Nucleophilic Substitution of Haloalkanes
Nucleophiles
- A nucleophile is an electron-rich species that can donate a pair of electrons
- ‘Nucleophile’ means ‘nucleus/positive charge loving’ as nucleophiles are attracted to positively charged species
- Nucleophilic refers to reactions that involve a nucleophile
- There are various different species which can behave as nucleophiles, and some make better nucleophiles than others
A hydroxide ion is a better nucleophile as it has a full formal negative charge whereas the oxygen atom in water only carries a partial negative charge
- A nucleophilic substitution reaction is one in which a nucleophile attacks a carbon atom which carries a partial positive charge
- An atom that has a partial negative charge is replaced by the nucleophile
- Haloalkanes will undergo nucleophilic substitution reactions due to the polar C-X bond (where X is a halogen)
Due to large differences in electronegativity between the carbon and halogen atom, the C-X bond is polar
General Mechanism for Nucleophilic Substitution
Hydrolysis of Haloalkanes
- The nucleophile in this reaction is the hydroxide, OH- ion
- An aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) with ethanol is used
- This reaction is very slow at room temperature, so the reaction mixture is warmed
- This is an example of a hydrolysis reaction and the product is an alcohol
- The rate of this reaction depends on the type of halogen in the haloalkane
- The stronger the C-X bond, the slower the rate of the reaction
- In terms of bond enthalpy, C-F > C-Cl > C-Br > C-I
- Fluoroalkanes do not react at all, but iodoalkanes have a very fast rate of reaction
The nucleophilic substitution mechanisms for the above reactions are as follows:
Nucleophilic Substitution with OH-
