INSPIRE Scholarship – Chemistry
Haloalkanes & Haloarenes
Purpose of this sheet – A concise, well‑structured guide that you can study in 30 min and use for quick revision before the exam.
1. Simple Explanation
| Term | What it is | Key feature |
|---|---|---|
| Haloalkane | An alkane in which one or more hydrogen atoms are replaced by a halogen (F, Cl, Br, I). | Usually linear or cyclic aliphatic compounds. |
| Haloarene | A benzene ring (aromatic) that carries one or more halogen atoms. | The halogen is part of an aromatic system, giving different reactivity compared to haloalkanes. |
Both classes are pivotal in organic synthesis: they are good leaving groups, initiate radical chains, and allow further functionalization.
2. Key Points
2.1 Haloalkanes
| Feature | Details |
|---|---|
| Classification by carbon skeleton | Primary, Secondary, Tertiary (based on the carbon attached to the halogen). |
| Leaving group ability | I > Br > Cl > F (generally). |
| Nomenclature | Prefix halo- + parent alkane + suffix – halo (e.g., 2‑bromobutane). |
| Reactivity trends | SN2 fastest on primary (back‑side attack), SN1 fastest on tertiary (ion formation). |
| Typical reactions | <ul><li>Substitution – SN1, SN2 (nucleophilic).</li><li>Elimination – E1, E2 (forming alkenes).</li><li>Radical halogenation – Cl₂, Br₂ in presence of H₂O₂ or light.</li><li>Finkelstein reaction – halide exchange (NaI + alkyl chloride). </li></ul> |
| Stereochemistry | SN2 gives inversion of configuration; SN1 gives racemization (unless neighboring groups lock the conformation). |
| Typical reagents | NaOH, KCN, NH₃, H₂O₂, NaI, AgNO₃. |
| Solvent effects | SN2 → polar aprotic (DMF, DMSO). SN1 → polar protic (MeOH, H₂O). |
Quick mnemonic for SN1 vs SN2
| Reaction | Carbocation stability | Mechanism | Product |
|---|---|---|---|
| SN1 | Tertiary > Secondary > Primary | 1 step (ion formation) | Racemic or equilibrium |
| SN2 | Primary > |