PREPARATION OF ALKYL HALIDES
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| 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 | ||
| REMEMBER: 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 | ||