Haloalkanes & Haloarenes
INSPIRE Scholarship – Chemistry Module
(Advanced University‑level notes, MIT‑style)
Goal of the module
- Build a deep, mechanistic understanding of halogenated organics.
- Connect first‑principle concepts (electronegativity, orbital theory, thermodynamics, kinetics) to observable chemical phenomena.
- Prepare for rigorous examinations and research projects in synthetic, physical, and medicinal chemistry.
Prerequisites
- General Organic Chemistry (alkanes, alkenes, aromatic chemistry).
- Basic Physical Chemistry (thermodynamics, kinetics, quantum chemistry).
- Basic Inorganic Chemistry (coordination, organometallics).
1. Definitions & Classification
| Class | General Formula | Key Features | Representative Examples |
|---|---|---|---|
| Haloalkanes | R–X (alkyl halide) |
Single halogen bonded to sp³ carbon. | Chloromethane, 1‑bromopropane, 2‑chlorobutane |
| Haloarenes | Ar–X (aryl halide) |
Halogen bonded to sp² aromatic carbon. | Chlorobenzene, 4‑bromophenol, 2‑fluoro‑pyridine |
Notation: X = F, Cl, Br, I.
Positioning: In haloarenes, ortho/para vs meta relative to substituents matters for reactivity.
2. Nomenclature (IUPAC)
-
Haloalkanes
- Parent chain → name of alkane.
- Halogen position → number (lowest possible).
- Halogen name → prefixed with halogen name (
fluoro-,chloro-,bromo-,iodo-). - Multiple halogens → use
di-,tri-, etc.
Example: 2‑bromopropane →
1,1,2‑tribromopropane(if three Br). -
Haloarenes
- Parent ring → benzene or heteroaromatic.
- Halogen position → number relative to a reference point (usually the heteroatom or a substituent).
- Halogen name as above.
Example: 4‑chlorobenzonitrile →
4‑chloro‑benzonitrile.
3. Physical Properties & Structure
| Property | Haloalkanes | Haloarenes |
|---|---|---|
| Polarity | Moderate; halogen electronegativity → dipole moment. | Strong dipole for C–X, but aromatic ring reduces overall polarity. |
| Boiling point | Increases with chain length & halogen size; Br > Cl > F. | Higher than alkanes due to π–π interactions and C–X bond strength. |
| Solubility | Generally soluble in organic solvents; low in water (except small haloalkanes). | Soluble in organic; limited in water. |
| Spectral signatures |
- ¹H NMR: upfield shift for protons α to halogen; splitting patterns via coupling constants J₁₀ ~ 7–10 Hz.
- ¹³C NMR: downfield shift (~5–10 ppm).
- ⁷⁹Br, ¹¹⁹Sn: EPR for radicals. |
- ¹³C NMR: characteristic C–X chemical shift ~120–140 ppm for aryl halides. | | Electronic structure |
- σ‑bond between C(sp³) and halogen: largely ionic.
- Halogen p-orbitals can overlap with C 2p for σ* antibonding. |
- C–X σ‑bond similar; π‑system remains largely unaffected unless deprotonated. |
4. Synthetic Routes to Haloalkanes
| Method | General Reaction | Key Features | Practical Notes |
|---|---|---|---|
| Halogenation of alkanes | R–H + X₂ → R–X + HX (radical). |
Requires UV or thermal initiation; produces radical chain. | Control stoichiometry to avoid over‑halogenation. |
| Substitution of alcohols | ROH + HX (e.g., HCl, HBr) → R–X + H₂O (Fischer–Speier). |
SN1 or SN2 depending on substrate. | Acidic medium protonates alcohol → better leaving group. |
| Reduction of alkyl halides | `R–X + 2e⁻ → R |