Preparation of Alkyl halides From Alcohols

Science > Chemistry > Organic Chemistry > Halogen Derivatives of Alkanes > Preparation of Alkyl halides From Alcohols

In the last two articles, we have studied the methods of preparation of alkyl halides from alkanes and alkenes. In this article, we shall study the preparation of alkyl halides from alcohols.

The Action of HX on Alcohols:

• The reaction is nucleophilic substitution.
• Methyl and primary alcohol undergo SN¹ mechanism while secondary and tertiary alcohols undergo SN² mechanism.
• Among halogen halides, HCl is the least reactive in nature. Because the chloride ion is a weaker nucleophile than bromide or iodide ions. Hence hydrogen chloride is mixed with anhydrous ZnCl2.
• In these reactions, anhydrous ZnCl2 not only acts as a dehydrating agent but also helps in the cleavage of C-O bond of the alcohol. ZnCl2 is Lewis acid and coordinates with the oxygen atom and thus weakens C-O bond. This results in the formation of carbocation which combines with Cl- ion to form chloroalkane. Tertiary alcohols are highly reactive hence for tertiary alcohol ZnCl2 is not required.
• This reaction is nucleophilic substitution. The stability of carbocations is of order tertiary > secondary > primary. Hence the order of reactivity also follows the same order tertiary > secondary > primary.
• The bond dissociation energy of H-X bond is of order H-Cl > H-Br > H-I. hence the reactivity of halogen acids follows the order HI > HBr > HCl.
• Unlike alkyl chlorides, the secondary and tertiary bromides and iodides cannot be obtained from their respective alcohols. It is because the secondary and tertiary alcohols on heating with concentrated H₂SO₄ undergo dehydration to form alkenes. Hence for these preparations dilute H₂SO₄ is used.

General Reaction:

R–OH  +            HX          →     R–X       +    H₂O

Alcohol     Halogen acid       alkyl halide

Order of Reactivity:

Tertiary alcohol > Secondary alcohol > Primary alcohol

and HI > HBr > HCl.

By Action of Haloacids: 

Preparation of Alkyl Chlorides:

General Reaction:

When alcohol is treated with Lucas reagent alkyl chloride is obtained. Lucas reagent is a solution of a concentrated hydrochloric acid with zinc chloride. This reaction is known as Groove’s process.

R–OH  +            HCl               R–Cl       +    H₂O

Alcohol     Conc.Hydrochloric acid     Alkyl chloride

Example – 1: Preparation of ethyl chloride (Chloroethane) from ethyl alcohol (Ethanol):

C₂H₅OH  +            HCl                C₂H₅Cl       +    H₂O

Ethyl alcohol   Conc.Hydrochloric acid       Ethyl chloride

Example – 2: (Preparation of isopropyl chloride (2-Chloropropane) from isopropyl alcohol (Propan-2-ol):

Example – 3: (Preparation of tert- Butyl chloride (2-Chloro-2-methylpropane) from tert- Butyl alcohol (2-Methylpropan-2-ol):

Preparation of Alkyl Bromides:

General Reaction:

When alcohol is treated with concentrated hydrobromic acid (NaBr + H2SO4) alkyl bromide is obtained.

R–OH  +            HBr                R–Br       +    H₂O

Alcohol    Hydrobromic acid               Alkyl chloride

Example – 1: Preparation of ethyl bromide (Bromoethane) from ethyl alcohol (Ethanol):

C₂H₅OH  +            HBr                C₂H₅Br       +    H₂O

Ethyl alcohol    Hydrobromic acid                          Ethyl bromide

Example – 2: (Preparation of isopropyl bromide (2-Bromopropane) from isopropyl alcohol (Propan-2-ol):

Example – 3: (Preparation of tert- Butyl bromide (2-Bromo-2-methylpropane) from tert- Butyl alcohol (2-Methylpropan-2-ol):

Preparation of Alkyl Iodides:

General Reaction:

When alcohol is refluxed with Potassium or sodium iodide with 95% phosphoric acid and heated, alkyl iodide is obtained.

R–OH  +     HI     R–I       +    H₂O

Alcohol    Hydroiodic acid         Alkyl iodide

Example – 1: Preparation of ethyl iodide (Iodoethane) from ethyl alcohol (Ethanol):

C₂H₅OH   +       HI      C₂H₅I   +    H₂O

Ethyl alcohol    Hydroiodic acid           Ethyl iodide

Example – 2: (Preparation of isopropyl iodide (2-Iodopropane) from isopropyl alcohol (Propan-2-ol):

By Action of Phosphorous Halides: 

• This method is suitable for preparation of primary and secondary alkyl halides.
• A good yield of tertiary alkyl halides cannot be obtained by this method.
• The reaction of an alcohol with PX₃ does not involve the formation of carbocation and usually occurs without rearrangement of the carbon skeleton.

Preparation of Alkyl Chlorides Using PCl₃:

General Reaction:

When alcohol is treated with phosphorous trichloride, alkyl chloride and phosphorous acid are obtained.

3 R-OH  +            PCl₃            3 R-Cl       +    H₃PO₃

Alcohol       phosphorous trihalide    alkyl halide    phosphorous acid

Example – 1: Preparation of ethyl chloride (Chloroethane) from ethyl alcohol (Ethanol):

3C₂H₅OH  +            PCl₃           3C₂H₅Cl       +    H₃PO₃

Ethyl alcohol   Phosphorous trichloride    ethyl chloride    Phosphorus acid

Both the products are in the liquid state and are separated by fractional distillation.

Example – 2: (Preparation of isopropyl chloride (2-Chloropropane) from isopropyl alcohol (Propan-2-ol):

Preparation of Alkyl Chlorides Using PCl₅:

General Reaction:

When alcohol is treated with phosphorous pentachloride, alkyl chloride and phosphoryl chloride (phosphorous oxychloride) are obtained.

ROH         +     PCl₅         RCl         +     POCl₃           +            HCl

Alcohol   Phosphorous pentachloride       Alkyl chloride    Phosphoryl chloride    Hydrogen chloride

Example – 1: Preparation of ethyl chloride (Chloroethane) from ethyl alcohol (Ethanol):

C₂H₅OH    +   PCl₅         C₂H₅Cl   +     POCl₃   +  HCl

Ethyl alcohol   Phosphorous pentachloride       Ethyl chloride    Phosphoryl chloride    Hydrogen chloride

Example – 2: (Preparation of isopropyl chloride (2-Chloropropane) from isopropyl alcohol (Propan-2-ol):

Preparation of Alkyl Bromides:

Unlike PCl₃, the compounds PBr₃ and PI₃ are not stable and hence they are to be prepared when they are to be used by treating bromine and iodine with red phosphorous. Compounds PBr₅ and PI₅ do not exist.

Alkyl bromides are prepared by the action of bromine, in presence of red phosphorus, on alcohols.  Phosphorus bromide PBr₃, which is unstable is formed as intermediates in the reaction.

3 R-OH  +        PBr₃            3 R-Br       +    H₃PO₃

Alcohol       phosphorous tribromide    alkyl bromide    phosphorous acid

Example – 1: Preparation of ethyl chloride (Chloroethane) from ethyl alcohol (Ethanol):

3C₂H₅OH  +        PBr₃          3C₂H₅Br   +    H₃PO₃

Ethyl alcohol   Phosphorous tribromide  Ethyl bromide  Phosphorus acid

Preparation of Alkyl Iodides:

Alkyl iodides are prepared by the action of iodine, in presence of red phosphorus, on alcohols.  Phosphorus iodide PBr₃, which is unstable is formed as intermediates in the reaction.

3 R-OH  +            PI₃            3 R-I       +    H₃PO₃

Alcohol       phosphorous triiodide    alkyl iodide    phosphorous acid

Example – 1: Preparation of ethyl iodide (Iodoethane) from ethyl alcohol (Ethanol):

3C₂H₅OH  +            PI₃            3C₂H₅I       +    H₃PO₃

Ethyl alcohol   Phosphorous triiodide          ethyl iodide    Phosphorus acid

By Action of Thionyl Chloride: 

This reaction is also known as Darzen’s Procedure.

General Reaction:

When alcohol is refluxed with thionyl chloride, alkyl chloride, sulphur dioxide and hydrogen chloride are obtained. Better yield is obtained if pyridine is added in a small amount.

ROH  +  SOCl₂        RCl  + SO₂ ↑­  + HCl  ­↑

Alcohol    Thionyl chloride              Alkyl chloride

Example – 1: Preparation of ethyl chloride (Chloroethane) from ethyl alcohol (Ethanol):

C₂H₅OH  +  SOCl₂        C₂H₅Cl  + SO₂ ↑­  + HCl  ­↑

Ethyl alcohol  Thionyl chloride                 Ethyl chloride

Example – 2: (Preparation of isopropyl chloride (2-Chloropropane) from isopropyl alcohol (Propan-2-ol):

The use of thionyl chloride for preparation of alkyl chlorides is most convenient because the other products of reaction (SO2 and HCI) being gases go off  (or can be expelled during distillation very easily). and hence the method requires no special purification/separation.

C₅H₁₁OH on bromination gives compound C₅H₁₁Br. Suggest all possible structures of alcohol and bromide.

Science > Chemistry > Organic Chemistry > Halogen Derivatives of Alkanes > Preparation of Alkyl halides From Alcohols