kt, in aqueous solutions, is of
the order of 10-12.
Kt
RPH(O)OH R − P − (OH)2
(A)
(B)
… (11)
Hence, two alternative broad mechanisms can be
formulated. Assuming in the first instance the
pentacoordinated tautomer (A) as the reactive
reducing species, the following mechanism may be
proposed which leads to the rate law (13).
ka
TEACC + RPH(O)OH → Products
… (12)
Rate = ka [Oxyacid]0 [TEACC]/ (1 + Kt )
… (13)
where [oxyacid]0 represents the initial concentration
of the oxyacid. Equation (13) can be reduced to
Eq. (14) as 1 >> Kt.
Rate = ka [Oxyacid]0 [TEACC]
… (14)
Another mechanism can be formulated assuming
the tricoordinated form (B) as the reactive reducing
species, Eq. (15), which leads to the rate
equation (16).
kb
TEACC + RP(OH)2 → Products
… (15)
Rate = kb Kt [Oxyacid]0 [TEACC]
… (16)
… (17)
Thus, the two rate equations conform to the
experimental rate law and are kinetically indistinguishable.
If Eqs. (11) and (15) represents the mechanism of
the oxidation of oxyacids of phosphorous then the
experimental specific rate constant, k2 = Kt kb, with
the value of Kt is of the order of 10−12. Therefore, the
value of rate limiting constant, kb, ranges between 108
and 109 dm3 mol−1 s−1. This value exceeds/equals the
rate constants of diffusion-controlled rate processes15.
Therefore, participation of tricoordinated form of the
oxyacids in the oxidation processes can be ruled out.
The absence of any effect of the radical scavenger
on the reaction rate and the failure to induce
polymerisation of acrylonitrile point against a
one-electron oxidation giving rise to free radicals. The
presence of a substantial kinetic isotope effect
confirms the cleavage of a P−H bond in the rate
determining step. Preferential cleavage of a P−H bond
in the rate determining step is likely in view of the
relatively high bond dissociation energy of the O-H
bond. The mean value of the bond dissociation energy
of an O−H bond is 460 kJ mol-1 (ref. 16) while that for
a P−H bond is 321 kJ mol-1 (ref. 17). Therefore, a
hydride ion mechanism may be proposed for the
oxidation of these oxyacids (Scheme 1). The proposed
mechanism involving the hydride ion transfer in the
rate determining step is also supported by the
observed major role of cation-solvating power of the
solvents.
It is of interest to compare the mode of oxidation of
lower oxyacids of phosphorus by PFC18, PCC19,
pyridinium bromochromate5 and TEACC. The
oxidation by PCC and TEACC exhibits second order
kinetics; first with respect to each reactant. The
oxidation by PFC exhibits a Michaelis-Menten type of
kinetics. The rate law, acid dependence, kinetic
isotope effect are similar in both the cases. In all the
three oxidations, excellent correlation was obtained in
terms of Swain’s equation, with the cation solvating
power of the solvents playing the major role.
The rate of oxidation follows the order: PPA > PA
> POA. The faster rate of PPA may be explained on
the basis of stabilisation of a positively polarized
306
INDIAN J CHEM, SEC A, MARCH 2010
phosphorus in the transition state by the phenyl group
through resonance. The slower rate of POA may well
be due to the electron withdrawing nature of hydroxyl
group causing an electron deficiency at the
phosphorus atom. This makes the removal of an anion
more difficult. A perusal of the activation parameters
in Table 2 reveals that mainly the entropy of
activation controls the reaction rates.
Acknowledgement
Thanks are due to the Council of Scientific and
Industrial Research, New Delhi, India for financial
support in the form of a Major Research Project
(No. 01(2014)/05/EMR-II).
References
1 Balasubramanian K & Prathiba V, Indian J Chem, 25B
(1986) 326.
2 Pandurangan A, Murugesan V & Palanichamy M, J Indian
Chem Soc, 72 (1995) 479.
3 Shiekh H N, Sharma M, Hussain M & Kalsotra B L, Oxid
Commun, 28 (2005) 887.
4 Saraswat S, Sharma V & Banerji K K, Indian J Chem, 40A
(2001) 583.
5 Grover A, Varshney S & Banerji K K, Indian J Chem, 33A
(1994) 622.
6 Dave I, Sharma V & Banerji K K, Indian J Chem, 41A
(2002) 493.
7 Vyas S & Sharma P K, Indian J Chem, 43A (2004) 1219.
8 Fratiello A & Anderson E W, J Am Chem Soc, 85 (1963)
519.
9 Battacharjee M N, Choudhary M K & Purkesyatha S,
Tetrahedron, 43 (1987) 5389.
10 Brown H C, Gundu Rao C & Kulkarni S U, J Org Chem, 44
(1979) 2809.
11 Vyas S & Sharma P K & Banerji K K, Indian J Chem, 40A
(2002) 1182.
12 Kaur R, Soni N & Sharma V, Indian J Chem, 45A (2006)
2241.
13 Soni N, Tiwari V & Sharma V, Indian J Chem, 47A (2008)
669.
14 Van Wazer J R, Phosphorous and its Compounds, Vol. I,
(Wiley, New York), 1958.
15 Vetter K J, Electrochemical Kinetics−Theoretical and
Experimental Aspects, (Academic Press, New York) 1967,
p. 511.
16 Louing E G & Laidler K J, Can J Chem, 38 (1990) 2367.
17 Gunn S R & Green L G, J Phys Chem, 65 (1961) 779.
18 Seth M, Mathur A & Banerji K K, Bull Chem Soc Japan, 63
(1992) 3846.
19 Moondra A, Mathur A & Banerji K K, J Chem Soc, Dalton
Trans, (1990) 2697.