Handbook of Solvents - George Wypych - ChemTech - Ventech!

830 Maw-Ling Wang
Solvent
Dielectric
constant (20 o C)
k×10 4 , Lmol -1 s -1
60 o C 70 o C 80 o C 90 o C 100 o C
N,N-Dimethylacetamide 37.8 0.29 1.06 4.29
N-Methylformamide 189.5 6.08 28.7 86.5
Data obtained from the work of Maccarone et al. 64
The BC-TP reaction shows better reactivity in protic or polar solvent since the activated
complex is more polar than both reactant molecules.
13.3.2 THREE-PHASE REACTIONS (TRIPHASE CATALYSIS)
As stated, the solid PTC is suitable for the industrial processes concerning the removal of
the catalyst from the reaction mixture and its economic recycle. The real mechanism of reaction
in a triphase catalysis is not completely understood. However, the reaction rate and
the conversion of reactant in a triphase catalysis (TC) is highly dependent on the
organiphilicity (hydrophilicity or hydroprobicity) of the polymer support of the catalyst and
the polarity of the organic solvent. Not only the partition of the organic to the aqueous solutions
is affected by the organophilicity of the polymer-supported catalyst, but also the concentration
distribution of the catalyst between two phases is influenced by the
organophilicity of the polymer-supported catalyst.
Ohtani et al. 82-86 used polystyrene-supported ammonium fluoride as a phase transfer
catalyst (triphase catalysis) for several base-catalyzed reactions, such as cyanoethylation,
Knoevenage reaction, Claisen condensation and Michael addition. The catalytic activity of
the polystyrene-supported ammonium fluid was comparable to that of tetrabutylammonium
fluoride (TBAF). The ionic loading and the ammonium structure of the fluoride polymers
hardly affected the catalytic efficiency. The reaction was fast in a non-polar solvent (e.g.,
octane or toluene) from which the rate-determining step of the base-catalyzed reaction is
2
very similar to that of the SN nucleophilic substitution reactions.
The solvent may affect the catalytic activity in several ways. The greater its swelling
power, the larger the volume fraction of catalytic occupied by the more mobile liquid, and
the swollen volume fraction of the more rigid polymer network. The degree of swelling and
the viscosity within the polymer matrix affect intraparticle diffusion rates. The solvent may
also affect intrinsic reactivity at the active sites. Experimentally, it is difficult to distinguish
solvent effect on diffusivity from solvent effects on reactivity. Tomoi and Ford 116 found that
the triphase catalysis followed pseudo-first-order rate law. The corresponding apparent rate
constant kapp decreases with solvent in the order: chlorobenzene > toluene > decane over
wide ranges of particle sizes and polymer crosslinking. The ability of the solvent to swell
the catalysts decreases in the same order.
13.3.2.1 The interaction between solid polymer (hydrophilicity) and the
organic solvents
In triphase catalysis, solvated resin supports are important carriers for solid-phase organic
synthesis in combinatorial chemistry. The physical properties of resin, resin swelling, dynamic
solvation, and solvated supports are important factors in affecting the synthesis. 160
However, these factors are also affected by solvent. Selective solvation of resin alters the local
reactivity and accessibility of the bound substrate and the mobility of the entrapped re-