- The mechanism for this reaction is nucleophilic substitution
- Hydroxide ions act as nucleophiles (a species with a negative charge or a lone pair of electrons that can be donated to form a covalent bond with a positive or 𝛿+ carbon atom)
- Common nucleophiles also include halide ions (Cl–), cyanide ions (CN–), ammonia (NH3) and water molecules
Let’s look at this mechanism in a little more detail ….
- The hydroxide ion is attracted to the 𝛅+ carbon atom (the C-Br bond is polar because bromine is slightly more electronegative than carbon, but becomes more polarised as the hydroxide ion approaches).
- The OH– ion attacks from the opposite side to the bromine and uses a lone pair of electrons to begin to form bond with the carbon atom.
- At the same time as the OH- is attacking, the bromine begins to leave, taking the pair of electrons from the C-Br bond with it.
- There is a short-lived, unstable transition state in which both the O and the Br are partially bonded to the carbon.
- Bromine leaves as an ion. The product molecule is inverted (turned inside out like an umbrella) compared with the original halogenoalkane reactant – this is a direct result of the direction of attack of the OH–.
Note:
The nucleophile always attacks the 𝛅+ carbon from the side opposite to the C-X bond because the empty molecular orbital which accepts the incoming electron pair from the nucleophile is orientated along the C-X bond axis.