Preparation of Haloalkanes
from Alcohols

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The most convenient method for the preparation of haloalkanes is from alcohols. Haloalkanes can be prepared from alcohols in the following ways :

By Reacting Alcohols with Halogen Acid

We get haloalkanes as main product when an alcohol derivative organic compound reacts with halogen acid.

Preparation of haloalkanes from alcohols

Preparation of Chloroalkanes

Primary and secondary alcohols react with hydrochloric acid gas in the presence of catalyst like anhydrous ZnCl₂ to form haloalkanes. Tertiary alcohols do not need catalyst because they are more reactive and react readily by conc. HCl.

Example : The following three reactions illustrate this method of preparation of chloroalkanes :

Preparation of primary chloroalkane from alcohol

Preparation of secondary chloroalkane from alcohol

Notice that anhydrous ZnCl₂ is not used in the above reaction because tertiary alcohol does not need any catalyst.

Preparation of Bromoalkanes

Bromoalkanes are prepared by reacting hydrogen bromide (HBr) with alcohols. HBr needed for this purpose can be generated using sodium or potassium bromide with sulphuric acid (H₂SO₄).

NaBr and H₂SO₄ react to produce HBr which reacts with alcohols.

Example : In the following reaction, bromoethane is prepared from ethanol :

Preparation of ethyl bromide from ethanol

Preparation of Iodoalkanes

Hydroiodic acid (HI) needed for the preparation of iodoalkanes can be generated in situ by the action of 95% phosphoric acid on potassium or soidum iodides (KI or NaI).

Predict the product

preparation of methyl iodide from methanol

By Reacting Alcohols with Phosphorus Trihalides

Chloroalkanes, bromoalkanes and iodoalkanes are prepared by this method. The general reaction is :

Preparation of haloalkanes from phosphorus trihalide

Example : Ethanol reacts with PCl₃ to form chloroethane :

Preparation of chloropropane from phosphorus trihalide

PBr₃ and PI₃ are not stable compounds and are usually prepared freshly by reaction of red phosphorus with bromine or iodine.

P₄ + 6Br₂ → 4PBr₃

P₄ + 6I₂ → 4PI₃

By Reacting Alcohols with Phosphorus Pentachlorides

Chloroalkanes can alternatively be prepared by reacting alcohols with phosphorus pentachloride.

Preparation of chloroalkanes from phosphorus pentahalide

Example : Ethanol reacts with PCl₅ to give chloroethane.

Preparation of chloroethane from phosphorus pentahalide

By Reacting Alcohols with Thionyl Chloride

Chloroalkanes are prepared by using thionyl chloride (SOCl₂ ).

Preparation of chloroalkanes from thionyl chloride (SOCl2)

Preparation of chloroalkanes using thionyl chloride is preferred because both the by-products are gases and escape out leaving behind the pure product.

Questions

Question : Why is sulphuric acid not used during the reaction of alcohols with KI?
Answer : Sulphuric acid (H₂SO₄) is an oxidising agent. It oxidises HI produced during the reaction to I₂.

2KI + H₂SO₄ → 2KHSO₄ + 2HI
2HI + H₂SO₄ → H₂O + I₂ + SO₂

To prepare an alkyl iodide, a non oxidising agent such as H₃PO₄ is used instead of sulphuric acid.

Question : Why don't we need anhydrous ZnCl₂ as catalyst when the alcohol is tertiary?
Answer : Since the stability of carbocation follows the order : 3° > 2° > 1°, the cleavage of C-O bond in tertiary alcohols occurs easily.

Question : Complete the reaction.

Reaction of cyclohexanol with thionyl chloride

Question : The ease with which an alcohol can be halogenated by treating it with HX in the presence of anhydrous ZnCl₂ is:

reactivity of alcohols with hydrogen halide