Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.3 Effects of organic solvents on phase-transfer catalysis 805
13.3.1.2 Effect of organic solvent on the reaction in various reaction
systems
(A) Synthesis of ether compound catalyzed by quaternary ammonium salts (NPTC)
One of the most useful synthesis applications of phase transfer catalysis (PTC) is in the
preparation of ether according to the following general equation
PTC
R′ X + ROX + OH ⎯ ⎯ →ROR′
+ H O + X
− −
2
[13.3.6]
where R and R� are the primary or secondary alkyl or aryl groups, X is a halide and the caustic
base is usually sodium or potassium hydroxide in the aqueous solution. The generally accepted
reaction mechanism is
[13.3.7]
It is important to consider that the alkoxide ion (RO - ) is a reactive nucleophilic but also
a strong base. It was shown that 10 -3 MC 6H 5(CH 3) 3N + OC 4H 9 is 1000 times more basic than
KOC 4H 9 (both in C 4H 9OH). Extracted alkoxide bases can be applied in principle to numerous
base-catalyzed reactions, e.g., oxidations, eliminations and isomerization. Better quantitative
understanding of the extraction of alkoxide into organic phase is important.
Dehmlow et al. 20 investigated the extraction of aqueous sodium hydroxide solution with organic
solvent containing various quaternary ammonium salts by mixing sodium hydroxide
with organic solvent containing R 4NX. After phase separation, titration of the organic phase
showed only traces of base presence if concentrated NaOH solution was employed and if Cl -
was the counter ion.
The Cl-OH exchange was found to be of the order 1-2% for all quaternary ammonium
chloride with chlorobenzene as solvent; i.e., 98% of the salts remained in the R 4NCl form.
However, upon addition of trace amounts of various alcohols, a dramatic change in the behavior
of the system was observed and significant amounts of base could be detected in the
organic phase. Table 13.3.3 shows the experimental results where 50% aqueous caustic solutions
were extracted by equal volumes of 0.1 M (C 8H 17) 4NBr in chlorobenzene containing
0.1 M of various alcohols.