NEET Chemistry Haloalkanes and Haloarenes / हैलोएल्केन तथा हैलोएरीन्स Notes - General Methods Of Alkyl Halides

 

General methods of preparation of Alkyl Halides

           

(1) From alkanes

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

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

This reaction proceed through free radical mechanism.

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

r The reactivity of the alkanes follows the order: \[3{}^\circ alkane\text{ }>~2{}^\circ alkane\text{ }>~1{}^\circ alkane\].

(ii) With sulphuryl chloride: \[R-H+S{{O}_{2}}C{{l}_{2}}\overset{hv}{\mathop{\xrightarrow[Organic\,peroxide{{(R'C{{O}_{2}})}_{2}}]{}}}\,R-Cl+S{{O}_{2}}+HCl\]

 

            Note: r In presence of light and trace of an organic peroxide the reaction is fast.

 

(2) From alkenes (Hydrohalogenation)

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

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

            r This addition take place by two mechanism,

Peroxide initiates free radical mechanism.

Markownikoff?s addition by electrophillic mechanism.

r From alkyne we cannot obtain mono alkyl halide.

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

           

(3) From alcohols

(i) By the action of halogen acids

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

 

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

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

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

r Concentrated HCl + anhy.\[ZnC{{l}_{2}}\] is known as lucas reagent.

(ii) Using \[\mathbf{PC}{{\mathbf{l}}_{\mathbf{5}}}\]and\[\mathbf{PC}{{\mathbf{l}}_{\mathbf{3}}}\]: \[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\]

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

 

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

unstable.

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

 

Note: r Reaction takes place through \[S{{N}^{2}}\] mechanism.

           

(4) From silver salt of carboxylic acids (Hunsdiecker reaction, Decarboxylation)

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

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

r Not suitable for chlorination because yield is poor.

r 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) From alkyl halide (Halide exchange method): \[R-X+NaI\underset{\text{Reflux}}{\mathop{\xrightarrow{\text{Acetone}}}}\,R-I+NaX(X=Cl,\,Br)\]

 

Note: r Alkyl fluorides cannot be prepared by this method. They can be obtained from corresponding chlorides by

the action of \[H{{g}_{2}}{{F}_{2}}\]or antimony trifluoride.

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

           

(6) Other method

(i) \[ROH\xrightarrow{KI,{{H}_{3}}P{{O}_{4}}\,\,\,\,\,\,}\]

(ii) \[ROH\underset{\text{Rydon method}}{\mathop{\xrightarrow{{{X}_{2}}+{{(PhO)}_{3}}P}}}\,\]

(iii) Dihalide \[\underset{HCl}{\mathop{\xrightarrow{Zn-Cu\,\,\,\,}}}\,\]\[R-X\]

(iv) \[RMgX\xrightarrow{{{X}_{2\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}}}\]