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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 de ned 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.
- Speci c solvation is where the solvent interacts with one of the ions (or molecules) in solution via a covalent interaction. Furthermore, there will be a speci c number of solvent molecules bound to each ion (or molecule), e.g., [Cu(NH 3 ) 4 ]2+^ and [Mg(H 2 O) 6 ]2+^ (Figure 1a).
- Non-speci c 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 de ned 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) speci c and (b) non-speci c solvation.
Table 1 shows the ability of three solvents to act with speci c and non-speci c solvation. The relative solvation ability of each solvent results in three dierent products from the dissolution of iron(III) chloride (FeCl 3 ).
∗Version 1.3: Jan 28, 2010 10:36 am US/Central †http://creativecommons.org/licenses/by/3.0/
Solvation DMSO (Me 2 SO) Pyridine (C 5 H 5 N) Acetonitrile (MeCN) Speci c Good Very good Poor Non-speci c Good Poor Moderately good
Table 1: The ability for speci c and non-speci c solvation.
Dissolution of FeCl 3 in DMSO results in the dissociation of a chloride ligand, (1), due to both the speci c solvation of the FeCl 2 + cation and the non-speci c 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).
In contrast, if FeCl 3 is dissolved in pyridine (py) the neutral Lewis acid-base complex is formed, (3), because while pyridine is a very good at speci c solvation (Table 1), it is poor at solvating the chloride anion.
In a similar manner, FeCl 3 (MeCN) 3 will be formed by the dissolution in acetonitrile, because although it is not good at speci c solvation, it is not su ciently good at non-speci c solvation to stabilize the chloride anion. However, since the FeCl 4 -^ anion has a lower charge density that Cl-, it can be supported by the non-speci c solvation of acetonitrile and thus a disproportionation reaction occurs, (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 = C 2 H 5. In each case the same general reaction takes place yielding the cation associated with the solvent.
In a similar manner, the weak basic character of water means the equilibrium reaction, (13), has a very small equilibrium constant, K. However, if the reaction is carried out in a strongly basic solvent such as ammonia the uride anion is stabilized, (14), and can be precipitated by cation exchange.
3.2 Salt stabilization through solvation
The following observations may be explained by a consideration of the solvation ability of the solvent.
- The reaction of silver nitrate with barium chloride in water yields silver chloride and barium nitrate, (15).
- The reaction of barium nitrate with silver chloride in ammonia yields barium chloride and silver nitrate, (16).
Since silver nitrate and barium nitrate are soluble in both solvents, the dierences must be due to dierences in the solubility of the chlorides in each solvent. A consideration of the relative stability of solid silver chloride versus the solvated species (Figure 2) shows that the enthalpy of solvation in water is less than the lattice energy. Thus, if silver chloride were present as Ag+^ and Cl-^ in water it would spontaneously precipitate. In contrast, the enthalpy of solvation in ammonia is greater than the lattice energy, thus solid AgCl will dissolve readily in liquid ammonia. The reason for the extra stabilization from the speci c solvation of the silver cation by the ammonia, i.e., the formation of the covalent complex [Ag(NH 3 ) 2 ]+.
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 BaCl 2 will dissolve readily in water. The stabilization of Ba2+(aq) occurs because water will have a larger sphere of non-speci c 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.
