Lewis properties, solvents

As an aside, benzotrifluoride is a slightly polar, non-Lewis basic solvent that has favorable properties for many kinds of organic reactions. A. Ogawa, D. P. Curran, Benzotrifluoride A useful alternative solvent for organic reactions currently conducted in dichloro-methane and related solvents , J. Org Chem. 1997, 62, 450. 

A book by Lewis tries to summarize some of the available literature on a number of the most commonly used solvents in chemistry (Lewis, 1994). Solvents may have many chemicophysical properties in common, but there are also many differences in chemical structure, and in metabolism, leading to the formation of different metabolites having different toxicity. These similarities and differences may explain some of... 

This chapter introduces the experimental work described in the following chapters. Some mechanistic aspects of the Diels-Alder reaction and Lewis-acid catalysis thereof are discussed. This chapter presents a critical survey of the literature on solvent ejfects on Diels-Alder reactions, with particular emphasis on the intriguing properties of water in connection with their effect on rate and selectivity. Similarly, the ejfects of water on Lewis acid - Lewis base interactions are discussed. Finally the aims of this thesis are outlined. 

Finally, the solvent also interacts with sites of the Lewis acid and the Lewis base that are not directly involved in mutual coordination, thereby altering the electronic properties of the complex. For example, delocalisation of charges into the surrounding solvent molecules causes ions in solution to be softer than in the gas phase . Again, water is particularly effective since it can act as an efficient electron pair acceptor as well as a donor. 

In Chapter 1 mechanistic aspects of Are Diels-Alder reaction are discussed. The literature on the effects of solvents and Lewis-acid catalysts on this reaction is surveyed. The special properties of water are reviewed and the effects of water on the Diels-Alder reaction is discussed. Finally, the effect of water on Lewis acid - Lewis base interactions is described. 

The thiazolium ring, as most heterocycloammoniums, is a Lewis acid conferring to the carbon atom in the 2-position the carbocationic property of adding the free pair of a base either organic or mineral that may be the molecule of solvent as ROH (Scheme 11). For many nuclei of suitable acidity, these equilibria can be observed in dilute solution by means of absorption spectra when species A and C possess different characteristics (24). For example, benzothiazolium and benzoxazolium in methanol and ethanol give at 10 mole liter 8 and 54% of the alkoxy derivatives for the former and 29 and 90% for the latter respectively. 

The purity of ionic liquids is a key parameter, especially when they are used as solvents for transition metal complexes (see Section 5.2). The presence of impurities arising from their mode of preparation can change their physical and chemical properties. Even trace amounts of impurities (e.g., Lewis bases, water, chloride anion) can poison the active catalyst, due to its generally low concentration in the solvent. The control of ionic liquid quality is thus of utmost importance. 

The types of ionic liquids shown in Figure 5.4 have been most extensively studied, especially ones based on chloroaluminate. Whilst these chloroaluminate materials also display useful Lewis acid properties they are highly air and moisture sensitive, which renders them relatively commercially unattractive. Newer ionic liquids containing C104 and NOa anions, for example, which are less air and moisture sensitive, are now being more widely studied, but these are less catalytically active. Other than lack of vapour pressure and catalytic properties there are several other features common to most ionic liquids that make them attractive reaction solvents. These include ... 

The described approach to this pharmaceutically important class of compounds [324] was also utilized by Bonnet-Delpon and coworkers one year later [325]. Interestingly, these authors employed hexafluoroisopropanol (HFIP) as solvent and were able to perform the domino process without adding any extra Lewis acid catalyst such as InCl3 due to the acidic properties of HFIP (pKa = 9.3) [326]. Besides di-hydrofuran or dihydropyran, they have also used acyclic enol ethers. 

The principal factors affecting solvent-ion interactions can be classified as ion-dipole, Lewis acid-base, hydrogen-bonding, solvent structural, and steric. The solvent obviously plays a major part in these interactions. Therefore, to interpret trends in conductance data, bulk solvent properties such as viscosity and dielectric constant should be considered. Table 1 lists selected physical properties for a number of organic solvents. 

The solvent classifications used here are (1) solvents possessing both Lewis acid and Lewis base properties and a dielectric constant (D) > 25 (2) solvents possessing both Lewis acid and Lewis base properties and D < 25 (3) solvents possessing only Lewis acid properties and D> 25 (4) same as(3) but D < 25 (5) solvents possessing only Lewis base properties and D> 25 (6) same as(5)but D < 25 (7) solvents possessing negligible Lewis acid or base properties and D > 25 and (8) same as (7) but D < 25. 

Association and mobilities are related in a complex way to the bulk properties of the solvent and solute. These properties include the charge density and distribution on the ions and the Lewis base properties, the strength and nature of the solvent molecule dipole, the hydrogen-bonding capability, and the intermolecular structure of the solvent. Some correlations can be made on the basis of mobility and association trends in series such as the halides and alkali metals within a single solvent others can be drawn between solvents for a given ion. It appears that conductance measurements provide a clear measure of the sum of ion-solvent interactions, but that other techniques must be used in conjunction with conductance if assessments of individual contributions from specific factors are to be made. 

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