Handbook of Solvents - George Wypych - ChemTech - Ventech!

804 Maw-Ling Wang
Table 13.3.2. Effect of organic solvents on
the distribution of (C 4H 9) 4N + Br - between
organic and aqueous phases
Solvent Extraction constant, EQBr*
C6H6 >10.0
C6H5Cl >10.0
o-C6H4Cl2 >10.0
n-C4H9Cl >10.0
Cl(CH2) 4Cl 3.33
Cl(CH2) 3Cl 0.34
Cl(CH2) 2Cl 0.16
ClCH2Cl 0.028
CHCl3 0.021
C2H5COC2H5 0.91
CH3COC2H5 0.071
n-C4H9OH 0.014
*E QBr=[Q + ] aq[Br - ] aq/[QBr] org
Data adopted from Brandstrom 4
phase may occur as the reaction proceeds.
This may have some effect on the reaction
rate of the organic-phase (may raise or
lower the value of the rate constant). The
changes in the polarity of the organic phase
may increase or decrease, causing almost
all catalyst cation-anion pairs to be partitioned
into the organic phase. This behavior
is evidenced in the cyanide displacement on
1-bromooctane catalyzed by tetra-nbutylphosphonium
bromide ((C 4H 9) 4P + Br - ,
or Q + Br - ). 107,109,110,111,114 The catalyst is only
sparingly soluble in 1-bromooctane, but is
substantially more soluble in aqueous sodium
cyanide solution, so that initially little
Q + CN - is in the organic phase and the displacement
reaction is slow. However,
tetra-n-butylphosphonium salts ((C 4H 9) 4
P + CN - , Q + CN - ) are more soluble in the
product 1-cyanooctane; therefore, as the
conversion of 1-bromooctane to
1-cyanooctane continues, increasing quantities
of the catalyst are taken into the organic
phase, and the reaction rate
accelerates. This behavior signals the
autocatalytic character of reaction.
Not only does the solvent affect the reaction rate, but it also determines the reaction
mechanism. In Starks’ extraction mechanism of PTC, most reacting compound transfers to
the bulk phase. However, reaction may occur at the interface of the two phases. For example:
hexachlorocyclotriphosphazene has been reported to react very slowly with
2,2,2-trifluoroethanol in an alkaline solution of NaOH/C 6H 5Cl two-phase system in the absence
of phase-transfer catalyst. 136-140 Since sodium 2,2,2-trifluoroethanoxide is not soluble
in chlorobenzene, the process probably proceeds at the interface region of the system. Similar
is the reaction of benzylation of isobutyraldehyde in the presence of
tetra-n-butylammonium iodide in an alkaline solution of NaOH/toluene, which is a
two-phase system. 37 Makosza interfacial mechanism 65-67 was employed to rationalize the
experimental results. The main reason is that the ammonium salt of the nucleophilic reagent
is not soluble in toluene.
Usually, the nucleophilic substitutions under NPTC condition are described by an
S N 2 -type reaction both in solid-liquid and liquid-liquid systems in which they can proceed at
the interface through the formation of cyclic adsorption complexes. 160 The activity of the
nucleophilic reagent in the organic phase is determined by the polarity of the organic solution
and the hydration in liquid-liquid system. In the solid-liquid system, the reaction is
highly affected by the organic solvent.