Fractional distillation distilling head

Fig. II, 17, 2 illustrates a fractional distillation unit f for use with glass helices. The column is provided with an electrically-heated jacket the resistance shown in the Figure may be replaced by a variable transformer. The still head is of the total-condensation variable take-off type aU the vapour at the top of the column is condensed, a portion of the condensate is returned to the column by means of the special stopcock (permitting of...

The separation of the isomers is carried out by a combination of fractional distillation and crystallization. In a fractional vacuum distillation step, the distillate, obtained at a head temperature of 96—97°C at 1.6 kPa (12 mm Hg), is fairly pure o-nitrotoluene and can be purified further by crystallization. The meta isomer is distilled from a mixture of m- and -nitrotoluene and can be purified further by additional distillation and crystallization steps. The bottoms product from the distillation steps is cooled in a crystallizer to obtain nitrotoluene. 

Packed fractional distillation columns run in the batch mode are often used for low-pressure drop vacuum separation. With a trayed column, the liquid holdup on the trays contributes directly to the hydraulic head required to pass through the column, and with twenty theoretical stages that static pressure drop is very high, e.g., as much as 100-200 mm Hg. 

The amount of fore-run and the yield depend on the efficiency of the fractionation. With a 7-cm. distilling head, a fore-... 

If no separation of isomers is required, as when the mixture is to be hydrogenated, the mixed esters may be obtained by distillation yield, 75-90 g. (71-85%, based on starting aldehyde). It is extremely difficult to separate the A2- from the A3-ester by fractional distillation, as the two boil only 7° apart however, by careful fractionation and refractionation through an 0.8 X 125-cm. simple Podbielniak column with partial-reflux head,4 it is possible to obtain 3 pure ethyl 4-ethyl-2-methyl-2-octenoate, b.p. 102-103°/4.8 mm., o 1.4478, and a nearly pure sample of ethyl 4-ethyl-2-methyl-3-octenoate, b.p. 94-95°/4.8 mm., ng 1-4393. 

A. 2-Methyl-2-nitropro]f)ane. To a well-stirred suspension of 650 g. (4.11 moles) of potassium permanganate in 3 1. of water, contained in a 5-1. three-necked flask fitted with a reflux condenser, a mechanical stirrer, a thermometer, and a 250-ml. dropping funnel, is added dropwise and with stirring over a 10-minute period, 100 g. (1.37 moles) of i-butylamine (Note 1). When the addition is complete, the reaction mixture is heated to 55° over a period of approximately 2 hours, and then the reaction mixture is maintained at 55° with continuous stirring for 3 hours. The dropping funnel and reflux condenser are replaced by a stopper and a still head fitted for steam distillation and the product is steam distilled from the reaction mixture (Note 2). The liquid product is separated from the denser water layer and then diluted with 250 ml. of ether and washed successively with two 50-ml. portions of aqueous 2M hydrochloric acid and with 50 ml. of water. After the ethereal solution has been dried over anhydrous magnesium sulfate, the solution is fractionally distilled at atmospheric pressure to remove the ether. The residual crude product (Note 3) amounts to 106-128 g. and is sufficiently pure for use in the next step. In a typical run, distillation of 124 g. of the crude product afforded 110 g. (78%) of the pure 2-methyl-2-nitrobutane as a colorless liquid, b.p. 127-128°, d 1.3992. The material slowly solidifies on standing to a waxy solid, m.p. 25-26° (Note 4). 

The reflux condenser is replaced by a Claisen distillation head, and the reaction mixture is distilled until the temperature of the vapor reaches 140°. The residue is transferred to a smaller flask and fractionated through a 30-cm. column packed with glass helices. A low-boiling fraction containing ethylidene diacetate and diethyl malonate is collected first, followed by 79-89.5 g. (68-77%) of diethyl ethylidenemalonate b.p. 102-106°/10 mm. %>5 1.4394 (Note 1). 

A. Diethyl propionylsuccinate (1). A solution of 412 g. (2.4 mole) of diethyl maleate (Note 1), 278 g. (4.8 mole) of freshly distilled propionaldehyde (Note 2), and 1.2 g. (0.0048 mole) of benzoyl peroxide in a normal 2-1. Pyrex flask is heated under reflux while undergoing irradiation with an ultraviolet lamp (Note 3). The initial reflux temperature is 60°. After 2 hours another 1.2 g. (0.0048 mole) of benzoyl peroxide is added. Strong reflux and irradiation are maintained throughout the entire reaction period. After 18 hours total time, the internal pot temperature reaches 68°. At this point the last 1.2 g. (0.0048 mole) of benzoyl peroxide is added, and the reaction is continued for a total of 30 hours, at which time the pot temperature reaches 74.5°. The reflux condenser is then replaced by a distillation head. The excess propionaldehyde (119 g.) is distilled under atmospheric pressure, b.p. 48-49°. Succinate 1 is distilled under reduced pressure. The main fraction, b.p. 145-151.5° (15-16 mm), provides 417-449 g. (75-81%) of product having sufficient purity for use in the next step (Note 4). 

Taking a small forerun serves to concentrate residual isophorone in this fraction. Care must also be taken not to overcool the distillation head which may cause crystallization of the hydroxy ketone throughout the system. 

To a mixture of anhyd ethylene glycol (250 mL) and anhyd KF (168 g, 3.0 mol) in a mechanically stirred 1-L flask fitted with a distillation head and a pressure-equalizing addition funnel was added dropwise l-bromopropan-2-one (1 102 g, 0.75 mol) at 160 C. The crude product 2 distilled from the mixture over a 70— 120 C boiling point range. The crude distillate was dried (K2C03) and then fractionally distilled yield 9.00 g (20%) bp 75-77 "C/760 Torr. 

Carbon Sulphidoselenide, CSSe, may be prepared by the action of carbon disulphide vapour on ferrous selenide at 650° C. The product, on fractional distillation through a 50-inch head-filled glass column, yields a deep yellow liquid as residue, which on repeated fractionation through the same column yields a middle fraction of the pure sulphidoselenide. It is obtained as a yellow oil of boiling-point 88-90° to 83-95° C. at 749-2 mm., and having a surface tension y =40-44 dynes/cm. at 20° C. 

The remaining diethyl ether is fractionally distilled off from the dimethyl-cadmium at atmospheric pressue (bp 34-35°C). Distillation is continued until pure dimethylcadmium has started to cross the still head (bp 105-106°C)—at which point the argon flow through the paraffin-oil bubbler is increased to a rapid purge while the oil bath is lowered quickly to prevent distillation of further product. The system is allowed to stand for several minutes to allow any dimethylcadmium in the Vigreux column to drain back down into the distillation flask and valve D is then closed. A small amount of dimethylcadmium will decompose during this distillation process, as witnessed by the cadmium metal residue formed in the distillation flask, but this is unavoidable and does not significantly affect the subsequent yield of product. 

In some special cases, such as the analysis of oil of lemon and the like, the fractional distillation is carried out in a flask with a three-bulb head [see Fig. 63). 

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