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14.3.5 Chemical
properties

            Haloalkane are one of the most
reactive classes of organic compounds. Their reactivity is due to the presence
of polar carbon – halogen bond in their molecules. The reactions of haloalkane
may be divided into the following types

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i)                  
Nucleophilic substitution reactions

ii)               
Elimination reactions

iii)             
Reaction with metals

iv)             
Reduction

 

 

 

 

1) Nucleophilic
substitution reactions

            As polar bond C?+ – X?-
is present in halo alkane, nucleophilic reagents are attracted by partially
positively charged carbon atoms and substitutes halogen. The substitution
reactions that take place through nucleophiles are called nucleophilic
substitution reactions. In short it is called SN reaction.

i)                  
Reaction
with aqueous alkali or moist silver oxide.(Hydrolysis)

boil

 

Haloalkane react with aqueous solution of KOH or
moist silver oxide (Ag2O/H2O) to form alcohols.

R – X + KOH(aq)               R – OH + KX                                                 (14.15)

Example

boil

 

            R – X + AgOH(moist)               R – OH + AgX                                         (14.17)

            Example

ii)               
Reaction
with alcoholic ammonia (Ammonolysis)

Haloalkane
react with alcoholic ammonia solution to form alkyl amines.

R  –  X  +  H  –  NH2
   R  –  NH2  + 
HX                                     (14.19)

Example

                        CH3 – CH2
– Br + H – NH2  CH3
– CH2 – NH2 + HBr              (14.20)

                        Bromo ethane                                   Ethyl amine

            However, with excess of halo alkane,
secondary and tertiary amines along with quartenary ammonium salts are obtained           

 

Ambident Nucleophiles

            Nucleophiles such as cyanide and
nitrite ion which can attack nucleophilic centre from two sites are called
ambident nucleophiles. These nucleophiles can attack from either of these sites
depending upon the reaction conditions and the reagent used.

iii)             
Reaction
with alcoholic KCN

Haloalkane
react with alcoholic KCN solution to form alkyl cyanides.

R
– X + KCN ? RCN + KX                                                            (14.22)

Example

                        CH3 – CH2
– Br + KCN ? CH3 – CH2 – CN + KBr                     (14.23)

                        Bromo ethane                                   Ethyl cyanide

iv)             
Reaction
with alcoholic AgCN

Haloalkane
react with alcoholic AgCN solution to form alkyl isocyanide.

R
– X + AgCN ? R – C + AgX                                                      (14.24)

Example

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