Preparing halo compounds

From alcohols, the most common route. The –OH is replaced by a halogen:

• HCl with anhydrous ZnCl₂ (together called Lucas reagent). The ZnCl₂ helps to remove water, and is needed for primary and secondary alcohols; a tertiary alcohol reacts with concentrated HCl alone. HBr or HI can be used too.
• PCl₅: R–OH + PCl₅ → R–Cl + POCl₃ + HCl.
• PCl₃: one PCl₃ converts three alcohol molecules: 3R–OH + PCl₃ → 3R–Cl + H₃PO₃. For bromides and iodides, red phosphorus with Br₂ or I₂ makes PBr₃ or PI₃ on the spot.
• Thionyl chloride, SOCl₂: R–OH + SOCl₂ → R–Cl + SO₂ + HCl. This is the best method for chlorides, because the other two products are gases and escape, leaving the haloalkane pure.

From alkanes, by free-radical halogenation with Cl₂ or Br₂ in light. This usually gives a mixture, one product for each different kind of hydrogen, so it is a poor way to make one pure compound. The chain also keeps going: as long as light and Cl₂ are there, CH₃Cl goes on to CH₂Cl₂, CHCl₃ and CCl₄.

From alkenes, by adding HX across the double bond. Markovnikov's rule decides the product: the H goes to the carbon that already has more H, and the halogen to the other one. The one exception: HBr in the presence of a peroxide adds the other way round (anti-Markovnikov, the peroxide effect), putting Br on the end carbon.

Halogen exchange:

• Finkelstein reaction: R–Cl or R–Br with NaI in dry acetone gives R–I. It works because NaCl and NaBr do not dissolve in acetone and drop out, pulling the reaction forward.
• Swarts reaction: R–Br or R–Cl heated with a metallic fluoride such as AgF, Hg₂F₂ or SbF₃ gives R–F.

Allylic and benzylic hydrogens are easy to replace: heating or light with Cl₂ halogenates those positions selectively.

Haloarenes are made by electrophilic substitution: benzene with Cl₂ and a Lewis acid catalyst (FeCl₃) gives chlorobenzene. They are also made from diazonium salts (Sandmeyer reaction, chapter 9).