3 сентября, 2015 
Pokraskin Distilling Off Solvents. In general, the solvent has a much lower boiling point than the product to be separated from it, and therefore a fractionating device is usually not required. There are, however, some substances which are volatile with steam and which are also carried over with the vapors of lower boiling solvents. In these cases, it is recommended that a simple fractionating head be used, e. g., a Hempel column filled with glass beads. The form of the distillation flask is of no consequence and frequently the solvent can be distilled out of the same container from which the main product is to be rectified later. For this purpose, the use of an unduly large flask can be avoided by first putting only a portion of the liquid into the flask, and adding more from a dropping funnel from time to time as the distillation proceeds. If the distillation residue is a solid at ordinary temperatures, its removal is facilitated if a wide-mouthed distillation vessel is used.
A steam bath or an electrically heated water bath is used as the heat source for the distillation of low boiling solvents, particularly those which are inflammable, such as alcohol, benzene, and especially ether. Larger quantities of these solvents should always be distilled in a special “ether room” in which the use of open flames is prohibited. If it is necessary to distill ether or the like on a water bath heated with an open gas flame, a closed receiver is used, connected tightly on one side to the condenser and on the other to a tube leading to the floor. In this way, uncondensed ether vapor is prevented from accumulating on the table top in the vicinity of the flame. Solvents which boil over 100°C. are usually distilled using an oil bath in which the oil is heated 20-30° higher than the boiling point of the solvent. Solvents with very high boiling points, such as nitrobenzene, are suitably distilled in vacuum, or steam distilled.
A solution which may have taken up some water is dried before
distilling off the solvent. The drying is accomplished by letting the solution stand for a few hours, or better, overnight, over one of the common drying agents (calcium chloride, zinc chloride, sodium sulfate, potassium carbonate, sodium hydroxide, potassium hydroxide, calcium oxide, etc.). A drying agent is selected which will not dissolve in the solvent or react in any way with the product present. Thus, calcium chloride or zinc chloride are unsuitable for use with amines, as are potassium carbonate or potassium hydroxide for use with acids. Sodium sulfate is indifferent and can almost always be used, but it should be freshly dehydrated in order to have good drying activity. Before distilling off the solvent, the drying agent is filtered off because, on warming, the absorbed water may be set free again.
When liquids are heated in smooth-walled containers, such as glass, boiling may be delayed until finally a sudden, violent boiling-up occurs. This “bumping” may blow out the stopper or throw some of the liquid over into the condenser, and cause not only a loss of material but also a fire in some cases. These difficulties are avoided by adding boiling stones (pieces of clay or pumice about the size of a pea) or boiling rods (wooden sticks about the thickness of a match which are placed upright in the flask and which extend up into the neck), not only in distillations, but also when boiling under reflux. The wooden rods have the advantage that they can be removed easily at the end of the distillation when crystallization of the residue is expected, but of course they are useful only with liquids which do not attack wood.
A spiral condenser is best for use with low boiling solvents since it gives very efficient cooling and occupies little space. If a spiral condenser is net available, a long, straight condenser, inclined downward, may be used satisfactorily. An ordinary Liebig condenser may be used for substances of somewhat higher boiling point, and for still higher boiling solvents, a simple air-cooled condenser is best.
The connection between the condenser and the flask should be made with a rubber stopper in most cases. However, it should be noted that many solvents attack rubber (benzene, ligroin, etc.), not only causing damage to the stopper, but also introducing impurities into the materials. In such cases, it is preferable to use cork stoppers which have been carefully rolled before boring. If the vapors attack both cork and rubber, the use of an apparatus with ground glass joints is necessary.
If water is the solvent, it is generally removed by evaporation rather than by distillation unless products are present which are volatile with steam. Evaporation should be done with constant stirring. The process can be accelerated still more by removing the steam rapidly by means
of an air stream. For this purpose, one can set up a wooden propeller to rotate over the surface of the liquid in a porcelain dish or the like, or, when the evaporation is being done in a beaker or similar container, an air stream can be blown or drawn through a glass tube with its open end near the liquid surface. Any of these methods will permit rapid evaporation, without boiling, at temperatures from 80 to 90°C. Such procedures should obviously not be used if the solution contains an easily oxidizable substance. If this is the case, evaporation should be done under vacuum.
Rectification. This process usually involves substances with high boiling points, so that intensive cooling is not necessary. A fractionating flask with a long side arm is best suited for use as the distillation flask because, with this, contact between the vapor and the rubber or cork stopper is minimized. The long side arm affords sufficient cooling in many cases, but if necessary a Liebig condenser or an air condenser can be attached, or a small jacket for water cooling can be installed on the side arm itself. If the distillate solidifies, the receiver should be cooled and the distilling flask should have a wide, somewhat shorter, side arm which does not clog so easily. Substances with very high boiling points should be distilled from flasks having the side arm attached close to the bulb of the flask, so that severe overheating of the vapor is not necessary.
A thermometer is always used in rectification. It should be placed so that the upper edge of the thermometer bulb is exactly opposite to the opening into the side arm. No other arrangement will give the correct temperature of the vapor going over. Regarding thermometer corrections, see the later section on testing the purity of products (page 41).
The portion of distillate coming over below the correct boiling point (remainder of solvent, unchanged starting material, etc.) is collected separately as the “forerun.” Any water which is present should be removed carefully before rectification, if not before distilling off the solvent, because water frequently delays the attainment of a sharp boiling point. The main fraction should come over within a temperature range of one degree. Any portion that distills over beyond this range of the main fraction is collected as the “afterrun,” which can often be redistilled to yield more of the pure material.
An oil bath is usually used for heating the distilling flask, and is to be recommended particularly for cases where an appreciable amount of residue remains at the end of the distillation, because excessive heating of such residues should be avoided. In other cases, the distilling
flask can be heated carefully with a free flame, or a hot air funnel can be employed.
Fractional Distillation. It is not possible to effect, by means of simple distillation, a separation of several products whose boiling points are not greatly different. In such cases, a fractionating tube, or fractionating column, must be used. The action of a fractionating column depends on the fact that the vapors ascending from the flask are in constant contact with the liquid mixture, causing the higher boiling constituents to be condensed and thus held back, while permitting the lower boiling substances to proceed into the condenser. This action takes place to a certain extent even when the vapors are passed through a long empty tube in which some condensation occurs on the side walls. The effect is greatly improved if the tube is alternately constricted and enlarged, or better, if the tube walls are indented with points, so that the partial condensation is favored and the contact area between vapor and liquid is increased. An apparatus of this type is quite suitable for the separation of substances whose boiling points do not lie too close together. It is of little use, however, if the boiling point difference is only a few degrees, as is the case with most mixtures of isomers. When this is the case, the contact surface between vapor and liquid must be increased still more by filling the column with a suitable packing. Raschig rings make a good packing. They are cylindrical rings of almost any material — usually glass for laboratory purposes — whose height is equal to their diameter, and which are put into the column in random orientation. In addition, the quantity of liquid flowing back through the column must be regulated by the use of a partial condenser installed in the top of the column. The partial condenser is constructed so that its cooling action can be regulated, while the rest of the column is insulated to prevent heat loss. The partial condenser can be cooled to a point depending on the boiling point of the substance, using water or other liquid, or air (see Fig. 37). The greater the proportion of die vapor condensed by the partial condenser, the more complete will be the separation of the components in the mixture, and, of course, the greater will be the length of time required for the distillation. The heating should always be uniform for fractional distillations, and the use of a water bath, an oil bath, or a graphite bath is recommended, the choice depending on the boiling point of the mixture.
With an efficient fractionating column, almost complete separation, for example, of a mixture of benzene, monochlorobenzene, and dichlorobenzene, can be achieved on a laboratory scale (see page 64). On the other hand, the isomeric dichlorobenzenes, whose boiling points differ
by only four or five degrees, cannot be separated completely, even on a large scale. In some cases involving liquid mixtures of isomers which are solids in the pure state, good separation is obtained by careful fractionation. The first and last fractions, each containing predominantly one or the other of the isomers, are cooled, whereupon the pure isomers are crystallized. The crystals are then separated from the residual liquid by centrifuging, the liquid residues being returned to the process (see the separation of 2,4- and 2,6-nitrochlorotoluenes, page 160).
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