Solvents, boiling points impurities

Dimethyl sulfoxide (DMSO) [14] Commercial products contain water and small amounts of dimethyl sulfide and dimethyl sulfone as impurities. In the purification, molecular sieve 5A (activated at 500 °C under argon for 16 h) is added and kept for several days to reduce water to < 10 ppm and other low boiling point impurities to < 50 ppm. Then the solvent is filtered and the filtrate is distilled over CaH2 at reduced pressure in a nitrogen atmosphere. 

The purification of solids by crystallisation is based upon differences in their solubility in a given solvent or mixture of solvents. In its simplest form, the crystallisation process consists of (i) dissolving the impure substance in some suitable solvent at or near the boiling point,... 

The most desirable characteristics of a solvent for recrystalhsation are (a) a high solvent power for the substance to be purified at elevated temperatures and a comparatively low solvent power at the laboratory temperature or below (6) it should dissolve the impurities readily or to only a very small extent (c) it should yield well-formed crystals of the purified compound and (d) it must be capable of easy removal from the crystals of the purified compound, i.e., possess a relatively low boiling point. It is assumed, of course, that the solvent does not react chemically with the substance to be purified. If two or more solvents appear to be equally suitable for the recrystallisation, the final selection will depend upon such factors as ease of manipulation, inflammability and cost. 

Naphthenic acids are viscous hquids, with phenohc and sulfur impurities present that are largely responsible for their characteristic odor. Their colors range from pale yeUow to dark amber. An odor develops upon storage of the refined acids. Naphthenic acids have wide boiling point ranges at high temperatures (250—350°C). They are completely soluble in organic solvents and oils but are insoluble (<50 mg/L) in water. Commercial naphthenic acids are available in... 

Butyl stearate [123-95-5] M 340.6, m 26.3 , d 0.861. Acidic impurities removed by shaking with 0.05M NaOH or a 2% NaHC03 soln, followed by several water washes, then purified by fractional freezing of the melt and fractional crystn from solvents with boiling points below 100°. 

Although the boiling-point method is able to dispose of a greater numbei of convenient solvents than are suitable for freezing-point determinations, it is never so accurate, mainly on account of the difficulty of avoiding fluctuations in the boiling-point, due to radiation, to the dripping of cold liquid from the condenser, to impure solvent, and to barometric fluctuations. 

Successful recrystallization of an impure solid is usually a function of solvent selection. The ideal solvent, of course, dissolves a large amount of the compound at the boiling point but very little at a lower temperature. Such a solvent or solvent mixture must exist (one feels) for the compound at hand, but its identification may necessitate a laborious trial and error search. Solvent polarity and boiling point are probably the most important factors in selection. Benzhydrol, for example, is only slightly soluble in 30-60 petroleum ether at the boiling point but readily dissolves in 60-90° petroleum ether at the boiling point. 

It may become necessary to work up solid products with the same solvent as used for crystallization. Cakes should not be allowed to dry out before washing, as air or inert gases result in evaporation of the solvent, and this may re.sult in the deposition of impurities, present in the dis.solved state, on the crystals. Such impurities when deposited are difficult to wash. In. some ca.ses washing of wet cake is done by replacing the solvent with another one, typically having a lower boiling point, so that in the final material loss of drying is within limits and the new solvent is more acceptable than the earlier one. 

The heat of decomposition (238.4 kJ/mol, 3.92 kJ/g) has been calculated to give an adiabatic product temperature of 2150°C accompanied by a 24-fold pressure increase in a closed vessel [9], Dining research into the Friedel-Crafts acylation reaction of aromatic compounds (components unspecified) in nitrobenzene as solvent, it was decided to use nitromethane in place of nitrobenzene because of the lower toxicity of the former. However, because of the lower boiling point of nitromethane (101°C, against 210°C for nitrobenzene), the reactions were run in an autoclave so that the same maximum reaction temperature of 155°C could be used, but at a maximum pressure of 10 bar. The reaction mixture was heated to 150°C and maintained there for 10 minutes, when a rapidly accelerating increase in temperature was noticed, and at 160°C the lid of the autoclave was blown off as decomposition accelerated to explosion [10], Impurities present in the commercial solvent are listed, and a recommended purification procedure is described [11]. The thermal decomposition of nitromethane under supercritical conditions has been studied [12], The effects of very high pressure and of temperature on the physical properties, chemical reactivity and thermal decomposition of nitromethane have been studied, and a mechanism for the bimolecular decomposition (to ammonium formate and water) identified [13], Solid nitromethane apparently has different susceptibility to detonation according to the orientation of the crystal, a theoretical model is advanced [14], Nitromethane actually finds employment as an explosive [15],... 

Oiling out is what it s called more work is what it means. Compounds usually oil out if the boiling point of the recrystallization solvent is higher than the melting point of the compound, though that s not the only time. In any case, if the oil solidifies, the impurities are trapped in the now solid oil, and you ll have to purify the solid again. 

Adding an impurity to a solvent makes its liquid phase more stable through the combined effects of boiling point elevation and freezing point depression. That s why you r irely see bodies of frozen salt water. The salt in the oceans lowers the freezing point of the water, making the liquid phase more stable and able to sustain temperatures slightly below 0°C. 

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