CBSE Class 12 Chemistry Notes: Haloalkanes – Prepration

1. i.   From Alcohols:-

It involves substitution of (-OH) group by halogen atom

 

       i.    The reactivity of halogen acids for given alcohol in the above reaction order HI >HBr>HCl

      ii.   Of alcohols for the halogen reactivity order is tertiary > secondary > primary.

Reasons of order of reactivity:-

Above reaction is nucleophilic substitution reaction in which (halide ion) Nucleophile attacks prolonated alcohol molecule

 

iii.      Nucleophilicity (tendency to donote e^–to carbon atom) follows order I^–> Br^–>Cl^–

iv.  Order of stability of carbocation is 3^o> 2^o> 1^o that’s why reactivity order follows the same as discussed above

1. Example

 

ii. Bromoalkanes are obtained by mixing a suitable alcohol with constant boiling hydro bromic acid (48%) in presen of con. H₂SO₄

2.  By Action of PhosPhorus Halides:-

They gives (80%) yield of haloalkenes

3. By Action of ThioxylChlorides:-
   

Reagent used (C₅H₅N) pyridine

 

METHODS OF PREPARATION OF HALOALKANES-II

 

4. From Hydrocarbons:-

i. From alkenes and alkynes:-

Order of reactivity HI >HBr>HCl> HF

 

ii. Markonikove’s Rule:-

ii.   In presence of the peroxide, the addition of HBr (notHCl and HI) to unsyimmetrical alkenes takes place contrary to markoniekove’srulemeas

1 – haloproduct will be major in quantity.

This is also called as peroxide or kharasch effect

 

Mechanism:-

It takes place by Free radical Mechanism

i. Chain Initiation Step:-

 

ii. Chain Propagation Step:-

 

 

iii. Chain Termination Step:-

 

• 5.   By Allylic Halogenation:-

 

• Such Reactions in which Halogenation occurs at allylic position no of an alkene are called allylic Halogenation reactions

 

6. From Alkanes By Free Radical Halogenation:-
i. 
ii.  

 

This reaction is called swartsreaction.

7. By Halogen Exchange:-

Iodoalkanes can be easily prepared from the corresponding chloroalkanes by heating with sodium iodide in acetone.

 

This reaction is called Finleelstein reaction.
1. FROM Aromatic Hydrocarbons/ Direct Halogenations:-

The reaction is carried out at law temperature 310 – 320 K in absence of sun light

 

Mechanism

     i. Generation of electrophile:-

 

   ii.  Formation of Complex (Carbocation intermediate):-

 

This step is slow Hence rate determining step.

iii. Loss of proton to give final product:-

 

 

 

METHODS OF PREPARATION OF HALOARENES-II

 

• 2. From Diazonium Salts:- (sandmeyer reaction):-

 

 

 

 

 

 

3. By Gattermann Reaction:-

4. From Silver Salt of Aromatic Acid/ Hansdieker reaction

 

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