Лако-красочные материалы — производство

It was pointed out in an earlier section that distillation in its various forms is generally the cheapest industrial method of purifi­cation. It is, of course, applicable only to those substances which can be volatilized without decomposition, and serves a useful purpose only if the impurities or by-products are sufficiently different in boiling point from the product being purified. Substances whose boiling points lie within a narrow range can often be separated sufficiently well for technical purposes by careful fractional distillation, although a com­plete separation may not be achieved. For example, technical benzene (b. p., 80°C.) cannot be freed, by distillation alone, from the thio­phene (b. p., 84°) which is always present as an impurity. Also, technical o-nitrotoluene (b. p., 220°) always contains some of the isomeric substances (b. p., 228° to 238°), although the amounts of the isomers present are so small that they do not interfere with the technical use of the product. If very high purity requirements are to be met, it may be necessary to combine distillation with other methods of purification.

Next to distillation, the most important purification method, especially in the laboratory, is recrystallization. In this operation, the substance to be purified is dissolved by heating with a suitable solvent, usually by boiling under reflux, and the hot solution is filtered to remove any undissolved impurities. The clear filtrate is cooled slowly, whereupon most of the main product crystallizes out, leaving the more soluble impurities in solution. If crystallization occurs slowly, it may be hastened by stirring. Some substances tend to form super­saturated solutions, and with these it may be necessary to induce

crystallization by scratching the walls of the container with a glass rod, or by seeding with a few small crystals of the substance. The crystallization can be brought to completion by cooling the solution in ice, or by allowing it to stand in the ice chest. A quantity of the solvent is employed that will dissolve the product when hot (except, of course, for the insoluble impurities) and that will hold the soluble impurities in solution in the cold so far as possible. If the product is quite impure, it may be desirable to wash it, before crystallization, with a cold solvent which is a good solvent for the impurities but which does not easily dissolve the main product. The use of animal charcoal, blood charcoal, activated carbon, or other strong adsorption agents, is often of value for removal of colored or colloidal impurities. The adsorption takes place better from a warm solution than in the cold, and requires some time; hence, the solution should be heated about 30 minutes with the decolorizing agent, then filtered hot. (Alkaline solutions cannot be decolorized with animal charcoal or activated carbon.)

The correct choice of solvent is of the utmost importance in determining the success of a recrystallization. If the literature gives no information on this point, the best solvent must be selected on the basis of test tube experiments. The first requirement, of course, is that the solvent must dissolve the substance more easily when hot than when cold, and it must allow most of the material to crystallize out on cooling. In addition, it should either not dissolve the impuri­ties, so that they can be removed in the hot filtration, or it should dissolve them so easily that they remain in solution and do not crystallize out with the main product. It is not always possible to find a solvent which fulfills these requirements, and it becomes neces­sary to repeat the crystallization, preferably with a change of solvent If there are several solvents which are equally suitable, preference is given to the one which has a moderate solvent action on the material. The quantity of material to be purified is another factor to be taken into consideration. If this quantity is quite large, say 200 grams, a fairly active solvent would be selected, so that about 1 liter of solvent, for example, would suffice to dissolve the material when hot. On the other hand, if the same selection of solvent were made in a research experiment where only 0.1 gram of the material was available, then the crystallization would be very difficult to carry out, since only 0.5 cc. solvent would be used. Under these circum­stances, it would be better to use a much less active solvent so that a quantity of 5—10 cc. could be used. In cases where one solvent

is too active and others are too inactive, the best results are often obtained by the use of a mixture of solvents, for example, water and alcohol, water and acetic acid, benzene and ligroin, or ether and chloroform. The use of a solvent mixture is resorted to only in necessary cases, because the results obtained are usually less satis­factory than when a single solvent is used. In addition, it is frequently difficult to work up the mother liquors when they contain more than one solvent.

The process of merely dissolving a substance in a solvent and then recovering the material by evaporation of the solvent does not constitute a recrystallization and does not effect a purification except insofar as less soluble impurities are removed by filtration. To be sure, one is frequently compelled to distill off part of the solvent after filtration in order to get full crystallization, but this concentration should never be carried to the point where the more soluble impuri­ties are not really dissolved. Substances which are difficultly soluble can be subjected to a sort of continuous dissolving and crystallization process, known as extraction. The raw product may be extracted with a low boiling solvent in a Soxhlet apparatus, in which the extraction thimble is placed above the extraction flask. For high boiling sol­vents, the Noll apparatus is used, in which the thimble is suspended in the neck of the flask.

More or less of the substance being purified always remains dis­solved in the recrystallization mother liquor, along with the soluble impurities. This material can be recovered by distilling off a part of the solvent, cooling, and filtering off the resulting crystals. The new filtrate can be concentrated again, and the process repeated as long as usable crystals are obtained. These mother liquor products are usually less pure than the first fraction, but they can be purified by subsequent recrystallization.

It should be noted that within a group of solvents having the same general character the solvent action runs parallel to the boiling point. Thus, a substance which dissolves only slightly in boiling benzene (b. p., 80°C.) is usually more soluble in boiling toluene (b. p., 111°), and still more soluble in boiling xylene (b. p., 140°) or chlorobenzene (b. p., 132°). Therefore, one is compelled to use high boiling solvents, such as trichlorobenzene, nitrobenzene, tetralin, aniline, phenol, etc., for recrystallizing very difficultly soluble compounds (frequently, e. g., with anthraquinone derivatives). The high boiling solvents have the advantage that they are good solvents at the boiling point, but poor solvents in the cold, so that the product crystallizes out quite com-

pletely. Unfortunately, the same solubility relations often hold also for the impurities, so that they too are dissolved in the hot solvent and separate on cooling. In such instances, no purification is accomplished. In general, a correctly performed recrystallization is an effective method of purification. It is, however, a rather costly operation on a technical scale, especially if it involves the use of organic solvents which must be recovered. The recovery of solvents is never accom­plished without some loss.

For this reason, preference is often given in industrial work to a process which might be called reprecipitation. This can be an effective method of purification if correctly carried out. By reprecipitation is meant the process of dissolving a product in a suitable solvent, either hot or cold, and then precipitating it by the addition of any other component which decreases the solubility. Solution in alcohol and reprecipitation by the addition of a second solvent, such as ether or water, might be taken as an example of this process. In industry, preference is given, for economic reasons, to those reprecipitation procedures which do not require the use of organic solvents, but which operate in purely aqueous media. Such procedures are exempli­fied by the process used so frequently in purifying sulfonic acids and dyes, consisting of dissolving the material in water and then precipi­tating it by adding some suitable salt —common salt, sodium sulfate, calcium chloride, ammonium sulfate, .etc. Usually, solution of the material is assisted by warming, and, if at all possible, the precipita­tion is carried out in the warm solution in order to obtain the material in a more coarsely granular form which is easier to filter. If the precipitation is sufficiently complete, the mixture may also be filtered warm; frequently, however, the product does not separate completely unless the mixture is cooled. Stirring during the cooling accelerates the crystallization and. makes it more uniform. Unless the solution is dear, it should be filtered before the addition of the pre­cipitating agent, and the latter should be used, if possible, in the form of a clear solution (e. g., a filtered saturated salt solution). In this way, the introduction of new impurities along with the precipitating agent is avoided. The precipitating agent is used in an amount which will precipitate the desired product completely without throwing out the impurities. After the product has separated completely, it is filtered with suction and washed on the funnel with a salt solution of the same concentration present in the mixture after precipitation. Th, us, if in the precipitation process, an aqueous solution was mixed with an equal volume of saturated salt solution, then a mixture of equal

volumes of saturated salt solution and water is used for washing the precipitate. In order to remove as much salt from the product as possible, it is often desirable to wash the product again with a some­what more dilute salt solution, and finally with a very small volume of cold water if the product is not too soluble. In the laboratory, a still further purification of sulfonic acids and their salts can often be achieved by a final washing of the precipitate with alcohol and then with ether. In this way, colored or tarry impurities are frequently removed. Furthermore, the precipitate dries more rapidly and with­out caking, so that a loose, light-colored product is obtained. Washing with alcohol should never be undertaken until a test has been made to show that the product is not appreciably soluble in alcohol. Most, but not all, sulfonates are sufficiently insoluble in alcohol. Some of them, however, are very easily soluble.

Another reprecipitation method commonly used is applicable to compounds which are difficultly soluble in water, but which form easily soluble salts. These include bases which dissolve in dilute acids and which are thrown out of solution by the addition of alkalis, as well as acidic substances, such as carboxylic acids, sulfonic acids, and phenols, which are dissolved in alkalis and thrown out by the addition of acids. This type of reprecipitation is effective in removing only those impurities which have no basic or acidic properties, and which, therefore, are not dissolved by the acid or alkali and are removed by filtration. Impurities having the same chemical properties as the main product are not removed to any extent by such a process. The effec­tiveness of the method is greatly increased, however, by two modifi­cations.

The first modification is fractional precipitation. As an example, a basic crude product ‘is dissolved in just the necessary amount of dilute hydrochloric acid. The alkali required to neutralize the acid is not added all at once, but first only a small portion of it, perhaps 5 or 10 per cent, is added, so that any less basic impurities which may be present are precipitated first. These less basic impurities are then filtered off, and on further neutralization of the filtrate an essentially pure base separates. Conversely, if the impurities are more strongly basic than the main product, they begin to precipitate only toward the end of the neutralization. In this case, a pure base is obtained by adding only 80 or 90 per cent of the quantity of alkali required to neutralize the hydrochloric acid. Obviously, the same method can be applied to compounds of an acidic nature. It gives good results where compounds of different basicities or acidities are to be separated, as

is often the case with mixtures of isomers. It was mentioned earlier, for example, that a nuclear ortho or para halogen greatly reduces the basicity of an amino group, but a halogen in the meta position has much less effect. Hence, if a reaction gives a m-chlorinated amine as the main product and a small amount of the ortho or para isomer as by-product, the latter can be removed by fractional precipitation.

In the second modification, which is of more general application, the dissolved salt is not transformed into the free base or acid, but is separated as such. Usually this is done, in the case of alkali metal salts of acids, by salting out, as described above. With amine salts, an excess of the acid used in dissolving the amine often effects the salt­ing out. For example, as was pointed out before, aniline hydrochloride is very soluble in water but quite insoluble in concentrated hydro­chloric acid. The difference is still greater with other amines. a-Naph — thylamine hydrochloride, for instance, is almost completely precipitated from its solutions by the addition of only a small excess of hydro­chloric acid. Instead of separating the easily soluble salt as such, it may be transformed into a less soluble salt, for example, by changing a hydrochloride into the sulfate, or an alkali metal salt into a barium salt or lead salt, if these are less soluble. Thus, the usual procedure for purifying benzidine is to dissolve the crude base in dilute hydro­chloric acid and then precipitate it as the sulfate by adding sodium sulfate. The sulfate of benzidine is practically insoluble and separates completely, while the sulfates of the isomeric bases formed as side products remain in solution. Obviously, the salts precipitated by any of these methods must be filtered off and washed before they are reconverted to the free compounds. In many cases, the salt as such is usable directly. Thus, anthranilic acid can be purified either by dis­solving in hydrochloric acid and precipitating its hydrochloride, or by dissolving in sodium carbonate solution and isolating the sodium salt. Compounds purified in this way are usually much more pure than those purified by solution in acid and precipitation with alkali or vice versa. In general, the purity is high enough for practical purposes. In cases where especially high purity is necessary, more complete puri­fication can be accomplished by recrystallizing or reprecipitating the isolated salt before converting it back to the free compound.

A related reprecipitation procedure, particularly valuable with difficultly soluble anthraquinone derivatives, consists in dissolving the crude product, which has weakly basic properties, in concentrated sulfuric acid. The acid is then diluted, by careful addition of water, just to the point where the sulfate separates out but is not decomposed

by hydrolysis. Ordinarily, this point is reached when the sulfuric acid content is about 70 or 80 per cent. The precipitated sulfate is filtered off on a sintered glass funnel and washed with acid of the same con­centration, and then finally converted back to the free base by stirring with a large volume of water.

In addition to those described, there are many other methods for purifying organic compounds. Examples which might be mentioned are sublimation, chromatography, and conversion into more crystal — lizable derivatives, such as amines into their acetyl, benzoyl, or other acyl derivatives, or acids into their chlorides, amides, esters, etc. These are the methods which are generally used in research labora­tories. Although they are used also in industrial laboratories, and the technical chemist should be familiar with them, it is beyond the scope of this book to treat them in detail.