Removal of Water from Reaction Mixtures

In many chemical condensation reactions like esterification, acetalization, ketali-zation, or etherification water is produced as an unwanted byproduct. All these reactions of the form 

The facilitated downstream purification may be even at least as economically important as the higher yield and conversion ratio. 

For a simulation and optimization of the coupled process the kinetics of the reaction and the performance of the membrane have to be known. An esterification reaction as in Eq. (30) can be described as a second-order reaction. 

The symbols in brackets relate to the concentration of the respective substances, ki and k2 are the reaction rate constants for the forward (esterification) and backward (hydrolysis) reaction, including their dependence on temperature and catalyst activity. For a given reaction the equilibrium constant is then given by 

When water is removed from the mixture the kinetic of water removal can be written as 

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Vapor permeation np Ap v/v Separation of azeotropic mixtures removal of water from reaction mixtures (reversible reactions)... 

Processes to produce boric acid esters are based on the azeotropic removal of water from a mixture of the appropriate alcohol, phenol, or glycol, and boric acid. A suitable hydrocarbon azeotroping agent is used to help remove the water. The water is removed continuously by using a condenser that allows continuous return of the solvent to the reaction vessel. Eor some borate esters, such as the glycol borates, distillation can result in decomposition. 

Benzyl 4, 6 -0-benzylidene-P-lactoside with five free hydroxyl groups was converted to the dibutylstannylene intermediate by azeotropic removal of water from its mixture with 2.5 molar equiv. dibutyltin oxide in benzene, the reaction with benzyl bromide in the presence of tetrabutylammonium bromide then gave the 2,3 -di-0-benzyl derivative in 52% yield [139]. When the 3, 4 -0-isopropylidene analog was treated with 1.2 molar equiv. only, the 2-O-benzyl derivative was the main product [150]. 

In Fig. 3.23 the conversion ratio of the wanted product (ester) and the water present in the reaction mixture are plotted over the reaction time for a given membrane area and two ratios of the educts. Without removal of water from the mixture by means of a membrane the wanted product C and water are produced at the same rate, and both concentrations in the reaction mixture increase until equilibrium is reached. When water is continuously removed through the membrane at a certain time the water content passes through a maximum, when the water is removed as fast as it is formed. The time to reach this point depends on the membrane area installed. The water content then goes down and eventually reaches a value close to zero, when the water is removed much faster than formed. 

It is occasionally necessary to carry out special operations in connection with a synthetic procedure. An example frequently encountered is the removal of water from a reaction mixture in order to alter equilibrium concentrations (for example, in the preparations of enamines). For this purpose, a Dean-Stark trap is employed as shown (Fig. A3.6). The reaction is carried out in a solvent that forms an azeotrope with water... 

The real value of aldol dehydration is that removal of water from the reaction mixture can be used to drive the aldol equilibrium tow ard product. Even though the initial aldol step itself may be unfavorable, as it usually is for ketones, the subsequent dehydration step nevertheless allows many aldol condensations to be... 

In a 3-I. flask, fitted with a stirrer and a fractionating column with condenser for downward distillation, are placed 912 g. of ethyl propane-1,1,2,3-tetracarboxylate (p. 29) and 950 cc. of a solution of equal volumes of pure concentrated hydrochloric acid and distilled water. A receiver with a side-tube is attached to the condenser this side-tube leads to a water trap. The mixture is boiled, with continual stirring, at such a rate that the alcohol is removed as fast as it is formed, but without undue removal of water from the flask (Note 1). The progress of the reaction can be followed by noting the rate at which carbon dioxide passes through the trap. When the temperature at the head of the column approaches ioo°, the flame is turned down so that very little liquid distils over. Heating is continued until evolution of carbon dioxide ceases (Note 2). 

The use of an entraining agent also has been suggested for the removal of water from the reaction mixture.109 110 For this method, either the reaction must be run under vacuum or at elevated temperatures. It should be very effective for use with the Zempl n procedure which uses mercuric salts with benzene under reflux (page 46) but so far the method has had only limited use. 

Like acetals, imines are unstable with respect to their parent carbonyl compound and amine, and must be formed by a method that allows removal of water from the reaction mixture. 

Perhaps the most commonly encountered equilibrium reactions are those involving water as a reactant or product. Driving such equilibria by using excess water (e.g. hydrolysis reactions) is easy, but driving equilibria by removing water (e.g. in ester or acetal formation) can be more difficult. An excellent device for the continuous removal of water from a reaction mixture is the Dean-Stark trap (Fig. 9.27). 

A detailed discussion of the influence of pressure, temperature, catalyst variations, and the removal of water from the reaction mixture, as well as the influence of different solvents on selectivity and reaction rates, may be found in [12]. For more details about the reaction mechanism and the chemistry of palladium-alkene-CO complexes cf. [13, 14, 17]. 

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