20.2 – Nucleophilic Substitution Reactions
20.2.1 – Explain why the hydroxide ion is a better nucleophile than water
A nucleophile has a lone pair of electrons and is attracted to a positive nucleus. OH– is a stronger nucleophile than H2O because it has a negative charge. Stronger nucleophiles are also stronger bases.
20.2.2 – Describe and explain how the rate of nucleophilic substitution in halogenoalkanes by the hydroxide ion depends on the identity of the halogen
Polarity
Depending on the electronegativity of the halogen, the polarity of the C-X bond will vary. A more electronegative halogen will cause a more polar bond. Greater polarity leads to greater reactivity.
Strength
Bond strength decreases from fluorine to iodine, so the C-F bond is stronger than the C-I bond, meaning that the C-I bond is easier to break. Since this is the largest factor in determining the rate of reaction, we can generalise that the relative rate of reaction is:
20.2.3 – Describe and explain how the rate of nucleophilic substitution in halogenoalkanes by the hydroxide ion depends whether the halogenoalkane is primary, secondary or tertiary
Reactions that use SN1 mechanisms are faster than those that use SN2 mechanisms. Tertiary halogenoalkanes tend to use SN1 mechanisms, whilst primary halogenoalkanes tend to use SN2 mechanisms. Secondary halogenoalkanes use a combination of the two. Therefore, the rate of reaction tends to be:
20.2.4 – Describe, using equations, the substitution reactions of halogenoalkanes with ammonia and potassium cyanide
Amines are produced in the reaction of a halogenoalkane and ammonia. The ammonia is present at high concentration. The reactants are heated under pressure. The halogen causes the halogenoalkane to be a polar molecule, making it susceptible to attack from a nucleophile like ammonia.
However, the primary amine also acts as a nucleophile, undergoing subsequent reactions to form secondary and tertiary amines.
20.2.5 – Explain the reactions of primary halogenoalkanes with ammonia and potassium cyanide in terms of the SN2 mechanism
SN2 reactions involve a single step, with transition state being formed. As a result, the rate is dependent on the concentration of both reactants.
The CN– ion acts as a nucleophile. It can react with a halogenoalkane to form a nitrile. It is usually added to the halogenoalkane in the form of potassium cyanide (KCN) with a solvent such as dimethylsulfoxide (CH3)2SO3 or ethanol. The reactants are heated under reflux.
This is also an SN2 reaction, hence it forms a transition state.
20.2.6 – Describe, using equations, the reduction of nitriles using hydrogen and a nickel catalyst
Once a nitrile has been formed, it can then be reduced into an amine in the presence of hydrogen. A nickel catalyst is used to speed up the reaction. For example, propanenitrile is reduced to propanamine according to the equation:
