Crystalline urea

Urea oxalate is also sparingly soluble in amyl alcohol and since urea is soluble in this alcohol, the property may be utilised in separating urea from mixtures. An aqueous extract of the mixture is rendered slightly alkaline with sodium hydroxide solution and extracted with ether this removes all the basic components, but not urea. The residual aqueous solution is extracted with amyl alcohol (to remove the urea) upon adding this extract to a solution of oxalic acid in amyl alcohol crystalline urea oxalate is precipitated. 

The non-linear optical (NLO) active aromatic chalcone l-(2-furyl)-3-(4-benzamidophenyl)-2-propene-l-one (FAPPO), shown in Fig.l, was found to give a rather big SHG activity of 1-2 order of magnitude higher than crystalline urea [6]. Due to their conjugated system all discussed NLO active molecules are approximately fiat. 

Morrison CA, Siddick MM, Camp PJ, Wilson CC (2005) Toward understanding mobile proton behavior from first principles calculation the short hydrogen bond in crystalline urea-phosphoric acid. J Am Chem Soc 127 4042 048... 

It was not strange that Woehler recognized at once this crystalline urea. He had started his career in science as a student of medicine and while competing for a prize for the best essay on the waste products found in urine, had come across urea. 

The method in Figure 17 often gives a product which contains traces of the corresponding (E,E)-diene. This isomer can be selectively removed from the (E., Z)-isomer, as described above, by formation of its Diels-Alder adduct with excess tetracyano-ethylene in tetrahydrofuran followed by chromatography on silica gel (cf. 17,18). Alternatively, the (E.,E)-isomer can be removed in many cases by the selective formation of its crystalline urea inclusion complex in methanol (cf. 13). 

One important host molecule among the inclusion compounds is urea. Ordinary crystalline urea is tetragonal, but when a guest is present, urea crystallizes in a hexagonal lattice, containing the guest in long channels (Fig. 3.1). ... 

This type of solid dispersion is distinguished from a simple eutectic mixture by the fact that the drug is precipitated out in an amorphous form. In a simple eutectic mixture, the drug is precipitated out in a crystalline form. An example of this is the precipitation of sulfathiazole in the amorphous form in crystalline urea. It is postulated that a drug with a propensity to supercooling has more tendency to solidify as an amorphous form in the presence of a carrier. 

Fig. 4 Diffuse reflectance spectra of crystalline urea illustrating the effect of particle size (for the top trace, the crystalline material was unprocessed from the supplier s container), whereas for the bottom spectrum, the substance was ground to finer particles.

Figure 1. Gradient trajectory map of the crystalline urea in the molecular plane. Bond-paths are indicated by heavy lines, those corresponding to the interaction surface crossing the plane are drawn by weaker, while those originating from a (3,-l-3)CP and terminating at a (3,-3)CP by the weakest lines. Reprinted with permission from Ref. [78].

The infinite-dilution heat of solution for solid urea (CH4N2O) in water at 25°C is reported in T/ie Chemical Engineer s Handbook to be —3609 cal/g. In the same book the heat of formation is reported to be —77.55 kcal/mol for liquid urea and —79.634 kcal/mol for crystalline urea. Compare the heat of melting of urea with its heat of solution. 

Fiffire 2. (a) Stereoview of the crystal structure of urea. Note that the hydrogen-bonded head-to-tail chains run parallel to the c axis and each O atom forms four N-H - O acceptor hydrogen bonds, (b) The hydrogen-bond framework in crystalline urea viewed approximately along the c axis. (Data from ref. 25b). 

Figure 22 (a) Crystalline urea unit cell, (b) Arrangement of urea molecules in the crystalline structure. 

Table 9 Cohesive Energy (A ° ) per Molecule of Crystalline Urea Calculated with the Experimental Lattice Parameters (in kJ/mol)...

Figure 24 a) Dependence of the BSSE on the number of neighboring atoms included in the CP correction for crystalline urea, with the LDA approximation (SVWN) and a 6-31G(d,p) basis set. (b) Urea molecule surrounded by a star of 63 neighboring ghost atoms. 

Table A2.1 Description of the Basis Sets Used for Crystalline Urea...

Ab Initio Approach to Molecular Crystals A Periodic Hartree-Fock Study of Crystalline Urea. 

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