Compound of Carbon Containing Halogen: Properties of Haloalkanes and Haloarenes (For CBSE, ICSE, IAS, NET, NRA 2022)

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Properties of Haloalkanes and Haloarenes

Nature of C-X Bond

In alkyl halides, the carbon – halogen bond is formed by the overlap of the hybrid orbital of carbon atom with the p-orbital of the halogen atom.

Nature of C-X Bond
  • Moving from fluorine to iodine, the size of the halogen atom increases and the overlap decreases. Hence, the C – X bond becomes longer and weaker on going from alkyl fluorides to alkyl iodides.
  • The halogens are more electronegative than carbon. Thus, the electron density along the C – X bond is displaced in the direction of the halogen. Thus, C-X bond is polar in nature.
  • The carbon atom bears a partial positive charge (+) and the halogen atoms bears a partial negative charge (–) .
Nature of C-X Bond

Physical Properties

  • The lower alkyl halides are gases at room temperature. The other alkyl halides containing up to are liquids having high boiling points.
  • Haloalkanes and haloarenes are moderately polar molecules C – X but are immiscible in water due to their inability to form hydrogen bond.
  • The melting and boiling points of haloalkanes and haloarenes are higher than those of their parent hydrocarbons.

This is due to:

  • The greater molecular mass and hence greater magnitude of van der Waals forces of attraction in halo compounds than in the parent hydrocarbons.
  • The existence of intermolecular dipole-dipole interaction.
Physical Properties
  • All monohalobenzenes are liquids at room temperature.
  • In dihalobenzene, the para isomers have the highest melting points. It is due to the greater symmetry that causes a better packing of molecule in the para isomer.
Physical Properties

Chemical Properties

Substitution Reaction

  • Substitution reactions are those in which an atom or a group of atoms from the reactant molecule is displaced by another atom or a group of atoms.
  • For example, treatment of chloroethane with sodium hydroxide, the chlorine atom of chloroethane is substituted by the hydroxyl group and ethanol is formed as a by-product.

  • In haloalkanes, the carbon atom carrying the halogen atom is electron deficient due to – I effect of halogen atom. This electron deficient carbon atom is susceptible to attack by a nucleophile.
  • Few examples of nucleophilic substitution reactions of haloalkanes.
Substitution Reaction

Mechanism of Nucleophilic Substitution Reaction

  • The nucleophilic substitution reactions could be either or type.
  • When the nucleophilic attacks the haloalkane, and simultaneously the leaving groups leaves then, the reaction is called nucleophilic substitution bimolecular i.e..
  • It is a one step process and the transition state involves two species.
  • The bond making and the bond breaking takes place simultaneously.
  • The nucleophile attacks from one side of the carbon atom whereas the leaving group leaves from the opposite direction.
  • Primary alkyl halides undergo substitution by mechanism.
  • When the nucleophilic attacks the haloalkane, and leave of leaving group involves two steps , the reaction is called nucleophilic substitution bimolecular i.e..
  • For example, in the hydrolysis of 2-bromo-2-methylpropane.
  • The rate determining step of this is the dissociation of the alkyl halide to alkyl cation and bromide ion.
Mechanism of Nucleophilic Substitution Reaction
  • This alkyl cation is a carbocation and its formation is a slow and rate determining step.
  • As this carbocation is formed, the nucleophile, which is water molecule attacks on it which is a fast step.
Mechanism of Nucleophilic Substitution Reaction

At last, the alkyl ox onium ion loses a proton to give the alcohol as the product.

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