Compound of Carbon Containing Halogen: Preparation of Haloalkanes (For CBSE, ICSE, IAS, NET, NRA 2022)

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Preparation of Haloalkanes

From Hydrocarbons

  • Direct halogenation of hydrocarbons takes place in the presence of sunlight or at high temperature in dark.
  • For example, chloroethane is prepared by monochlorination of ethane.

  • This reaction follows a free radical mechanism.
  • Bromo derivatives of alkanes are prepared by direct bromination.

  • Direct iodination is not possible with iodine as the reaction is reversible.
  • Direct Fluorination is not possible because due to high reactivity of fluorine the reaction cannot be controlled.

From Alcohols

Alcohols are converted into haloalkanes by treating with,

  • Hydrogen halides
  • Phosphorus halides
  • Thionyl chloride

Reaction with Hydrogen Halides

Hydrogen halides react with an alcohol in presence of a dehydrating agent such as anhydrous zinc chloride to produce a haloalkane.

For example, chloroethane

Bromoethane is obtained by refluxing ethanol with HBr in presence of little concentrated as the catalyst.

Reaction with Phosphorus Halides

Haloalkanes are prepared by the reaction of an alcohol with a phosphorus halide .

Reaction with Thionyl Chloride

Thionyl chloride is another reagent which reacts with an alcohol to yield a chloroalkane.

Both the by-products, and are gases, the purification of final is not required.

Preparation of Haloarenes

From Aromatic Hydrocarbons

  • Haloarenes are obtained by direct halogenation of aromatic hydrocarbons in the presence of a catalyst.
  • Mostly, iron filings or iron (III) halide is used as the catalyst.
From Aromatic Hydrocarbons

The direct iodination of aromatic hydrocarbons is not a useful reaction since the HI produced reduces the aryl iodide back to the aromatic hydrocarbon.

From Aromatic Hydrocarbons

In the presence of an oxidizing agent such as nitric acid, iodic acid , mercury oxide, the HI produced is either oxidized to iodine or is eliminated as mercuric iodide and iodobenzamide is obtained.

Fluorobenzene cannot be prepared by direct fluorination of aromatic hydrocarbons since the reaction is very violent and cannot be controlled.

From Diazonium Salts

Benzene diazonium salt is formed by treating an aromatic primary amine with and dil. at low temperature. The process is known as diazotization.

From Aromatic Hydrocarbons
  • Diazonium salts are highly reactive compounds.
  • They are used in the preparation of a large number of arene derivatives.
  • When a diazonium salt is treated with copper (I) chloride or copper (I) bromide , the corresponding haloarenes is obtained.
  • This reaction is known as Sandmeyer reaction.
  • It is used for introducing chloro or bromo group in the benzene ring.
From Diazonium Salts

Haloarenes can also be prepared by reacting benzene diazonium chloride with copper powder in presence of corresponding halogen acid. This reaction is called Gattermann reaction and shown below:

From Diazonium Salts

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