Haloalkanes (alkyl halides) are derived from alkanes by replacing one or more H atoms with halogens (F, Cl, Br, I). Haloarenes have halogen directly on a benzene ring. These are key industrial solvents, refrigerants, and synthetic building blocks.
Classification
- By substitution class: Primary (1°) — halogen on C bonded to one C; Secondary (2°) — two C; Tertiary (3°) — three C.
- By hybridisation: Alkyl halides (sp3 C–X, reactive); vinyl/aryl halides (sp2 C–X, less reactive due to resonance).
Nature of C–X Bond
The C–X bond is polar covalent (X is more electronegative). Bond polarity: C–F > C–Cl > C–Br > C–I. Bond strength decreases and bond length increases from F to I. Reactivity order: RI > RBr > RCl > RF.
Methods of Preparation
- 1.From alcohols: ROH + SOCl2 → RCl + SO2 + HCl (no rearrangement)
- 2.From alkenes (Markovnikov): CH3CH=CH2 + HBr → CH3CHBrCH3
- 3.Finkelstein reaction: RI + NaCl -(acetone)→ RCl + NaI (halogen exchange)
- 4.Sandmeyer (aryl halides): ArN2^+Cl^- + CuCl → ArCl + N2
Key Reactions
- Nucleophilic Substitution:
- SN2: One-step backside attack, rate = k[RX][Nu^-], inversion of configuration (Walden inversion). Favoured by 1° halides and polar aprotic solvents (DMSO).
- SN1: Two-step via carbocation, rate = k[RX], racemisation. Favoured by 3° halides and polar protic solvents.
Elimination: Alcoholic KOH + 2° or 3° halides → alkenes (Saytzeff rule: more substituted alkene major).
Grignard reagent: RX + Mg -(dry ether)→ RMgX (nucleophilic carbon, key synthetic tool).
Wurtz reaction: 2RX + 2Na → R–R + 2NaX.
Aryl halides are far less reactive toward SN because the C–X bond has partial double-bond character (resonance). Electron-withdrawing groups (–NO2) at ortho/para activate them.
Worked Examples
IUPAC name of CH3–CHBr–CH2–CH3.
Step 1: Longest chain = 4C (butane). Step 2: Br at C-2.
Answer: 2-bromobutane.
Why does SN2 give inversion of configuration?
The nucleophile attacks from the back of the C–X bond; the three remaining groups flip (umbrella inversion).
Answer: Complete inversion — Walden inversion; product is the opposite enantiomer.
SN1 reactivity order: CH3Cl, (CH3)2CHCl, (CH3)3CCl.
SN1 rate depends on carbocation stability: 3° > 2° > 1°.
Answer: CH3Cl < (CH3)2CHCl < (CH3)3CCl.
CH3Br + Mg -(dry ether)→ ?
Answer: CH3MgBr (methylmagnesium bromide, Grignard reagent).
Sandmeyer preparation of chlorobenzene.
Aniline + NaNO2/HCl (0-5°C) → C6H5N2^+Cl^-; then + CuCl → C6H5Cl + N2.
Answer: Chlorobenzene.
Why does C6H5Cl not react with NaOH(aq) at room temperature?
Resonance gives the C–Cl bond partial double bond character; sp2 C is less electrophilic; ring repels nucleophile.
Answer: Aryl halides are inert to ordinary SN reactions.
2-Bromobutane + alcoholic KOH → ?
Alcoholic KOH is a strong base, favours E2. Saytzeff rule: more substituted alkene preferred.
Answer: But-2-ene (major), but-1-ene (minor).
Common mistakes
> Watch out: Aqueous KOH → substitution; alcoholic KOH → elimination. Primary halides prefer SN2; tertiary prefer SN1. Aryl halides need EWG (–NO2 at ortho/para) to undergo nucleophilic substitution.
Summary
Haloalkanes react via SN1/SN2 or elimination depending on structure and conditions. Grignard reagents are vital synthetic intermediates. Aryl halides are resistant to SN due to resonance but can undergo electrophilic substitution. Reactivity order: RI > RBr > RCl > RF.