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Thursday, 23 February 2017

Haloalkanes and Haloarenes - III


Physical Properties of Haloarenes:
  • These are generally colourless liquids or crystalline solids.
  • These are heavier than water.
  • Melting and boiling points of haloarenes
  • Melting and boiling points of haloarenes are nearly the same as those of alkyl halides containing the same number of carbon atoms.
  • The boiling points of monohalogen derivatives of benzene are in the order: iodo > bromo > chloro > fluoro
  • For the same halogen atom, the melting and boiling points increase as the size of the aryl group increases.
  • The melting point of para isomer is quite higher than that of ortho or meta isomers. This is due to the fast that is has symmetrical structure and therefore, its molecules can easily pack loosely in the crystal lattice. As a result intermolecular forces of attraction are stronger and therefore, greater energy is required to break its lattice and it melts at higher temperature.
Chemical properties of haloalkanes:

Nucleophilic substitution reaction:
Mechanism of Nucleophilic Substitution Reaction:

SN1 Mechanism
First order reaction.
Rate =   k [RX] [Nu]
Racemic mixture
One step reaction
Order: CH3X < 10< 20 < 30 SN2 Mechanism
Second order reaction
Rate = k [RX]
Inversion of configuration
Two step reaction
Order: CH3X > 10 > 20 > 30
  • R - X + aq. KOH ® R - OH + KX
  • R -X+ NH3  ® R - NH 2 + HX
  • R - X + KCN ® R - CN + KX
  • R - X + AgCN ® R - NC + KX
Elimination reaction: Dehydrohalogentaion(  - elimination):

When a haloalkane with β-hydrogen atom is heated with alcoholic solution of potassium hydroxide, there is elimination of hydrogen atom from β-carbon and a halogen atom from the α-carbon atom. As a result, an alkene is formed as a product. Zaitsev rule (also pronounced as Saytzeff) is followed.It states that “In dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms.”

Reaction with metals:


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