- 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 3o> 2o> 1o that’s why reactivity order follows the same as discussed above
- 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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Class 12 Chem Haloalkanes and Haloarenes All Topic Notes Class 12 Chemistry All Chapters Notes
