Surface recrystallisation

If a compound has been recrystallised from petrol, benzene, etc.y some freshly cut shavings of clean paraffin wax should be added to the calcium chloride in (A) or to the sodium hydroxide in D, The surface of the wax absorbs organic solvent vapours (particularly the hydrocarbons) and the last trace of such solvents is thus readily removed from the recrystallised material. 

If the solid does not dissolve in the cold solvent gently heat the mixture over a micro-Bunsen burner or in a small water-ba until the liquid boils. Continue to add o-i ml. portions of solvent until the solid dissolves. [If more than about i ml. of solvent is required, the solvent is considered unsatisfactory.] If a clear solution is obtained, cool the tube and scratch it below the surface of the solution with a very fine glass rod and proceed as suggested on p. 16. In general, the products from the choice of solvent investigation are not discarded but added to the main bulk of the crude product for recrystallisation. 

In order to dry the crystals, the Buchner funnel is inverted over two or three thicknesses of drying paper (i.e., coarse-grained, smooth surfaced Alter paper) resting upon a pad of newspaper, and the crystalline cake is removed with the aid of a clean spatula several sheets of drying paper are placed on top and the crystals are pressed flrmly. If the sheets become too soiled by the mother liquor absorbed, the crystals should be transferred to fresh paper. The disadvantage of this method of rapid drying is that the recrystallised product is liable to become contaminated with the Alter paper flbre. 

The ether is allowed to evaporate on the water-bath and th e nitrile is then hydrolysed by continuing to heat it on the water-bath with the addition of 4—5 times its volume of cone, hydrochloiic acid until crystals appear on the surface. Water is added and the hot liquid decanted and filteied from any oil. On cooling, the crystals are filtered, washed vvith a little cold ivaterand dried. A fuither quantity can be extiacted from the filtrate with ether. It may be recrystallised from benzene. Yield, 10—15 grms. 

Much of the difficulty in demonstrating the mechanism of breakaway in a particular case arises from the thinness of the reaction zone and its location at the metal-oxide interface. Workers must consider (a) whether the oxide is cracked or merely recrystallised (b) whether the oxide now results from direct molecular reaction, or whether a barrier layer remains (c) whether the inception of a side reaction (e.g. 2CO - COj + C)" caused failure or (d) whether a new transport process, chemical transport or volatilisation, has become possible. In developing these mechanisms both arguments and experimental technique require considerable sophistication. As a few examples one may cite the use of density and specific surface-area measurements as routine of porosimetry by a variety of methods of optical microscopy, electron microscopy and X-ray diffraction at reaction temperature of tracer, electric field and stress measurements. Excellent metallographic sectioning is taken for granted in this field of research. 

In a 2-litre round-bottomed flask, equipped with a double surface condenser, place 60 g. of trimetliylene dicyanide (Section 111,114) and 900 g. of 50 per cent, sulphuric acid (by weight). Reflux the mixture for 10 hours and allow to cool. Saturate the solution with ammonium sulphate and extract with four 150 ml. portions of ether dry the ethereal extracts with anhydrous sodium or magnesium sulphate. Distil off the ether on a water bath the residual glutaric acid (69 g.) crystallises on cooling and has m.p. 97-97-5°. Upon recrystallisation from chloroform, or benzene, or benzene mixed with 10 per cent, by weight of ether, the m.p. is 97 -5-98°. 

In a 1 litre round-bottomed flask provided with an efficient double surface condenser, place 40 g. (39 ml.) of aniline, 50 g. (40 ml.) of carbon sulphide (CAUTION inflammable) (1), and 50 g. (63-5 ml.) of absolute ethyl alcohol (2). Set up the apparatus in the fume cupboard or attach an absorption device to the top of the condenser (see Fig. II, 8, 1) to absorb the hydrogen sulphide which is evolved. Heat upon an electrically-heated water bath or upon a steam bath for 8 hours or until the contents of the flask solidify. When the reaction is complete, arrange the condenser for downward distillation (Fig. II, 13, 3), and remove the excess of carbon disulphide and alcohol (CAUTION inflammable there must be no flame near the receiver). Shake the residue in the flask with excess of dilute hydrochloric acid (1 10) to remove any aniline present, filter at the pump, wash with water, and drain well. Dry in the steam oven. The yield of crude product, which is quite satisfactory for the preparation of phenyl iso-thiocyanate (Section IV,95), is 40-45 g. Recrystallise the crude thiocarbanilide by dissolving it, under reflux, in boiling rectified spirit (filter through a hot water funnel if the solution is not clear), and add hot water until the solution just becomes cloudy and allow to cool. Pure sym.diphenylthiourea separates in colourless needles, m.p. 154°. 

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