Dean and Stark traps

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). 

The apparatus is assembled as shown and the reaction is conducted in a solvent which forms an azeotrope with water (almost invariably a 

A Dean-Stark trap can also be used to remove volatile alcohols, such as methanol and ethanol, which are miscible with many organic solvents. They can nevertheless be removed by placing SA molecular sieves (Section 4.3) in the trap, in order to absorb the alcohol. An alternative is to use a Soxhlet extractor containing molecular sieves. 

On a small scale, simply placing some activated sieves in the reaction flask is a convenient means of removing water. This method is effective in driving equilibria and is also used to promote reactions which are adversely affected by water. 

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Dichlorophthalic anhydride [4466-59-5] M 189-191 , 191-191.5 , b 339 . Boil in xylene (allowing any vapours which would contain H O to be removed, e.g. Dean and Stark trap), which causes the acid to dehydrate to the anhydride and cool. Recryst from xylene [Villiger Chem Ber 42 3539 1909 Fedoorow Izv Akad Nauk SSSR Otd Khim Nauk 397 1948, Chem Abstr 1585 1948]. 

HCIO4). Refluxed with benzene (6mL/g) in a flask fitted with a Dean and Stark trap until all the water was removed azeotropically (ca 4h). The soln was cooled and diluted with dry pentane (4mL/g of AgC104). The ppted AgC104 was filtered off and dried in a desiccator over P2O5 at 1mm for 24h [Radell, Connolly and Raymond J Am Chem Soc 83 3958 1961]. It has also been recrystallised from perchloric acid. [Caution due to EXPLOSIVE nature in the presence of organic matter.]... 

A Dean and Stark trap 2 modified to provide for continuous return of the heavier organic layer is most convenient. Such a trap can be constructed like a small Wehrli extractor.3... 

To a 500 ml, three-necked, round-bottomed flask equipped with a Dean and Stark trap, condenser, and stirrer is added a mixture of 52.5 gm (0.60 mole) of morpholine, 49 gm (0.5 mole) of cyclohexanone, 100 ml of toluene, and 0.5 gm ofp-toluenesulfonic acid. The contents are refluxed for approximately 4 hr to remove the water of reaction. The product mixture is cooled and then distilled to afford 59 gm (71 %) of the enamine, b.p. 117°-120°C (10 mm), n 5 1.5122-1.5129. Yields of enamine have been reported to vary from 71 to 80%. 

This preparation is divided into three steps (1) the reaction of phosphorus (V) chloride with ammonium chloride in an inert solvent, (2) the rapid distillation from the solvent and from the nondistillable, rubber-forming higher polymers, and (3) the fractionation of the trimeric and tetra-meric compounds. The reaction apparatus consists of a 5-1. three-necked flask with standard-taper joints fitted with a thermometer and a motor-driven, heavy-duty stirrer, and heated by an electric mantle. For the preliminary removal of water from the solvent a Dean and Stark trap is used at a side joint in conjunction with a long water-jacketed condenser, the latter having a calcium chloride drying tube at its upper end. 

A solution of ethyl cyanoacetate (20 mmol), /i-dodecane (0.3 g) and the carbonyl compound (20 mmol) in cyclohexane or toluene (25 cm ) containing AMP-silica (0.5 g, ca. 2 mol% amino groups) was heated to reflux and the water removed using a Dean and Stark trap. The reaction was monitored by GC and the product isolated by filtration of the catalyst followed by normal workup. 

Round bottomed flask (500 ml) with magnetic stirrer bar Dean and Stark trap Reflux condenser... 

Procedure. Place methyl-4,6-0-benzylidene-(3-D-glucopyranoside (8.00 g, 28.3 mmol), dibutyltin oxide (8.41 g, 33.8 mmol) and benzene (400 ml) in a round-bottomed flask equipped with a Dean and Stark trap and a reflux condenser. Stir the solution at reflux for 16 h with azeotropic removal of water. Allow... 

Primary alcohols give better yields of esters than secondary alcohols and tertiary alcohols, and phenols react only to a very small extent. Acid catalysts are used in small amounts. The mixture is refluxed for several hours and the equilibrium is shifted to the right by the use of a large excess of either the alcohol or add and the removal of water. Azeotropic distillation of water, the use of a Dean and Stark trap, or a suitable drying agent helps to increase the rate of reaction. No acid catalysts are required for the preparation of esters of formic acid or of benzyl alcohol. 

A simple esterification employing azeotropic removal of water by means of a Dean and Stark trap can be used in an introductory organic laboratory course. This technique is widely used in industry for the preparation of polyesters. The preparation of y-chloropropyl acetate in 93-95% yield and of n-amyl acetate in 71% yield have been described. 

To the round-bottomed flask are added 15 gm (0.25 mole) of glacial acetic acid, 17.6 gm (0.20 mole) of -amyl alcohol, 30 ml of benzene, and 0.15 gm of p-toluenesulfonic acid catalyst. A Dean and Stark trap is filled with benzene and the contents of the flask are refluxed for 1 hr. The water that is produced remains in the trap as a bottom layer. The reaction mixture is extracted with sodium bicarbonate solution in order to remove the excess acid, washed with water, and then with a saturated sodium chloride solution. The organic layer is fractionated in order to remove the benzene-water azeotrope and the residue is transferred to another flask containing a small loose plug of steel wool in the neck. The product is isolated by simple distillation in 71% yield (20.7 gm), bp 14r-146°C, 1.4012. 

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