Nucleophilic Substitution Reactions
- In nucleophilic substitution reactions involving halogenoalkanes, the halogen atom is replaced by a nucleophile
- The strength of any nucleophile depends on its ability to make its lone pair of electrons available for reaction
- The hydroxide ion, OH-, is a stronger nucleophile than water because it has a full negative charge
- This means that it has a readily available lone pair of electrons
- A water molecule only has partial charges, δ+ and δ-
- This means that its lone pair of electrons is less available than the hydroxide ions
- The lone pairs of electrons in a water molecule are still available to react
Lewis structures of the hydroxide ion and water molecule - illustrating the lone pairs of electrons and charges within their structures
Exam Tip
In general:
- A negatively charged ion will be a stronger nucleophile than a neutral molecule
- A conjugate base will be a stronger nucleophile than its corresponding conjugate acid
- e.g. the hydroxide ion is a stronger nucleophile than water