Crystallization of benzene

The diazonio group of one zwitterion is stabilized by intermolecular interactions with the carboxylato oxygens of two neighbouring zwitterions. The same type of coordination is observed in crystals of benzene diazonium chloride, tribromide, and tetrafluoroborate (Andresen and Romming, 1962 Romming, 1963 Cygler et al., 1982). 

Observation of H h-Pressure Crystallization of Benzene fi om Benzene—Cyclohexane Mixture... 

In typical organic crystals, molecular pairs are easily sorted out and ab initio methods that work for gas-phase dimers can be applied to the analysis of molecular dimers in the crystal coordination sphere. The entire lattice energy can then be approximated as a sum of pairwise molecule-molecule interactions examples are crystals of benzene [40], alloxan [41], and of more complex aziridine molecules [42]. This obviously neglects cooperative and, in general, many-body effects, which seem less important in hard closed-shell systems. The positive side of this approach is that molecular coordination spheres in crystals can be dissected and bonding factors can be better analyzed, as examples in the next few sections will show. 

Some diamagnetic crystals (graphite, bismuth, naphthalene and other aromatic substances) show prohounced diamagnetic anisotropy. The observed anisotropy of crystals of benzene derivatives correspond to the molar diamagnetic susceptibility —54 X 10 with the field direction perpendicular to the plane of the benzene ring and —37 X lO"6 with it in the plane. This molecular anisotropy has been found to be of some use in determining the orientation of the planes of aromatic molecules in crystals.1... 

Solvent choice is also governed by another important factor, the ease of solvent removal. Solvents with low boiling points are preferred because their removal is easy. A third consideration in selecting a solvent is the temperature at which the solvent solidifies. Benzene was once widely used in recrystallization, but when placed in an ice bath, crystals of benzene would also precipitate (benzene crystallizes at 6 Celsius). A final consideration in choosing a solvent is reactivity. Obviously a solvent that reacts with a solid cannot be used for recrystallization. 

If mixture X is eouled, crystals of benzene will form. 

As already mentioned, in some molecular crystals, a hindered or nearly free rotation of entire molecules is observed, e.g. of benzene molecules in crystals of benzene, or of molecular groups, e.g. of CH3 groups in crystals of methyl naphthalene. These motions are stochastic and are termed pseudorotations or reorientations. These two terms denote the two limiting approximations, that of free rotation and that of a fixed orientation of the molecules or molecular groups. Experimental methods which have proved useful for the investigation of these stochastic motions are nuclear-spin magnetic resonance (NMR) [24] and quasielastic neutron scattering [35, 36]. 

Crystals of benzene chromium tricarbonyl are monoclinic with a = 6.17, h = 11.07, c = 6.57 A, 101.5°, and the density is 1.650gcm. Calculate the volume of the unit cell and the number of molecules per unit cell. 

BHC is manufactured by chlorination of benzene in the presence of ultra-violet light. The gamma-isomer is obtained from the crude mixture by selective crystallization, and forms colourless crystals, m.p. I13" C. U.S. production 1980 400 tonnes. 

The tribromobenzene obtained in this way should be entirely free from unchanged tribromoaniline. To test its purity, dissolve a small quantity in hot dry benzene and pass in hydrogen chloride gas from a Kipp s apparatus no trace of crystals of tribromoaniline hydrochloride should appear. Note also that although the m.p.s of the two compounds are almost identical, that of the recrystallised product from the above preparation is considerably depressed by admixture with tribromoaniline. 

To obtain the free acid, dissolve the potassium salt in 50 ml. of cold water, filter the solution if a small undissolved residue remains, and then boil the clear solution gently whilst dilute sulphuric acid is added until the separation of the acid is complete. Cool the solution and filter off the pale orange-coloured crystals of the benzilic acid wash the crystals on the filter with some hot distilled water, drain well, and then dry in a desiccator. Yield of crude acid, 4 g. Recrystallise from benzene (about 50 ml.) to which a small quantity of animal charcoal has been added, filtering the boiling solution through a preheated funnel fitted w ith a fluted filter-paper, as the benzilic acid readily crystallises as the solution cools alternatively, recrystallise from much hot water. The benzilic acid is obtained as colourless crystals, m.p. 150°. 

Iodine solutions. Dissolve i crystal of iodine in diethyl ether and note the brown colour. Aromatic hydrocarbons e.g. benzene) give purple solutions. 

The acetamide often contains a minute amount of impurity having an odour resembling mice excrement this can be removed by washing with a small volume of a 10 per cent, solution of ethyl alcohol in ether or by recrystallLsation. Dissolve 5 g. of impure acetamide in a mixture of 5 ml. of benzene and 1 5 ml. of dry ethyl acetate warm on a water bath until all is dissolved and cool rapidly in ice or cold water. Filter oflF the crystals, press between Alter paper and dry in a desiccator. The unpleasant odour is absent and the pure acetamide melts at 81°. Beautiful large crystals may be obtained by dissolving the acetamide (5 g.) in warm methyl alcohol (4 ml.), adding ether (40 ml.) and allowing to stand. 

To prepare pure anhydrous o-benzoylbenzoic acid, dissolve the air-dried (or the moist) product in about 175 ml. of benzene contained in a 500 ml. round-bottomed flask fitted with a reflux condenser and heat on a water bath. Transfer the benzene solution to a separatory funnel, run oflF any water present, and dry with anhydrous magnesium sulphate. Concentrate the benzene solution to about 75 ml. and add light petroleum, (b.p. 60-80°) to the hot solution until a slight turbidity is produced. Allow to cool spontaneously to room temperature, then cool in ice to about 5°, collect the crystals and dry. The yield of pure, anhydrous o-benzoylbenzoic acid, m.p. 128°, is 32 g. 

The soiution is aliowed to cool and the crystals of the P2P-bisulfite addition compound are then separated by vacuum filtration, washed with a little clean dH20 then washed with a couple hundred mLs of ether, DCM or benzene. The filter cake of MD-P2P-bisulfate is processed by scraping the crystals into a flask and then 300mL of either 20% sodium carbonate solution or 10% HCi soiution are added (HCI works best). The soiution is stirred for another 30 minutes during which time the MD-P2P-bisulfite complex will be busted up and the P2P will return to its happy oil form. The P2P is then taken up with ether, dried and removed of the solvent to give pure MD-P2P. Whaddya think of that ... 

Ethyl bis-(2,4-dinitrophenyl) acetate (indicator) the stock solution is prepared by saturating a solution containing equal volumes of alcohol and acetone with the indicator pH range colorless 7.4-9.1 deep blue. This compound is available commercially. The preparation of this compound is described by Fehnel and Amstutz, Ind. Eng. Chem., Anal. Ed. 16 53 (1944), and by von Richter, Ber. 21 2470 (1888), who recommended it for the titration of orange- and red-colored solutions or dark oils in which the endpoint of phenol-phthalein is not easily visible. The indicator is an orange solid which after crystallization from benzene gives pale yellow crystals melting at 150-153.5°C, uncorrected. 

Methyllithium. MethyUithium [917-54 ] CH Li, crystallizes from benzene or hexane solution giving cubic crystals that have a salt-hke constitution (128). Crystalline methyllithium molecules exist as tetrahedral tetramers (129). Solutions of methyllithium are less reactive than those of its higher homologues. Methyllithium is stable for at least six months in diethyl ether at room temperature. A one-molar solution of methyllithium in tetrahydrofuran (14 wt %) and cumene (83 wt %) containing 0.08 M dimethyknagnesium as stabilizer loses only 0.008% of its activity per day at 15°C and is nonpyrophoric (117). 

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