Handbook of Solvents - George Wypych - ChemTech - Ventech!

822 Maw-Ling Wang
13.3.1.4 Effect of the volume of organic solvent and water on the reactions
in various reaction systems
Figure 13.3.7. Effect of the amount of water on the apparent
rate constant (k app);4gof2,4,6-tribromophenol, 0.9 g
of KOH, 0.6 mL of benzyl bromide, 50 mL of H 2O, 50
mL of chlorobenzene, 40 o C. (Adapted from Ref. [147],
by permission.)
In general, the concentration of reactants in
the aqueous phase is decreased by increased
amount of water. The addition of
water probably also decreases the concentration
of the intermediate product in the organic
phase. Hence, both the mass transfer
rate and the degree of hydration with the anion
are decreased, which also decreases the
reaction rate. However, this argument is not
necessarily correct. Figure 13.3.7 indicates
that the conversion in the two-phase reaction
is not affected by the amount of water
added. 147 Wang and Yang 135 studied the effects
of the volume ratio of water to chlorobenzene
on the conversion for the reaction
of 4-bromophenol and allyl bromide in an
alkaline solution of KOH/chlorobenzene at
50 o C under phase-transfer catalytic conditions.
The reaction followed the
pseudo-first-order rate law and the
corresponding apparent rate constant
decreased gradually when
the water content was increased,
as shown in Figure 13.3.8. 135 The
reason was that the concentration
of the intermediate product
tetra-n-butylammonium
phenoxide (ArOQ, or the active
catalyst) in the aqueous phase decreased
with the increase in the
amount of water. The mass transfer
rate of the intermediate product
(or the active catalyst) from
Figure 13.3.8. Effect of the volume ratio of water to chlorobenzene
the aqueous phase to the organic
phase decreased when a large
on the conversion; 1.568 g of 4-bromophenol, 1.0 g of KOH, 0.7 g of
allyl bromide, 0.2 g of TBAB catalyst, 50 mL of chlorobenzene,
50
amount of water was used. In addition,
the dilution effect led to reduction
of the reaction rate in the
aqueous phase.
In general, a higher concentration of the intermediate product
o C. (Adapted from Ref. [135], by permission.)
(tetra-n-butylammonium alkoxide, or the active catalyst, ArOQ) in the aqueous phase enhances
the reaction rate. This is due to a large concentration gradient across the interface in
transferring the species from the aqueous phase to the organic phase. For the reaction of
allyl bromide and 2,4-dibromophenol in synthesizing 2,4-dibromophenyl allyl ether in an