Crystallization following carbon

Scheme 9.28). Despite the modest enantioselectivity, we knew from our previous work that upgrade of enantiomeric purity was possible via preferential removal of the less soluble racemic material by crystallization. In this manner, after a carbon treatment to achieve acceptable levels of residual rhodium, racemic 1 was removed by partial crystallization, followed by subsequent isolation of 1 as a crystalline solid in 72% yield from 22, and in 98.5% ee. 

The yellow pentanuclear dianion, [Ni5(CO)i2]2-, is rather labile and has been isolated in a pure state only as the bis(triphenylphosphine)iminium salt by crystallization under carbon monoxide in anhydrous solvents. In wet solvents, it reacts readily with carbon monoxide to give a mixture of tetracarbonylnickel and an unstable hydride derivative presently formulated as [Ni(CO)3H] (t = 18.3), by comparison of its IR spectrum with those of [Ni(CO)3X] (X = Cl, Br, I) (31). This reaction contrasts with that of [Ni5(CO)i2]2 with water under nitrogen when the red dianion [Ni6(CO)i2]2- is formed. The reaction proceeds with formation of tetracarbonylnickel, hydrogen and traces of carbon monoxide and its stoichiometry is believed to be the following ... 

Directions (a) Put on a glass plate a lump of crystallized sodium carbonate, upon another plate a lump of crystallized sodium sulphate and upon a third plate a few crystals of potassium chlorate. Put into a watch glass a lump of calcium chloride and into another a piece of sodium hydroxide. Label these substances and set them away on the shelf in your desk until the next exercise. Then answer the following questions. 

In the troposphere, the combustion of an aircraft s fuel results in carbon dioxide and water vapor in its exhaust. In the stratosphere, however, the water vapor instantly freezes, the result being a jet trail of ice crystals following the aircraft. The stratosphere is where ozone gas (O3) accumulates. Ozone s concentration is exceptionally low, but its presence is still crucial in that it absorbs much of the harmful ultraviolet (UV) radiation from the Sun and prevents that radiation from reaching Earth s surface. By absorbing UV radiation, ozone causes the stratosphere to be warmer than it would be otherwise. It is still colder than the troposphere, but warmer than would be predicted by extrapolating the rate of decrease in temperature with elevation that occurs in the troposphere. 

To explore the difficulties in practical implementation of the above concepts, mixed matrix membranes, with 20% molecular sieves (by volume), were prepared by solution deposition on top of a porous ceramic support. The ceramic supports used were Anodise membrane filters which had 200 A pores that open into 2000 A pores and offer negligible resistance to gas flow. Initially the molecular sieve media, zeolites (4A crystals) or carbon molecular sieves, was dispersed in the solvent, dichloromethane, to remove entrapped air. After two hours, Matrimid was added to the mixture, and the solution was stirred for four hours. The solutions used varied in polymer content from 1-5 wt %. The solution was then deposited on top of the ceramic support, and the solvent was evaporated in a controlled manner. The membranes were then dried overnight at 90°C under vacuum. This was followed by a reactive intercalation post treatment technique 15) to eliminate defects. This technique involves imbibing a reactive monomer (e.g. diamine) from an inert solvent (e.g. heptane) into any micro defects. Next, a second reactive monomer (e.g. acid chloride) was introduced to reactively close defects by forming a low permeability polymer. The membranes were dried again to remove the inert solvent. Individual membrane thickness was determined by weight gain and varied from 5 to 25 Jim. 

By far the most important cluster structures are those composed of C atoms. The field of carbon clusters was bom with the discovery of a perfectly symmetric and exquisitely beautiful stmcture consisting of 60 C atoms [207]. The discoverers of this structure, R.R Curl, H.W. Kroto and R.E. Smalley, were awarded the 1996 Nobel prize for Chemistry. The subsequent production of this cluster in macroscopic quantities [208], made possible both its detailed study as weU as its use in practical applications such as the formation of C60 crystals. Following the discovery of C6o several other clusters composed of C atoms were found, as well as interesting variations of these structures. We will mention briefly these other clusters and stmctures, but will concentrate the discussion mostly on Ceo, which remains the center of attention in this field. 

The crude o-phenylenediamine may be converted into the dihydrocliloride and the salt purified in the following manner. Dissolve it in 60 ml. of concentrated hydrochloric acid and 40 ml. of water containing 2 g. of stannous chloride, and treat the hot solution with 2-3 g. of decolourising carbon. Filter, add 100 ml. of concentrated hydrochloric acid to the hot colourless filtrate, and cool in a freezing mixture of ice and salt. Collect the colourless crystals of the dihydrochloride on a Buchner or sintered glass funnel, wash with a small volume of concentrated hydrochloric acid, and dry in a vacuum desiccator over sodium hydroxide. The yield is 61 g. 

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