Halogenoalkanes react with nucleophiles in nucleophilic substitution reactions. The halogen atom in the molecule is replaced by the nucleophile and the halogen leaves as a halide ion.
Nucleophile: a negatively charged ion or neutral molecule that donates a pair of electrons to an electron acceptor forming a covalent bond
Common nucleophiles include halide ions (Cl–), cyanide ions (CN–), ammonia (NH3) and water molecules:
Fluorine, chlorine and bromine are all more electronegative than carbon so the C-X bond is polar and the 𝛅+ carbon is open to attack.
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.
- The bromine leaves as a bromide ion. Halide ions make good leaving groups because they are stable.
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–.
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.
Practice questions
- Pentan-2-ol can be prepared by gently heating 2-bromopentane under reflux with aqueous sodium hydroxide. A student was asked to draw the mechanism for the reaction. Their answer is shown below:
Explain why the examiner awarded this answer NO marks.
- Iodoethane reacts with ammonia to form ethylamine. In the first step of the mechanism ammonia behaves as a nucleophile, attacking the carbon of the C-I bond, and the C-I bond breaks.
(a) What is a nucleophile? Explain why ammonia is able to behave as a nucleophile.
(b) Name the type of bond fission that occurs when the C-X bond breaks.
(c) Outline the full mechanism for the reaction between iodoethane and ammonia using curly arrows, showing all relevant charges, dipoles and lone pairs of electrons.
Answers
- The student’s answer got no marks because:
• OH– uses the lone pair of electrons to attack the 𝛅+ carbon of the C-Br bond so curly arrow should move from lone pair on the oxygen to the 𝛅+ carbon.
• OH– attacks from the opposite side to the bromine atom.
• Dipole not shown on C-Br bond.
• When the C-Br bond breaks the pair of electrons moves from the bond (not the carbon atom) to the bromine atom.
• No Br– product shown.
2. (a) A nucleophile is a negatively charged ion or neutral molecule with a lone pair of electrons that can be donated to a partially positive carbon atom to form a covalent bond.
Ammonia can behave as a nucleophile because the nitrogen atom has a lone pair of electrons that can be donated.
(b) Heterolytic fission
(c)
