How haloalkanes are prepared from alcohols?

Question

This is the common method for preparing haloalkanes in laboratory. Alcohols can be converted to haloalkanes by substitution of - OH group with a halogen atom.

Different reagents can be used to get haloalkanes from alcohols as described below:
    
Reaction by Halogen Acids: alcohols can be converted into haloalkanes by action with halogen acids. The reaction, in general, can be represented as
 

Chlorolakanes are obtained by treating alcohols with HCl in the presence of anhydrous zinc chloride. The anhydrous ZnCl₂ acts as dehydrating agent and thus favours the forward reaction (Le Chateliar principle). The mixture of HCl and anhydrous ZnCl₂ is known as Lucas reagent.
 

Bromoalkanes can also be obtained in an identical manner by heating alcohols with KBr or NaBr and conc. H₂SO₄.HBr is generated in situ (during the reaction) by reaction of KBr or NaBr with conc. H₂SO₄.
 
 KBr + H₂SO₄    KHSO₄ + HBr

C₂H₅OH + HBr    C₂H₅Br + H₂O

The reaction can also be carried out with conc. Solution (48%) of hydrobromic acid. Iodoalkanes are obtained by heating alcohols withKI and 95% H₃PO₄ (phosphoric acid).

 

C₂H₅OH + HI    C₂H₅I + H₂O

The reaction can also be carried out with conc. solution (57%) of hydroiodic acid.

Reactivity of halogen acids towards this reaction is: HI > HBr > HCl

It is because of the fact that the bond dissociation energy of HI is less than that of HBr which in turn is less than that of HCl.

Reactivity of alcohols towards this particular reaction is: tertiary > secondary > primary

It is because of the fact that greater the number electron releasing groups on -carbon atom of alcohol more is the polarity of C-OH bond. Consequently greater is the ease with which it cleaves.
    
Reaction by phosphorus halides: Chlorolakanes are obtained by reaction of alcohols with PCl₃ or PCl₅.
 


 

Bromo and iodo alkanes are obtained by reaction of alcohols with a mixture of red phosphorus and Br₂ or I₂. The function of red phosphorus and Br₂ or I₂ is to get PBr₃ and PI₃ during the course of the reaction. PBr₃ and PI₃ being relatively unstable have to be prepared either in situ or just before use. They cannot be accumulated for long periods. For example,
              
P4 + 6Br2    4PBr3

  
                   
P₄ + 6I₂    4PI₃    

 

The alkyl halides are obtained in excellent yield (80%) in the reaction. The phosphorus halide method is better than halogen acid method because
    
Certain alcohols such as, secondary and tertiary alcohols tend to dehydrate in the presence of halogen acids.
    
Intermediate carbocation formed can undergo rearrangement in the presence of acids.
    
Reaction by Thionyl Chloride: in this method the alcohol is refluxed with thionyl chloride in the presence of a small amount of pyridine. Refluxing is the process of heating a volatile liquid in a flask fitted with condenser so that the vapours are condensed back into the flask.

The reaction of straight chain primary alcohols in the presence or absence of pyridine is called Darzen's procedure. Pyridine is used in the reaction to neutralize hydrogen chloride in the reaction.



The above methods cannot be applied for conversion of phenols to aryl halides because carbon-oxygen bond in phenols has partial double bond character due to resonance and hence is stronger and difficult to break.