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Handbook of Solvents - George Wypych - ChemTech - Ventech!

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810 Maw-Ling Wang<br />

reacts with phenoxide (PhO - ) or alkoxide (RO - ) ion in the aqueous phase, as shown in Figure<br />

13.3.2. 143 The reaction rate in the aqueous phase is also enhanced. The mechanism <strong>of</strong> the reaction<br />

rate <strong>of</strong> alkyl halide (or allyl halide, RX) and phenoxide (PhO - ), both existing in the<br />

aqueous phase with PEG help, is different from that in presence <strong>of</strong> quaternary ammonium<br />

salt.<br />

Figure 13.3.2 Dependence <strong>of</strong> the concentration <strong>of</strong> allyl chloride in<br />

the aqueous phase on the amount <strong>of</strong> PEG-1500 added, V org=V aq=50<br />

mL, 30 o C (Adapted from Ref. [143], by permission.)<br />

[13.3.9]<br />

The reaction catalyzed by<br />

PEG can be carried out either in a<br />

homogeneous phase or in a<br />

two-phase solution. The alkyl halide<br />

usually serves as the reactant<br />

as well as the solvent. The reaction<br />

proceeds because organic-phase<br />

reactant dissolves in<br />

an organic solvent in the presence<br />

<strong>of</strong> PEGs. Dichloromethane,<br />

chlorobenzene, ethyl ether, cyclohexane<br />

and n-decane are frequently<br />

used solvents.<br />

The reaction mechanism <strong>of</strong><br />

two-phase catalytic reaction by<br />

PEG includes formation <strong>of</strong> a complex<br />

<strong>of</strong> PEG with cation. 141,142 This<br />

is different than the reaction catalyzed<br />

by quaternary salts. Table<br />

13.3.8 shows the initial reaction<br />

rate using PEG, ((-r) i,PEG) and the initial reaction rate without using PEG, ((-r) i,B) in various<br />

organic solvents. Both (-r) i,B and (-r) i,PEG decrease when the polarity <strong>of</strong> the organic solvent<br />

increases. The maximum reaction rate is obtained with n-decane, which has the lowest polarity,<br />

as the protic solvent. Same results were obtained from the work <strong>of</strong> Landini et al. 55 on<br />

the reaction <strong>of</strong> n-octylmethylene sulfonate and bromide ion in a homogeneous phase with<br />

C 16H 33P + (C 4H 9) 3Y - as PTC. Wang and Chang 142 made a reasonable explanation for this peculiar<br />

phenomena, i.e., the transition state possesses a higher degree <strong>of</strong> dispersity <strong>of</strong> electric<br />

charge than does the ground state. Increasing the polarity <strong>of</strong> the solvent increases the relative<br />

activation energy between the transition state and the reactants. Hence the reaction rate<br />

is decreased.

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