Haloalkanes

Haloalkanes

  1. CH3Cl, CH3Br, C2H5Cl are all gases at room temperature and pressure. The other haloalkanes are liquids with boiling points related to molar mass.
  2. They are all immiscible with water.
  3. General formula: CnH2n+1X (where X = halogen atom)
  4. Polarity: haloalkanes are polar due to the inductive effect of the halogen atom. Since the halogens are more electronegative than carbon, they have a greater share of the electrons in the C-X bond.

There are three distinct molecular environments for the halogen atom:

Exercise Which of the following haloalkanes will have the greatest polarity?

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The third example (tertiary halogenoalkane) has the greatest polarity. This is because the positive carbon ion (carbocation) is stabilised by the inductive effect of the three other bound carbons.

The polarity of the C-X bond results in haloalkanes being much more reactive than their parent alkanes. Therefore they are of greater importance industrially.

Types of reaction occurring at C-X bond:

  1. Nucleophilic substitution reactions
  2. Elimination reactions

Nucleophilic substitution

The inductive effect of the halogen atom results in a positive charge on the carbon atom to which it is attached. Hence, this carbon atom is readily attacked by nucleophiles.

a) Attack by OH- (or water): Hydrolysis

The haloalkanes are attacked only slowly by water. The rate is much faster, but a poor yield is obtained if the haloalkane is refluxed with aqueous sodium or potassium hydroxide.

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b) Formation of nitriles: attack by CN-

If a haloalkanes is refluxed with an alcoholic solution of KCN, an acid nitrile is formed: R-C=N

C2H5Br + CN- → C2H5CN + Br-

bromoethane propanonitrile

c) Attack by ammonia NH3: Products are amines.

If a haloalkane is heated with an alcoholic solution of ammonia in a sealed tube, a mixture of products is formed. The mixture consists of amines and amine salts.

Mechanism for production of primary amines.

Nucleophilic attack by ammonia:

Abstraction of a proton (taken by a base)

Elimination reactions

When haloalkanes are heated with aqueous solution of potassium or sodium hydroxide, the major product is the alcohol, produced by nucleophilic displacement of the halogen by OH-.

If the reaction conditions are changed so that the haloalkane is heated with concentrated alcoholic potassium hydroxide, the major product is an alkene due to the elimination of hydrogen halide.

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