Solvents for reactions

Because acetylene is a far weaker acid than water and alcohols these substances are not suitable solvents for reactions involving acetylide ions Acetylide is instantly converted to acetylene by proton transfer from compounds that contain —OH groups... 

The solubility of various gases in ionic liquids is extremely important in evaluating ILs as solvents for reactions, separations, and materials processing. There are a number of viable techniques for measuring gas solubilities in ILs, including the... 

Although very corrosive, it can be stored and shipped in steel or common alloys at commercial concentrations. In some chemical processes, it is used simply as an acid while in others it is used as a dehydrating agent to remove water, as an agent to increase the rate of a chemical reaction (catalyst), or as a solvent for reactions in which it remains unchanged. It rarely ends up in the final product. Waste sulfuric acid can be recycled. 

Especially for large-scale work, esters may be more safely and efficiently prepared by reaction of carboxylate salts with alkyl halides or tosylates. Carboxylate anions are not very reactive nucleophiles so the best results are obtained in polar aprotic solvents45 or with crown ether catalysts.46 The reactivity order for carboxylate salts is Na+ < K+ < Rb+ < Cs+. Cesium carboxylates are especially useful in polar aprotic solvents. The enhanced reactivity of the cesium salts is due to both high solubility and minimal ion pairing with the anion 47 Acetone is a good solvent for reaction of carboxylate anions with alkyl iodides48 Cesium fluoride in DMF is another useful... 

HCN is the most preferred cyanide source in cyanohydrin synthesis. Besides HCN, several different cyanide sources, like potassium cyanide, are being used in biotransformation. Alternative methods for the safe handling of cyanides on a laboratory scale are, for instance, to use cyanide salts in solution. These solutions can be acidified and used as the aqueous layer in two-phase systems or the HCN can be extracted into the organic layer with the desired solvent for reactions in an organic phase. After the reaction, excess cyanide can readily be destroyed with iron(II) sulfate, or iron(III) chloride or bleach. Cyanide can also be presented in the form of organic cyano, such as acetone cyanohydrin [46] or cyanoformates. However, as claimed by Effenberger, some results could not be reproduced [47]. 

For qualitative investigations there is considerable latitude in experimental procedure. There are few limitations on solvent for reactions studied in the liquid phase, although standard considerations of susceptibility to radical attack must obviously be taken into account. With polar solvents it may be desirable to replace the normal silica sample tube with a flat cell, although spin-adduct concentrations are usually sufficiently great for this to be circumvented by the use of capillary tubes. 

Fluorous compounds with appropriate melting and boiling points can be used as solvents for reactions. Fluorous compounds that have been used as solvents for a variety of reactions include perfluoromethylcyclohexane (PP2) and perfluoro-hexane (Figure 3.1). 

Supercritical fluids (SCFs) are best known through their use for the decaffeination of coffee, which employs supercritical carbon dioxide (scCC ). In this chapter, we will demonstrate that SCFs also have many properties that make them interesting and useful reaction media. Firstly, the physical properties of SCFs will be explained, then the specialist equipment needed for carrying out reactions under high temperatures and pressures will be described. Finally, we will discuss issues relevant to the use of SCFs as solvents for reactions. 

Interest in the uses of HMPT has also been maintained, but a warning has been issued (by the E. I. du Pont de Nemours Company and the U.S. National Institute for Occupational Safety and Health) about its potential acute toxicity. HMPT has been used in the synthesis of 2,4-bis(dimethylamino)qui nolines,9 8 as a solvent for reactions between carbonyl compounds and sulphur," for the conversion of iV-benzylcarbox-amides into 3-phenylpropionitriles,100 in reactions between metals or organometallic compounds with a variety of organic substrates,101 and as a solvent for alkylation reactions of /J-keto-esters and related compounds in which the alkylation reaction is accompanied by de(alkoxycarbonylation) (Scheme 7).102... 

TABLE 7.2 Effects of the Lewis Acid in Different Solvents for Reaction (1)... 

Ionic liquids, having per definition a melting point below 100 °C, and especially room temperature ionic liquids (RTIL) have attracted much interest in recent years as novel solvents for reactions and electrochemical processes [164], Some of these liquids are considered to be green solvents [165]. The scope of ionic liquids based on various combinations of cations and anions has dramatically increased, and continuously new salts [166-168] and solvent mixtures [169] are discovered. The most commonly used liquids are based on imidazolium cations like l-butyl-3-methylimidazolium [bmim] with an appropriate counter anion like hexafluorophos-phate [PFg]. Salts with the latter anion are moisture stable and are sometimes called third generation ionic liquids. 

One of the main impetuses for using ILs for gas separations and as a solvent for reactions involving permanent gases is that most ILs have extremely low-vapor pressures at normal operating conditions. Thus, one will not lose any of the solvent in the purified gas sfream or in fhe products. Another attractive feature is that ILs are highly tunable by varying the cation, anion, and substituents. Thus, they can be tailored for specific applications to optimize selectivity, capacities, reactant or product solubilities, and rates. 

Transition metal complex catalyzed reactions exhibit different outcomes depending on whether they are conducted in water or not. Chao-Jim Li highlighted the greening of chemical reactions, focusing on the use of water as an alternative solvent for reactions involving metals, R = CH2Ph (Li, 1996) (see fig. 6.8). 

Pickel, K.H., Steiner, K. Supercritical Fluids Solvents for Reactions. In Proceedings 3rd Int. Symposium on Supercritical Fluids, Strasbourg, I.S.A.S.F. Nancy, France, 1994, 3, 25-29. 

Nonetheless, understanding high-pressure phase behavior is vitally important to evaluating C02 as a potential replacement solvent for reactions. Certainly, commercial reactions are not run at dilute conditions, so the solubility of the reactants, products, and catalysts in the C02 (if one desires to run the reaction as a single-phase system) will frequently be the key factor in determining the economic viability of the C02-based reaction system. 

Figure 7.10 Potential energy profile showing the effect of the solvent for reaction between uncharged reactants— charge-separated activated complex...

When an ion is solvated the solvent molecules are packed around the ion, and occupy a smaller volume than they would in pure solvent. For reaction between two ions of opposite charge, formation of the activated complex releases solvent molecules from their solvation sheaths, thereby increasing their volume compared... 

Ethers are also commonly used as solvents for reactions involving strongly basic reagents because they do not have acidic hydrogens but are still polar enough to dissolve the reagents. Ethers that are used include diethyl ether, /etrafrydro/iiran (THF),... 

A major use of ethers in the organic laboratory is as solvents for reactions. Ethers are nonpolar enough to dissolve many organic compounds, and the electrons on the oxygen can interact with alkali metal cations to help solubilize salts. In addition, ethers are nonacidic and are not very reactive. For these reasons they are especially useful in reactions involving strongly basic reagents. In addition to diethyl ether, other ethers that are commonly used as solvents are 1.2-dimethoxyethane (DME) and the cyclic ethers tetrahydrofuran (THF) and 1,4-dioxane ... 

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