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ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2016, Vol. 90, No. 9, pp. 1760–1765. © Pleiades Publishing, Ltd., 2016.
Kinetics of Ruthenium(III) Catalyzed and Uncatalyzed Oxidation
of Monoethanolamine by N-Bromosuccinimide1
R. Venkata Nadha,*, B. Syama Sundarb, and P. S. Radhakrishnamurtic
aGITAM University—Bengaluru Campus, Karnataka 561203, India
b
Yogi Vemana University, Kadapa 516216, India
c
Department of Chemistry, TJPS (PG) College, Guntur-522006, India
*e-mail: doctornadh@yahoo.co.in
Received April 14, 2015
Abstract—Kinetics of uncatalyzed and ruthenium(III) catalyzed oxidation of monoethanolamine by N-bro-
mosuccinimide (NBS) has been studied in an aqueous acetic acid medium in the presence of sodium acetate
and perchloric acid, respectively. In the uncatalyzed oxidation the kinetic orders are: the first order in NBS,
a fractional order in the substrate. The rate of the reaction increased with an increase in the sodium acetate
concentration and decreased with an increase in the perchloric acid concentration. This indicates that free
amine molecules are the reactive species. Addition of halide ions results in a decrease in the kinetic rate,
which is noteworthy. Both in absence and presence of a catalyst, a decrease in the dielectric constant of the
medium decreases the kinetic rate pointing out that these are dipole—dipole reactions. A relatively higher
oxidation state of ruthenium i.e., Ru(V) was found to be the active species in Ru(III) catalyzed reactions. A
suitable mechanism consistent with the observations has been proposed and a rate law has been derived to
explain the kinetic orders.
Keywords: kinetics, oxidation, ruthenium(III), catalysis, monoethanolamine, N-bromosuccinimide
DOI: 10.1134/S0036024416090296
INTRODUCTION
Plants, animals, and vegetation emit volatile
amines. Naturally aminoalcohols are produced by
biodegradation amino acids [1]. Monoethanolamine
(MEA) finds extensive applications in the synthesis of
surfactants, pharmaceuticals, and as addition agents in
the metal finishing industry [2]. Compared to other
amines, MEA is cheap, has a high absorption capacity
and reacts quickly with gaseous CO2 [3, 4]. Hence,
MEA is the benchmark solvent in carbon capture and
storage (CCS), a technology aimed at reducing CO2
emissions in large combustion industries [5]. To accu-
rately assess the environmental impact of CCS, a bet-
ter knowledge on the reactions of MEA is necessary.
Considerable attention has been centered on the
chemistry of N-halomines, because of their versatility
in behaving as mild oxidants, halogenating agents, and
N-anions, which act as both bases and nucleophiles.
As a result these reagents react with a wide range of
functional groups [6–9].
Literature collection shows that kinetics of oxida-
tion of amino alcohols was studied using different
oxidants like vanadium(V) as an oxidant in aqueous
perchloric acid medium [10], potassium diperiodato-
cupate(III) in alkaline medium [11], N-bromosuc-
cinimide in alkaline medium in absence and in pres-
ence of non-ionic micellar aggregates [12]. Similarly,
kinetics of oxidation of aminoalcohols by chloramine-T
was studied in perchloric acid medium in presence of
palladous chloride as catalyst [13] and in alkaline
medium [14, 15].
The above literature survey shows that various oxi-
dizing agents have studied oxidations of amino acids
but catalyzed oxidations using transition metals, as
catalysts have not been studied in detail by using co-
oxidants like N-halo compounds. The present investi-
gation has been under taken using N-bromosuccinim-
ide (NBS) as an oxidant to clarify whether oxidations
of monoethanolamine are catalyzed by transition
metal catalysts like Ru(III) and Os(VIII) or whether
the catalysis is selective to transition metal catalysts
like Ru(III). Hence a systematic investigation has been
done in acetic acid—perchloric acid mixtures to throw
light on the nature of reaction orders and mechanistic
sequences.
EXPERIMENTAL
The reagents employed were monoethanolamine
(Loba sample), N-bromo succinimide (GR, S.Merck
Grade), ruthenium(III) chloride (Johnson Mathey,1
The article is published in the original.
CHEMICAL KINETICS
AND CATALYSIS
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 9 2016
KINETICS OF RUTHENIUM(III) CATALYZED 1761
London), osmium tetroxide (Johnson and Mathey
Co., London), mercuric acetate (E.Merck G.R.),
other reagents used were of analytical grade reagents.
All the solutions were prepared by doubly distilled
water. Stock solutions of N-bromosuccinimide were
prepared in pure acetic acid and standardized iodo-
metrically.
Kinetic studies were carried out in a perchloric acid
medium under pseudo first order conditions with a
large excess of monoethanolamine over NBS. The
progress of reaction was followed by determining
N-bromosuccinimide concentrations iodometrically
in aliquots withdrawn after suitable time intervals [16].
Compared to the rates of present reaction, self decom-
position rates of NBS were negligibly smaller under
the conditions employed. The rate constants remained
practically unaltered in air or in a de-aerated atmo-
sphere. Only a representative set of the average values
of kinetic data in aqueous acetic acid medium was pre-
sented here. First order rate constants were evaluated
from the linear plots (r2
> 0.993) of log [unreacted
DCICA] against time. Rate constants and other deter-
mined values were reproducible to ±2%.
RESULTS AND DISCUSSION
Uncatalyzed Oxidations
Effect of reactants on the rate. Linear plots of
log(a – x) versus time and constancy of k1 values at
different N-bromosuccinimide concentrations are
confirming unit dependence with respect to N-bro-
mosuccinimide (Table 1). Plots of log k1 versus log [S]
are linear with a slope of nearly 0.5 indicates a frac-
tional order dependence on substrate concentration.
Addition of sodium acetate results in increase in
kinetic rate. Evidently this means that free amine mol-
ecules are the reactive species. A decrease of rate of
reaction was observed with an addition of the reaction
product (succinimide) indicating that succinimide is
involved in a fast pre-equilibrium to the rate-deter-
mining step.
Effect of added salt. Potassium halides are added to
the reaction mixture and the reaction rates are deter-
mined (Table 1). Addition of potassium bromide did
not increase the kinetic rate as expected but rather
decreased the rate. By an addition of halide ions in
oxidation of aminoalcohols with chloramines-T, a
decrease in rate of reaction was observed in perchloric
acid medium [13], whereas, no effect of added halide
ions was observed in alkaline buffer medium [15, 17].
Finding in the present case seems to be at variance to
the earlier published work [18, 19], where in molecular
bromine formed on the addition of bromide to the sys-
tem was taken to be the more reactive species than
Table 1. Rate constants for uncatalyzed oxidation of mon-
oethanolamine by N-bromosuccinimide in acetic acid—
sodium acetate medium. Common conditions: [NBS] = 1.0 ×
10–3
M [Monoethanolamine] = 1.0 × 10–2
M, [AcONa] =
0.1 M, Temp. = 40°C, solvent = 20% AcOH : 80% H2O
(vol/vol)
Variant Conc. of oxidant, M k1 × 104
, min–1
Oxidant 0.5 × 10–3 17.36
1.0 × 10–3
17.65
2.0 × 10–3
16.74
4.0 × 10–3
16.48
Monoethanol-
amine
0.125 × 10–2
9.79
0.25 × 10–2
10.79
0.50 × 10–2 12.27
1.00 × 10–2 17.65
2.00 × 10–2 26.59
Sodium acetate 2.5 × 10–2
6.69
5.0 × 10–2
9.73
10.0 × 10–2 17.65
20.0 × 10–2
28.55
40.0 × 10–2
62.58
80.0 × 10–2 106.10
Added halide 0 17.65
0.02 M KBr 6.29
0.02 M KCl 11.67
Acetic acid com-
position
5 : 95 25.27
10 : 90 20.74
20 : 80 17.65
40 : 60 8.77
60 : 40 6.60
Os(VIII) 0.0 17.65
6.40 × 10–5
M 17.89
12.80 × 10–5
M 17.78
19.20 × 10–5M 17.91
25.60 × 10–5
M 17.98
Temperature 40 17.65
50 43.85
60 120.39
1762
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 9 2016
VENKATA NADH et al.
N-halo compounds and hence acceleration was
expected. The present result of stopping the rate may
be due to the following reasons:
1. There is no doubt that molecular bromine is
formed on the addition of higher concentration of
Br‒
ion.
2. The question to be decided is whether bromine
in the molecular form is always a powerful oxidant
whatever may be the substrate. The observed differen-
tial rate points to the fact that in the present reactions
molecular bromine is probably not a powerful oxidant
compared to N-halo compound or Br+
or HOBr the
likely species in this reaction.
This seems to be quite in order as seen from the
work of de la Mare and our earlier work where in it has
been observed that NBS or HOBr or H2OBr+ or Br+
are more reactive than molecular bromine in partially
aqueous systems with different substrates [20–22].
Hence in the present investigation as well extending
the argument to aliphatic amino acid, molecular bro-
mine, which is formed, is not effective compared to
the other reactive species. So, it is not surprising that
the addition of Br–
did not result in increase in rate. It
was observed that change in the concentration of
added mercuric acetate had negligible effect on the
rate. The function of added mercuric acetate is there-
fore only to fix up Br-
formed in the course of the reac-
tion as HgBr2 or .
Nature of the species in the present investigation. It
can be stated that the likely species are NBS or Br+ or
H2OBr+ or HOBr depending upon the experimental
conditions employed. It is difficult to distinguish
between Br+
and H2OBr+
as the energy barrier
between these two is much smaller. It is evident that
Br+
formation requires that Br+
should be in triplet
state. The conversion of singlet to triplet in 4p electron
shell of bromine is much easier compared to 3p elec-
tron shell of chlorine [19]. The possibility of the spe-
cies Br+
or H2OBr+
is generally in acidic conditions.
But in the present study most of the experimentation
has been done in acetic acid - sodium acetate mixtures
where in the acidity is not very large due to pH being
3.5 to 4.8. This finally settles the probable likely spe-
cies as NBS and not Br+
or H2OBr+
in this investiga-
tion.
Effect of variation of solvent composition. In order
to determine the effect of dielectric constant (polarity)
of the medium on rate, the oxidation of monoethanol-
amine by NBS was studied in aqueous acetic acid mix-
tures of various compositions (Table 1). The rate of the
reaction decreased with increase in the percentage of
acetic acid in the mixture. In other words, decrease in
the dielectric constant of the medium decreases the
rate of reaction. This indicates that there is a charge
development in the transition state involving a more
polar activated complex than the reactants [23, 24].
2
4HgBr −
Amis showed that in a straight line plot of logarithm
kobs versus 1/D, a positive slope indicates a positive
ion–dipole reaction, while a negative slope indicates
the involvement of two dipoles or a negative ion-
dipole reaction [25]. In this investigation a plot of log-
arithm kobs versus 1/D, give straight lines with negative
slopes; these results clearly support the involvement of
two dipoles in the rate determining step.
For the dipole-dipole type of reaction, Laidler and
Eyring treatment [26] can be applied. Laidler and
Eyring derived an expression for the free energy trans-
fer of a polar molecule with a dipole moment from a
vacuum to a medium of dielectric constant D. This
equation is in the following form for a molecule of
radius r which has symmetrical charge distribution.
Introducing a non electrostatic term, this becomes
For the reaction under consideration, the equation for
specific velocity constant k will be
A plot of logk versus will be linear if non-elec-
trostatic terms are negligibly small. In the present
investigation the plots of logk1 versus are also
linear confirming dipole—dipole nature of reaction.
The two neutral molecules, which are participating in
the reaction, are NBS and amine.
Test for free radicals. To test for free radicals, the
reaction mixture containing stabilizer free acryloni-
trile was kept for 24 h in an inert atmosphere [27]. On
diluting the reaction mixture by methanol and no pre-
cipitate was observed. It is indicating that there is no
intervention of free radicals in the reaction.
Differential protonation in ethylamine, benzylamine,
and monoethanolamine in acid medium. It is well
known that the primary amines get protonated in
acidic media, hence protonation is the factor of
utmost importance in amine derivatives. The free
amine molecules are the reactive species as there is
fractional dependence of acidity in the oxidation of
ethylamine and benzylamine by N-bromosuccinimide
in aqueous acetic acid—perchloric acid medium [22].
For monoethanolamine the presence of hydroxyl
group makes the protonation difficult at the amine end
due to the electron with drawing nature of hydroxy
group. Hence it appears that the acidity dependence
has to be traced to the protonation of NBS predomi-
μ
3
1ln .
2 1
DF kT
Dr
μ −⎡ ⎤= β = −
⎢ ⎥⎣ ⎦+
3
1ln .
2 1
DF kT
Dr
μ −⎡ ⎤= β = − + φ
⎢ ⎥⎣ ⎦+
0
22 2
3 3 3
1 1ln ln
2 1
* *
.mA B A B M
A B m
kT Dk x k
h kT D
kTr r r
−⎡ ⎤ ⎡ ⎤= −
⎢ ⎥⎢ ⎥ ⎣ ⎦⎣ ⎦ +
⎡ ⎤μμ μ φ + φ − φ⎡ ⎤
× + + +⎢ ⎥ ⎢ ⎥⎣ ⎦⎣ ⎦
1
2 1
D
D
−
+
1
2 1
D
D
−
+
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 9 2016
KINETICS OF RUTHENIUM(III) CATALYZED 1763
nantly. In summary what is postulated is, in simple
amines protonation of the amine takes place to large
extent in preference to protonation of the NBS. In
monoethanolamine electron availability is reduced at
the nitrogen in amino group due to the presence of
hydroxyl group, so the protonation probably takes
place at the NBS in preference to the monoethanol-
amine. This is utilized in postulating a different mech-
anistic pathway for oxidation of monoethanolamine
compared to ethylamine and benzylamine by N-bro-
mosuccinimide though oxidation of all these sub-
strates were carried out in the same set of experimental
conditions.
Comparison of rates of ethylamine and monoetha-
nolamine. The kinetic rates are higher for oxidation of
monoethanolamine compared to earlier work of ethyl-
amine oxidation under similar conditions [22]. Strictly
speaking a discussion of structural reactivity is not
very ideal as the sites of oxidation in these compounds
are different. In the former it is the alcoholic group
that is attacked where as amine group is oxidized in the
latter. But a broad comparison is relevant as alcohol
oxidations are faster than amine oxidations in aliphatic
series.
Effect of osmium(VIII). Osmium(VIII) catalyst has
no effect on the rate of oxidation of monoethanol-
amine by N-bromosuccinimide in aqueous acetic
acid—sodium acetate buffer medium (Table 1).
Rate and activation parameters. The effect of tem-
perature on the rate of the reaction was studied in the
range (313–333 K) and the results were shown in
Table 2. From Arrhenius plot, the value of energy of
activation ( ), enthalpy ( ), entropy ( ),
and free energy ( ) were computed. Large negative
value of entropy ( ) are observed, which can be
attributed to the severe restriction of solvent molecules
(electrostriction) around the transition state [28]. It
also indicates that the complex is more ordered than
the reactants [29]. The observed modest activation
energy and sizeable entropy of activation supports a
complex transition state in the reaction.
Product analysis. In alkaline buffer medium, form-
aldehyde, formic acid and ammonia were the reported
products in the oxidation of monoethanolamines by
sodium N-bromobenzenesulfonamide [17], sodium
N-chloro-p-toluenesulphonamide [15]. In the present
case, non-formation of formic acid and ammonia was
confirmed from negative tests for chromotropic acid
procedure [30] and Nessler’s reagent test [31], respec-
≠
ΔE ≠
ΔH ≠
−ΔS
≠
ΔG
≠
−ΔS
tively. Formation of 2-aminoethanol as product was
confirmed from the pmr peaks at δ 1.5, 3.7, and 9.7,
respectively corresponding to protons of amine, meth-
ylene and aldehyde groups.
Mechanism of uncatalyzed reaction:
NH2 CH2 CH2 OH + NBS Complex C,
Complex C NH2CH2CHO + H+ + Br–
+ Succinimide.
Rate law:
Rate = k[Complex]
= kK[Substrate] [NBS] → (1),
[NBS]T = [NBS] + [Complex]
= [NBS] + K[NBS] [S] = [NBS] {1 + K[S]} → (2)
from (1) and (2),
rate = → (3).
The above rate law explains the first order in oxi-
dant and fractional order in substrate.
Ru(III) Catalyzed Reactions
Ruthenium(III) chloride accelerates the oxidation
of monoethanolamine by NBS in aqueous acetic
acid—perchloric acid medium and the kinetic features
are as follows. Plots of log(a – x) versus time for the
disappearance of NBS are linear indicating first order
in NBS and increase in concentration of NBS yields
fairly constant first order rate constants confirming
the first order dependence on NBS (Table 3). Increase
in concentration of ethanolamine increased the
kinetic rate and plot of log k1 versus log [S] are linear
with a slope nearly 0.33 indicating fractional order on
substrate concentration. Increase of concentration of
Ru(III) has no effect on the reaction kinetics in the
range of [Ru(III)] studied indicating apparent zero
order on the catalyst Ru(III). The effect of acid con-
centration on the reaction kinetics has been studied
and the first order rate constants are recorded in Table 3.
The rate constants decrease with increase in concen-
tration of perchloric acid. A linear plot with a slope
‒1.09 indicates the inverse unit dependence on [H+
].
As the reaction rate decreased with increase in H+
,
probably the free substrate molecules are the predom-
inant species. The reactions have been carried out at
various solvent compositions to find the effect of
K
⎯→←⎯
k⎯→
+
T[NBS] [S]
1 [S]
kK
K
Table 2. Arrhenius parameters at 313 K for uncatalyzed oxidation of monoethanolamine by N-bromosuccinimide in acetic
acid—sodium acetate medium
Substrate , kJ mol–1
, kJ mol–1
, J K–1
mol–1
, kJ mol–1
Monoethanolamine 81.4 78.8 9.1 79.9 103.8
≠
ΔE ≠
ΔH 10log PZ ≠
−ΔS
≠
ΔG
1764
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 9 2016
VENKATA NADH et al.
change in dielectric constant. Increase in acetic acid
percentage decreases the rate, showing that decrease
in dielectric constant decreases the reaction rate indi-
cating that dipole-dipole reactions as the major reac-
tion in between free substrate and free NBS molecules.
The reactions are carried out at four different tempera-
tures 40, 50, and 60°C and reaction rate constants are
recorded in. Plot of logk1 versus 1/T is linear. Arrhe-
nius parameters are calculated (Table 4).
Nature of Ru(III) species. Electronic spectrum of
Ru(III) chloride shows that probably it exists as
[Ru(H2O)6]3+
under acid conditions [18, 32, 33] while
the species like [RuCl5.H2O]2–
, [RuCl(H2O)5]2+
[RuCl4(H2O)2]1–
, [RuCl3(H2O)3], [RuCl2(H2O)4]+1
,
[RuCl(H2O)5]2+
, and [Ru(H2O)6]3+
are present in
aqueous solutions. The most active species is
[Ru(H2O)6]3+
, in acid medium which is formed due to
the following equilibrium:
[Ru (H2O)5OH]2+
+ H+
[Ru(H2O)6]3+
.
Mechanism and rate law for the Ru(III) catalyzed
oxidation of monoethanolamine by N-bromosuccinim-
ide. The sequence of reactions is given below:
NBS + H+
NBSH+
,
NBS + Ru(III) Ru(V) + Succinimide,
Ru(V) + S complex C,
C Products,
Rate = kC =kK2 [Ru(V)][S]
= kK1K2 [NBS] [Ru(III)] [S],
[NBS]T = [NBS] + [NBSH+
] + [Ru(V)] + [C]
= [NBS] +K[H+
] [NBS] + K1[NBS] [Ru(III)]
+ K1K2[S][NBS][Ru(III)]
= [NBS] {1 + K[H+] + K1[Ru(III)] (1 + K2[S])},
Rate =
The above rate law explains the first order depen-
dence on NBS, fractional order in substrate, inverse
unit order in [H+
] and apparent zero order in Ru(III).
CONCLUSIONS
In summary, in the oxidation of monoethanol-
amine by NBS in aqueous acetic acid medium the fol-
lowing points can be delineated.
K⎯⎯→←⎯⎯
1K
⎯⎯→←⎯⎯
2K
⎯⎯→←⎯⎯
k⎯→
+
+ + +
1 2 T
1 2
[NBS] [Ru(III)][S]
.
1 [H ] [Ru(III)](1 [S])
kK K
K K K
Table 3. Rate constants for Ru(III) catalyzed oxidation of
monoethanolamine by N-bromosuccinimide in acetic
acid—perchloric acid medium. Common conditions:
[NBS] = 1.0 × 10–3
M [monoethanolamine] = 10.0 ×
10‒3 M, [Hg(OAc)2] = 2.0 × 10–2 M, [H+] = 1.0 M, solvent =
20% AcOH : 80% H2O (vol/vol) [Ru(III)] = 3.8 × 10–5
M,
Temp. = 40°C
Variant [Variant], M
k1 × 104,
min–1
[NBS] 0.5 × 10–3
290.93
1.0 × 10–3
324.95
2.0 × 10–3
307.84
4.0 × 10–3
307.91
[Monoethanolamine] 0.625 × 10–3
113.87
1.25 × 10–3
166.97
2.5 × 10–3 211.60
5.0 × 10–3 273.62
10.0 × 10–3 324.95
20.0 × 10–3 450.82
40.0 × 10–3 555.85
[Ru(III)] 1.9 × 10–5 353.01
3.8 × 10–5 324.95
7.6 × 10–5 331.90
15.2 × 10–5 348.68
[H+] 0.25 390.40
0.5 344.79
1.0 324.95
2.0 134.30
4.0 78.57
Solvent composition
AcOH : H2O (vol %/vol %)
5 : 95 (vol/vol) 522.20
10 : 90 (vol/vol) 408.73
20 : 80 (vol/vol) 324.95
40 : 60 (vol/vol) 238.98
60 : 40 (vol/vol) 80.76
Temperature 40°C 324.35
50°C 619.36
60°C 1253.87
Table 4. Arrhenius parameters at 313 K for Ru(III) cata-
lyzed oxidation of monoethanolamine by N-bromosuccin-
imide in acetic acid—perchloric acid medium
,
kJ mol–1
,
kJ mol–1
,
J K–1
mol–1
,
kJ mol–1
57.4 54.8 7.3 114.1 91.7
≠
ΔE ≠
ΔH
10log PZ
≠
−ΔS G ≠
Δ
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 9 2016
KINETICS OF RUTHENIUM(III) CATALYZED 1765
(a) Monoethanolaine can be classified as sub-
strates which do not show any catalysis with Os(VIII).
The reason obviously is the absence of complex for-
mation directly between Os(VIII) with alanine.
(b) The presence of catalysis with Ru(III) and
absence of catalysis with Os(VIII) in NBS oxidations
has to be traced to different factors. The factor other
than complexation obviously has to be a more power-
ful oxidant species like Ru(V) which accelerates the
NBS oxidations.
Hence, Ru(V) participation is responsible for the
catalysis observed in the compound like monoetha-
nolamine in NBS oxidations.
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Kinetics of Ruthenium(III) Catalyzed and Uncatalyzed Oxidation of Monoethanolamine by N-Bromosuccinimide1

  • 1. 1760 ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2016, Vol. 90, No. 9, pp. 1760–1765. © Pleiades Publishing, Ltd., 2016. Kinetics of Ruthenium(III) Catalyzed and Uncatalyzed Oxidation of Monoethanolamine by N-Bromosuccinimide1 R. Venkata Nadha,*, B. Syama Sundarb, and P. S. Radhakrishnamurtic aGITAM University—Bengaluru Campus, Karnataka 561203, India b Yogi Vemana University, Kadapa 516216, India c Department of Chemistry, TJPS (PG) College, Guntur-522006, India *e-mail: doctornadh@yahoo.co.in Received April 14, 2015 Abstract—Kinetics of uncatalyzed and ruthenium(III) catalyzed oxidation of monoethanolamine by N-bro- mosuccinimide (NBS) has been studied in an aqueous acetic acid medium in the presence of sodium acetate and perchloric acid, respectively. In the uncatalyzed oxidation the kinetic orders are: the first order in NBS, a fractional order in the substrate. The rate of the reaction increased with an increase in the sodium acetate concentration and decreased with an increase in the perchloric acid concentration. This indicates that free amine molecules are the reactive species. Addition of halide ions results in a decrease in the kinetic rate, which is noteworthy. Both in absence and presence of a catalyst, a decrease in the dielectric constant of the medium decreases the kinetic rate pointing out that these are dipole—dipole reactions. A relatively higher oxidation state of ruthenium i.e., Ru(V) was found to be the active species in Ru(III) catalyzed reactions. A suitable mechanism consistent with the observations has been proposed and a rate law has been derived to explain the kinetic orders. Keywords: kinetics, oxidation, ruthenium(III), catalysis, monoethanolamine, N-bromosuccinimide DOI: 10.1134/S0036024416090296 INTRODUCTION Plants, animals, and vegetation emit volatile amines. Naturally aminoalcohols are produced by biodegradation amino acids [1]. Monoethanolamine (MEA) finds extensive applications in the synthesis of surfactants, pharmaceuticals, and as addition agents in the metal finishing industry [2]. Compared to other amines, MEA is cheap, has a high absorption capacity and reacts quickly with gaseous CO2 [3, 4]. Hence, MEA is the benchmark solvent in carbon capture and storage (CCS), a technology aimed at reducing CO2 emissions in large combustion industries [5]. To accu- rately assess the environmental impact of CCS, a bet- ter knowledge on the reactions of MEA is necessary. Considerable attention has been centered on the chemistry of N-halomines, because of their versatility in behaving as mild oxidants, halogenating agents, and N-anions, which act as both bases and nucleophiles. As a result these reagents react with a wide range of functional groups [6–9]. Literature collection shows that kinetics of oxida- tion of amino alcohols was studied using different oxidants like vanadium(V) as an oxidant in aqueous perchloric acid medium [10], potassium diperiodato- cupate(III) in alkaline medium [11], N-bromosuc- cinimide in alkaline medium in absence and in pres- ence of non-ionic micellar aggregates [12]. Similarly, kinetics of oxidation of aminoalcohols by chloramine-T was studied in perchloric acid medium in presence of palladous chloride as catalyst [13] and in alkaline medium [14, 15]. The above literature survey shows that various oxi- dizing agents have studied oxidations of amino acids but catalyzed oxidations using transition metals, as catalysts have not been studied in detail by using co- oxidants like N-halo compounds. The present investi- gation has been under taken using N-bromosuccinim- ide (NBS) as an oxidant to clarify whether oxidations of monoethanolamine are catalyzed by transition metal catalysts like Ru(III) and Os(VIII) or whether the catalysis is selective to transition metal catalysts like Ru(III). Hence a systematic investigation has been done in acetic acid—perchloric acid mixtures to throw light on the nature of reaction orders and mechanistic sequences. EXPERIMENTAL The reagents employed were monoethanolamine (Loba sample), N-bromo succinimide (GR, S.Merck Grade), ruthenium(III) chloride (Johnson Mathey,1 The article is published in the original. CHEMICAL KINETICS AND CATALYSIS
  • 2. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 9 2016 KINETICS OF RUTHENIUM(III) CATALYZED 1761 London), osmium tetroxide (Johnson and Mathey Co., London), mercuric acetate (E.Merck G.R.), other reagents used were of analytical grade reagents. All the solutions were prepared by doubly distilled water. Stock solutions of N-bromosuccinimide were prepared in pure acetic acid and standardized iodo- metrically. Kinetic studies were carried out in a perchloric acid medium under pseudo first order conditions with a large excess of monoethanolamine over NBS. The progress of reaction was followed by determining N-bromosuccinimide concentrations iodometrically in aliquots withdrawn after suitable time intervals [16]. Compared to the rates of present reaction, self decom- position rates of NBS were negligibly smaller under the conditions employed. The rate constants remained practically unaltered in air or in a de-aerated atmo- sphere. Only a representative set of the average values of kinetic data in aqueous acetic acid medium was pre- sented here. First order rate constants were evaluated from the linear plots (r2 > 0.993) of log [unreacted DCICA] against time. Rate constants and other deter- mined values were reproducible to ±2%. RESULTS AND DISCUSSION Uncatalyzed Oxidations Effect of reactants on the rate. Linear plots of log(a – x) versus time and constancy of k1 values at different N-bromosuccinimide concentrations are confirming unit dependence with respect to N-bro- mosuccinimide (Table 1). Plots of log k1 versus log [S] are linear with a slope of nearly 0.5 indicates a frac- tional order dependence on substrate concentration. Addition of sodium acetate results in increase in kinetic rate. Evidently this means that free amine mol- ecules are the reactive species. A decrease of rate of reaction was observed with an addition of the reaction product (succinimide) indicating that succinimide is involved in a fast pre-equilibrium to the rate-deter- mining step. Effect of added salt. Potassium halides are added to the reaction mixture and the reaction rates are deter- mined (Table 1). Addition of potassium bromide did not increase the kinetic rate as expected but rather decreased the rate. By an addition of halide ions in oxidation of aminoalcohols with chloramines-T, a decrease in rate of reaction was observed in perchloric acid medium [13], whereas, no effect of added halide ions was observed in alkaline buffer medium [15, 17]. Finding in the present case seems to be at variance to the earlier published work [18, 19], where in molecular bromine formed on the addition of bromide to the sys- tem was taken to be the more reactive species than Table 1. Rate constants for uncatalyzed oxidation of mon- oethanolamine by N-bromosuccinimide in acetic acid— sodium acetate medium. Common conditions: [NBS] = 1.0 × 10–3 M [Monoethanolamine] = 1.0 × 10–2 M, [AcONa] = 0.1 M, Temp. = 40°C, solvent = 20% AcOH : 80% H2O (vol/vol) Variant Conc. of oxidant, M k1 × 104 , min–1 Oxidant 0.5 × 10–3 17.36 1.0 × 10–3 17.65 2.0 × 10–3 16.74 4.0 × 10–3 16.48 Monoethanol- amine 0.125 × 10–2 9.79 0.25 × 10–2 10.79 0.50 × 10–2 12.27 1.00 × 10–2 17.65 2.00 × 10–2 26.59 Sodium acetate 2.5 × 10–2 6.69 5.0 × 10–2 9.73 10.0 × 10–2 17.65 20.0 × 10–2 28.55 40.0 × 10–2 62.58 80.0 × 10–2 106.10 Added halide 0 17.65 0.02 M KBr 6.29 0.02 M KCl 11.67 Acetic acid com- position 5 : 95 25.27 10 : 90 20.74 20 : 80 17.65 40 : 60 8.77 60 : 40 6.60 Os(VIII) 0.0 17.65 6.40 × 10–5 M 17.89 12.80 × 10–5 M 17.78 19.20 × 10–5M 17.91 25.60 × 10–5 M 17.98 Temperature 40 17.65 50 43.85 60 120.39
  • 3. 1762 RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 9 2016 VENKATA NADH et al. N-halo compounds and hence acceleration was expected. The present result of stopping the rate may be due to the following reasons: 1. There is no doubt that molecular bromine is formed on the addition of higher concentration of Br‒ ion. 2. The question to be decided is whether bromine in the molecular form is always a powerful oxidant whatever may be the substrate. The observed differen- tial rate points to the fact that in the present reactions molecular bromine is probably not a powerful oxidant compared to N-halo compound or Br+ or HOBr the likely species in this reaction. This seems to be quite in order as seen from the work of de la Mare and our earlier work where in it has been observed that NBS or HOBr or H2OBr+ or Br+ are more reactive than molecular bromine in partially aqueous systems with different substrates [20–22]. Hence in the present investigation as well extending the argument to aliphatic amino acid, molecular bro- mine, which is formed, is not effective compared to the other reactive species. So, it is not surprising that the addition of Br– did not result in increase in rate. It was observed that change in the concentration of added mercuric acetate had negligible effect on the rate. The function of added mercuric acetate is there- fore only to fix up Br- formed in the course of the reac- tion as HgBr2 or . Nature of the species in the present investigation. It can be stated that the likely species are NBS or Br+ or H2OBr+ or HOBr depending upon the experimental conditions employed. It is difficult to distinguish between Br+ and H2OBr+ as the energy barrier between these two is much smaller. It is evident that Br+ formation requires that Br+ should be in triplet state. The conversion of singlet to triplet in 4p electron shell of bromine is much easier compared to 3p elec- tron shell of chlorine [19]. The possibility of the spe- cies Br+ or H2OBr+ is generally in acidic conditions. But in the present study most of the experimentation has been done in acetic acid - sodium acetate mixtures where in the acidity is not very large due to pH being 3.5 to 4.8. This finally settles the probable likely spe- cies as NBS and not Br+ or H2OBr+ in this investiga- tion. Effect of variation of solvent composition. In order to determine the effect of dielectric constant (polarity) of the medium on rate, the oxidation of monoethanol- amine by NBS was studied in aqueous acetic acid mix- tures of various compositions (Table 1). The rate of the reaction decreased with increase in the percentage of acetic acid in the mixture. In other words, decrease in the dielectric constant of the medium decreases the rate of reaction. This indicates that there is a charge development in the transition state involving a more polar activated complex than the reactants [23, 24]. 2 4HgBr − Amis showed that in a straight line plot of logarithm kobs versus 1/D, a positive slope indicates a positive ion–dipole reaction, while a negative slope indicates the involvement of two dipoles or a negative ion- dipole reaction [25]. In this investigation a plot of log- arithm kobs versus 1/D, give straight lines with negative slopes; these results clearly support the involvement of two dipoles in the rate determining step. For the dipole-dipole type of reaction, Laidler and Eyring treatment [26] can be applied. Laidler and Eyring derived an expression for the free energy trans- fer of a polar molecule with a dipole moment from a vacuum to a medium of dielectric constant D. This equation is in the following form for a molecule of radius r which has symmetrical charge distribution. Introducing a non electrostatic term, this becomes For the reaction under consideration, the equation for specific velocity constant k will be A plot of logk versus will be linear if non-elec- trostatic terms are negligibly small. In the present investigation the plots of logk1 versus are also linear confirming dipole—dipole nature of reaction. The two neutral molecules, which are participating in the reaction, are NBS and amine. Test for free radicals. To test for free radicals, the reaction mixture containing stabilizer free acryloni- trile was kept for 24 h in an inert atmosphere [27]. On diluting the reaction mixture by methanol and no pre- cipitate was observed. It is indicating that there is no intervention of free radicals in the reaction. Differential protonation in ethylamine, benzylamine, and monoethanolamine in acid medium. It is well known that the primary amines get protonated in acidic media, hence protonation is the factor of utmost importance in amine derivatives. The free amine molecules are the reactive species as there is fractional dependence of acidity in the oxidation of ethylamine and benzylamine by N-bromosuccinimide in aqueous acetic acid—perchloric acid medium [22]. For monoethanolamine the presence of hydroxyl group makes the protonation difficult at the amine end due to the electron with drawing nature of hydroxy group. Hence it appears that the acidity dependence has to be traced to the protonation of NBS predomi- μ 3 1ln . 2 1 DF kT Dr μ −⎡ ⎤= β = − ⎢ ⎥⎣ ⎦+ 3 1ln . 2 1 DF kT Dr μ −⎡ ⎤= β = − + φ ⎢ ⎥⎣ ⎦+ 0 22 2 3 3 3 1 1ln ln 2 1 * * .mA B A B M A B m kT Dk x k h kT D kTr r r −⎡ ⎤ ⎡ ⎤= − ⎢ ⎥⎢ ⎥ ⎣ ⎦⎣ ⎦ + ⎡ ⎤μμ μ φ + φ − φ⎡ ⎤ × + + +⎢ ⎥ ⎢ ⎥⎣ ⎦⎣ ⎦ 1 2 1 D D − + 1 2 1 D D − +
  • 4. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 9 2016 KINETICS OF RUTHENIUM(III) CATALYZED 1763 nantly. In summary what is postulated is, in simple amines protonation of the amine takes place to large extent in preference to protonation of the NBS. In monoethanolamine electron availability is reduced at the nitrogen in amino group due to the presence of hydroxyl group, so the protonation probably takes place at the NBS in preference to the monoethanol- amine. This is utilized in postulating a different mech- anistic pathway for oxidation of monoethanolamine compared to ethylamine and benzylamine by N-bro- mosuccinimide though oxidation of all these sub- strates were carried out in the same set of experimental conditions. Comparison of rates of ethylamine and monoetha- nolamine. The kinetic rates are higher for oxidation of monoethanolamine compared to earlier work of ethyl- amine oxidation under similar conditions [22]. Strictly speaking a discussion of structural reactivity is not very ideal as the sites of oxidation in these compounds are different. In the former it is the alcoholic group that is attacked where as amine group is oxidized in the latter. But a broad comparison is relevant as alcohol oxidations are faster than amine oxidations in aliphatic series. Effect of osmium(VIII). Osmium(VIII) catalyst has no effect on the rate of oxidation of monoethanol- amine by N-bromosuccinimide in aqueous acetic acid—sodium acetate buffer medium (Table 1). Rate and activation parameters. The effect of tem- perature on the rate of the reaction was studied in the range (313–333 K) and the results were shown in Table 2. From Arrhenius plot, the value of energy of activation ( ), enthalpy ( ), entropy ( ), and free energy ( ) were computed. Large negative value of entropy ( ) are observed, which can be attributed to the severe restriction of solvent molecules (electrostriction) around the transition state [28]. It also indicates that the complex is more ordered than the reactants [29]. The observed modest activation energy and sizeable entropy of activation supports a complex transition state in the reaction. Product analysis. In alkaline buffer medium, form- aldehyde, formic acid and ammonia were the reported products in the oxidation of monoethanolamines by sodium N-bromobenzenesulfonamide [17], sodium N-chloro-p-toluenesulphonamide [15]. In the present case, non-formation of formic acid and ammonia was confirmed from negative tests for chromotropic acid procedure [30] and Nessler’s reagent test [31], respec- ≠ ΔE ≠ ΔH ≠ −ΔS ≠ ΔG ≠ −ΔS tively. Formation of 2-aminoethanol as product was confirmed from the pmr peaks at δ 1.5, 3.7, and 9.7, respectively corresponding to protons of amine, meth- ylene and aldehyde groups. Mechanism of uncatalyzed reaction: NH2 CH2 CH2 OH + NBS Complex C, Complex C NH2CH2CHO + H+ + Br– + Succinimide. Rate law: Rate = k[Complex] = kK[Substrate] [NBS] → (1), [NBS]T = [NBS] + [Complex] = [NBS] + K[NBS] [S] = [NBS] {1 + K[S]} → (2) from (1) and (2), rate = → (3). The above rate law explains the first order in oxi- dant and fractional order in substrate. Ru(III) Catalyzed Reactions Ruthenium(III) chloride accelerates the oxidation of monoethanolamine by NBS in aqueous acetic acid—perchloric acid medium and the kinetic features are as follows. Plots of log(a – x) versus time for the disappearance of NBS are linear indicating first order in NBS and increase in concentration of NBS yields fairly constant first order rate constants confirming the first order dependence on NBS (Table 3). Increase in concentration of ethanolamine increased the kinetic rate and plot of log k1 versus log [S] are linear with a slope nearly 0.33 indicating fractional order on substrate concentration. Increase of concentration of Ru(III) has no effect on the reaction kinetics in the range of [Ru(III)] studied indicating apparent zero order on the catalyst Ru(III). The effect of acid con- centration on the reaction kinetics has been studied and the first order rate constants are recorded in Table 3. The rate constants decrease with increase in concen- tration of perchloric acid. A linear plot with a slope ‒1.09 indicates the inverse unit dependence on [H+ ]. As the reaction rate decreased with increase in H+ , probably the free substrate molecules are the predom- inant species. The reactions have been carried out at various solvent compositions to find the effect of K ⎯→←⎯ k⎯→ + T[NBS] [S] 1 [S] kK K Table 2. Arrhenius parameters at 313 K for uncatalyzed oxidation of monoethanolamine by N-bromosuccinimide in acetic acid—sodium acetate medium Substrate , kJ mol–1 , kJ mol–1 , J K–1 mol–1 , kJ mol–1 Monoethanolamine 81.4 78.8 9.1 79.9 103.8 ≠ ΔE ≠ ΔH 10log PZ ≠ −ΔS ≠ ΔG
  • 5. 1764 RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 9 2016 VENKATA NADH et al. change in dielectric constant. Increase in acetic acid percentage decreases the rate, showing that decrease in dielectric constant decreases the reaction rate indi- cating that dipole-dipole reactions as the major reac- tion in between free substrate and free NBS molecules. The reactions are carried out at four different tempera- tures 40, 50, and 60°C and reaction rate constants are recorded in. Plot of logk1 versus 1/T is linear. Arrhe- nius parameters are calculated (Table 4). Nature of Ru(III) species. Electronic spectrum of Ru(III) chloride shows that probably it exists as [Ru(H2O)6]3+ under acid conditions [18, 32, 33] while the species like [RuCl5.H2O]2– , [RuCl(H2O)5]2+ [RuCl4(H2O)2]1– , [RuCl3(H2O)3], [RuCl2(H2O)4]+1 , [RuCl(H2O)5]2+ , and [Ru(H2O)6]3+ are present in aqueous solutions. The most active species is [Ru(H2O)6]3+ , in acid medium which is formed due to the following equilibrium: [Ru (H2O)5OH]2+ + H+ [Ru(H2O)6]3+ . Mechanism and rate law for the Ru(III) catalyzed oxidation of monoethanolamine by N-bromosuccinim- ide. The sequence of reactions is given below: NBS + H+ NBSH+ , NBS + Ru(III) Ru(V) + Succinimide, Ru(V) + S complex C, C Products, Rate = kC =kK2 [Ru(V)][S] = kK1K2 [NBS] [Ru(III)] [S], [NBS]T = [NBS] + [NBSH+ ] + [Ru(V)] + [C] = [NBS] +K[H+ ] [NBS] + K1[NBS] [Ru(III)] + K1K2[S][NBS][Ru(III)] = [NBS] {1 + K[H+] + K1[Ru(III)] (1 + K2[S])}, Rate = The above rate law explains the first order depen- dence on NBS, fractional order in substrate, inverse unit order in [H+ ] and apparent zero order in Ru(III). CONCLUSIONS In summary, in the oxidation of monoethanol- amine by NBS in aqueous acetic acid medium the fol- lowing points can be delineated. K⎯⎯→←⎯⎯ 1K ⎯⎯→←⎯⎯ 2K ⎯⎯→←⎯⎯ k⎯→ + + + + 1 2 T 1 2 [NBS] [Ru(III)][S] . 1 [H ] [Ru(III)](1 [S]) kK K K K K Table 3. Rate constants for Ru(III) catalyzed oxidation of monoethanolamine by N-bromosuccinimide in acetic acid—perchloric acid medium. Common conditions: [NBS] = 1.0 × 10–3 M [monoethanolamine] = 10.0 × 10‒3 M, [Hg(OAc)2] = 2.0 × 10–2 M, [H+] = 1.0 M, solvent = 20% AcOH : 80% H2O (vol/vol) [Ru(III)] = 3.8 × 10–5 M, Temp. = 40°C Variant [Variant], M k1 × 104, min–1 [NBS] 0.5 × 10–3 290.93 1.0 × 10–3 324.95 2.0 × 10–3 307.84 4.0 × 10–3 307.91 [Monoethanolamine] 0.625 × 10–3 113.87 1.25 × 10–3 166.97 2.5 × 10–3 211.60 5.0 × 10–3 273.62 10.0 × 10–3 324.95 20.0 × 10–3 450.82 40.0 × 10–3 555.85 [Ru(III)] 1.9 × 10–5 353.01 3.8 × 10–5 324.95 7.6 × 10–5 331.90 15.2 × 10–5 348.68 [H+] 0.25 390.40 0.5 344.79 1.0 324.95 2.0 134.30 4.0 78.57 Solvent composition AcOH : H2O (vol %/vol %) 5 : 95 (vol/vol) 522.20 10 : 90 (vol/vol) 408.73 20 : 80 (vol/vol) 324.95 40 : 60 (vol/vol) 238.98 60 : 40 (vol/vol) 80.76 Temperature 40°C 324.35 50°C 619.36 60°C 1253.87 Table 4. Arrhenius parameters at 313 K for Ru(III) cata- lyzed oxidation of monoethanolamine by N-bromosuccin- imide in acetic acid—perchloric acid medium , kJ mol–1 , kJ mol–1 , J K–1 mol–1 , kJ mol–1 57.4 54.8 7.3 114.1 91.7 ≠ ΔE ≠ ΔH 10log PZ ≠ −ΔS G ≠ Δ
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