Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.3 Effects of organic solvents on phase-transfer catalysis 807
Table 13.3.4 Effect of solvent on the rate of
reaction of thiophenoxide and bromooctane
Catalyst Solvent k×10 3 M -1 s -1
(C4H9) 4N + I -
C7H16 0.02
(C4H9) 4N + I -
C6H4Cl2 88
(C3H7) 4N + Br -
C6H4Cl2 0.45
C6H5CH2(C2H5) 3N + Br - C6H4Cl2 0.04
C8H17(C2H5) 3N + Br -
C6H4Cl2 28
(C6H5) 4P + Br -
C7H16 0.0093
(C6H5) 4P + Br -
C6H4Cl2 47
(C6H4) 4P + Cl -
C6H4Cl2 180
Data obtained from Herriott and Picker 36
Figure 13.3.1 Effects of the organic solvents on the conversion
of CH 2Br 2; 6.88×10 -2 mol of 1-butanol, 1.5 molar ratio
of 1-butanol/1-octanol, 30 g of KOH, 2.76×10 -2 mol of
CH 2Br 2, 3.11×10 -3 mol of TBAB catalyst, 10 mL of water, 50
mL of organic solvent, 1020 rpm, 50 o C (Adapted from Ref.
[145], by permission.)
used PTC technique to synthesize formaldehyde
acetals from alcohol and dichloromethane
in the aqueous phase.
Wang and Chang 144-146 employed the
PTC technique to synthesize formaldehyde
acetals from the reaction of alcohol
and dibromomethane in an alkaline solution
of KOH/organic solvent. Alcohol
(ROH) first reacted with KOH so as to
form potassium alkoxide (ROK) in the
aqueous phase. ROK further reacted
with quaternary ammonium salt (QBr)
in the aqueous phase to produce quaternary
ammonium alkoxide (QOR) which
is more soluble in the organic solvent.
Dibromomethane reacted with QOR in
the next step to form the desired product
dialkoxymethane CH 2(OR) 2 in the organic
phase, as shown in Equation
[13.3.7].
Dibromomethane, which possesses
weak dipole moment, may form a weak
dipole-dipole bond with the organic solvent.
However, this bond does not significantly
affect the reaction rate. QOR
solvates in a polar organic solvent. This
solvation results in less energy in the
nucleophilic agent than that in the transition
state compound. The activation energy
therefore becomes high due to the
solvation of QOR with a highly polar
solvent which is unfavorable in the present
reaction system. The low polarity
solvent neither solvates QOR, nor separates
tetra-n-butylammonium ion (Q + )
from the alkoxide ion ( - OR). Thus, the
reactivity in low polar solvent is low.
Organic solvents of appropriate polar-
ity, such as chlorobenzene or dibutyl ether, are the best solvents to obtain a high yields of
various alcohols, as shown in Table 13.3.5. As shown in Figure 13.3.1, similar results were
obtained in synthesis of unsymmetric acetals under PTC conditions. 144-146 The values of k app,
in which the reaction follows a pseudo-first-order kinetic rate law, are 4.59×10 -3 , 4.58×10 -3 ,
8.17×10 -3 , and 1.47×10 -2 min -1 for reaction of CH 2Br 2 with butanol and octanol in cyclohexane,
n-decane, dibutyl ether and chlorobenzene, respectively.