Diphenylurea, crystallization

If it is heated alone at 160°, ammonia comes off rapidly, the aniline and phenylisocyanate may be detected by their odor but they combine in large part to form 5ym.-diphenyliu ea which crystallizes out from the hot liquid, and cyanic acid may be detected by means of silver nitrate in the water with which the apparatus is washed out after the experiment. When an aqueous solution of phenylurea is distilled, aniline may be detected in the distillate and 5ym.-diphenylurea crystallizes from the boiling liquid. By heating urea with the amine or with its hydrochloride at 160°, or by refluxing urea in aqueous solution with the amine or its hydrochloride, we have prepared sym.-6i-o- and />-tolyl, di-a- andjS-naphthyl, dimethyl, di-w-butyl, di-w- and i5 -amyl and dibenzylureas in satisfactory yield. 

Diphenylurea Crystallization. 1,3-hfsphenylurea (13) is the parent compound of a large family of derivatives, most of which do not cocrystallize with guest molecules (Etter et al. 1990). Even when put into solution with strong hydrogen bond acceptors, e.g., dimethyl sulfoxide (DMSO), triphenylphosphineoxide (TPPO) and tetrahydrofuran (THF), most diphenyl ureas crystallize with other molecules of the same kind in a connectivity pattern viewed as is shown below (14), instead of forming cocrystals (e.g., 15). 

If the phosgene is mixed with halogens, e.g., chlorine or bromine, the latter may oxidise the anihne and so render the diphenylurea crystals impure. In this case it is necessary to pass the gas through a reagent which will remove them. This is carried out by inserting before the vessel containing the aniline a tube containing cotton wool soaked in a concentrated solution... 

Dissolve 12 g. of aniline hydrochloride and 6 g. of urea in 50 ml. of warm water, and then filter the solution through a fluted filter to remove any suspended impurities which may have been introduced with the aniline hydrochloride. Transfer the clear filtrate to a 200 ml. conical flask, fit the latter with a reflux water-condenser, and boil the solution gently over a gauze for about hours. Crystals of diphenylurea usually start to separate after about 30-40 minutes boiling. Occasionally however, the solution becomes supersaturated with the diphenylurea and therefore remains clear in this case, if the solution is vigorously shaken after about 40 minutes heating, a sudden separation of the crystalline diphenyl compound will usually occur. The further deposition of the crystals during the re-... 

The insoluble residue of diphenylurea from the original filtration is chemically almost pure. It may be recrystallised from hot rectified spirit or ethanol, a process which will be necessary if the material contains fragments of porcelain. When using either of these solvents, however, the hot solution should be filtered at the pump using a small Buchner funnel and flask which again have been preheated by the filtration of some of the hot solvent, as the solution when cooled rapidly deposits the diphenylurea. iSym-Diphenylurea (or carbanilide) is thus obtained as fine colourless crystals, m.p. 237° yield, 1-1 5 S ... 

Example. Add a solution of 0 5 ml. of benzyl alcohol in 5 ml. of petroleum (b.p. 100-120 ) to a similar solution of 0 5 ml. of phenylisocyanate, and boil the mixture gently under reflux for 20 minutes. Filter hot if necessary from any insoluble diphenylurea, and cool. Filter off the crystalline urethane, and recrystallise from the petroleum colourless crystals, m.p. 76 . 

Summary TNPU is prepared in a two step process starting with the formation of a mixture of tetranitro and trinitro isomers of N,N -diphenylurea. These isomers are prepared by reacting 99% nitric acid with N,N -diphenylurea in the presence of concentrated sulfuric acid. The resulting reaction mixture is treated with ice water to precipitate the nitro isomers, which are then collected by filtration, washed, and then dried. The second step is merely an extra nitration process carried out at a higher temperature. The dry nitro isomers are treated with 99% nitric acid and concentrated sulfuric acid at 90 Celsius, and then the resulting TNPU is then separated by treating the reaction mixture with ice water. The precipitated crystals are then collected by filtration, washed, and then dried. 

Jeong and co-workers reported that diphenylureas 41a-d formed foldamer complexes with chloride and sulfate. Crystal structure of complex 41a-S04 (Fig. 5.34) showed that the sulfate ion was located in the middle of the cavity. 

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