Revise Halogenoalkanes for Cambridge 9701 (15.1)

The key to understanding the chemistry of halogenoalkanes is the carbon-halogen bond (C-X). As halogens are more electronegative than carbon, the electron pair in the covalent bond is drawn closer to the halogen. This creates a polar bond, with the carbon atom being electron-deficient (δ+) and the halogen atom being electron-rich (δ−). This δ+ carbon is an electrophilic centre, making it susceptible to attack by nucleophiles (electron-pair donors).

This is the characteristic reaction of halogenoalkanes. A nucleophile attacks the partially positive carbon atom, and the halogen is displaced as a halide ion ($X^−$), which is known as a leaving group. The overall process is the substitution of the halogen by the nucleophile.

The way a substitution reaction happens is called its mechanism. There are two main pathways: SN1 and SN2. The path taken depends primarily on the structure of the halogenoalkane: primary (1°), secondary (2°), or tertiary (3°).

SN2 stands for Substitution Nucleophilic Bimolecular. The '2' (bimolecular) indicates that the rate of reaction depends on the concentrations of two species: the halogenoalkane and the nucleophile. It is a single-step process. The nucleophile attacks the δ+ carbon atom from the side opposite to the leaving group (the halogen). This is called 'backside attack'. As the nucleophile bonds, the carbon-halogen bond breaks simultaneously. This occurs via a high-energy transition state where the carbon is momentarily bonded to five groups. This mechanism results in an 'inversion of configuration' at the carbon centre. Primary halogenoalkanes favour this mechanism because there is little steric hindrance to prevent the nucleophile from attacking.

SN1 stands for Substitution Nucleophilic Unimolecular. The '1' (unimolecular) indicates that the rate of reaction depends only on the concentration of one species: the halogenoalkane. This is a two-step mechanism.

Step 1 (slow, rate-determining): The C-X bond breaks by heterolytic fission to form a planar carbocation intermediate and a halide ion. This is the slow step because it requires energy to break the bond.

Step 2 (fast): The nucleophile rapidly attacks the flat carbocation. Since the carbocation is planar, the nucleophile can attack from either the top or the bottom face with equal probability. If the original carbon was a chiral centre, this results in a 50:50 mixture of two enantiomers, known as a racemic mixture.

Tertiary halogenoalkanes favour this mechanism because the resulting tertiary carbocation is relatively stable, stabilised by the positive inductive effect of the three electron-donating alkyl groups.

The rate of nucleophilic substitution is also influenced by the identity of the halogen. The reaction involves breaking the carbon-halogen bond. The weaker the bond, the faster the reaction. Bond strength decreases down Group 17.

Bond Enthalpy: C-F > C-Cl > C-Br > C-I Rate of Reaction: R-I > R-Br > R-Cl > R-F

In an elimination reaction, a small molecule (usually H-X) is removed from the halogenoalkane to form an alkene. This reaction is favoured by using a strong base in a non-polar solvent. The typical conditions are heating the halogenoalkane with a hot, concentrated solution of sodium or potassium hydroxide in ethanol. Here, the hydroxide ion acts as a base, removing a proton ($H^+$) from a carbon atom adjacent to the one bonded to the halogen. Simultaneously, the halogen leaves as a halide ion and a C=C double bond is formed.

$CH_3CH_2Br + KOH \xrightarrow{\text{ethanol, heat}} CH_2=CH_2 + KBr + H_2O$