Haloalkanes and Haloarenes
Haloalkanes and Haloarenes tests nomenclature, nucleophilic substitution (SN1/SN2), elimination, reactivity and the special inertness of aryl halides. Mechanism-and-reason questions dominate.
Core rules
- SN1: two steps, carbocation intermediate, first-order, favoured by 3° substrate and polar protic solvent.
- SN2: one step, backside attack, second-order, favoured by 1° substrate and polar aprotic solvent.
- Reactivity toward SN1: 3° > 2° > 1°; toward SN2: 1° > 2° > 3°.
- E1 and E2 compete; strong bulky base favours E2 and less substituted alkene.
- Aryl and vinyl halides are inert toward SN1/SN2 because the C–X bond has partial double-bond character and backside attack is blocked.
Worked example
Explain why 2-bromo-2-methylpropane reacts rapidly with aqueous NaOH by an SN1 mechanism. Reasoning: the substrate is tertiary, so it forms a stable tertiary carbocation after bromide leaves. The rate-determining step is ionisation and depends only on the substrate. Water/OH- then attacks the carbocation; because the intermediate is planar, the product can be racemic.
Traps
- Do not use SN2 for a tertiary halide as the main pathway.
- Allylic and benzylic halides are unusually reactive because their carbocations are resonance-stabilised.
- Aryl halides can undergo nucleophilic substitution only under drastic conditions or via benzyne.
- Saytzeff’s rule predicts the more substituted alkene, not the less substituted one.