Concept 01 / 06Haloalkanes and Haloarenes
Haloalkanes and Haloarenes: Classification, C-X Bond, Preparation and Physical Properties
Classification of alkyl and aryl halides (mono, di, poly; primary, secondary, tertiary; allylic, benzylic, vinylic, aryl), nature of the C-X bond, preparation of haloalkanes from alcohols, hydrocarbons and by halogen exchange (Finkelstein, Swarts), preparation of haloarenes (electrophilic substitution, Sandmeyer and Gattermann reactions), and physical properties.
Replacing hydrogen of a hydrocarbon by halogen gives haloalkanes (alkyl halides, R-X) or haloarenes (aryl halides, Ar-X).
Classification. By number of halogens (mono-, di-, poly-); by the carbon bearing X in C-X compounds: primary (), secondary (), tertiary (), allylic (X on carbon next to C=C), benzylic (X on carbon next to a ring); in C-X compounds: vinylic (X on a C=C carbon) and aryl (X on a ring carbon). Dihalides can be geminal (same carbon, alkylidene halides) or vicinal (adjacent carbons, alkylene dihalides).
Nature of the C-X bond (see figure). Halogen is more electronegative than carbon, so the bond is polar (). Down the group, the halogen's size increases, so the C-X bond becomes longer and weaker: C-F () to C-I (). Dipole moment is highest for (the shorter bond of compensates its larger charge separation).
Preparation of haloalkanes.
From alcohols (the OH is replaced): with concentrated HCl and anhydrous (Lucas reagent; alcohols react readily); with HBr, or with ; with (), (), red P with or ; best with thionyl chloride, (the by-products are gases, leaving pure RCl). Reactivity of alcohols: .
From hydrocarbons: free radical halogenation of alkanes (gives mixtures); addition of HX or to alkenes (Markovnikov, or anti-Markovnikov with HBr and peroxide).
Halogen exchange: Finkelstein reaction, or (dry acetone; NaCl and NaBr precipitate, driving the equilibrium); Swarts reaction, (also , , ).
Preparation of haloarenes.
Electrophilic substitution: (dark); toluene gives o- and p-chlorotoluene. Iodination is reversible and needs an oxidant (, ); fluorination is too violent.
From diazonium salts: Sandmeyer reaction, (X = Cl, Br); Gattermann reaction with Cu powder and HX; iodobenzene simply by warming with KI.
Physical properties. Boiling points are higher than those of parent hydrocarbons (greater molar mass and dipole-dipole forces), and increase RF < RCl < RBr < RI and with chain length; branching lowers the boiling point (1-bromobutane , 2-bromobutane , 2-bromo-2-methylpropane ). Bromo, iodo and polychloro compounds are denser than water. Haloalkanes are only very slightly soluble in water (they cannot form enough hydrogen bonds to compensate for breaking water's hydrogen bonds). Among dichlorobenzenes, the para isomer has a much higher melting point () than ortho () and meta (), because its symmetry lets it pack better in the crystal.
Key Takeaways:
- C-X bond: polar; length increases and enthalpy decreases from F to I.
- gives pure alkyl chlorides (gaseous by-products).
- Finkelstein (NaI/acetone) makes iodides; Swarts (AgF, ) makes fluorides.
- Sandmeyer: with gives ArX.
- Pitfall: aryl halides cannot be made from phenols by the reagents that convert alcohols to alkyl halides (the C-O bond of phenol is too strong).
Brain-Teaser 1: Classify: (a) , (b) , (c) , (d) , (e) .
Solution: (a) allylic (primary); (b) benzylic (primary); (c) vinylic; (d) tertiary alkyl; (e) aryl.
Brain-Teaser 2: Why is thionyl chloride preferred to for converting ethanol to chloroethane?
Solution: With the by-products and HCl are gases that escape, leaving almost pure chloroethane; with , liquid (b.p. ) must be separated by distillation.
Brain-Teaser 3: Arrange in increasing boiling point: bromomethane, bromoform, chloromethane, dibromomethane.
Solution: Chloromethane < bromomethane < dibromomethane < bromoform. Boiling point increases with molar mass and polarisability (more and heavier halogens give stronger van der Waals forces).
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