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Solvation

Solvation describes the interaction of solvent with dissolved molecules. Both ionized and neutral molecules interact strongly with solvent, with solvation involving any of bond formation, hydrogen bonding or Van der Waals forces. The strength and nature of these interactions influence many properties of the solute - for example solubility and reactivity.

Protic solvents that contain at least one hydrogen atom bound to an oxygen or nitrogen, are particularly notorious for solvent-solute interactions due to the presence of a labile hydrogen. When protic solvents are used in nucleophilic reactions this positively polarized hydrogen can create hydrogen bonds with the negatively charged nucleophile.

On the left are two ions in protic solvent: one ion is negatively charged and one ion is positively charged. 6 water molecules have surrounded each of the ions. Hydrogen bonds have formed between water’s hydrogen atoms and the negatively charged anion. This anion is a nucleophile and is caged by water molecules. Hydrogen bonds have formed between water’s oxygen atom and the positively charged cation. On the right are two ions in an aprotic solvent: one ion is negatively charged and one ion is positively charged. The negatively charged anion is not surrounded by any solvent molecules therefore it is exposed. The positively charged cation is surrounded by 4 acetone molecules. The oxygen in acetone is interacting with the positively charged cation.

Figure 1 Left: Charged species solvated in protic solvent (water); Right: Solvent interactions in an aprotic solvent (acetone) and charged species in solution.

Molecules or ions that are surrounded by these solvent molecules are said to be solvated. The solvent forms a 'shell' around the nucleophile and this reduces the nucleophile's ability to attack an electrophile. In other words - solvation decreases nucleophilicity.

These attractive solvent interactions can negatively affect the outcome of nucleophilic reactions. Aprotic solvents - those without a labile hydrogen, and therefore unable to form hydrogen bonds with ions in solution - should be selected to allow the nucleophile to freely initiate the reaction.

Referred from:

Article
Solvent types
Article
The SN2 Reaction