10. HALOGEN DERIVATIVES - part 05 - Chemical properties C

10. HALOGEN DERIVATIVES - part 05 - Chemical properties

Chemical properties :
1 Laboratory test of haloalkanes :
  • Haloalkanes are of neutral type in aqueous medium. 
  • On warming with aqueous sodium or potassium hydroxide the covalently bonded halogen in haloalkane is converted to halide ion.
  •  When this reaction mixture is acidified by adding dilute nitric acid and silver nitrate solution is added a precipitate of silver halide is formed which confirms presence of halogen in the original organic compound.
2 Nucleophilic substitution reactions of haloalkanes :
  • When a group bonded to a carbon in a substrate is replaced by another group to get a product with no change in state of hybridization of that carbon the reaction is called substitution reaction. 
  • The C-X bond in alkyl halides is a polar covalent bond and the carbon in C-X bond is positively polarized. 
  • C-X carbon is an electrophilic centre. 
  • It has, therefore, a tendency to react with a nucleophile.
  • Alkyl halides react with a variety of nucleophiles to give nucleophilic substitution reactions(SN). 
  • The reaction is represented in general form as shown below.
  • When a substrate reacts fast it is said to bereactive. 
  • The reactivity of alkyl halides in SN reactiondepends upon two factors, namely -
  1. substitution state (10, 20 or 30) of the carbon and 
  2. Nature of the halogen.
  • The order of reactivity influenced by these two factors is as shown below.
          tertiary alkyl halide (30) > secondary alkyl halide (20) >primary alkyl halide (10) and
                                                    R - I > R - Br > R - Cl

Do you know ?
  • Cyanide ion is capable of attacking through more than one site (atom).
  • Such nucleophiles are called ambident nucleophiles. 
  • KCN is predominantly ionic and provides cyanide ions. 
  • Both carbon and nitrogen are capable of donating electron pair. 
  • C-C Bond being stronger than C-N bond, attack occurs through carbon atom of cyanide group forming alkyl cyanides as major product. 
  • However AgCN (Ag-C ≡ N) is mainly covalent compound and nitrogen is free to donate pair of electron.
  • Hence attack occurs through nitrogen resulting in formation of isocyanide.
  • Another ambident nucleophile is nitrite ion, which can attack through ‘O’ or ‘N.
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