Vol. 26, No. 16 (2014)
Kinetics Studies of Ru(III) Catalyzed Oxidation of p-Hydroxy Benzoic Acid 5123
variation in its concentration range 5 × 10-2 to 9 × 10-2 mol dm-3.
A negative [Cl-] effect was observed. Addition of PTS did not
influence the rate. this is also evident from nearly constant
kobs values at different PTS. Variation of ionic strength (I) from
2 × 10-2 to 5.50 ×10-2 mol dm-3 shows negligible effect on rate
of reaction. The reaction was studied in 30-40 °C temperature
range and free energy of activation (∆G*) calculated from the
rate measurements at these temperatures are given in Table-3.
CH3
K1
Na
Cl
+
Ru(III)
R
N
- - - (i)
Ru(III)
SO2NClNa
Na
Cl
R
N
K2
Na
Cl
+ H+
- -(ii)
R
N
+ C6H4(OH)COOH
Ru—O=C—OH
C6H4(OH)
(S)
Ru(III)
(X)
k
X + H2O
CH3C6H4SO2NH2 + CO2 + Ru(III) +
C6H4(OH)2 + NaCl (iii)
The reaction scheme of above chemical reaction in which
TABLE-3
Slow
ACTIVATION PARAMETERS OF [Ru(III)] CATALYZED
OXIDATION OF[p-HBA] BY [RN NaCl] IN ACIDIC MEDIUM
UNDER THE CONDITION OF TABLES 1 AND 2 AT 35 °C
oxidant chloramine-T first interact with catalyst Ru(III) and
form complex (C1). Then complex (C1) react with p-hydroxy
benzoic acid to form intermediate (X) with the removal of H+.
Finally intermediate (X) with H2O give product and catalyst
Ru(III) regenerate. Step (iii) is slow and rate determining step.
In the light of aforesaid reaction scheme, the rate of reaction
may be written in terms of rate of loss of [Chloramine-T] as
eqn 1.
Parameters
Ea (kJ mol–1)
∆S# (JK–1 mol–1)
∆H# (kJ mol–1)
∆G# (kJ mol–1)
A × 10–13 (mol–1 dm3 s–1)
Values
73.68
–17.42
71.12
76.49
9.18
Chloramine-T behaves like a strong electrolyte25 and
ionizes in aqueous solutions as follows:
d[CAT]
−
= k[X]
(1)
dt
RN NaCl
RNCl- + Na+
From step (1), we have
where R = CH3C6H4SO2
Chloramine-T in acidic medium exists as following
equilibria-
2RN HCl
[C1]
K1 =
[CAT][Ru(III)]
From step (2), we have
(2)
RNCl2 + RNH2
RNHCl + HOCl
RNH2 + HOCl
(a)
(b)
(c)
RNCl2 + H2O
RNHCl + H2O
[X][H+ ]
[C1][S]
K2 =
Thus in the acidified solution of chloramine-T, the possible
oxidizing species are RNHCl, RNCl2, HOCl and CAT as such.
If RNCl2 were to be reactive species, the rate law would require
second order depedence on CAT, which is contrary to the
experimental observations. If HOCl was to be reactive species,
a first order retardation of the rate by added para-toluene
sulphonamide would be expected which is again contrary to
the observed zero effect.
K2[C1][S]
[H+ ]
[X] =
(3)
where S = p-hydroxy benzoic acid. By comparing eqns 2 and
3, we have
K1K2[CAT][Ru(III)][S]
[X] =
(4)
[H+ ]
On substituting the value of [X] from eqn 4 in eqn 1, we
have eqn 5.
Further if RNHCl was to be reactive species, then frac-
tional order with respect to (PTS) would be required which is
also contrary to observed zero effect of (PTS). Hence CAT itself
may be taken as real reactive species of chloramine-T.
Ruthenium(III) chloride is reported26,27 to exist as [RuCl6]3-
in acidic medium. Ruthenium(III) chloride exist as [Ru(H2O)6]3+
in dil. HCl28. Such type of aqueous complexes are known to
exist in the form of [Ru(H2O)5OH]2+ in equilibrium29 with hexa
aquo form.
d[CAT]
dt
kK1K2[CAT][Ru(III)][S]
+
[H ]
-
=
(5)
The rate expression eqn 5 fully explains all observed
kinetic data. It explains first order kinetics with respect to
oxidant, first order with respect to substrate, first order with
respect to catalyst and inverse first order kinetics with respect
to [H+] i.e. medium. Thus, the mechanism seems to be valid.
[Ru(H2O)6]3+
[Ru(H2O)5OH]2+ + H+
[RuCl6]3- which is in equilibrium with [Ru(H2O)5OH]2+
in the following manner30.
REFERENCES
1. H.S. Singh, A.K. Sisodia, S.M. Singh, R.K. Singh and R.N. Singh, J.
Chim. Phys. Phys.-Chim. Biol., 73, 283 (1976).
[RuCl6]3+ + H2O
[RuCl5.H2O]
2. Puttaswamy and N. Vaz, J. Chem. Sci.., 113, 325 (2001).
3. D.H. Bremner, in ed.: J.S. Pized, Synthetic Reagents, Ellish Horwood
Limited, England, vol. 6 (1985).
This equilibrium in right direction would be highly
favoured in acidic medium, hence [RuCl5.H2O]2+ has been
taken as real effective catalytic species in acidic medium.
Considering the reactive species of chloramine-T and
RuCl3 in aqueous acidic medium as well as the observed kinetic
results, the following reaction scheme is suggested.
4. S.P. Mushran, R.M. Mehrotra and R. Sanehi, J. Indian Chem. Soc., 51,
594 (1974).
5. A. Kumar and R.A. Singh, J. Indian Chem. Soc., 89, 1671 (2012).
6. A. Berka, J. Vulterin and J. Zyka, Newer Redox Titrants, Pergamon
Press, Elmsford, New York, vol. 1, p. 37 (1965).