JEE Main & Advanced Chemistry Haloalkanes and Haloarenes General methods of preparation of Alkyl Halides

General methods of preparation of Alkyl Halides

Category : JEE Main & Advanced

(1) From alkanes

 

(i) By halogenation :

 

\[\underset{\text{Ethane}}{\mathop{{{C}_{2}}{{H}_{6}}}}\,\] (Excess) +\[C{{l}_{2}}\xrightarrow{hv}\underset{\text{Ethyl}\,\text{chloride (Major}\,\text{product)}}{\mathop{{{C}_{2}}{{H}_{5}}Cl}}\,+HCl\]

 

\[\underset{\text{Propane}}{\mathop{C{{H}_{3}}C{{H}_{2}}C{{H}_{3}}}}\,\underset{UV\,\text{light}}{\mathop{\xrightarrow{C{{l}_{2}}}}}\,\underset{1-\text{Chloropropane (45 }\!\!%\!\!\text{ )}}{\mathop{C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}Cl}}\,\,\underset{\text{2- Chloropropane (55 }\!\!%\!\!\text{ )}}{\mathop{\underset{\,Cl}{\mathop{\underset{|}{\mathop{+\ C{{H}_{3}}CHC{{H}_{3}}}}\,}}\,}}\,\]

 

This reaction proceed through free radical mechanism.

 

  • Order of reactivity of \[{{X}_{2}}\] for a given alkane is, \[{{F}_{2}}>C{{l}_{2}}>B{{r}_{2}}>{{I}_{2}}\].

 

  • The reactivity of the alkanes follows the order :

 

 3°alkane >  2°alkane >  1°alkane.

 

(ii) With sulphuryl chloride :

 

\[R-H+S{{O}_{2}}C{{l}_{2}}\underset{Organic\,peroxide{{({R}'C{{O}_{2}})}_{2}}}{\mathop{\xrightarrow{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,hv\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}}}\,R-Cl+S{{O}_{2}}+HCl\]

 

  • This reaction is a fast due to in presence of light and trace of an organic peroxide.

 

(2) From alkenes (Hydrohalogenation by Electrophillic addition)

 

\[\underset{\text{But}-\text{2}-\text{ene}}{\mathop{C{{H}_{3}}-CH=CH-C{{H}_{3}}+HBr}}\,\to \]\[\underset{\text{2-Bromobutane}}{\mathop{C{{H}_{3}}C{{H}_{2}}-\underset{Br\,\,\,\,\,\,\,\,}{\mathop{\underset{|}{\mathop{C}}\,H-}}\,C{{H}_{3}}}}\,\]

 

Addition of HBr to alkene in the presence of organic peroxide take place due to peroxide effect or Kharasch's effect.

 

This addition take place by two mechanism, Peroxide initiates free radical mechanism.

 

Markownikoff’s addition by electrophillic mechanism.        

 

The order of reactivity of halogen acids is, \[HI>HBr>HCl\].

 

(3) From alcohols

 

(i) By the action of halogen acids

 

Groove’s process

 

\[\underset{\text{Alcohol}}{\mathop{R-OH}}\,+H-X\underset{300{}^\circ C}{\mathop{\xrightarrow{Anhy.\,ZnC{{l}_{2}}}}}\,\underset{\text{Haloalkane}}{\mathop{RX}}\,+{{H}_{2}}O\]

 

  • The reactivity order of \[HX\] in the above reaction is : \[HI>HBr>HCl>HF\].

 

  • Reactivity order of alcohols \[3{}^\circ >2{}^\circ >1{}^\circ >MeOH\].

 

  • 2° and 3° alcohols undergo \[{{S}_{{{N}^{1}}}}\]; where as 1° and MeOH undergo \[{{S}_{{{N}^{2}}}}\]mechanism.

 

  • Concentrated HCl + anhy. ZnCl2 is known as lucas reagent.

 

 (ii) Using PCl5 and PCl3

 

\[C{{H}_{3}}C{{H}_{2}}OH+\underset{\begin{smallmatrix} \text{Phosphorus } \\ \text{pentachloride} \end{smallmatrix}}{\mathop{PC{{l}_{5}}}}\,\xrightarrow{{}}\underset{\text{Chloroethane}}{\mathop{C{{H}_{3}}C{{H}_{2}}Cl}}\,+\underset{\begin{smallmatrix} \text{Phosphorus} \\ \text{Oxychloride} \end{smallmatrix}}{\mathop{POC{{l}_{3}}}}\,+HCl\]

 

\[3C{{H}_{3}}C{{H}_{2}}OH+PC{{l}_{3}}\xrightarrow{{}}\underset{\text{Chloroethane}}{\mathop{3C{{H}_{3}}C{{H}_{2}}Cl}}\,+\underset{\text{Phosphorus}\,\text{acid}}{\mathop{{{H}_{3}}P{{O}_{3}}}}\,\]

 

  • Bromine and iodine derivatives cannot be obtain from the above reaction, because \[PB{{r}_{5}}\] or \[P{{I}_{5}}\] are unstable.

 

  • This method gives good yield of primary alkyl halides but poor yields of secondary and tertiary alkyl halides.

 

 

(iii) By the action of thionyl chloride (Darzan's process) : Reaction takes place through \[S{{N}^{2}}\] mechanism.

 

\[C{{H}_{3}}C{{H}_{2}}OH+SOC{{l}_{2}}\xrightarrow{\text{Pyridine}}\]\[C{{H}_{3}}C{{H}_{2}}Cl+S{{O}_{2}}+HCl\]

 

(4) From silver salt of carboxylic acids (Hunsdiecker reaction, Decarboxylation by Free radical mechanism) 

 

\[R-\underset{O\,\,\,}{\mathop{\underset{||}{\mathop{C}}\,-}}\,O-Ag+Br-Br\underset{\text{Decarboxylation}}{\mathop{\xrightarrow{CC{{l}_{4}}}}}\,R-Br+C{{O}_{2}}\uparrow +AgBr\downarrow \]

 

  • The reactivity of alkyl group is \[1{}^\circ >2{}^\circ >3{}^\circ \]

 

  • Only bromide are obtained in good yield.

 

  • Not suitable for chlorination because yield is poor.

 

  • In this reaction iodine forms ester instead of alkyl halide and the reaction is called Birnbourn-Simonini reaction,

 

\[2R-COOAg+{{I}_{2}}\xrightarrow{{}}RCOO{R}'+2C{{O}_{2}}+2AgI\].

 

(5) By Finkelstein reaction (Halide exchange method) : \[R-X+NaI\underset{\text{Reflux}}{\mathop{\xrightarrow{\text{Acetone}}}}\,R-I+NaX(X=Cl,\,Br)\]

 

  • Alkyl fluorides can not be prepared by this method. They can be obtained from corresponding chlorides by the action of \[H{{g}_{2}}{{F}_{2}}\]or antimony trifluoride. (swart reaction)

 

\[2C{{H}_{3}}Cl+H{{g}_{2}}{{F}_{2}}\to \underset{\text{Methyl fluoride}}{\mathop{2C{{H}_{3}}F+H{{g}_{2}}C{{l}_{2}}}}\,\]

 

(6) Other method

 

                                                            



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