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Xe ¼ equilibrium conversion, k1 ¼ second order rate reaction, V
¼ reactor volume, F ¼ molar ow rate, CA0 ¼ concentration of A
at the entrance of reactor, X ¼ conversion.
A plot of this kinetic equation using the experimental results
is presented in Fig. 5 (second order reversible kinetics). The
equilibrium conversion required was estimated by method
described elsewhere.35 However a more detailed kinetic inves-
tigation should be made before conclusive evidence could be
obtained. Such a kinetic investigation is under study in this
laboratory in order to determine the accurate rate equation of
this reaction.
Conclusions
Herein we demonstrated an efficient equilibrium conversion
method for halogen exchange (chloride to bromide) using
molten alkyl phosphonium catalyst without any additional
material usage in multiple cycles in a long continuous ow.
Alkyl phosphonium catalyst reactivity screened under various
optimization parameters viz; loading (2–20%), supported over
(silica or alumina) for halogen exchange reaction. The silica and
alumina based phase-transfer catalyst easily regenerated and
reused several times without loss of their activity. The products
of the exchange reactions could be simply separated by distil-
lation under normal pressure.
Fig. 5 Plot of variables for the second order reversible kinetics for the
halide exchange reaction (4), over 10% tetrabutyl phosphonium/Al2O3
at 148 ꢀC. Symbols are for different experiments.
Therefore from Table 2 it is very clear that support (SiO2 or
Al2O3) is not playing active role in halogen exchange reaction. A
possible pathway for the exchange reaction over the alkyl
phosphonium catalyst is summarized in the following scheme.
Conflicts of interest
RCl + QBr / RBr + QCl
R0Br + QCl / R0Cl + QBr
(1)
(2)
There are no conicts of interest in the present research work.
Acknowledgements
The overall catalytic cycle is summarized by the reaction;
RCl + R0Br / RBr + R0Cl
The authors warmly acknowledge Dr Leonardo Mendelovici for
his help to accomplish the present research work.
(3)
The proposed catalytic sequence is supported by the results
obtained in a series of experiments (Fig. 4) in which the alkyl
phosphonium bromide catalyst was reacted rst with the alkyl
chloride converting the alkyl phosphonium bromide into the
chloride and producing the respective alkyl bromide (reaction
(1)). The alkyl phosphonium chloride was then reacted with the
alkyl bromide according to reaction (2).
The reaction between 1,2 dichloro ethane (EDC) and 1,2
dibromoethane (EDB) was used to illustrate the proposed
mechanism because of its relative simplicity giving only one
possible reaction product according to the equation;
References
1 T. D. Sheppard, Org. Biomol. Chem., 2009, 7, 1043–1052.
2 Y. Mizukami, Z. Song and T. Takahashi, Org. Lett., 2015, 17,
5942–5945.
3 J. San Filippo, A. F. Sowinski and L. J. Romano, J. Org. Chem.,
1975, 40, 3295–3296.
4 M. Namavari, N. Satyamurthy, M. E. Phelps and J. R. Barrio,
Tetrahedron Lett., 1990, 31, 4973–4976.
5 C.-L. Sun and Z.-J. Shi, Chem. Rev., 2014, 114, 9219–9280.
6 R. Bloch, Chem. Rev., 1998, 98, 1407–1438.
7 C. Fischer and B. Koenig, Beilstein J. Org. Chem., 2011, 7, 59–
74.
8 T. Kihlberg and B. Langstrom, J. Org. Chem., 1999, 64, 9201–
9205.
9 R. Viswanathan, E. N. Prabhakaran, M. A. Plotkin and
J. N. Johnston, J. Am. Chem. Soc., 2003, 125, 163–168.
10 A. S. Guram, R. A. Rennels and S. L. Buchwald, Angew. Chem.,
Int. Ed. Engl., 1995, 34, 1348–1350.
BrC2H4Br + ClC2H4Cl / 2BrC2H4Cl
(4)
Additionally evidence supporting the proposed catalytic
cycle could be obtained from preliminary kinetic investigation
carried out in this laboratory. From the results obtained, the
reaction seems to follow a second order reversible kinetics,
according to the equation (Fig. 5);
˚
¨
ꢀ
ꢁ
Xeð1 ꢂ 2XÞ
Xe ꢂ X
1
2 V
F
ln
þ X ¼ 2k1
ꢂ 1 CA0
Xe
11 K. W. Anderson, T. Ikawa, R. E. Tundel and S. L. Buchwald, J.
Am. Chem. Soc., 2006, 128, 10694–10695.
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