Purification, liquids fractional distillation

Classical methods for separation and purification include fractional distillation of liquids and recrystallization of solids, and these two methods are routinely included in the early portions of laboratory courses in organic chemistry. Because they are capable of being adapted to work on a large scale, fractional distillation and recrystallization are the preferred methods for purifying organic substances in the pharmaceutical and chemical industries. 

One of the most widely applicable and most commonly used methods of purification of liquids or low melting solids (especially of organic chemicals) is fractional distillation at atmospheric, or some lower, pressure. Almost without exception, this method can be assumed to be suitable for all organic liquids and most of the low-melting organic solids. For this reason it has been possible in Chapter 4 to omit many procedures for purification of organic chemicals when only a simple fractional distillation is involved - the suitability of such a procedure is implied from the boiling point. 

Where larger quantities (upwards of Ig) are required, most of the impurities should be removed by preliminary treatments, such as solvent extraction, liquid-liquid partition, or conversion to a derivative (vide supra) which can be purified by crystallisation or fractional distillation before being reconverted to the starting material. The substance is then crystallised or distilled. If the final amounts must be in excess of 25g, preparation of a derivative is sometimes omitted because of the cost involved. In all of the above cases, purification is likely to be more laborious if the impurity is an isomer or a derivative with closely similar physical properties. 

Common impurities found in aldehydes are the corresponding alcohols, aldols and water from selfcondensation, and the corresponding acids formed by autoxidation. Acids can be removed by shaking with aqueous 10% sodium bicarbonate solution. The organic liquid is then washed with water. It is dried with anhydrous sodium sulfate or magnesium sulfate and then fractionally distilled. Water soluble aldehydes must be dissolved in a suitable solvent such as diethyl ether before being washed in this way. Further purification can be effected via the bisulfite derivative (see pp. 57 and 59) or the Schiff base formed with aniline or benzidine. Solid aldehydes can be dissolved in diethyl ether and purified as above. Alternatively, they can be steam distilled, then sublimed and crystallised from toluene or petroleum ether. 

The purification of diethyl ether (see Chapter 4) is typical of liquid ethers. The most common contaminants are the alcohols or hydroxy compounds from which the ethers are prepared, their oxidation products (e.g. aldehydes), peroxides and water. Peroxides, aldehydes and alcohols can be removed by shaking with alkaline potassium permanganate solution for several hours, followed by washing with water, concentrated sulfuric acid [CARE], then water. After drying with calcium chloride, the ether is distilled. It is then dried with sodium or with lithium aluminium hydride, redistilled and given a final fractional distillation. The drying process should be repeated if necessary. 

Electronic Grade Silicon (EGS). As the first step in the production of electronic grade silicon (EGS), an impure grade of silicon is pulverized and reacted with anhydrous hydrochloric acid, to yield primarily tricholorosilane, HSiClg. This reaction is carried out in afluidizedbed at approximately 300°C in the presence of a catalyst. At the same time, the impurities in the starter impure silicon reactto form their respective chlorides. These chlorides are liquid at room temperature with the exception of vanadium dichloride and iron dichloride, which are soluble in HSiCl3 at the low concentration prevailing. Purification is accomplished by fractional distillation. 

The simplest problem that presents itself in the preparation of pure chemical materials is the purification of an impure specimen. In the case of many liquids, this is accomplished by fractional distillation in the case of some solids, it may be effected by the process of sublimation but for the most part, it is done by crystallization. For the present, attention will be directed to this last process. 

In the manufacture of ethylene dibromide, gaseous ethylene is brought into contact with bromine by various methods, allowing for dissipation of the heat of reaction (100—102). Free acids are neutralized and the product may be fractionally distilled for purification. Typical specifications call for a clear liquid with 99.5% purity min sp gr (25/25°C), 2.170—2.180 boiling range, 130.4—132.4°C APHA color, 200 max water, 200 ppm max acidity as HC1, 0.0004 wt % max and nonvolatile matter, 0.0050 wt % max. 

---