Haloalkanes and Haloarenes
Replacing hydrogen atom(s) of a hydrocarbon by halogen atom(s) gives alkyl halides (haloalkanes, R-X) when the halogen is on an sp3 carbon and aryl halides (haloarenes) when it is on an sp2 carbon of an aromatic ring.
Classification
- By number of halogens: mono, di (gem/vicinal), poly halogen compounds.
- By carbon type: allylic (sp3 C next to C=C), benzylic (sp3 C attached to ring), vinylic (sp2 C of C=C), aryl (sp2 ring C).
- Alkyl halides are 1 degree, 2 degree or 3 degree by the carbon bearing the halogen.
Nature of the C-X bond
Halogen is more electronegative than carbon, so the bond is polarised (C is delta+, X is delta-). Down the group, bond length increases (C-F 139 to C-I 214 pm) while bond enthalpy decreases (C-F strongest).Preparation
- From alcohols: with HX (3>2>1 reactivity), PCl3/PBr3, or SOCl2 (preferred - SO2 and HCl escape, pure product).
- From hydrocarbons: free radical halogenation of alkanes (poor, gives mixtures); addition of HX (Markovnikov) or X2 to alkenes.
- Halogen exchange: Finkelstein (R-Cl/Br + NaI in dry acetone gives R-I); Swarts (gives R-F using AgF, SbF3 etc.).
- Haloarenes: electrophilic halogenation of arenes (Lewis acid catalyst) or Sandmeyer reaction from diazonium salts.
Reactions of haloalkanes
- Nucleophilic substitution - via SN1 (two steps, carbocation, 3>2>1, racemisation) or SN2 (one step, 1>2>3, inversion).
- Elimination (alcoholic KOH, beta-elimination, Zaitsev product).
- Reaction with metals - Grignard reagents (RMgX), Wurtz reaction.