4130
Ind. Eng. Chem. Res. 1999, 38, 4130-4132
CORRESPONDENCE Comments on “Kinetic Study for Synthesizing Dibenzyl Phthalate via Solid-Liquid Phase-Transfer Catalysis” Daniel R. Pilipauskas† Searle/Monsanto, 4901 Searle Parkway, Skokie, Illinois 60077
Sir: In the recent article “Kinetic Study for Synthesizing Dibenzyl Phthalate via Solid-Liquid PhaseTransfer Catalysis,”1 the authors report the investigation of various tetraalkylammonium and phosphonium solid-liquid phase-transfer catalysts to promote the esterification of solid dipotassium phthalate and benzyl bromide in several organic solvents. The major focus of the report is the study of tetrabutylammonium bromide (TBAB) catalysis in chlorobenzene. The authors provide a rich collection of data and interpretations of the catalytic process, but have not provided a sufficiently complete interpretation of the data. Two aspects of this report require further discussion: (1) how TBAB facilitates the solid-liquid phase-transfer process and (2) the nature of the rate enhancement by TBAB. Two mechanisms have been proposed in the literature to explain how solid-liquid phase-transfer catalysis (SLPTC) facilitates the transfer of an anionic reactant from the solid phase to the reaction medium.2 In heterogeneous solubilization, the SLPTC salt Q+X- acts at the surface of the solid K+Y- to exchange the anion X- for Y-. The new catalyst-anion pair Q+Y- and the byproduct salt K+X- enter the reaction medium. After reaction with an electrophile (RX), the catalyst Q+X- is regenerated. This mechanism has been proposed for flat phase-transfer catalysts such as crown ethers. Alternatively, when catalysts are too bulky to approach the solid surface, a homogeneous solubilization mechanism must operate for catalysis to occur. In this mechanism the solid must have sufficient solubility in the reaction medium for the salt K+Y- to dissolve and undergo anion exchange with the catalyst Q+X-. The authors in the current study propose that the heterogeneous solubilization mechanism is operating in their system when they attribute the role of TBAB to “...solvate and react with some parts of the solid ArK2 to produce ArQ2 (Ar ) phthalate, Q ) TBA) at the film near the solid-liquid interface.” This is at odds with the notion that bulky catalysts, such as the tetraalkylammonium cation, are not able to approach the solid surface.2 The case for sufficient solubility in the homogeneous solubilization mechanism is supported by the authors’ observation that dipotassium phthalate can react to a small extent in the absence of catalysis. These two facts and previous studies of the SLPTC-mediated esterification of solid sodium acetate/benzoate3 and potassium acetate2 point toward a homogeneous solubilization mechanism. Reanalysis of the authors’ data requires modification of their reaction mechanism. †
E-mail:
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The authors use eq 1 to describe the reaction at the K1
ArK2(org) + 2QX(org) {\} ArQ2(org) + 2KX(org) (1) solid surface and eq 2 to describe the distribution of K2
K+X-(org) {\} KX(solid)
(2)
potassium bromide between solution and solid phases, where “org” denotes the stagnant boundary layer around the solid dipotassium phthalate (ArK2). QX refers to TBAB and ArQ2 is the di(tetrabutylammonium) phthalate. However, a description of a homogeneous solubilization mechanism requires the addition of eq 3 for the K3
ArK2(solid) {\} ArK2(org)
(3)
dissolution of dipotassium phthate.2,3 Adding eq 3 for the dissolution of dipotassium phthalate to eqs 1 and 2 (“org” now denotes the bulk solution) completes the homogeneous solubilization model for TBAB and dipotassium phthalate. The overall equilibrium expression for this process is given in eq 4. Alternatively, the 2
K1K2 K3 )
[ArQ2][KBr(solid)]2 [ArK2(solid)][QX]2
(4)
equilibrium expression can be expressed in terms of the solubility constants for ArK2 and KBr (eq 5), where Ks
K1KsArK2 2
)
KsKBr
[ArQ2] [QX]2
(5)
is the solubility constant of the solids. With eq 5 the potassium bromide effect on the concentration of ArQ2 can be explored. The authors observe that adding solid KBr to the reaction mixture has an effect on the steady-state concentration of ArQ2. For a small amount of potassium bromide (
