Crystallisation fractional

A single crystallisation operation performed on a solution or a melt may fail to produce a pure crystalline product for a variety of reasons including  

Re-crystallisation from a solution or a melt is, therefore, widely employed to increase crystal purity. 

Explain how fractional crystallisation may be applied to a mixture of sodium chloride and sodium nitrate given the following data. At 293 K, the solubility of sodium chloride is 36 kg/100 kg water 

In this way, relatively pure NaNC 3, depending on the choice of conditions and particle size, has been separated from a mixture of NaN03 and NaCl. 

An alternative approach is to note that points C and B represent 59 and 88 kg NaNO3/100 kg water and assuming CB to be a straight line, then by similar triangles  

The process of fractional crystallisation is employed to separate two or more substances, all of which are soluble in the solvent used. When only two substances are present, it is often possible to find, by preliminary tests, a solvent which, when used in suitable quantity, will dissolve the whole of 

When commencing a fractional crystallisation, preliminary tests similar to those described under Crystallisation are first carried out, and the crystals which separate during such tests examined with a lens. The crystals which form first are either the least soluble or most abundant constituent of the mixture. If a second or further type of crystal appears, its shape and time of formation relative to cooling should be noted it is often necessary to filter off the first crop while the mixture is still warm. 

When dealing with a finely powdered or amorphous mixture, it is often useful to examine a small portion placed on a watch-glass under the microscope. The action of a few drops of various solvents (hot or cold) can be examined in this position, and valuable information—which might not be obvious to the naked eye—perhaps gained concerning the solubility or insolubility of some constituent in a particular solvent. 

No definite plan can be given which will suit all examples of fractional crystallisation. The scheme outlined in Text-book of Inorganic Chemistry, Yol. IV., p. 324 (J. Newton Friend), when applicable, affords a convenient method of marking and recording the various fractions involved in a fractional crystallisation it also avoids the accumulation of a vast number of small crops and mother liquors — 

This method was found very useful for the purification of d-a-phenyl-ethylamine Z-malate (p. 408). 

---

Originally, general methods of separation were based on small differences in the solubilities of their salts, for examples the nitrates, and a laborious series of fractional crystallisations had to be carried out to obtain the pure salts. In a few cases, individual lanthanides could be separated because they yielded oxidation states other than three. Thus the commonest lanthanide, cerium, exhibits oxidation states of h-3 and -t-4 hence oxidation of a mixture of lanthanide salts in alkaline solution with chlorine yields the soluble chlorates(I) of all the -1-3 lanthanides (which are not oxidised) but gives a precipitate of cerium(IV) hydroxide, Ce(OH)4, since this is too weak a base to form a chlorate(I). In some cases also, preferential reduction to the metal by sodium amalgam could be used to separate out individual lanthanides. 

Fractional crystallisation is occasionally employed. The mixture is dissolved in a suitable solvent, the whole frozen, and then allowed to melt slowly in a centrifuge in order that the successive fractions may be removed as they are formed. The various melts are then fractionally distilled. If necessary, the fractional crystallisation may be repeated. 

Commercial benzene may contain thiophene C H S, b.p. 84°, which cannot be separated by distillation or by fractional crystallisation. The presence of thiophene may be detected by shaking 3 ml. of benzene with a solution of 10 mg. of isatin in 10 ml. of concentrated sulphuric acid and allowing the mixture to stand for a short time a bluish-green colouration is produced if thiophene is present. The thiophene may be removed from benzene by any of the following methods —... 

Ak2o has been iastmmental ia developiag a new process for the stereospecific synthesis of 1,4-cyclohexane diisocyanate [7517-76-2] (21). This process, based on the conversion of poly(ethylene terephthalate) [25038-59-9] circumvents the elaborate fractional crystallisation procedures required for the existing -phenylenediamine [108-45-2] approaches. The synthesis starts with poly(ethylene terephthalate) (PET) (32) or phthaUc acid, which is converted to the dimethyl ester and hydrogenated to yield the cyclohexane-based diester (33). Subsequent reaction of the ester with ammonia provides the desired bisamide (34). The synthesis of the amide is the key... 

The mixed monocyclic aromatics are called BTX as an abbreviation for ben2ene, toluene, and xylene (see Btxprocessing). The benzene and toluene are isolated by distillation, and the isomers of the xylene are separated by superfractionation, fractional crystallisation, or adsorption (see Xylenes and ethylbenzene). Bensene is the starting material for styrene (qv), phenol (qv), and a number of fibers and plastics. Toluene (qv) is used to make a number of... 

Froth Flotation. Froth flotation (qv) of potassium chloride from sylvinite ores accounts for ca 80% of the potassium chloride produced in North America and about 50% of the potassium chloride in Europe and the CIS. Fractional crystallisation and heavy-media processing account for the remaining amounts produced. Froth flotation has been described (6,16,17). 

Fractional crystallisation is based on favorable solubiUty relationships. Potassium chloride is much more soluble at elevated temperatures than at ambient temperatures in solutions that are saturated with sodium and potassium chlorides. Sodium chloride is slightly less soluble at elevated temperatures than at ambient temperatures in solutions that are saturated with KCl and NaCl. Working process temperatures are usually 30—110°C. The system,... 

Table 3 fists cycloaliphatic diamines. Specific registry numbers are assigned to the optical isomers of /n t-l,2-cyclohexanediamine the cis isomer is achiral at ambient temperatures because of rapid interconversion of ring conformers. Commercial products ate most often marketed as geometric isomer mixtures, though large differences in symmetry may lead to such wide variations in physical properties that separations by classical unit operations are practicable, as in Du Font s fractional crystallisation of /n t-l,4-cyclohexanediamine (mp 72°C) from the low melting (5°C) cis—trans mixture. 

The polyamide copolymer of dodecanoic acid with methylenedi(cyclohexylamine) (MDCHA, PACM) was sold as continuous filament yam fiber under the tradename QIANA. As late as 1981, over 145,000 t was produced using high percentages, typically 80%, of trans, trans MDCHA isomer. The low melting raffinate coproduct left after t,t isomer separation by fractional crystallisation was phosgenated to produce a Hquid aUphatic diisocyanate marketed by Du Pont as Hylene W. Upon terrnination of their QIANA commitment, Du Pont sold the urethane intermediate product rights to Mobay, who now markets the 20% trans, trans—50% cis, trans—30% cis, cis diisocyanate isomer mixture as Desmodur W. In addition to its use in polyamides and as an isocyanate precursor, methylenedi (cyclohexyl amine) is used directiy as an epoxy curative. The Hquid diamine mixture identified historically as PACM-20 is marketed as AMICURE PACM by Anchor Chemical for performance epoxies. 

By fractional crystallisation of the mixture, either by direct freezing or by dissolving in a suitable... 

Because phenols are weak acids, they can be freed from neutral impurities by dissolution in aqueous N sodium hydroxide and extraction with a solvent such as diethyl ether, or by steam distillation to remove the non-acidic material. The phenol is recovered by acidification of the aqueous phase with 2N sulfuric acid, and either extracted with ether or steam distilled. In the second case the phenol is extracted from the steam distillate after saturating it with sodium chloride (salting out). A solvent is necessary when large quantities of liquid phenols are purified. The phenol is fractionated by distillation under reduced pressure, preferably in an atmosphere of nitrogen to minimise oxidation. Solid phenols can be crystallised from toluene, petroleum ether or a mixture of these solvents, and can be sublimed under vacuum. Purification can also be effected by fractional crystallisation or zone refining. For further purification of phenols via their acetyl or benzoyl derivatives (vide supra). 

Difficult to distil because it is a solid at ambient temperatures. Purified by fractional crystallisation and sublimation. 

Iodine monobromide [7789-33-5] M 206.8, m 42°. Purified by repeated fractional crystallisation from its melt. 

The constitution of laudanosine was determined by Pictet and Athanasescu/ who prepared it by reducing papaverine methochloride with tin and hydrochloric acid and deracemising the dZ-laudanosine (iV-methyltetrahydropapaverine) so obtained, by fractional crystallisation of the quinate. Laudanosine must, therefore, be represented by formula... 

Gadamer by fractional crystallisation of dZ-tetrahydroberberine bromocamphorsulphonate, isolated a laevorotatory alkaloid identical... 

Caryophyllene nitrosochloride, (CjgHgJjN OoCL, is obtained when a mixture of the sesquiterpene, alcohol, ethyl acetate, and ethyl nitrite is cooled in a freezing mixture, and then treated with a saturated solution of hydrochloric acid in alcohol. The reaction mass is allowed to stand on ice for an hour and is then exposed to sunlight. Thus prepared it melts at about 158° to 163°, and can be separated into two compounds, one being that of a-caryophyllene and the other that of yS-caryophyllene Deussen s sesquiterpenes of natural caryophyllene from clove oil), a-caryophyllene nitrosochloride melts at 177", and /3-caryophyllene nitrosochloride at 159°. They can be separated by fractional crystallisation. The corresponding a-nitrolbenzylamine melts at 126° to 128°, and the /3-nitrolbenzylamine at 172° to 173°. The bimolecular formula given above is probable but not certain. 

No crystalline hydrochlorides eould be obtained from either santalene. tt-Santalene forms a liquid dihydrochloride of optical rotation -n 6°, when dry hydrochloric acid is passed through its ethereal solution. It also forms a crystalline nitrosochloride melting with decomposition at 122°, and a nitrol-piperidide melting at 108° to 109°. /3-santalene forms corresponding compounds, the dihydrochloride having a rotatory power -H 8°. It forms, however, two isomeric nitrosochlorides, CigHj NOCl. They may be separated by fractional crystallisation from alcohol. One melts at 106°, the other at 152°. The corresponding nitrol-piperidides melt at 105° and 101° respectively. 

Faolini and Divizia have succeeded in partially resolving inactive linalol into its optically active isomers hut only to the extent of optical rotations of + 1° 70 and — 1° 60 respectively. Linalol was converted into its acid phthalate, and an alcoholic solution of this compound was treated with the equivalent quantity of strychnine. By fractional crystallisation the laevo-rotatory salt, yielding dextro-rotatory linalol, separates first, leaving the more soluble dextro-rotatory strychnine salt, which yielded laevo-rotatory linalol in the mother liquor. 

No isomer appeared to be present in savin oil, since no separation could be effected by conversion of the hydrogen phthalate into its strychnine salt, and fractional crystallisation thereof. The strychnine salt, CggH OgNj, melts at 200° to 201°. 

Pickard, Lewcock and Yates have prepared fenchyl alcohol by the reduction of laevo-rotatory. On conversion into its hydrogen phthalate and fractionally crystallising the magnesium and cinchonine salts, they obtained a fraction, which on saponification yielded Za w-fenchyl alcohol, having a specific rotation of - 15 5°, which is probably the correct value for this figure. 

Its optical rotation has usually been recorded as from - 2° to - 3", but recently Pickard, Lewcock, and Yates have prepared f-isopulegol by fractional crystallisation of the magnesium and cinchonine salts of the hydrogen phthalate, and found its specific rotation to be - 22-2°. 

The most reliable work on the isomeric menthols is that of Pickard and Littlebury.2 Starting from the mixture of alcohols which Brunei had obtained by the reduction of thymol, and which he had described under the name of thymomenthol, they isolated from it about 60 per cent, of isomeric menthols, 30 per cent, of menthones, and several other compounds. They prepared the phthalic acid esters and converted them into their magnesium and zinc salts, which were then fractionally crystallised. 

Thymol frequently occurs associated with carvacrol, its wtfio-isomer, and may be separated therefrom by fractional crystallisation of the phenyltirethanes, that of carvacrol being much less soluble in petroleum ether than that of thymol. 

Some of the artificial musks of commerce are mixtures of two or more of the bodies above described, a fact which is easily demonstrated by fractionally crystallising the specimen, when the fractions will show altered melting-points. The principal adulterant of artificial musk is acetanilide. This can, of course, be easily detected by the phenyl-isocyanide reaction or by dissolving it out with hot water. 

Explain how fractional crystallisation may be applied to a mixture of sodium chloride and sodium nitrate, given the following data. At 290 K, the solubility of sodium chloride is 36 kg/100 kg water and of sodium nitrate 88 kg/100 kg water. Whilst at this temperature, a saturated solution comprising both salts will contain 25 kg sodium chloride and 59 kg sodium nitrate/100 parts of water. At 357 K these values, again per 100 kg of water, are 40 and 176, and 17 and 160 kg respectively. 

This stereospecific oxidation does not occur for all dioximes, probably due to isomerisation of the dioxime during the reaction or to different reaction mechanisms involved in the use of different oxidants. When the lipophilic-hydrophilic balance of the two furoxan isomers is appropriate, they are easily separated by chromatography or fractional crystallisation. For example, the synthesis of 4-hydroxymethyl-3-furoxancarboxamide (CAS 1609), one of the most promising furoxancarboxamide vasodilators (see later), passes through the intermediate formation of a mixture of the two isomeric methyl hydroxymethylfuroxancarboxylic esters, which can easily be separated by recrystallisation from isopropyl acetate [18]. 

---