Tuesday, February 5, 2008

IIT JEE Revision - Alkyl and Aryl Halides - Methods of preparation

Methods of preparation

1. From hydrocarbons
a) from alkanes: halogens react with alkanes in the presence of uv light to form haloalkanes.
b) from alkenes: by the electrophylic addition of halogen acids (HBr, HCl, or HI)

Markownikov rule and anti Markownikov rule are applicable in the reaction between alkene and halogen acids.

2. From alcohols:

This is the most widely used method for the preparation of haloalkanes in the laboratory.

Methods preparing alkyl halides from alcohols

(i) By the action of halogen acids on alcohols

(a) Chloro alkanes: Primary and secondary alcohols form chloroalkanes when hydrochloric acid gas is passed through alcohol in the presence of anhydrous zinc chloride (Groove's process).

ZnCl2 is a Lewis acid.

(b) Tertiary alcohols are very reactive and therefore, they react readily with conc. HCl even in the absence of zinc chloride.

(c) Bromo alkanes: Bromoalkanes are obtained by heating an alcohol with hydrobromic acid (48%) in the presence of a little conc. H2SO4 whihc acts as a catalyst.

Hydrobromic acid (HBR) can be generated in situ (during the reaction itself) by the action of conc. H2SO4 on KBr or NaBr.

(d) Iodoalkanes: are obtained by heating alcohols with constant boiling hydroiodic acid (57%).

Hydroiodic acid (HI) can be generated in situ (during the reaction itself) by the action of phosporic acid on potassium iodide.

Note: Unlike alkyl chlorides, secondary andd tertiary bromides and iodides cannot be prepared from respective alcohols because the secondary and tertiary alcohols on heating with H2SO4 undergo dehydration to form alkenes.

Hydrogen fluoride is the least reactive of the hydrogen halides and the preparation of fluroalkanes is not practical by using alcohol and hydrogen fluoride.

(ii) Reaction of alcohols with phosphorus halides

Phosphorus halides such as PCl5, PCl3, PBr3 and PI3 react with alcohols to form corresponding haloalkanes.

(a) Chloroalkanes can be prepared by the action of phosphorus pentachloride or phosphorous trichloride on alcohols.

(b)Bromoalkanes and iodoalkanes are prepared by the action of phosphorous bromide and phosphorous tri-iodide respectively on alcohols.

phosphorous bromide and phosphorous iodide are not very stable compounds. Hence there are generally prepared in situ (during the reaction) by the action of red phosphorous on

Haloalkanes : Chloroethane, Bromoethane, Iodoethane

Reaction with ZnCl2/conc.-HCl

This is a reaction or test to distinguish various categories of alcohols and is termed Lucas test.

In this test, an alcohol is treated with an equimolar mixture of concentrated hydrochloric acid and anhydrous ZnCl2 (called Lucas reagent).

Alcohols get converted into alkylhalides. As alkyl halides are insoluble in water, their presence is indicated by the appearance of turbidity in the reaction mixture. Br2 or I2.

(iii) By the action of thionyl chloride on alcohol

Chloroalkanes can be prepared from alcohols by refluxing alcohols with thionyl chloride in the presence of pyridine.

3. By Halide Exchange

This reaction is particularly useful for preparing iodoalkanes.

The iodoalkanes are obtained by heating chloro or bromo alkanes with a concentrated solution of sodium iodide in acetone.

The reaction gives the best results with primary halides. This reaction is known as Finkelstein reaction.

Fluoro alkanes are difficult to prepare by other methods. They are prepared by treating alkyl chlorides or bromides with salts such as mecurous fluoride (Hg2F2), silver fluoride (AgF), cobalt fluoride (CoF3) or antimony trifluoride (SbF3). This reaction is known as Swarts reaction.

cobalt fluoride (CoF3) or antimony trifluoride (SbF3) are used when the organic halides contain two or three halogen atoms on the same carbon atom.

4. Preparation of alkyl halides from silver salts of acids.

The silver salts of the carboxylic acids dissolved in CCl4 are decomposed by bromine to form bromoalkanes. This reaction is called Borodine Hundsdiecker reaction.

Chloroalkanes can also be obtained by this method by using Cl2 instead of Br2, but the yield of chloroalkanes is very poor.

Iodo alkanes cannot be obtained by this reaction.

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