Academic literature on the topic 'Chemical reactions. Liquid crystals. Solvents'

The use of compressed carbon dioxide as a reaction medium, either as a liquid or a supercritical fluid (sc CO2), offers the opportunity not only to replace conventional hazardous organic solvents but also to optimize and potentially control the effect of solvent on chemical synthesis. Although synthetic chemists, particularly those employing catalysis, may be relative latecomers to the area of supercritical fluids, the area of catalysis in carbon dioxide has grown significantly since around 1975 to the point that a number of excellent reviews have appeared (Baiker et al., 1999; Buelow et al., 1998; Jessop and Leitner, 1999; Jessop et al., 1995c, 1999; Morgenstern et al., 1996). Developing and understanding catalytic processes in dense-phase carbon dioxide could lead to “greener” processing at three levels: (1) solvent replacement, (2) improved chemistry (e.g., higher reactivity, selectivity, less energy), and (3) new chemistry (e.g., use of CO2 as a C-1 source). In this chapter, we will highlight a number of examples from the literature in homogeneous and heterogeneous transition-metal catalysis, as well as the emerging area of biphasic catalysis in H2O/sc CO2 mixtures. The intent is to provide an illustrative rather than a comprehensive overview to four classes of catalytic transformations: acid catalysis, reduction via hydrogenation, selective oxidation catalysis, and catalytic carbon–carbon bond-forming reactions. The reader is referred to other chapters in this book and other reviews (King and Bott, 1993) for discussion of uncatalyzed reactions, phase-transfer catalysis, polymerization, and radical reactions in sc CO2. From a synthetic chemist’s viewpoint, sc CO2 has a number of potential advantages that one would like to capitalize upon. • Solvent Replacement Carbon dioxide is a nontoxic, nonflammable, inexpensive alternative to hazardous organic solvents. Simple solvent replacement will not be a sufficient driver for all chemical reactions; however, as described below, the use of carbon dioxide could lead to better chemistry for certain reactions. • Gas Miscibility Gases such as H2, O2, and CO are sparingly soluble in liquid solvents but they are highly miscible with sc CO2.

During the 1990s, the chemical industry has focused on ways to reduce and prevent pollution caused by chemical synthesis and manufacturing. The goal of this approach is to modify existing reaction conditions and/or to develop new chemistries that do not require the use of toxic reagents or solvents, or that do not produce toxic by-products. The terms “environmentally benign synthesis and processing” and “green chemistry” have been coined to describe this approach where the environmental impact of a process is as important an issue as reaction yield, efficiency, or cost. Most chemical reactions require the use of a solvent that may serve several functions in a reaction: for example, ensuring homogeneity of the reactants, facilitating heat transfer, extraction of a product (or by-product), or product purification via chromatography. However, because the solvent is only indirectly involved in a reaction (i.e., it is not consumed), its disposal becomes an important issue. Thus, one obvious approach to “green chemistry” is to identify alternative solvents that are nontoxic and/or environmentally benign. Supercritical carbon dioxide (sc CO2) has been identified as a solvent that may be a viable alternative to solvents such as CCl4, benzene, and chloroflurocarbons (CFCs), which are either toxic or damaging to the environment. The critical state is achieved when a substance is taken above its critical temperature and pressure (Tc, Pc). Above this point on a phase diagram, the gas and liquid phases become indistinguishable. The physical properties of the supercritical state (e.g., density, viscosity, solubility parameter, etc.) are intermediate between those of a gas and a liquid, and vary considerably as a function of temperature and pressure. The interest in sc CO2 specifically is related to the fact that CO2 is nontoxic and naturally occurring. The critical parameters of CO2 are moderate (Tc = 31 °C, Pc = 74 bar), which means that the supercritical state can be achieved without a disproportionate expenditure of energy. For these two reasons, there is a great deal of interest in sc CO2 as a solvent for chemical reactions. This chapter reviews the literature pertaining to free-radical reactions in sc CO2 solvent.

Microwave heating is widely used to accelerate organic reactions in the chemistry field. However, the effect of microwaves on chemical reaction has not yet been well characterized at the molecular level. In this review chapter, microwave heating processes of liquid crystals and an ethanol-hexane mixed solution under microwave irradiation were experimentally and theoretically investigated using in situ microwave irradiation nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulation, respectively. The temperature of the solution under microwave irradiation was estimated from a chemical shift calibrated temperature (CSC-temperature) which was determined from the temperature dependence of the 1H chemical shift. The CSC-temperatures of CH2 and CH3 non-polar protons of ethanol reflect the bulk temperature of a solution by the thermal microwave effect. The lower CSC-temperature of the OH polar protons in ethanol and much higher CSC-temperature of H-C=N (7′) and CH3-O (α’) protons of N-(4-methoxybenzyliden)-4-butylaniline with respect to the bulk temperature are attributed to the non-thermal microwave effects. According to the MD simulation under microwave irradiation, the number of hydrogen bonds increased in the ethanol-hexane mixed solution as a result of a non-thermal microwave effect. It is concluded that a coherently ordered low entropy state of polar molecules is induced by a non-thermal microwave effect. The ordered state induces molecular interaction, which may accelerate the chemical reaction rate between molecules with polar groups.

There are many situations in organic synthesis where it is desirable to bring about reaction between reactants present in two (or more) immiscible phases. Agents known as phase-transfer catalysts are used for this purpose. Their role in initiating or accelerating such reactions has been proven extensively since the early seventies, and the principles of their operation are being increasingly understood [see Weber and Gokel, 1977; Reuben and Sjoberg, 1981; Frechet, 1984; Freedman, 1986; Goldberg, 1992 (English translation); Dehmlow and Dehmlow, 1993; Starks et al., 1994; Yufit, 1995; Sasson and Neumann, 1997; Naik and Doraiswamy, 1998]. To date, an estimated 500 different commercial chemical processes (mostly for small volume chemicals) using about 5-25 million pounds per annum of phase-transfer catalysts have been reported (Starks et al., 1994), and well over 6,500 compounds have been synthesized in the laboratory using PTC (Keller, 1979, 1986). A large number of industrial applications of phase-transfer catalysis are found in the pharmaceutical, agrochemical, and fine chemicals industries. Additionally, it is now being increasingly used in processes related to the environment, in process modifications for eliminating the use of solvents, and in reactions related to the treatment of poisonous effluents. Not surprisingly, then, there has been a constant stream of publications and patents every year. Phase-transfer catalysis (PTC) is an area that has largely been the province of the preparatory organic chemist (defined broadly to include organometallic and polymer chemists). It is only since the early eighties that the engineering aspects of phase-transfer catalysis are being explored, including such traditional features as mass and heat transfer and reactor design. Our main objective is to present a brief but coherent engineering analysis of PTC, following an introduction to its basic principles. When two reactants are present in two different, immiscible liquid phases (usually one aqueous and the other organic), they can often be brought together by addition of a solvent that is both water-like and organic-like (e.g., ethanol, which derives its hydrophilic nature from its hydroxyl group and its lipophilicity from the ethyl group). However, the rate enhancement tends to be limited due to excessive solvation of the nucleophile.