Choosing the Right Solvent: A Guide to Solvation and Its Impact on Chemical Reactions, Study notes of Chemistry

Download Choosing the Right Solvent: A Guide to Solvation and Its Impact on Chemical Reactions and more Study notes Chemistry in PDF only on Docsity! Connexions module: m33076 1 Choosing a Solvent ∗ Andrew R. Barron This work is produced by The Connexions Project and licensed under the Creative Commons Attribution License † The choice of solvent is an important parameter for any chemical reaction. The following provides a guide to some of the consideration to be made in choosing a solvent to ensure the desired reaction occurs. 1 Solvation Solvation may be dened as the interaction between the solvent and the solute, however, two general classes of solvation have dierent consequences to the stability of either reagents or products in a chemical reaction, and hence the potential of a reaction to occur. • Specic solvation is where the solvent interacts with one of the ions (or molecules) in solution via a covalent interaction. Furthermore, there will be a specic number of solvent molecules bound to each ion (or molecule), e.g., [Cu(NH3)4] 2+ and [Mg(H2O)6] 2+ (Figure 1a). • Non-specic solvation is as a result of van der Waals or dipole-dipole forces between the solvent and an ion (or molecule). There will be no dened number of interactions and the solvent...ion interaction will be highly uxional, e.g., while water solvates the chloride ion (Figure 1b) the number of water molecules around each anion is not xed. Figure 1: Examples of (a) specic and (b) non-specic solvation. Table 1 shows the ability of three solvents to act with specic and non-specic solvation. The relative solvation ability of each solvent results in three dierent products from the dissolution of iron(III) chloride (FeCl3). ∗Version 1.3: Jan 28, 2010 10:36 am US/Central †http://creativecommons.org/licenses/by/3.0/ http://cnx.org/content/m33076/1.3/ Connexions module: m33076 2 Solvation DMSO (Me2SO) Pyridine (C5H5N) Acetonitrile (MeCN) Specic Good Very good Poor Non-specic Good Poor Moderately good Table 1: The ability for specic and non-specic solvation. Dissolution of FeCl3 in DMSO results in the dissociation of a chloride ligand, (1), due to both the specic solvation of the FeCl2 + cation and the non-specic solvation of the Cl- anion. In fact, the good solvation properties of DMSO means that depending on the concentration (and temperature) a series of dissociations may occur, (2). (1) (2) In contrast, if FeCl3 is dissolved in pyridine (py) the neutral Lewis acid-base complex is formed, (3), because while pyridine is a very good at specic solvation (Table 1), it is poor at solvating the chloride anion. (3) In a similar manner, FeCl3(MeCN)3 will be formed by the dissolution in acetonitrile, because although it is not good at specic solvation, it is not suciently good at non-specic solvation to stabilize the chloride anion. However, since the FeCl4 - anion has a lower charge density that Cl-, it can be supported by the non-specic solvation of acetonitrile and thus a disproportionation reaction occurs, (4). (4) 2 Interference by the solvent Rather than solvating a molecule or ion, the solvent can take an active and detrimental role in the synthesis of a desired compound. 2.1 Solvolysis The archetypal solvolysis reaction is the reaction with water, i.e., hydrolysis, (5). However, solvolysis is a general reaction, involving bond breaking by the solvent. Thus, the reaction with ammonia is ammonolysis, (6), the reaction with acetic acid is acetolysis, (7), and the reaction with an alcohol is alcoholysis, (8) where Et = C2H5. In each case the same general reaction takes place yielding the cation associated with the solvent. (5) (6) http://cnx.org/content/m33076/1.3/ Connexions module: m33076 5 Figure 2: Enthalpy of solvation of silver chloride in water and ammonia in comparison to the lattice energy. As may be seen from Figure 3, the opposite eect occurs for barium chloride. Here the enthalpy of solvation in ammonia is less than the lattice energy. Thus, if barium chloride were present as Ba2+ and Cl- in ammonia it would spontaneously precipitate. In contrast, the enthalpy of solvation in water is greater than the lattice energy, thus solid BaCl2 will dissolve readily in water. The stabilization of Ba 2+(aq) occurs because water will have a larger sphere of non-specic solvation as a consequence of having two lone pairs, allowing interaction with the Ba2+ as well as other water molecules (Figure 4). Figure 3: Enthalpy of solvation of barium chloride in water and ammonia in comparison to the lattice energy. http://cnx.org/content/m33076/1.3/ Connexions module: m33076 6 Figure 4: Schematic representation of the extended water solvation (aquation) sphere about Ba2+ cation. 4 Electron transfer reactions A consideration of the oxidation, (17), and reduction, (18), reactions that occur for pure water at neutral pH (where [H+] = 10-7) would suggest that water will not tolerate oxidants whose E0 is greater than 0.82 V nor tolerate reductants whose E0 is less than -0.41 V. (17) (18) Thus, while water has a fair range to support redox reactions it is not very good at the extremes with strong reducing agents or strong oxidizing agents. Liquid ammonia is an excellent solvent for very strong reducing agents because of the stabilization of solvated electrons, i.e., [e-(NH3)6]. In contrast, hydrochloric acid is a good solvent for reactions involving very strong oxidizing agents. http://cnx.org/content/m33076/1.3/