Crystallisation from

Concentrated sulphuric acid has a strong affinity for water and great heat is evolved on mixing hence the acid must be added to water to dilute it. Because of this affinity, the acid can be used to dry gases with which it does not react, for example oxygen, chlorine, sulphur dioxide, and is used in desiccators. It will remove water of crystallisation from some compounds, for example... 

A somewhat similar reaction is the power of sulphur oxide dichloride to remove water of crystallisation from hydrated chlorides, the hydroxyl groups of the water molecule reacting as do those in the acid molecules in the above reaction. 

Most iodine produced commercially comes from the sodium iodate(V) remaining after sodium nitrate has been crystallised from Chile saltpetre. The iodatefV) is first reduced to iodide by blowing sulphur dioxide into the solution (or by addition of sodium sulphite) ... 

Although the complex ion [MnClg] is unstable, salts such as K2[MnF6] (containing the octahedral hexafluoromanganate(IV) ion) are much more stable and can be crystallised from solution. 

Hydrated cobalt III) sulphate, Co2(S04)3. JSHjO is obtained when cobalt(II) sulphate is oxidised electrolytically in moderately concentrated sulphuric acid solution it is stable when dry but liberates oxygen from water. Some alums, for example KCo(S04)2.12H,0 can be obtained by crystallisation from sulphuric acid solutions. In these and the sulphate, the cation [CofHjO) ] may exist it is both acidic and strongly oxidising. 

The s)mthesis of zeolites is traditionally performed by crystallisation from a sol-gel mixture comprising reagents such as silica, sodium aluminate, sodium hydroxide and water. Another key component of the sol-gel mixture is a base whose main role is to regulate the pH of the mixture. If an organic base is used then a templating effect may also be observed... 

A) A solid substance has crystallised from a solution, and it is necessary to separate the crystals (i.e., the solute) from the cold mother-liquor by filtration. 

Sometimes the crude substance may contain an insoluble impurity, and on cooling the solution it may be difficult to judge how much of the solid matter is merely undissolved impurity and how much is solute which has subsequently crystallised from solution. To avoid this difficulty, the hot solution should be filtered, and should thus always be absolutely clear before cooling is attempted. Therefore filter the hot solution into a clean tube through a very small fluted filter-paper contained in a correspondingly small glass funnel, which should have had its stem cut off as that shown in Fig. 6, p. 12 (and for the same reason). Unless the upper part of the filter is cut awav to reduce its size to a minimum, a large proportion of the solution will remain held mechanically in the pores of the paper itself and only a few drops of clear filtrate will be obtained. 

Acetamide is thus obtained as a colourless crystalline solid, which has a characteristic odour of mice, stated to be due to the presence of small quantities of methylacetamide, CH3CONHCH3. The acetamide can be purified and rendered odourless by re-crystallisation from acetone, and then has m.p. 82°, b.p. 223°. If this recrystallisation is contemplated, the distilled material should be collected directly into a small weighed beaker or conical flask, so that the solidified acetamide can be readily broken up and removed. 

A further crystallisation from 15 ml. of hydrochloric acid gives the sulphonic acid, 5 5 g., m.p. 105°, almost devoid of deliquescent properties. 

If desired, the 3,5-dinitrobenzoic acid may be re crystallised from water as almost colourless rhombs, m.p. 204 . 

Dissolve 1 g. of anthracene in 10 ml. of glacial acetic acid and place in 50 ml. bolt head flask fitted with a reflux water-condenser. Dissolve 2 g. of chromium trioxide in 2 ml. of water and add 5 ml. of glacial acetic acid. Pour this solution down the condenser, shake the contents of the flask and boil gently for 10 minutes. Cool and pour the contents of the flask into about 20 ml. of cold water. Filter off the crude anthraquinone at the pump, wash with water, drain well and dry. Yield, 1 g. Purify by re crystallisation from glacial acetic acid or by sublimation using the semi-micro sublimation apparatus (Fig. 35, p. 62, or Fig. 50, p. 70). 

Add 4 g. of malonic acid to 4 ml. of pyridine, and then add 3 1 ml. of crotonaldehyde. Boil the mixture gently under reflux over an asbestos-covered gauze, using a small Bunsen flame, for 40 minutes and then cool it in ice-water. Meanwhile add 2 ml. of concentrated sulphuric acid carefully with shaking to 4 ml. of water, cool the diluted acid, and add it with shaking to the chilled reaction-mixture. Sorbic acid readily crystallises from the solution. Filter the sorbic acid at the pump, wash it with a small quantity of cold water and then recrystallise it from water (ca, 25 ml.). The colourless crystals, m.p. 132-133°, weigh ro-i-2 g. 

Acetylation. Heat i g. of />-nitrophenol with 5 ml. of an acetic acid-acetic anhydride mixture under reflux for 15 minutes. Pour into water the solid acetate separates. Filter, wash with water and re-crystallise from ethanol m.p. 77 5°. This treatment usually leaves o-nitrophenol unchanged. The addition, however, of about 0 5 ml. of cone. H2SO4 to the acetylating mixture gives the o-derivative, m.p. 40°. 

Solid organic compounds when isolated from organic reactions are seldom pure they are usually contaminated with small amounts of other compounds ( impurities ) which are produced along with the desired product. Tlie purification of impure crystalline compounds is usually effected by crystallisation from a suitable solvent or mixture of solvents. Attention must, however, be drawn to the fact that direct crystallisation of a crude reaction product is not always advisable as certain impurities may retard the rate of crystallisation and, in some cases, may even prevent the formation of crystals entirely furthermore, considerable loss of... 

If the crude substance contains an insoluble impurity, difficulty may be experienced at a later stage in estimating how much solute has crystallised from the cold solution. The hot solution should therefore be filtered into another tube through a very small fiuted filter paper contained in a small short-stemmed funnel. The solution must always be clear before cooling is attempted. 

The condensation products are almost insoluble in water, but can be crystallised from dilute alcohol. Dimedone is therefore a good reagent for the detection and characterisation of aldehydes. 

Semicarbazones. Dissolve 1 g. of semicarbazide hydrochloride and 1 5g. of crystallised sodium acetate in 8-10 ml. of water add 0 - 5-1 g. of the aldehyde or ketone and shake. If the mixture is turbid, add alcohol (acetone-free) or water until a clear solution is obtained shake the mixture for a few minutes and allow to stand. Usually the semicarbazone crystallises from the cold solution on standing, the time varying from a few minutes to several hours. The reaction may be accelerated,... 

With phenylalanine and tyrosine, the sodium salt of the derivative is sparingly soluble in water and separates during the initial reaction. Acidify the suspension to Congo red the salts pass into solution and the mixture separates into two layers. The derivative is in the etheresil lay and crystallises from it within a few minutes. It is filtered off and recrystaUised. 

The following give abnormal results when treated with chlorosulphonio acid alone, preferably at 50° for 30-60 minutes —fluobenzene (4 4 -difluorodiplienyl-sulphone, m.p. 98°) j iodobenzene (4 4 -di-iododiphenylsulplione, m.p. 202°) o-diclilorobenzene (3 4 3. -4 -tetrachlorodiphenylsulphone, m.p. 176°) and o-dibromobenzene (3 4 3 4 -tetrabromodiphenylsulphone, m.p. 176-177°). The resulting sulphones may be crystallised from glacial acetic acid, benzene or alcohol, and are satisfactory for identification of the original aryl halide. In some cases sulphones accompany the sulphonyl chloride they are readily separated from the final sulphonamide by their insolubility in cold 6N sodium hydroxide solution the sulphonamides dissolve readily and are reprecipitated by 6iV hydrochloric acid. 

The high sodium ion concentration results in facile crystallisation of the sodium salt. This process of salting out with common salt may be used for recrystallisation, but sodium benzenesulphonate (and salts of other acids of comparable molecular weight) is so very soluble in water that the solution must be almost saturated with sodium chloride and consequently the product is likely to be contaminated with it. In such a case a pure product may be obtained by crystallisation from, or Soxhlet extraction with, absolute alcohol the sul-phonate is slightly soluble but the inorganic salts are almost insoluble. Very small amounts of sulphones are formed as by-products, but since these are insoluble in water, they separate when the reaction mixture is poured into water ... 

In general, however, the diacetyl derivatives are unstable in the presence of water, undergoing hydrolysis to the mono-acetyl compound, so that when they (or a mixture of mono- and di-acetyl derivatives) are crystallised from an aqueous solvent, e.g., dilute alcohol, only the mono-acetyl derivative is obtained. A further disadvantage of the use of acetic anhydride in the absence of a solvent is that all the impm-ities in the amine are generally present in the reaction product. Heavily substituted amines, t.g., 2 4 6-tribromoaniline, react extremely slowly with acetic anhydride, but in the presence of a few drops of concentrated sulphuric acid as catalyst acetylation occurs rapidly, for example ... 

Benzidine crystallises from water as the monohydrate the m.p. of the latter is unsatisfactory, since water is lost gradually above about 95°,... 

The solution will then contain the free acid and the hydrochloride of the base either of these may separate if sparingly soluble. If a sohd crystallises from the cold solution, filter, test with sodium bicarbonate solution compare Section 111,85, (i) and compare the m.p. with that of the original compound. If it is a hydrolysis product, examine it separately. Otherwise, render the filtrate alkahne with sodium hydroxide solution and extract the base with ether if the presence of the unchanged acyl canpound is suspected, extract the base with weak acid. Identify the base in the usual manner (see Section IV, 100). The acid will be present as the sodium salt in the alkaline extract and may be identified as described in Section IV,175. 

The acid may also be crystallised from glacial acetic acid. 

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