Nucleophilic Substitution

What happens in a nucleophilic substitution reaction?

A nucleophile attacks a polar molecule, kicks out a functional group and settles itself down in its place.

What is the general equation for the nucleophilic substitution of an alkane?

CH₃CH₂X + Nu^- → CH₃CH₂Nu + X^-

What is the overall equation for the reaction of a halogenoalkane (R-X) with OH^-?

R-X + OH^- → ROH + X^-

What is the mechanism for the nucleophilic substitution of bromoethane with a hydroxide ion?

1. The C-Br bond is polar. The partially positive C attracts a lone pair of electrons from the OH^- ion.

2. The OH^- ion acts as a nucleophile, attacking the slightly positive carbon atom.

3. A new bond forms because the C and the OH^- ion, making an alcohol.

4. The C-Br bond breaks. Both the electrons from the bond are taken by the Br.

What are nitriles?

Nitriles have CN groups. The carbon atom and nitrogen atom are held together with a triple bond. They are derived from hydrogen cyanide.

What is the reaction of bromoethane with potassium cyanide?

• CH₃CH₂Br + KCN → CH₃CH₂CN + KBr

• The ethanolic potassium cyanide dissociates for form a K⁺ ion and a CN^- ion.

• KCN → K⁺ + CN^-

What is the mechanism for the nucleophilic substitution of bromoethane and potassium cyanide under reflux?

The lone pair of electrons on the cyanide ion attacked the partially positive carbon, the C-Br bond breaks and the bromine leaves.

What are amines?

• An amine has the structure R₃N. The R groups can be hydrogens or another group.

• In amines, the nitrogen always has a lone pair

What is the mechanism for the nucleophilic substitution reaction of bromoethane with ethanolic ammonia?

• The first step is the same as the other mechanisms. The nitrogen atom donates the lone pair of electrons to the carbon atom to create a bond

• The nitrogen atom was neutral to begin with, so nitrogen is left with a positive charge

• In the second step another ammonia molecule removes a hydrogen from the NH₃ group to form an ammonium ion and an amine

• The ammonium ion can react with the bromide ion to form ammonium bromide

How does reactivity differ for different halogenoalkanes?

• The carbon-halogen bond strength decides reactivity as for the reaction to occur this bond needs to break

• The C-F bond is the strongest so fluoroalkanes undergo nucleophilic substitution reactions more slowly than other halogenoalkanes.

• The C-I bond has the lowest bond enthalpy, so it is easier to break. This means iodoalkanes are substituted more quickly.