The distilling flask

The distilling flask. To minimise superheating of the liquid (due to the absence of minute air bubbles or other suitable nuclei for forming bubbles of vapour), and to prevent bumping, one or more of the following precautions should be taken  

For simple distillations a Claisen flask (see, for example, Quickfit and Quartz Ltd cataloque of interchangeable laboratory glassware, Kontes Glass Co, Vineland, New Jersey, cat.no TG-15, Normschiff, Wertheim, Germany, Embell Scientific, Murwillumbah, NSW 2484, Australia) is often used. This flask is, essentially, a round-bottomed flask to the neck of which is joined another neck carrying a side arm. This second neck is sometimes extended so as to form a Vigreux column. 

For heating baths, see Table 2 (p 33). For distillation apparatus on a semi-micro scale see Quickfit, Kontes and other glassware catalogues (above). 

The reflux ratio should be high (i.e. the ratio of drops of liquid which return to the distilling flask and the drops which distil over), so that the distillation proceeds slowly and with minimum disturbance of the equilibria in the column. 

The hold-up of the column should not exceed one-tenth of the volume of any one component to be separated. 

Heat loss from the column should be prevented but, if the column is heated to offset this, its temperature must not exceed that of the distillate in the column. 

Heat input to the still-pot should remain constant. 

For distillation under reduced pressure there must be careful control of the pressure to avoid flooding or cessation of reflux. 

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10 mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

Steam distillation. When two immmiscible liquids distil, the sum of their (independent) partial pressures is equal to the atmospheric pressure. Hence in steam distillation, the distillate has the composition 

Azeotropic distillation. In some cases two or more liquids form constant-boiling mixtures, or azeotropes. Azeotropic mixtures are most likely to be found with components which readily form hydrogen bonds or are otherwise highly associated, especially when the components are dissimilar, for example an alcohol and an aromatic hydrocarbon, but have similar boiling points. 

Examples where the boiling point of the distillate is a minimum (less than either pure component) include  

Water with ethanol, n-propanol and isopropanol, fert-butanol, propionic acid, butyric acid, pyridine, methanol with methyl iodide, methyl acetate, chloroform, 

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The thermometer should be so arranged that the top of the bulb is just level with the centre of the side-arm of the distilling-flask. 

Distillation. If the impurities in a liquid are non-volatile, the liquid may be purified by direct distillation, the impurities remaining in the distilling-flask. This process is therefore essentially the same as a simple distillation for boiling-point determination, and has been already described on pp. 7-9. 

Concentrate each of the two solutions (or eluates) to about 20 ml, by distilling off the greater part of the benzene, the distilling-flask being immersed in the boiling water-bath. Then pour the concentrated solution into an evaporating-basin, and evaporate the remaining benzene (preferably in a fume-cupboard) in the absence of free flames, i.e., on an electrically heated water-bath, or on a steam-bath directly connected to a steam-pipe. Wash the dry residue from the first eluate with petrol and then dry it in a desiccator pure o-nitroaniline, m.p. 72°, is obtained. Wash the second residue similarly with a small quantity of benzene and dry pure />--nitroaniline, m.p. 148" , is obtained. Record the yield and m.p. of each component. 

Hydrolysis of Acetoxime. Place about i g. of the recrystallised oxime in a small distilling-flask (50 ml.), add 10 ml. of dilute HjSO, and heat gently until about half the solution has distilled over. Test [a] the aqueous distillate for acetone by the iodoform reaction (p.346), b) the residual solution in the distilling-flask for hydroxylamine by... 

A considerable amount of the formic acid, however, still remains behind in the distilling-flask as the unhydrolysed monoformate. Therefore, if time allows, dilute the residue in the flask with about an equal volume of water, and then steam-distil, the monoformate ester being thus completely hydrolysed and the formic acid then driven over in the steam. Collect about 400 ml. of distillate. Add this distillate to that obtained by direct heating of the reaction mixture and then treat with lead carbonate as described above. Total yield of lead formate is now about 40 g. 

Filter the dried ethereal solution, and then distil off the ether from a small flask, using precisely similar apparatus and the same method as those described in the preparation of aniline (Fig. 64, p. 163 see also Fig. 23(E), p. 45) and observing the same precautions. When the ether has been removed, fit the distilling-flask to a short air-condenser, and distil the benzonitrile, collecting the fraction boiling between 187" and 191°. Yield, 16-5 g. (16 ml.). 

In order to save the cost of the distilling-flask (which after the above treatment is useless for further work), the mixture may be distilled from a small retort made by blowing a suitable bulb on the end of a rather thick-walled tube of about 8-10 mm. internal diameter the tube is then bent through almost 90° a few cm. above the bulb, cut to a suitable length, and the Open end fitted into the boiling ube as before. 

In the distilling-flask remains a very small high-boiling fraction of benzoic anhydride (b.p. 366 ), formed by the dehydrating action of the thionyl chloride on the benzoic acid ... 

The theory of the process is discussed in Sections 1,1-1,3. The apparatus of Fig. 11,12,1 may be used when moderate quantities of the substance are available. A is a 50 ml. distilling flask attached by a cork to a Liebig condenser B, upon the end of which an adapter C is fitted D is a receiver for collecting the distillate. The thermometer E is fitted into the neck of the distilling flask by means of a well-bored cork the bulb of the thermometer should be in the centre of the neck of the flask and slightly below (ca. 5 mm.) the level of the side tube. The flask may be heated on a wire gauze with asbestos centre or preferably in an air bath (Fig. 77, 5, 3). 

The liquid should be poured into the distilling flask, preferably through a funnel the stem of which extends below the side arm, and a few fragments of unglazed porous porcelain ( porous pot, see Section 1,2)... 

Solvents with boiling points below 90-95°. A steam bath or water bath should be employed. Alternatively, the apparatus of Fig. 77,13, 3, but with a Alter flask as receiver, may be used the end of the rubber tubing attached to the tubulure is either placed in the sink or allowed to hang over the bench. If a distillation is ultimately to be conducted in the flask from which the solvent is removed, the apparatus depicted in Fig. 77,13, 4 is recommended the distilling flask may be replaced by a Claisen flask or a Claisen flask with fractionating side arm, particularly if the subsequent distillation is to be conducted under diminished pressure. 

In order to protect the oil in the oil pump from contamination, some form of trap must be interposed between the distillation apparatus and the pump in order to prevent vapours from the distilling flask, etc. passing... 

If time is limited, the boiling point determinations by the distilling flask method for these liquids may be omitted. 

A further quantity of wopropyl iodide, only slightly less than that obtained in the first distillation, may be prepared by combining the residues in the distilling flask, adding 30 g. (38 ml.) of isopropyl alcohol, and repeating the distillation. Finally, the residues should be distUled and the 67 per cent, constant boiling point acid recovered. 

Into a 500 ml. three-necked flask, provided with a mechanical stirrer, a gas inlet tube and a reflux condenser, place 57 g. of anhydrous stannous chloride (Section 11,50,11) and 200 ml. of anhydrous ether. Pass in dry hydrogen chloride gas (Section 11,48,1) until the mixture is saturated and separates into two layers the lower viscous layer consists of stannous chloride dissolved in ethereal hydrogen chloride. Set the stirrer in motion and add 19 5 g. of n-amyl cyanide (Sections III,112 and III,113) through the separatory funnel. Separation of the crystalline aldimine hydrochloride commences after a few minutes continue the stirring for 15 minutes. Filter oflF the crystalline solid, suspend it in about 50 ml. of water and heat under reflux until it is completely hydrolysed. Allow to cool and extract with ether dry the ethereal extract with anhydrous magnesium or calcium sulphate and remove the ether slowly (Fig. II, 13, 4, but with the distilling flask replaced by a Claisen flask with fractionating side arm). Finally, distil the residue and collect the n-hexaldehyde at 127-129°. The yield is 19 g. 

An iron saucepan with a layer of sand at the bottom is quite satisfactory the distilling flask is immersed in the bath to within 2 cm. of the side arm, and two asbestos boards, cut to fit the neck of the flask, rest on top of the bath (as in Fig. II, 5, 3).. Somewhat better rosiilts are obtained if the bath is filled with nickel shot. 

Place 100 g. of adipic acid in a 750 ml. round-bottomed flask and add successively 100 g. (127 ml.) of absolute ethyl alcohol, 250 ml. of sodium-dried benzene and 40 g. (22 ml.) of concentrated sulphuric acid (the last-named cautiously and with gentle swirling of the contents of the flask). Attach a reflux condenser and reflux the mixture gently for 5-6 hours. Pour the reaction mixture into excess of water (2-3 volumes), separate the benzene layer (1), wash it with saturated sodium bicarbonate solution until eflfervescence ceases, then with water, and dry with anhydrous magnesium or calcium sulphate. Remove most of the benzene by distillation under normal pressure until the temperature rises to 100° using the apparatus of Fig. II, 13, 4 but substituting a 250 ml. Claisen flask for the distilling flask then distil under reduced pressure and collect the ethyl adipate at 134-135°/17 mm. The yield is 130 g. 

Place a mixture of 25 5 g. of n-valerio acid (Sections 111,83 and 111,84), 30 g. (37 -5 ml.) of dry n-propyl alcohol, 50 ml. of sodium-dried benzene and 10 g. (5-5 ml.) of concentrated sulphuric acid in a 250 ml. round-bottomed flask equipped with a vertical condenser, and reflux for 36 hours. Pour into 250 ml. of water and separate the upper layer. Extract the aqueous layer with ether, and add the extract to the benzene solution. Wash the combined extracts with saturated sodium bicarbonate solution until effervescence ceases, then with water, and dry with anhydrous magnesium sulphate. Remove the low boiling point solvents by distillation (use the apparatus of Fig. II, 13,4 but with a Claisen flask replacing the distilling flask) the temperature will rise abruptly and the fi-propyl n-valerate will pass over at 163-164°. The yield is 28 g. 

Remove the ether using the apparatus shown in Fig. II, 13, 4 except that a 200 ml. Claisen flask replaces the distilling flask depicted in the diagram. Distil the residual ester vmder diminished pressure (Fig. II, 20, 1) and collect the ethyl malonate at 92-94°/16 mm. The yield is 105 g. 

Tribromoaniline. Assemble the apparatus depicted in Fig. 7F, 47, 1. The distilling flask B has a capacity of 100 ml. and the bolt-head flask A (which may be replaced by a flat-bottomed flask) is 1 litre. Into the flask place 10 g. of aniline, 100 ml. of water and 10 ml. of concentrated hydrochloric acid shake until the aniline has dissolved and dilute with 400 ml. of water. 

Assemble the apparatus shown in Fig. 1 V, 67, 1 this is self-explanatory. The distilling flask has a capacity of 250 ml. and the beaker contains 150 ml. of 10 per cent, sodium hydroxide solution. All corks must fit well and should be coated with paraflSn wax (by dipping into molten wax, and allowing to drain). Place half of the yield of the dry phenyldiazonium fluoborate in the distilling flask. Heat the solid gently with a small luminous flame at one point near its surface until decomposition begins withdraw the flame and allow the reaction to continue... 

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