Crystal of P-

The p-dibromobenzene formed as a by-product in the above reaction usually solidifies when the undistilled residue obtained in the first distillation is chilled. It may then he isolated by adding about lo ml. of methylated spirit and some animal charcoal to the flask, boiling for a few minutes, and filtering hot. On cooling the filtrate in ice-water, crystals of p-dibromobenzene, m.p. 89°, separate recrystallise a second time if necessary to obtain colourless crystals. 

Method 1. Arrange the flask containing the reaction mixture for steam distillation as in Fig. II, 40, 1. Proceed with the steam distillation until crystals of p-dibromobenzene appear in the condenser. Change the receiver and continue with the distillation until all the p-dibromobenzeiie has passed over from time to time run out the water from the condenser so that the crystals melt and run down into the receiver. Reject the residue in the flask. Transfer the first distillate to a separatory funnel, wash it with a httle water, and dry the lower layer with a little anhydrous magnesium sulphate or anhydrous calcium chloride filter. Distil slowly from a small distilling flask use a wire gauze or an air bath (Fig. II, 5, 3). Collect the fraction which passes over at 150-170° pour the residue (R), while it is still hot, into a small beaker or porcelain basin for the isolation of p-dibromobenzene. Redistil the fraction of b.p. 150-170° and collect the bromobenzene at 154-157° (3). The yield is 60 g. 

To recover the free base, dissolve the hydrochloride in the minimum volume of boiling alcohol, add concentrated ammonia solution dropwise until a clear solution results and the blue colour has become fight brown. Add water carefully untU a cloudiness appears, warm on a water bath untU the cloudiness just disappears, and allow to cool. Yellow crystals of p-amino-azobenzene separate on coofing. 

In a flask the chemist mixes 50g piperonal into 200mL glacial acetic acid, then adds 45mL nitroethane and 17g ammonium acetate. The solution is then refluxed 4 hours and takes on the color of yellow to yellow-orange. After 4 hours and cooling, yellowish crystals of p-nitropropene will spontaneously form. If not, the solution can be diluted with 50ml of dHjO and chilled in an ice bath for an hour to form the crystals with some slushy glacial acetic acid and water intermixed. The mass of crystals is broken up and plopped into a Buchner funnel to be vacuum filtered. The filter cake is washed with a little extra acetic acid or water. All of the filtrate is saved. 

The increase in Ca is initiated rapidly and begins to recover after 1 min. The order of potency correlates fairly well with the solubilities of these compounds in organic solvents (37) and their abilities to accumulate in phospholipid vesicles (38), i.e., 6>y>a>p, but not with their insecticidal activity (y 6>a p 39). At these concentrations, crystals of p-, a-, and y-HCH were evident in the cell suspensions when we made simultaneous measurements of the right-angle light scatter, indicating that the order of aqueous solubilities is 6>y>a>p. However, stimulation by 6-HCH at concentrations below its aqueous solubility limit shows a typical dose dependency of the response (Figure 10). 

Each needle-like crystal of p-FeOOH is known to consist of a bundle of much thinner suhcrystals. 

Figure 37. (a) Packing arrangement of p-bromobenzoic anhydride, viewed along the c axis. (b) Crystals of p-bromobenzoic anhydride after amisotropic reaction with ammonia. (Reproduced by kind permission of I. C. Paul and D. Y. Curtin.)... 

Recently, Curtin and Paul (114) have used the pyroelectric effect for the direct assignment of the absolute structure of crystals of p-bromobenzoic anhydride. 

Maeda, H. Maeda,Y. (1996) Atomic force microscopy studies for investigating the smecti-tic structures of colloidal crystals of P-FeOOH. Langmuir 12 1446-1452... 

Watson, I. (1979) Iron oxide pigments. Colour-frd competitors between natural and synthetic. Minerals in Industry, Aug., 43—51 Watson, J.H.L., Cardell, R.R.Jr. Heller, W. (1962) The internal structure of colloidal crystals of P-FeOOH and remarks on their assemblies in Schiller layers. J. Phys. Chem. 66 1757-1763... 

Figure 4.12 View of the chain polymer in the crystal of p l Au ( 1 ) ] 31.

If the residue in the flask after the steam distillation is filtered hot, and cooled, crystals of p-hydroxydiphenyl separate. It is formed by the coupling of a portion of the phenol first formed with undecomposed diazonium compound. 

Fig. 1 Crystals of p-nitroaniline obtained from different solvents by evaporation, showing multiple morphologies, but no polymorphism.

Figure 2. Single crystals of p-aminobenzoic acid (on the left), 3,5-dinitrobenzoic acid (on the right), and their 1 1 cocrystal (center). The cocrystal is acentric and grows as large chunky crystals from methanol.

Cognate preparations. p-Nitrobenzaldehyde. This preparation is an example of the Sommelet reaction in which the hexaminium salt is isolated. Dissolve llg (0.13mol) of hexamethylenetetramine in 70ml of chloroform (CAUTION) and add 11.4 g (0.067 mol) of p-nitrobenzyl chloride or 14.4 g of p-nitrobenzyl bromide (Expt 6.28). Heat the mixture under reflux on a steam bath for 4 hours a precipitate gradually separates. Replace the reflux condenser by a condenser set for distillation and distil off about 35 ml of solvent. Add 35 ml of acetone, cool in ice, collect the precipitate by suction filtration and dry it in the air. Heat the hexaminium salt thus obtained under reflux for 1 hour with 100 ml of 50 per cent acetic acid then add 100 ml of water and 25 ml of concentrated hydrochloric acid and continue the refluxing for 5-10 minutes. Cool the solution in ice, collect the crystals of p-nitrobenzaldehyde and dry them in a vacuum desiccator. The yield is 6.4 g (63%), m.p. 106 °C. The p.m.r. spectrum is noted in Expt 6.117. 

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