Halogenoalkanes: properties, reactions, mechanisms

Halogenoalkanes (Alkyl Halides)

Halogenoalkanes are organic compounds in which one or more hydrogen atoms of an alkane have been replaced by a halogen (F, Cl, Br, I). They are central to many synthetic routes and feature prominently in the Cambridge International AS & A Level Chemistry (9701) syllabus (2025‑27).

1. Classification

Note: In the AS/A‑Level syllabus the reaction outcomes (SN1, SN2, E1, E2) are only required for alkyl halides (primary, secondary, tertiary). Vinyl and aryl halides are discussed only in the optional “organic” section.

2. Physical properties

• State: colourless liquids or gases; higher members become oily.
• Boiling point: rises with molecular weight and with the size of the halogen (I > Br > Cl > F).
• Polarity (and hence water solubility): F > Cl > Br > I. More polar halogenoalkanes are more soluble.
• Density: usually slightly greater than water but lower than the corresponding alkane.
• Odour: often characteristic (e.g., chloroform has a sweet smell).

3. IUPAC nomenclature (halogen‑substituted alkanes)

1. Identify the longest carbon chain that contains the carbon bearing the halogen.
2. Number the chain to give the carbon attached to the halogen the lowest possible locant.
3. Use the appropriate halo‑prefix (fluoro‑, chloro‑, bromo‑, iodo‑) and place the locant before it.
4. If more than one different halogen is present, list them alphabetically (e.g., 1‑chloro‑2‑bromo‑propane).
5. If several identical halogens are present, use the di‑, tri‑, tetra‑ prefixes (e.g., 1,2‑dichloroethane).

4. Preparation of halogenoalkanes

4.1 Free‑radical halogenation of alkanes

• Photochemical or thermal initiation produces halogen radicals (Cl·, Br·).
• Overall: CH₄ + Cl₂ →[hv] CH₃Cl + HCl
• Reaction is selective for the most substituted (tertiary > secondary > primary) C–H bond because of radical stability.

4.2 Substitution of alcohols

• With hydrogen halides (HX) – usually HCl or HBr in the presence of ZnCl₂ (Lucas reagent) for tertiary alcohols:

  R–CH₂OH + HBr → R–CH₂Br + H₂O

• With reagents that convert –OH into a good leaving group:
  • Thionyl chloride (SOCl₂): R–CH₂OH + SOCl₂ → R–CH₂Cl + SO₂ + HCl
  • Phosphorus pentachloride (PCl₅): R–CH₂OH + PCl₅ → R–CH₂Cl + POCl₃ + HCl
  • Phosphorus tribromide (PBr₃): 3 R–CH₂OH + PBr₃ → 3 R–CH₂Br + H₃PO₃

  These reactions must be performed under anhydrous conditions to avoid hydrolysis.

4.3 Displacement of sulfonate (good leaving) groups

• Tosylates (OTs) or mesylates (OMs) are excellent leaving groups. Example (Finkelstein‑type exchange):

  R–OTs + NaI → R–I + NaOTs   (SN2, polar aprotic solvent e.g., acetone)

4.4 Halide exchange (Finkelstein reaction)

• Conversion of a chloride or bromide into an iodide:

  R–Cl + NaI → R–I + NaCl↓   (dry acetone drives the reaction by precipitating NaCl)

• Works best for primary and some unhindered secondary substrates.

4.5 Nucleophilic substitution on an existing halide

• Replacing the halide with another nucleophile (e.g., azide, cyanide, thiolate) under SN2 conditions:

  R–Br + NaN₃ → R–N₃ + NaBr

5. Reactions of halogenoalkanes

5.1 Nucleophilic substitution

SN2 – bimolecular, concerted

• Typical substrates: primary halides; secondary halides if sterically unhindered.
• Strong nucleophile in a polar aprotic solvent (e.g., NaI/acetone, KCN/DMF).
• Overall:

  R–CH₂–X + Nu⁻ → R–CH₂–Nu + X⁻

• Back‑side attack → Walden inversion of configuration.
• Rate law: rate = k[substrate][nucleophile] (second order).
• Leaving‑group ability: I⁻ > Br⁻ > Cl⁻ > F⁻.

SN1 – unimolecular, stepwise

• Typical substrates: tertiary halides; some secondary halides that can form a relatively stable carbocation.
• Weak nucleophile, polar protic solvent (water, ethanol, aqueous acid).
• Mechanism:

  R₃C–X  →[slow]  R₃C⁺  +  X⁻   (carbocation formation)
  R₃C⁺   +  Nu⁻  →  R₃C–Nu       (fast capture)
  

• Carbocation stability governs rate: tertiary > secondary > primary.
• Rate law: rate = k[substrate] (first order).
• Planar carbocation → attack from either side → racemic mixture when the carbon is chiral.

5.2 Elimination reactions

E2 – bimolecular, concerted

• Strong, unhindered base (e.g., NaOEt, NaOMe, t‑BuOK) with a good leaving group.
• Anti‑periplanar geometry between the β‑hydrogen and the leaving group is required.
• Overall:

  R–CH₂–CH₂–X + Base⁻ → R–CH=CH₂ + X⁻ + Base–H

• Rate law: rate = k[substrate][base] (second order).
• Favoured for primary and secondary substrates when a strong base is present; also competes with SN2.

E1 – unimolecular, stepwise

• Weak base, polar protic solvent; proceeds via the same carbocation as SN1.
• Mechanism:

  R₃C–X  →[slow]  R₃C⁺  +  X⁻
  R₃C⁺   +  Base⁻  →  R₃C=CH₂  +  Base–H
  

• Rate law: rate = k[substrate] (first order).
• Product distribution follows Zaitsev’s rule (more substituted alkene preferred) unless a bulky base forces the Hofmann product.
• Competes with SN1; the dominant pathway depends on base strength and solvent polarity.

5.3 Other characteristic reactions

• Grignard reagent formation (organomagnesium halide):

  R–X + Mg →[dry ether] R–MgX

  Requires anhydrous ether, a clean Mg surface, and often a small iodine crystal to initiate the reaction.
• Wurtz coupling (C–C bond formation):

  2 R–X + 2 Na → R–R + 2 NaX

  Best with primary halides; secondary/tertiary substrates give mixtures of coupled products and elimination side‑products.
• Halogen–metal exchange (preparation of organolithium reagents):

  R–Br + n‑BuLi → R–Li + n‑BuBr

  Performed at low temperature in anhydrous ether.
• Silver nitrate test (AgNO₃) – qualitative identification:

  R–X + Ag⁺ → R⁺ + AgX↓

  A white precipitate of AgCl or AgBr indicates a halide that can ionise (typically primary/secondary alkyl chlorides and bromides). Aryl halides do not give a precipitate under these conditions.
• Hydrolysis (oxidation to alcohols):

  R–X + 2 NaOH → R–OH + NaX + H₂O

  Proceeds via an SN1 or SN2 pathway depending on the substrate.
• Reduction to alkanes:
  • Zn/H⁺ (or Zn/AcOH) – replaces the halogen with hydrogen.
  • Na/liq. NH₃ – Birch‑type reduction (mainly for aryl halides, not required for AS/A‑Level).

6. Factors influencing reactivity

7. Typical Cambridge AS & A‑Level examination questions

1. Predict the major product when 2‑bromo‑2‑methylpropane reacts with aqueous NaOH at 25 °C. Explain why the reaction follows an SN1 pathway.
2. Explain why 1‑bromobutane undergoes an SN2 reaction with NaI in dry acetone, whereas 2‑bromobutane does not give a comparable substitution.
3. Write a detailed mechanism (curved‑arrow notation) for the preparation of ethylmagnesium bromide from ethyl bromide.
4. Compare the rate laws for SN1 and SN2 reactions and describe an experimental method (e.g., varying nucleophile concentration) that would allow you to distinguish the two mechanisms.
5. Describe how the AgNO₃ test can be used to differentiate between a primary alkyl chloride and an aryl chloride.
6. Outline the hydrolysis of an alkyl bromide to the corresponding alcohol using aqueous NaOH, indicating any stereochemical consequences for a chiral substrate.
7. Given the substrate tert‑butyl bromide and a strong base (t‑BuOK), predict the major product and justify whether E2 or SN2 predominates.

8. Suggested revision diagrams (to be drawn by the student)

• Energy‑profile diagram contrasting the single‑step transition state of SN2 with the two‑step profile of SN1 (including the carbocation intermediate).
• Stereochemical outcome: Walden inversion in SN2 versus racemisation in SN1.
• Anti‑periplanar geometry required for an E2 elimination (showing the β‑hydrogen and leaving group alignment).
• Carbocation stabilisation by hyper‑conjugation and the +I effect of alkyl groups.

9. Quick reference table – reaction outcome predictions