solvation

Examples of solvation in the following topics:

• Solvent Effects

  • Solvation of nucleophilic anions markedly influences their reactivity.
  • Polar, protic solvents such as water and alcohols solvate anions by hydrogen bonding interactions, as shown in the diagram below.
  • These solvated species are more stable and less reactive than the unsolvated "naked" anions.
  • Polar, aprotic solvents such as DMSO (dimethyl sulfoxide), DMF (dimethylformamide) and acetonitrile do not solvate anions nearly as well as methanol, but provide good solvation of the accompanying cations.

• Solvent Effects

  • The second factor important in the stabilization of ions, which also resists their intimate recombination, is called solvation.
  • Anions are best solvated by hydrogen-bonding solvents; cations are generally solvated by binding to nucleophilic sites on a solvent molecule Two-dimensional diagrams illustrating the primary solvation shell about Na(+) and Cl(–) are shown here.
  • Thus solvation stabilizes and insulates an ion, helping the enthalpic change, whereas the same solvation adds order and structure to the ionic species at the cost of lowering entropy.
  • However this argument fails to consider the ordering of solvent molecules taking place in the solvation of these ions.
  • Because of their greater charge density, small ions and highly charged ions, such as F– and Ca2+, require greater solvation than large or singly charged ions, such as Na+ or Cl–.

• Intermolecular Forces and Solutions

  • In order to form a solution, the solute must be surrounded, or solvated, by the solvent.
  • After dissolution occurs, solvation follows.
  • Many intermolecular forces can contribute to solvation, including hydrogen bonding, dipole-dipole forces, and Van Der Waals forces.
  • Another common example of these forces at work is an ion-dipole interaction, which arises when water solvates ions in solution.
  • In this case, the anion Cl- is solvated by the positive dipoles of water, which are represented by hyrogen atoms.

• Nucleophilicity & Basicity

  • Solvation of nucleophilic anions markedly influences their reactivity.
  • Polar, protic solvents such as water and alcohols solvate anions by hydrogen bonding interactions, as shown in the diagram below:
  • These solvated species are more stable and less reactive than the unsolvated "naked" anions.
  • Polar, aprotic solvents such as DMSO (dimethyl sulfoxide), DMF (dimethylformamide) and acetonitrile do not solvate anions nearly as well as methanol, but provide good solvation of the accompanying cations.

• Water Solubility

  • Water dissolves many ionic salts thanks to its high dielectric constant and ability to solvate ions.

• Nucleophilic Addition Reactions & Reduction

  • In the first, sodium dissolves in the pure liquid to give a deep blue solution consisting of very mobile and loosely bound electrons together with solvated sodium cations (first equation below).

• Amines

  • Amines are bases, and their basicity depends on the electronic properties of the substituents (alkyl groups enhance the basicity; aryl groups diminish it), steric hindrance, and the degree of solvation of the protonated amine.
  • The solvation of protonated amines changes upon their conversion to ammonium compounds.

• Specialized Equilibrium Constants

  • As described previously, hydrogen ions actually exist as solvated hydronium ions in aqueous solutions.

• Heat of Solution

  • Solute-solvent attractive bond formation (the exothermic step in the process of solvation) is indicated by dashed lines.

• The Brønsted-Lowry Definition of Acids and Bases

  • Problems arise with this conceptualization because Arrhenius's definition is limited to aqueous solutions, referring to the solvation of aqueous ions, and is therefore not inclusive of acids dissolved in organic solvents.