Nucleophilic Substitution of Haloalkanes (OCR AS Chemistry A): Revision Note

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:

• A curly arrow goes from a lone pair on the nucleophile (e.g. :OH^-) to the carbon atom of the C-X bond, which carries a partial positive charge (δ+)

• A second curly arrow goes from the C-X bond to the halogen, so the bond breaks by heterolytic fission and the halogen leaves as the halide ion X^-

• For example, bromoethane and hydroxide ions give ethanol: CH₃CH₂Br + OH^- → CH₃CH₂OH + Br^-

Nucleophilic Substitution with OH^-