Dean-Stark apparatus, - trap azeotropic

Dean-Stark apparatus, -separator, - trap (s. a. Water entrainment, azeotropic)... 

Secondary amines react with ketones that contain an H atom in the a-position through an addition and subsequent El elimination to form enamines (Figure 9.29). In order for enamines to be formed at all in the way indicated, one must add an acid catalyst. In order for them to be formed completely, the released water must be removed (e.g. azeotropically). The method of choice for preparing enamines is therefore to heat a solution of the carbonyl compound, the amine, and a catalytic amount of toluenesulfonic acid in cyclohexane to reflux in an apparatus connected to a Dean-Stark trap. Did someone say Le Chateher ... 

Preparation of the reagent [70] A solution of PEG monomethyl ether 89 (MW = 750 5.88 g, 7.8 mmol) in benzene (20 mL) was dried azeotropically for 24 h in an apparatus fitted with a Dean-Stark trap and subsequently added dropwise to a solution of chlorosulfonyl isocyanate (88) (1.10 g, 7.8 mmol) in dry benzene (20 mL). The mixture was stirred at room temperature for 1 h, then concentrated to dryness. A solution of this residue in benzene (35 mL) was added dropwise to a solution of triethylamine (2.5 mL, 17.3 mmol) in benzene (15 mL). The mixture was stirred for 30 min at room temperature, then filtered, and the solid was dried to yield polymer-supported Burgess reagent 91 (6.2 g, 82%). 

After all the components have been thoroughly dried in an oven, the apparatus is assembled as shown. Dried xylene is added to the Morton flask and heated to reflux any traces of residual moisture are removed by distilling over several milliliters and removing any azeotrope that forms in the Dean-Stark trap. Anhydrous rerr-butyl alcohol is added to the flask, followed by potassium metal. The solution is stirred, and after all the potassium has reacted and the solution is boiling vigorously, slow addition of diethyl tetradecan-l,14-dicarboxylate from the Hershberg funnel is started. The solvent vapors, which condense and fall into the dilution chamber, dilute the diester and carry it into the Morton flask where the reaction occurs. An ethanol-xylene mixture is removed via the Dean-Stark trap at approximately the same rate as the solution is added from the dropping funnel. A slow continuous addition of the ester maximizes the probability of cyclization a typical reaction time is 24 hr. 

The dehydration of 34 to 29 is a reversible process that is driven to completion by removing the water from the reaction mixture. This is conveniently done by using toluene as the solvent for the reaction. Water and toluene form an azeotrope (Sec. 4.4), so azeotropic distillation allows the continuous separation of water as dehydration occurs. To minimize the amount of solvent that is required for distillations of this type, a Dean-Stark trap (Fig. 18.15) is commonly used. Because such traps are often not available in the undergraduate laboratory, an operational equivalent may be devised by assembling the apparatus in a way such that water, but not toluene, can be prevented from returning to the reaction flask (Fig. 18.16). 

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