Unit 21: Halogenoalkanes — Long Questions

E2 (Elimination Bimolecular) single-step, concerted mechanism — the base removes the β-hydrogen while the leaving group (halide) departs at the same time, directly forming the C=C double bond. It is a bimolecular reaction; rate depends on concentrations of both the alkyl halide and the base: Rate = k[alkyl halide][base], second order overall. It is favoured by primary halogenoalkanes (and strong bases).

E1 (Elimination Unimolecular) two-step mechanism — Step 1 is the slow, reversible ionization of the alkyl halide to form a carbocation (rate-determining step); Step 2 is the fast removal of a β-hydrogen by the base to give the alkene. It is a unimolecular reaction; rate depends only on the alkyl halide concentration: Rate = k[alkyl halide], first order overall. It is favoured by tertiary halogenoalkanes, since they form stable carbocations.

| Feature | E2 | E1 |
|---|---|---|
| Number of steps | One (concerted) | Two |
| Intermediate | None | Carbocation |
| Rate law | Rate = k[substrate][base] | Rate = k[substrate] |
| Order of reaction | Second order | First order |
| Molecularity | Bimolecular | Unimolecular |
| Favoured substrate | Primary halogenoalkanes | Tertiary halogenoalkanes |

Chloroethane (a halogenoalkane) the C–Cl bond is a simple, polar single bond. Because of the electronegativity difference between C and Cl, the carbon carries a partial positive charge and acts as an electrophile. This makes the carbon readily attacked by nucleophiles, so chloroethane easily undergoes nucleophilic substitution — e.g. it reacts with aqueous NaOH under reflux to give ethanol (and ethene by elimination).

Chlorobenzene (a halogenoarene) the lone pair of electrons on the chlorine atom is delocalised into the aromatic ring through resonance with the ring's π system. This gives the C–Cl bond partial double-bond character, making it shorter and much stronger than the C–Cl bond in chloroethane. As a result, chlorobenzene does not react with aqueous NaOH under normal conditions and is, in general, far less reactive than chloroethane towards nucleophilic substitution. The same electron delocalisation, however, increases the electron density on the ring, making chlorobenzene more reactive than benzene itself towards electrophilic substitution.

Conclusion Halogenoalkanes (chloroethane) are considerably more reactive towards nucleophiles than halogenoarenes (chlorobenzene) because of the weaker, purely σ, more polar C–Cl bond in the former compared with the stronger, partially double-bonded C–Cl bond in the latter.

Nucleophilic substitution in alkyl halides involves two processes: departure of the leaving group (breaking the C–X bond) and attack of the nucleophile (forming the C–Nu bond). Depending on whether these happen together or in separate steps, the mechanism is Sɴ2 or Sɴ1.

Sɴ2 (Nucleophilic Substitution Bimolecular) This is a single-step, concerted mechanism. As the nucleophile begins attacking the electrophilic carbon, the C–X bond simultaneously starts breaking, so bond formation and bond breaking occur to an equal extent at the same time. The nucleophile attacks from the side opposite to the leaving group, passing through a transition state in which both the incoming nucleophile and the departing leaving group are partially bonded to carbon. Since this single, rate-determining step involves two species (the substrate and the nucleophile), the reaction is bimolecular and second order: Rate = k[alkyl halide][nucleophile]. Primary halogenoalkanes always follow Sɴ2 because low steric hindrance allows easy nucleophilic approach (example: bromomethane + OH⁻ → methanol + Br⁻).

Sɴ1 (Nucleophilic Substitution Unimolecular) This is a two-step mechanism. Step 1 is the slow, reversible ionization of the alkyl halide (typically in aqueous acetone or aqueous ethanol) to form a carbocation intermediate — this is the rate-determining step, since it involves breaking a covalent bond. Step 2 is the fast attack of the nucleophile on the planar carbocation to give the product. Because only the substrate takes part in the rate-determining step, the reaction is unimolecular and first order: Rate = k[alkyl halide]. Tertiary halogenoalkanes always follow Sɴ1 because they form the more stable tertiary carbocation, and their three bulky alkyl groups also sterically block backside attack needed for Sɴ2 (example: 2-bromo-2-methylpropane → tertiary carbocation → 2-methyl-2-propanol).

Secondary halogenoalkanes can follow either mechanism, depending on the solvent used.