10.5 - Halogenoalkanes
10.5.1: Describe, using equations, the substitution reactions of halogenoalkanes with sodium hydroxide.
Sodium hydroxide provides hydroxide ions. The reaction is usually carried out in aqueous ethanol to allow the halogenoalkane to dissolve, as they are not soluble in water.
The hydroxide ions attack the halogenoalkane at the partially positive (because of the difference in electronegativity between carbon and halogens) carbon.
This causes the halogen to break off leaving the OH group attached to the carbon.
The overall effect is substitution of the halogen for an alcohol group.
Notice that the sodium ions are spectators and are not involved.
10.5.2: Explain the substitution reactions of halogenoalkanes with sodium hydroxide in terms of SN1 and SN2 mechanisms.
The type of mechanism (the actual collisions and order of bond cleavage) employed for the substitution of halogenoalkanes by nucleophiles depends on whether the halgenoalkane is primary, secondary or tertiary.
The hydroxide ion attacks the partially positive carbon of the C-X bond (where X is the halogen)
This produces a five bonded carbon transition state. This is a logical necessity and cannot be identified.
The transition state then loses the halogen atom as a halide ion, returning the carbon to its normal tetrahedral condition.
This mechanism is said to be SN2, (substitution, nucleophilic, bimolecular)
The attack by the reagent is nucleophilic
The overall effect of the mechanism is substitution
There are two particles colliding in the rate determining step = bimolecular.
Tertiary halogenoalkanes undergo substitution via a different mechanism.
The first step is heterolytic bond fission of the C-X bond. This is the rate determing step of the mechanism.
This can occur because it gives rise to an intermediate structure called a tertiary carbonium ion (carbo-cation). This structure is relatively stabilised compared to its primary counterpart by the positive electron inductive effect of three alkyol groups attached to the carbon atom holding the positive charge.
The positive carbonium ion is then atttacked by the nucleophilic hydroxide ion forming the alcohol.
The mechanism is said to be SN1 (substitution, nucleophilic, unimolecular) as only one particle is involved in the rate determining step.