General methods of preparation of Alkyl Halides.

(1) From alkanes

hv (i) By halogenation: C2 H 6 (Excess) + Cl 2  C 2 H 5 Cl  HCl Ethane Ethyl chloride (Major product)

Cl 2

CH 3CH 2CH 3 UV light  CH 3CH 2CH 2Cl  CH 3CHCH 3 Propane 1Chloroprop ane (45%) | Cl 2- Chloroprop ane (55%)

This reaction proceed through free radical mechanism.

Note: Order of reactivity of X 2 for a given alkane is, F2  Cl2  Br2  I2 .

The reactivity of the alkanes follows the order : 3°alkane > 2°alkane > 1°alkane.

hv (ii) With sulphurylchloride: R  H  SO 2 Cl2   R  Cl  SO 2  HCl Organic peroxide (R'CO 2 )2

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

(2) From alkenes (Hydrohalogenation)

CH 3  CH  CH  CH 3  HBr CH 3CH 2  C H  CH 3  Electrophillic addition. But 2ene | Br 2-Bromobutan e Note: 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.

From alkyne we cannot obtain mono alkyl halide.

The order of reactivity of acids is, HI  HBr  HCl .

(3) From alcohols

(i) By the action of halogen acids

Anhy . ZnCl 2 Groove’s process R  OH  H  X   RX  H 2O Alcohol 300 C

Note: The reactivity order of HX in the above reaction is : HI  HBr  HCl  HF .

Reactivity order of alcohols 3  2  1  MeOH .

2° and 3° alcohols undergo SN 1 ; where as 1° and MeOH undergo SN 2 mechanism.

Concentrated HCl + anhy. ZnCl 2 is known as lucas reagent.

(ii) Using PCl5 and PCl3 : CH 3CH 2OH  PCl5 CH 3CH 2Cl POCl 3  HCl Phosphorus Chloroetha ne Phosphorus pentachlor ide Oxychlorid e

3CH 3CH 2OH  PCl3 3CH 3CH 2Cl H 3 PO3 Chloroetha ne Phosphorus acid Note: and derivatives cannot be obtain from the above reaction, because PBr5 or PI5 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

Pyridine (Darzan's process) CH 3 CH 2 OH  SOCl 2  CH 3 CH 2 Cl  SO 2  HCl

Note: Reaction takes place through SN 2 mechanism.

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

CCl 4 (Free radical mechanism) R  C O  Ag  Br  Br   R  Br  CO 2  AgBr  || Decarboxyl ation O

Note: The reactivity of alkyl group is 1  2  3

Not suitable for chlorination because yield is poor.

In this reaction iodine forms instead of alkyl halide and the reaction is called Birnbourn-

Simoninireaction, 2R  COOAg  I2  RCOO R  2CO 2  2AgI .

(5) From alkyl halide (Halide exchange method): R  X  NaI Acetone  R  I  NaX (X  Cl, Br) Reflux

Note: Alkyl fluorides cannot be prepared by this method. They can be obtained from corresponding chlorides by the action of Hg 2 F2 or antimony trifluoride.

2CH 3 Cl  Hg 2 F2  2CH 3 F  Hg 2 Cl 2 Methyl fluoride

(6) Other method

(i) ROH KI,H3 PO4   (ii) ROH X 2 (PhO)3P  Rydon method

(iii) Dihalide ZnCu  R  X HCl

X (iv) RMgX 2  

(v) ROR PCl5  