Kinetics and mechanism of the oxidation of lower oxyacids of phosphorus by hexamethylenetetramine bromine

Article abstract of DOI:10.1002/(SICI)1097-4601(2000)32:3<165::AID-KIN6>3.0.CO;2-M

The oxidation of lower oxyacids of phosphorus by hexamethylenetetramine bromine (HABR) in glacial acetic acid resulted in the formation of corresponding oxyacids with phosphorus in a higher oxidation state. The reaction exhibited 2:1 stoichiometry. The reaction is first order with respect to HABR. Michaelis-Menten-type kinetics were observed with respect to the acids. The formation constant of the phenylphosphinic acid-HABR complex also has been determined spectrophotometrically. The thermodynamic parameters for the complex formation and the activation parameters for their decomposition were calculated. The reaction showed the presence of a substantial kinetic isotope effect. It is proposed that the HABR itself is the reactive oxidizing species. It has been shown that the pentacoordinated tautomer of the phosphorus oxyacid is the reactive reductant. A suitable mechanism has been proposed.

Full text of DOI:10.1002/(SICI)1097-4601(2000)32:3<165::AID-KIN6>3.0.CO;2-M

Kinetics and Mechanism of
the Oxidation of Lower
Oxyacids of Phosphorus by
Hexamethylenetetramine
Bromine
SATISH K. MEHLA, SEEMA KOTHARI, KALYAN K. BANERJI
Department of Chemistry, J.N.V. University, Jodhpur 342 005, India
Received 14 June 1999; accepted 8 November 1999
ABSTRACT: The oxidation of lower oxyacids of phosphorus by hexamethylenetetramine bro-
mine (HABR) in glacial acetic acid resulted in the formation of corresponding oxyacids with
phosphorus in a higher oxidation state. The reaction exhibited 2:1 stoichiometry. The reaction
is first order with respect to HABR. Michaelis–Menten-type kinetics were observed with respect
to the acids. The formation constant of the phenylphosphinic acid–HABR complex also has
been determined spectrophotometrically. The thermodynamic parameters for the complex
formation and the activation parameters for their decomposition were calculated. The reaction
showed the presence of a substantial kinetic isotope effect. It is proposed that the HABR itself
is the reactive oxidizing species. It has been shown that the pentacoordinated tautomer of
the phosphorus oxyacid is the reactive reductant. A suitable mechanism has been proposed.
᭧
2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 165–170, 2000
INTRODUCTION
idizing and reducing species. A suitable mechanism
has been proposed.
Hexamethylenetetramine bromine (HABR) has been
reported as a synthetic reagent for the oxidation of
alcohols to carbonyl compounds [1]. We have under-
taken a study of the kinetics and mechanism of oxi-
dation by HABR. Four reports have been issued from
our laboratory on the mechanistic aspects of the oxi-
dation of alcohols [2,3], aldehydes [4], and diols [5].
In this article, the kinetics of oxidation of three lower
oxyacids of phosphorus, namely, phosphorous (POA),
phosphinic (PA), and phenylphosphinic (PPA) acids,
by HABR in glacial acetic acid have been reported.
Attempts have been made to identify the reactive ox-
EXPERIMENTAL
Material
The phosphorus oxyacids were commercial products
(Fluka) and were used as supplied. HABR was pre-
pared by the reported method [1] and its purity was
checked by an iodometric method and melting point
determination. Contrary to the earlier report [1], we
found that in acetic acid solutions, the active bromine
content of this complex is 2 mol/mol of the reagent.
The P9H bonds in PA and POA were deuterated by
repeatedly dissolving the acid in deuterium oxide
Correspondence to: Kalyan K. Banerji
Contract grant sponsor: University Grants Commission (India)
(BARC, 99.4%) and evaporating water and the excess
᭧
2000 John Wiley & Sons, Inc.
of deuterium oxide in vacuo [6]. This isotopic purity

1
66
MEHLA ET AL.
of the deuterated PA and POA, as determined from
their nuclear magnetic resonance (NMR) spectra, was
ter the completion of the reaction. The [PPA]/[HABR
consumed] was found to be 2.10 Ϯ 0.08.
9
2 Ϯ 5% and 91 Ϯ 5%, respectively. Acetic acid was
refluxed with chromium trioxide and acetic anhydride
for 6 h and then distilled.
RESULTS
Oxidation of the phosphorus oxyacids exhibited a
2 : 1 stoichiometry and the overall reaction can, there-
fore, be written as Eq. (1).
Kinetic Measurements
The reactions were studied under pseudo-first-order
conditions by keeping an excess (20 times or more) of
the acid over HABR. The solvent was glacial acetic
acid. The reactions were studied at constant tempera-
ture (Ϯ0.1 K) and were followed by monitoring the
decrease in the [HABR] spectrophotometrically at 380
nm for up to 80% reaction. Beer’s law was found to
be valid for HABR within the concentration range
used in our experiments. Pseudo-first-order rate
constants, kobs were evaluated from linear plots
2RH PO ϩ (CH ) N B r ϩ 4CH COOH !:
2
2
2 6
4
4
3
2RH PO ϩ (CH ) N ϩ 4HBr ϩ 4(CH CO) O
2
3
2 6
4
3
2
(1)
where R ϭ H, OH, or Ph.
The reactions were found to be first order with re-
spect to HABR. Furthermore, the pseudo-first-order
rate constants do not depend on the initial concentra-
tion of HABR. The rate of the reaction increases with
an increase in the concentration of the oxyacid (Table
I). The order with respect to PA, PPA, and POA, as
determined by the log–log plot, is 0.45 Ϯ 0.04,
(r Ͼ 0.9990) of log [HABR] against time. Duplicate
kinetic runs showed that the rates were reproducible to
within Ϯ3%.
0
1
.47 Ϯ 0.05, and 0.44 Ϯ 0.04, respectively. A plot of
/kobs vs. 1/[oxyacid] was linear with an intercept on the
Spectral Analysis
rate ordinate (Fig. 1). Thus, Michaelis–Menten-typeki-
netics were observed with respect to the substrate. This
leads to the postulation of the following overall mech-
anism, Eqs. (2) and (3), and the rate law (4).
Ultraviolet visible (UV-vis) spectra of HABR, hexa-
methylenetetramine (HXA), bromine, and the reaction
mixture were obtained on an HP diode-array rapid
scanning spectrophotometer (Model 8452A) with a
scanning speed of 600 nm s . To determine the for-
mation constant of the intermediate, spectra of reaction
mixtures containing a constant [HABR] and varying
K
Ϫ1
Oxyacid ϩ HABR C
R
D [complex]
(2)
[PPA] also were obtained. The solvent was glacial ace-
tic acid and the temperature was 313 Ϯ 1 K. For the
individual compounds the blank was glacial acetic
acid and for the reaction mixture the blank was a so-
lution of the oxyacid in glacial acetic acid. The time
gap between preparation of the reaction mixture and
recording of the spectrum was Ͻ10 s.
Stoichiometry
The oxidation of phosphorus(I) and -(III) oxyacids
leads to the formation of corresponding phospho-
rus(III) and -(V) oxyacids.
Reaction mixtures were prepared containing a
known excess of PA or POA. On completion of the
reaction, the amount of POA formed in the oxidation
of PA and the residual reductant in the oxidation of
POA were determined by the reported method [7].
Several determinations indicated that the [POA
formed]/[HABR] and [consumed POA]/[HABR] in
the case of PA and POA are 2.09 Ϯ 0.06 and
1
.96 Ϯ 0.05, respectively. To determine the stoichi-
ometry of the oxidation of PPA, a known excess of
HABR was treated with PPA and the residual HABR
was determined spectrophotometrically at 380 nm af-
Figure 1 A plot of 1/[PA] vs. 1/kobs in the oxidation by
HABR. (Conditions as in Table I.)

KINETICS/MECHANISM OF OXIDATION OF LOWER OXYACIDS OF PHOSPHORUS
167
Table I Rate Constants for the Oxidation of Oxyacids of Phosphorus by HABR at 303 K
1
03 kobs/(s )
Ϫ1
3
1
(
0 [HABR]
[Oxyacid]
(mol dm )
Ϫ3
Ϫ3
mol dm )
PA
PPA
POA
1
1
1
1
1
1
1
0
0
2
3
4
.0
.0
.0
.0
.0
.0
.0
.5
.8
.0
.0
.0
0.05
0.10
0.20
0.50
0.80
1.00
1.00
0.20
0.20
0.20
0.20
0.20
1.40
2.25
3.42
4.85
5.29
5.58
5.60a
3.46
3.51
3.44
3.60
3.50
3.20
5.26
8.05
11.7
12.4
13.5
13.0a
7.91
7.93
8.00
7.88
8.10
0.40
0.63
0.96
1.34
1.45
1.55
1.54a
0.92
0.90
0.97
0.88
0.86
a
Contained 0.005 mol dm 3 acrylonitrile.
Ϫ
k
2
constants of the complexes of ordinary and deuterated
oxyacids have similar values but the rates of decom-
position of the complexes showed the presence of a
substantial kinetic isotope effect (k /k ϭ 5.94 and
[
Complex] 9
9
: products
(3)
Rate ϭ k K [oxyacid][HABR]/(1 ϩ K [oxyacid])
2
(4)
H
D
5
.90 for PA and POA, respectively, at 303 K).
The dependence of kobs on the concentration of ox-
yacid was studied at different temperatures and the val-
Spectral Analysis
ues of K and k were evaluated from the double recip-
2
A perusal of UV-vis spectra of HABR (0.001 mol
rocal plots. The thermodynamic parameters for the
complex formation and activation parameters of the de-
composition of the complexes, at 298 K, were calcu-
Ϫ3
dm ) and an equivalent amount of bromine (0.002
Ϫ3
mol dm ), in AcOH at Ϸ313 Ϯ 1 K, showed that the
difference in the nature of the spectra of HABR and
bromine was not very striking but their optical densi-
ties showed variations. HXA had no appreciable ab-
sorption in this range. Moreover, the spectrum of
HABR did not show any change in the experimental
time period (ca. 2 h). When a solution of HABR in
acetic acid was evaporated to dryness under reduced
pressure, HABR was recovered unchanged. This con-
firmed that HABR retained its integrity in acetic acid.
A perusal of the spectra of HABR and of
HABR ϩ PPA showed that there is a distinct increase
in the absorbance of HABR on the addition of PPA
(Fig. 2). Additionally, the value of the absorbance in-
creases with an increase in the concentration of PPA.
This supports the postulation of the formation of an
intermediate complex in a pre-equilibrium. The for-
lated from the values of K and k , respectively, at dif-
2
ferent temperatures (Tables II and III).
Induced Polymerization of Acrylonitrile
The oxidation of PA, in an atmosphere of nitrogen,
failed to induce the polymerization of acrylonitrile.
Also, an addition of acrylonitrile had no effect on the
rate (see Table I).
Kinetic Isotope Effect
To ascertain the importance of the cleavage of the
P9H bond in the rate-determining step, the oxidation
of deuterated PA and POA was studied. Results re-
corded in Tables II and III showed that the formation
Table II Formation Constants and Thermodynamic Parameters for the Oxyacid–HABR Complexes
3
Ϫ1
K/dm mol
ꢀ
H
ꢀS
Ϫ
1
Ϫ1
Ϫ1
Acid
PA
303 K
313 K
323 K
(kJ mol )
(J mol K )
5.43
5.00
5.76
5.46
5.88
3.90
3.54
4.30
3.80
4.49
2.98
2.50
3.38
2.86
3.33
Ϫ26.9 Ϯ 0.6
Ϫ30.7 Ϯ 0.3
Ϫ24.2 Ϯ 0.5
Ϫ28.8 Ϯ 0.9
Ϫ25.6 Ϯ 0.7
Ϫ67 Ϯ 2
Ϫ80 Ϯ 1
Ϫ58 Ϯ 2
Ϫ73 Ϯ 3
Ϫ62. Ϯ 2
PPA
POA
DPA
DPOA

1
68
MEHLA ET AL.
Table III Rate Constants for the Decomposition of the Oxyacid–HABR Complexes and Their Activation Parameters
3
Ϫ1
1
0 k /s
2
Ha
a
ꢀ
ꢀS
Ϫ1
Ϫ
1
Ϫ1
Acid
PA
303 K
313 K
323 K
27.1
60.0
6.90
4.26
1.18
(kJ mol )
(J mol K )
6.53
16.0
1.77
1.10
0.30
5.94
5.90
13.0
32.8
3.71
2.30
0.65
5.65
5.71
55.3 Ϯ 1.3
51.3 Ϯ 1.0
52.9 Ϯ 1.1
52.6 Ϯ 1.1
53.3 Ϯ 2.1
Ϫ105 Ϯ 4
Ϫ111 Ϯ 3
Ϫ124 Ϯ 3
Ϫ129 Ϯ 4
Ϫ137 Ϯ 7
PPA
POA
DPA
DPOA
k /k
6.36 (PA)
5.85 (POA)
H
D
D
k /k
H
mation constant of the HABR–PPA complex was de-
termined spectrophotometrically from the absorbance
of solutions containing a constant initial [HABR] and
varying initial [PPA] by a double reciprocal plot. The
value of the formation constant, K, at 313 K was found
shown by HABR. The oxidation by bromine, under
similar reaction conditions, is much faster than that by
HABR (ca. five times) and the order with respect to
PA is one. Furthermore, an addition of bromide ions
retards the oxidation.
3
Ϫ1
to be 2.6 dm mol , which compares favorably with
the value of 3.5 obtained kinetically (cf. Table II).
DISCUSSION
Effect of Hexamethylenetetramine
In solutions, HABR may dissociate to form molecular
bromine [reaction (5)].
The rate of oxidation of oxyacids is not affected by
the addition of HXA (Table IV).
(
CH ) N B r CD (CH ) N ϩ 2Br
(5)
2
6
4
4
2 6
4
2
Effect of Bromide Ion
However, nil effect of both bromide ions and HXA
on the rate showed that the equilibrium in reaction (5)
is not participating in the oxidation process. Moreover,
the isolation of unchanged HABR, from a solution in
acetic acid, indicated that HABR retained its integrity
in acetic acid. Therefore, it is proposed that the reac-
tive oxidizing species is HABR itself. The different
kinetic pictures obtained in the oxidation of PA by
bromine and HABR support this postulation.
The addition of bromide ion has no effect on the rate
of oxidation of oxyacids (Table V).
Oxidation by Bromine
For the purpose of comparison, the oxidation of PA
by bromine was studied. It was found that the oxida-
tion by bromine exhibits kinetics different than those
Nothing much is known about the structure of
HABR. For the sake of convenience, we have shown
the reactive part as 9N
B
rBr. However, an al-
ϩ
Ϫ
3
ternative form can be Br9N B r T. here is no evi-
dence for any of these suggestions.
Lower oxyacids of phosphorus are reported to exist
in two tautomeric forms [8,9]. The predominant spe-
cies is the pentacoordinated form (A). The value [10]
of the equilibrium constant, K , in aqueous solutions
t
Ϫ12
is of the order of 10 . Although in acetic acid solu-
tion, the value of K might be different, the order is
likely to be the same.
t
Ϫ
3
Figure 2 UV-vis spectra of (A) 0.001 mol dm HABR;
Ϫ3
Ϫ3
(
0
B) 0.001 mol dm HABR ϩ 0.018 mol dm PPA; (C)
.001 mol dm HABR ϩ 0.020 mol dm PPA; (D) 0.001
mol dm HABR ϩ 0.0246 mol dm PPA; (E) 0.001 mol
dm HABR ϩ 0.026 mol dm PPA; (F) 0.001 mol dm
HABR ϩ 0.045 mol dm PPA; (G) 0.001 mol dm
HABR ϩ 0.090 mol dm PPA; temperature: 313 Ϯ 1 K;
Ϫ3
Ϫ3
H
Ϫ3
Ϫ3
K
t
Ϫ3
Ϫ3
Ϫ3
R9P"O
R9P9OH
(6)
Ϫ3
Ϫ3
OH
(A)
OH
(B)
Ϫ3
solvent: acetic acid.

KINETICS/MECHANISM OF OXIDATION OF LOWER OXYACIDS OF PHOSPHORUS
169
Table IV Effect of Hexamethylenetetramine on the Oxidation of Phosphinic Acid by HABRa
3
Ϫ3
1
1
0 [HXA]/mol dm
0.0
4.85
0.5
4.80
1.0
4.91
2.0
4.84
3.0
4.75
5.0
5.05
3
Ϫ1
0 kobs/s
a
HABR] ϭ 0.001 mol dm 3; [PA] ϭ 0.50; and T ϭ 303 K.
Ϫ
[
Hence, two alternative broad mechanisms can be
imental rate law, Eq. (4), and are kinetically indistin-
guishable.
In the plots of 1/kobs vs. 1/[substrate], values of the
slope and the intercept, using Eqs. (10) and (14), are
as follows:
formulated. By assuming in the first instance that pen-
tacoordinated tautometer (A) is the reactive reducing
species, the following mechanism, Eqs. (6)–(8), can
be written, which leads to the rate law (9).
Ϫ
1
K
a
Slope ϭ (K k )
a 2a
HABR ϩ RPH(O)OH C
R
D [complex]
(7)
(8)
(9)
and
and
k
2a
[
Complex] 9
k K [oxidant] [RHP(O)OH]
0
9: products
Intercept ϭ 1/k2a
2
a
a
Rate Ϫ
1
ϩ K ϩ K [RHP(O)OH]
Ϫ1
t
a
0
Slope ϭ (K K k )
t b 2b
where [RHP(O)OH] represents the initial concentra-
tion of the phosphorus oxyacid. Because 1 ϾϾ K , Eq.
0
t
(
9) is reduced to Eq. (10).
k K [oxidant] [RHP(O)OH]
0
Intercept ϭ 1/k2b
It is thus seen that k ϭ k ϭ k , which means that
2a
a
Rate ϭ
(10)
2
2a
2b
1
ϩ K [RHP(O)OH]0
the rate constant for the disproportionation of the com-
plex is not affected by the reactive form of the oxyacid.
However, the values of the equilibrium constant from
the rate laws (10) and (14) are K ϭ K ϭ K K re-
a
Another mechanism can be formulated assuming
the tricoordinated form (B) as the reactive reducing
species, Eqs. (6), (11), and (12).
a
t
b
1
2
spectively. Therefore, K ϭ 10 K or K for PA at
b
b
3
Ϫ1
3
03 K should have a value of ca. 5 ϫ 1012 dm mol .
K
b
Polyhalides are not reported to form a highly stable
series of complexes. In the oxidation of PA with ben-
zyltrimethylammonium dichloroiodate [11] and ben-
zyltrimethylammonium tribomide [12], the complex
formation was not detected, implying that if the com-
plex is formed the formation constants have very small
values. HABR has been found to form complexes with
alcohols [2] and aldehydes [4] but the values of for-
mation constants range from 4 to 7 dm mol . Thus,
the very high value of the formation constant of the
HABR–oxyacid complex is improbable, and it is
highly unlikely that form (B) is involved in the oxi-
HABR ϩ RP(OH) C
R
D [complex]
(11)
(12)
2
k
2b
[
Complex] 99 : products
The corresponding rate law is given by Eq. (13). In
view of the relation 1 ϾϾ K , Eq. (13) is reduced to Eq.
t
(14).
3
Ϫ1
k2b K K [oxidant] [RHP(O)OH]
0
t
b
Rate ϭ
Rate ϭ
(13)
(14)
1
ϩ K ϩ K K [RHP(O)OH]
t b t 0
k2b K K [oxidant] [RHP(O)OH]
0
b
t
dation process. The very small value of K means that
t
1
ϩ K K [RHP(O)OH]
b t 0
in 0.05 mol oxyacid the concentration of form (B) is
too small to form a sufficient amount of the interme-
diate complex.
Thus, the two rate equations conform to the exper-
Table V Effect of Bromide Ion on the Oxidation of Phosphinic Acid by HABRa
3
Ϫ
Ϫ3
1
1
0 [Br ]/mol dm
0.0
4.85
0.5
5.08
1.0
4.89
2.0
4.84
3.0
4.99
5.0
5.05
3
Ϫ1
0 kobs/s
a
HABR] ϭ 0.001 mol dm 3; [PA] ϭ 0.50; and T ϭ 303 K.
Ϫ
[

1
70
MEHLA ET AL.
OH
K
RPH(O)OH ꢁ Br Br N9
R9P"O Br Br N9
H
k2
#
OH
ꢁ
ꢂ
R9P(O)OH ꢁ HBr ꢁ Br ꢁ N9
R9P"O Br
H
Br N9
ꢁ
ꢁ2AcOH
ꢁ
RP(O)OH
RP(O)(OH) ꢁ Ac O ꢁ H
2 2
Scheme I
MECHANISM
ion more difficult. A perusal of the activation param-
eters in Table III revealed that the reaction rates are
controlled mainly by the entropy of activation.
The absence of any effect of the radical scavenger on
the reaction rate and the failure to induce polymeriza-
tion of acrylonitrile point against a one-electron oxi-
dation give rise to free radicals. The presence of a
substantial kinetic isotope effect confirms the cleavage
of a P9H bond in the rate-determining step. A pref-
erential cleavage of a P9H bond in the rate-deter-
mining step is likely in view of the relatively high
bond dissociation energy of the O9H bond. The
mean value of the bond dissociation energy of an
Thanks are due to the University Grants Commission (India)
for financial assistance.
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1
2
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Ϫ
1
O9H bond [13] is 460 kJ mol , while that for a
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Ϫ1
P9H bond [14] is 321 kJ mol . Thus, the experi-
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