**2.3 C-Alkylation**

698 Mass Transfer - Advanced Aspects

As a consequence, the preferential extraction of –OCH2CF3 into the organic phase by the quaternary cation catalyst is responsible for the efficiency of the reaction. The apparent extraction constant, *Eapp,,* is thus an index for reflecting the mass transfer effect. Thus, a larger value of *Eapp* implies that the two-phase reaction is dominated by the effects of mass

Influence of solvents on the rate of the reaction was examined by employing seven different solvents under PTC conditions (Table 3). The order of relative activities of the solvents is dichloroethane > chlorobenzene > dichloromethane > benzene > toluene > chloroform > hexane. Higher values of *k0.5/2* imply a significant influence of the mass transfer on the

From the Arrhenius plot of *k0,app* of *vs.* 1/T for different initial concentration ratios of NaOH and HOCH2CF3, the activation energy, *Ea*, was obtained and presented in Table 4. Thus, the effects of mass transfer and chemical reaction kinetics on the conversion depend highly on

**HOCH2CF3 (g) NaOH (g) Ea (kcal/mol)**  14 2.5 11.4 7 7 19.0 7 3 7.1 Table 4. Energy of activation at various amounts of NaOH and HOCH2CF3. (Adapted from

0.006

0.005

0.004

K f,app

(min ) -1

0.003

0.002

200 400 600 800 1000 1200 1400

Agitated rate (rpm)

Fig. 5. Dependence of the apparent reaction rate constants *kr,app* and *kf,app* on the agitated rate; 7 g of HOCH2CF3,0.0059 mol of (NPCl2)3, 3 g of NaOH, 20 mL of H20, 0.175 mequiv of catalyst, 50 mL of chlorobenzene, 20 °C. (Adapted from Ref. [59], by permission)

Further, the kinetics and the mass transfer behaviors of synthesizing polytrifluoroethoxycyclotriphosphazene from the reaction of 2,2,2-trifluoroethanol with hexachlorocyclotriphosphazene by triphase catalysis in an organic solvent / alkaline solution were studied [59]. In general, the reaction mechanism of the triphase catalysis is: (1) mass transfer of reactants from the bulk solution to the surface of the catalyst pellet,

the reactant concentrations of NaOH and HOCH2CF3.

transfer.

reaction rate.

Ref. [56], by permission).

K r,app

(min ) -1

0.05

0.04

0.03

0.02

The essential condition for a reaction to occur is the effective collision of reactant molecules, even in the phase transfer catalysis system. Recently, Vivekanand and Blakrishnan [29] investigated the effect of varying stirring speed on the rate of the reaction of C-alkylation of dimedone by dibromoethane in the range 200–800 rpm under PTC conditions (Scheme 2). The experimental results show that the rate constants increase with the increase of stirring speed from 200 to 600 rpm. Further increase in the speed of agitation had practically no effect on the rate of reaction (Fig. 6). This is because the interfacial area per unit volume of dispersion increased linearly with increasing the stirring speed till 600 rpm is reached and there after there is no significant increase in the interfacial area per unit volume of dispersion with the corresponding increases in the speed. Consequently, increasing the stirring speed changes the particle size in the dispersed phase. At stirring speeds of 700 and 800 rpm, nearly constant rate constant values were observed. This is not because the process is necessarily reaction rate-limited, but because the mass transfer has reached a constant value. Thus, Fig. 6 is indicative of an interfacial mechanism rather than Starks' extraction mechanism. Chiellini et al. [60] observed a continuous increase in the rate of ethylation of PAN, even up to stirring speeds of 1950 rpm, for which an interfacial mechanism was proposed. Similar observations were made under various phase transfer catalytic reactions and an interfacial mechanism was proposed [41,61–64].

Scheme 2. C-Alkylation of dmedone under PTC conditions
