Apparatus, Dean-Stark Azeotropes

The preparation of imines, enamines, nitroalkenes and N-sulfonylimines proceeds via the azeotropic removal of water from the intermediate in reactions that are normally catalyzed by p-toluenesulfonic acid, titanium(IV) chloride, or montmorillonite K 10 clay. A Dean-Stark apparatus is traditionally used which requires a large excess of aromatic hydrocarbons such as benzene or toluene for azeotropic water elimination. 

Use of Heptane Azeotrope in a Dean-Stark Apparatus (Z. Liu and A. Taylor, Unpublished Results)... 

The reaction from an enamine is initiated by the addition of a trace of a strong acid, e.g. /7-toluenesulfonic acid (TsOH, 4-methylbenzene-sulfonic acid), to the ketone and pyrrolidine in a solvent such as toluene. When the mixture is at reflux in a Dean-Stark apparatus, water is liberated and is removed through azeotropic distillation, leaving the enamine in the reaction vessel. After a follow-up reaction between the enamine and a suitable electrophile, an iminium salt is produced that liberates both the a-substituted ketone and pyrrolidine when it is treated with aqueous acid (Scheme 6.20). 

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 ... 

When a mixture of toluene and water boils, the vapour produced is a constant ratio mixture of toluene vapour and water vapour known as an azeotrope. If this mixture is condensed, the liquid toluene and water, being immiscible, separate out into two layers with the water below. By using a Dean Stark apparatus, or Dean Stark head, the toluene layer can be returned to the reaction mixture while the water is removed. Reactions requiring removal of water by distillation are therefore often carried out in refluxing toluene or benzene under a Dean Stark head. 

The water content is determined by azeotropic distillation in the Dean-Stark apparatus. ... 

Increase the temperature ofthe oil-bath until the solution begins to reflux. Collect the initial fraction in the Dean-Stark apparatus. This is the water/ toluene azeotrope and removal of this fraction will ensure a dry toluene solution of poly(ethylene glycol). Allow the solution to cool. 

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%). 

One of the first reactions carried out in perfluorinated solvents was the transesterification described by Zhu in 1993 [10]. He used the perfluorinated solvent FC-77 (mainly perfluoro-2-butyltetrahydrofuran) for an azeotropic distillation of the formed methanol or propanol in the transesterification of methyl- or pro-pylesters 1 with different alcohols 2 using a Dean-Stark apparatus Eq. (1). 

Washing and extraction steps are made easier or even suppressed. In the case of equilibrated reactions leading to light polar molecules (MeOH, EtOH or H2O), equilibrium can be easily shifted by a simple heating just above the boiUng points or under reduced pressure. With the usual procedure this operation is impeded by the presence of solvent necessitating an azeotropic distillation using a Dean-Stark apparatus [Eq. (2)]. 

Before extraction, soil and sediment samples may be dried, for example, by freeze-drying — provided that volatile compounds are not to be analyzed — or by mixing with anhydrous sodium sulfate and extraction in a Soxhlet apparatus. It should, however, be noted that it has frequently been found advantageous to add low concentrations of water, and this is consistent with the finding that addition of water to dry soils inhibits sorption of PAHs (Karimi-Lotfabad et al. 1996). If wet samples are to be analyzed directly, acetonitrile, propan-2-ol, or ethanol may be employed first, and these may be valuable in promoting the chemical accessibility of substances sorbed onto components of the matrices the analyte may then be extracted into water-immiscible solvents and the water phase discarded. Alternatively, if the analyte is sufficiently soluble in, for example, benzene, the water may be removed azeotropically in a Dean Stark apparatus and the analyte then extracted with the dry solvent. Analytes may, however, be entrapped in micropores in the soil matrix so that, for example, recovery of even the volatile 1,2-dibromoethane required extraction with methanol at 75°C for 24 h (Sawhney et al. 1988). 

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). 

Typical procedure. (4R,5S)-4-MethYl-5-phenyloxazolidin-2-one [487] To a solution of (IS,2J )-norephedrine (40 g, 0.26 mol) in toluene (400 mL) was added diethyl carbonate (37 mL, 0.32 mol). The mixture was heated to reflux (under Ar) while 40 mL of solvent was removed through the use of a Dean-Stark apparatus. The mixture was allowed to cool for 20 min, and then sodium methoxide (1 g) was added. Upon reheating, an EtOH/toluene azeotropic mixture was removed at 75-77 °C. After 3 h, the reaction was complete and the temperature of the mixture had increased to 125 °C. The mixture was left to stand at room temperature for 16 h, whereupon (4R,5S)-4-methyl-5-phenyloxazdidin-2-one 40.6 g) crystallized and could be collected. The solvent was removed from the filtrate in vacuo and the residue was redissolved in EtOAc (250 mL). This solution was washed with brine (50 mL) and a precipitate was removed by filtration. The solvent was then removed in vacuo and toluene (50 mL) was added to the residue. Removal of the toluene by distillation yielded oily crystals of the oxazolidinone, which were washed with Et20 to afford 4.5 g (total 45 g, 97%). 

A soln. of 2-acetonaphthenone, 2-acetonaphthoxime (cis-methyl), and a little p-toluenesulfonic acid monohydrate in anhydrous benzene refluxed 3 days with azeotropic water entrainment using a Dean-Stark apparatus 2-( -naphthyl)-4-methylbenzo[h] quinoline. Y 40%. Limitations s. A. Rosenthal, J. Org. Ghem. 26,1638 (1961). 

Although this method proved successful for 4-keto-L-proline methyl ester 48, it was decided to investigate the conventional azeotropic dehydration method. Derivative 48 and pyrrolidine (1.2 equiv) were heated under reflux in benzene, collecting the water produced using a Dean and Stark apparatus. A quantitative yield of the corresponding enamine 52 was obtained after only 30 min (Scheme 19). Again, full assignment of NMR spectra was complicated by the apparent presence of rotameric structures. 

Moisture and water content are important parameters in the processing and sale of materials from foodstuffs to petroleum based fuels. Azeotropic distillation is commonly used to determine water content using apparatus developed by Dean and Stark and an immiscible liquid such as toluene as the codistillate. 

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