Carbon dioxide solvent properties

Carbon dioxide exhibits properties typical of hydrocarbon solvents, such as toluene however, for basic molecules, such as pyrrole, CO2 provides more hydrogen bonding basicity than do hydrocarbon solvents. No significant difference in polarity can be detected between the liquid and supercritical phases. It is hoped that this view of CO2 as a hydrocarbon solvent with unusual properties (infinite compressibility in the supercritical state, low surface tension and viscosity, low... 

Chemical Properties The formation of salts with acids is the most characteristic reaction of amines. Since the amines are soluble in organic solvents and the salts are usually not soluble, acidic products can be conveniendy separated by the reaction with an amine, the unshared electron pair on the amine nitrogen acting as proton acceptor. Amines are good nucleophiles reactions of amines at the nitrogen atom have as a first step the formation of a bond with the unshared electron pair of nitrogen, eg, reactions with acid anhydrides, haUdes, and esters, with carbon dioxide or carbon disulfide, and with isocyanic or isothiocyanic acid derivatives. 

Conventional nitrocellulose lacquer finishing leads to the emission of large quantities of solvents into the atmosphere. An ingeneous approach to reducing VOC emissions is the use of supercritical carbon dioxide as a component of the solvent mixture (172). The critical temperature and pressure of CO2 are 31.3°C and 7.4 MPa (72.9 atm), respectively. Below that temperature and above that pressure, CO2 is a supercritical fluid. It has been found that under these conditions, the solvency properties of CO2 ate similar to aromatic hydrocarbons (see Supercritical fluids). The coating is shipped in a concentrated form, then metered with supercritical CO2 into a proportioning airless spray gun system in such a ratio as to reduce the viscosity to the level needed for proper atomization. VOC emission reductions of 50% or more are projected. 

The combination of ionic liquids with supercritical carbon dioxide is an attractive approach, as these solvents present complementary properties (volatility, polarity scale.). Compressed CO2 dissolves quite well in ionic liquid, but ionic liquids do not dissolve in CO2. It decreases the viscosity of ionic liquids, thus facilitating mass transfer during catalysis. The separation of the products in solvent-free form can be effective and the CO2 can be recycled by recompressing it back into the reactor. Continuous flow catalytic systems based on the combination of these two solvents have been reported [19]. This concept is developed in more detail in Section 5.4. 

Above the critical temperature and pressure, a substance is referred to as a supercritical fluid. Such fluids have unusual solvent properties that have led to many practical applications. Supercritical carbon dioxide is used most commonly because it is cheap, nontoxic, and relatively easy to liquefy (critical T = 31°C, P = 73 atm). It was first used more than 20 years ago to extract caffeine from coffee dichloromethane, CH2C12, long used for this purpose, is both a narcotic and a potential carcinogen. Today more than 10s metric tons of decaf coffee are made annually using supercritical C02. It is also used on a large scale to extract nicotine from tobacco and various objectionable impurities from the hops used to make beer. 

SFE is used mainly for nonpolar compounds [e.g. polychlorinated biphenyls (PCBs)]. Typically, small aliquots of soil (0.5-10 g) are used for extraction. The extraction solvent is a supercritical fluid, most commonly carbon dioxide, which has properties of both a liquid and gas. The supercritical fluid easily penetrates the small pores of soil and dissolves a variety of nonpolar compounds. Supercritical carbon dioxide extracts compounds from environmental samples at elevated temperature (100-200 °C) and pressure (5000-10 000 psi). High-quality carbon dioxide is required to minimize... 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

Preparation, characterization, and properties of silicon-containing triorganotin fluorides, both symmetrical and unsymmetrical, were investigated. It was observed that introduction of a trimethylsilyl group in the alkyl chain results in a considerable enhancement of solubility in various nonpolar solvents including dense carbon dioxide. 

Kho, Y.W., Conrad, D.C. and Knutson, B.L. (2003) Phase equilibria and thermophysical properties of carbon dioxide-expanded fluorinated solvents. Fluid Phase Equilibria, 206 (1-2), 179-193. 

---