Preparation of haloalkanes

From alcohols

The -OH group is replaced by a halogen. Order of reactivity of alcohols: 3° > 2° > 1°.

1. 1
   R-OH + HCl → R-Cl + H₂O

   Conc HCl with anhydrous ZnCl₂ for 1° and 2° alcohols. A 3° alcohol only needs shaking with conc HCl at room temperature
2. 2
   R-OH + HBr → R-Br + H₂O

   Constant-boiling 48% HBr, or NaBr with H₂SO₄, which makes HBr in the flask
3. 3
   3R-OH + PX₃ → 3R-X + H₃PO₃ (X = Cl, Br, I)

   PBr₃ and PI₃ are made in situ (inside the reaction flask) from red phosphorus and Br₂ or I₂
4. 4
   R-OH + PCl₅ → R-Cl + POCl₃ + HCl
5. 5★
   R-OH + SOCl₂ → R-Cl + SO₂↑ + HCl↑

   Preferred method: both by-products are gases and escape, so the alkyl chloride is left pure

★ key step

From hydrocarbons

1. 1
   CH₃CH₂CH₂CH₃ + Cl₂ → CH₃CH₂CH₂CH₂Cl or CH₃CH₂CHClCH₃, + HCl

   Free radical halogenation, in UV light or on heating. It gives a mixture of isomers and polyhalides that is hard to separate, so the yield of any one compound is poor
2. 2
   CH₂=CH₂ + HI → CH₃CH₂I

   Adding HX to a symmetrical alkene gives one product
3. 3★
   CH₃CH=CH₂ + HI → CH₃CHICH₃ (major) or CH₃CH₂CH₂I (minor)

   Markovnikov's rule: the H of HX goes to the double-bond carbon that already has more H atoms, so 2-iodopropane is the major product
4. 4
   CH₂=CH₂ + Br₂ → BrCH₂CH₂Br

   Br₂ in CCl₄. The reddish-brown colour of bromine disappears: a test for unsaturation. The product, 1,2-dibromoethane, is a vic-dibromide

★ key step

Halogen exchange

1. 1★
   R-X + NaI → R-I + NaX (X = Cl, Br)

   Finkelstein reaction, in dry acetone. NaCl or NaBr does not dissolve in dry acetone and precipitates, which pulls the reaction forward (Le Chatelier's principle)
2. 2★
   CH₃Br + AgF → CH₃F + AgBr

   Swarts reaction: heat the alkyl chloride or bromide with a metallic fluoride such as AgF, Hg₂F₂, CoF₂ or SbF₃

★ key step

Finkelstein vs Swarts

Both swap one halogen for another. Remember the products: Finkelstein gives I, Swarts gives F.