1) The phase-transfer catalyst (Q+X–) is usually a quaternary ammonium halide
(R 4 N+X–) such as tetrabutylammonium halide (CH 3 CH 2 CH 2 CH 2 ) 4 N+X–.
2) The phase-transfer catalyst causes the transfer of the nucleophile (e.g. CN–) as an
ion pair [Q+CN–] into the organic phase.
3) This transfer takes place because the cation (Q+) of the ion pair, with its four
alkyl groups, resembles a hydrocarbon in spite of its positive charge.
i) It is said to be lipophilic –– it prefers a nonpolar environment to an aqueous
one.
4) In the organic phase the nucleophile of the ion pair (CN–) reacts with the organic
substrate RX.
5) The cation (Q+) [and anion (X–)] then migrate back into the aqueous phase to
complete the cycle.
i) This process continues until all of the nucleophile or the organic substrate has
reacted.
- An example of phase-transfer catalysis:
- The nucleophilic substitution reaction of 1-chlorooctane (in decane) and sodium
cyanide (in water):
CH 3 (CH 2 ) 6 CH 2 Cl (in decane) aqueous RNa^4 NCN, 1 05 oC CH 3 (CH 2 ) 6 CH 2 CN
+Br−
i) The reaction (at 105 °C) is complete in less than 2 h and gives a 95% yield of
the substitution product.
- Many other types of reactions than nucleophilic substitution are also amenable to
phase-transfer catalysis.
- Oxidation of alkenes dissolved in benzene can be accomplished in excellent
yield using potassium permanganate (in water) when a quaternary ammonium
salt is present.
