Chemistry 11 Notes for Chemistry Notes

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Preparation Of Alkyl Halides

Mono haloalkanes or Alkyl halides can be prepared by a number of methods. Some important synthetic methods are as follows:
Method no. 1
By using alcohol
As a main reagent
A number of reagents such as halogen acids, PCl5, PCl3, SOCl2 etc. react with aliphatic alcohols to produce corresponding alkyl halides.

From halogen acid
Alcohols react with halogen acids (HCl ,HBr or HI) to produce alkyl halides.
R-OH + HX ® R-X + H2O
CH3-OH + HCl® CH3-Cl + H2O (in the presence of ZnCl2/H2SO4)
C2H5OH + HI® C2H5I + H2O
C2H5OH + HBr® C2H5Br + H2O
CH3-CH2-CH2-OH + HCl ® CH3-CH2-CH2-Cl + H2O

From phosphorus penta chloride (PCl5)
When alcohols are treated with phosphorus penta chloride, corresponding alkyl chlorides are obtained.
CH3-CH2-OH + PCl5 ® CH3-CH2-Cl + POCl3 + HCl

From thionyl chloride (SOCl2)
CH3-CH2-OH +SOCl2 ® CH3-CH2-Cl + SO2 + HCl
Above reaction is carried out in the presence of Pyridine. Pyridine absorbs HCl as it is formed.

From phosphorus
Tri bromide (PBr3)
3C2H5OH + PBr3 ® 3C2H5Br + H3PO3
From phosphorus
Tri iodide (PI3)
3CH3OH + PI3 ® 3CH3I + H3PO3

Method no. 2
By the halogenation of alkanes
Alkanes react with chlorine and bromine in presence of sunlight and undergo photo chemical series of substitution reactions to yield alkyl halides.
CH4 + (Cl2) ® CH3Cl + CH2Cl2 + CHCl3 + CCl4

This is not an appropriate method for the laboratory preparation of alkyl halides because the resultant mixture contains polyhalogen derivatives which are difficult to separate.

Method no. 3
From alkene
We know that alkenes are unsaturated hydrocarbons.Therefore, when alkenes are treated with halogen acid, addition reaction takes place and alkyl halides are formed. Additionof HX takes place according to markonikov's principle.
CH2 = CH2 + HCl ® CH3 – CH2 – Cl
CH3-CH=CH2 + HBr ® CH3-CHBr-CH3
The order of reactivity of halogen acid is as follows:
HI > HBr > HCl

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