Preparation of Haloalkanes.

The general method of preparation of haloalkane.

1. From alkanes Halogenation.

Alkanes are good starting material to prepare haloalkanes. When they are treated with sunlight, heat or suitable catalyst, haloalkanes are obtained. This reaction is called halogenation.

The reaction of an alkane with halogen in presence of sunlight is called photo halogenation.

example.

$$CH_3-CH_3+Cl_2\xrightarrow{hv}CH_3CH_2Cl =(Chloro \,ethane)+HCl$$

Sometimes a mixture of the product is obtained on halogenation of alkanes.

The rate of abstraction of hydrogen in halogenation is 3⁰H>2⁰H>1⁰H

The reactivity of halogen in reaction follows the order

$$F_2>Cl_2>Br_2>I_2$$

Important question.

Fluoro,iodo and bromo alkanes are generally not prepared by halogenation. Why?

Fluoro is not generally prepared by halogenation because it is a highly reactive element and forms carbon as.

$$2CH_4+4F_2\longrightarrow2C+8HF$$

$$C_2H_4+3F_2\longrightarrow2C+6HF$$

Bromo is not generally prepared by halogenation because the reactions are very slow and takes much time.

Iodo is not prepared by halogenation because the reaction is reversible. But in presence of oxidising agent like HIO₃, HNO₃, haloalkane can be formed.

$$CH_4+I_2\rightleftharpoons CH_3I+HI$$

$$HIO_3+5HI\longrightarrow3H_2O+3I_2$$

2. From alkenes. Markovnikovs rule.

Alkenes undergo electrophilic addition with halogen acid to give haloalkanes.

$$CH_2=CH_2+HBr→CH_3-CH_2Br$$

The addition of halogen acids to an unsymmetrical alkene forms two different haloalkanes of which major product is predicted according to markivnikov's rule.

It states that " The electronegativity part of an unsymmetrical reagent to be added across a double of an unsymmetrical alkene goes to that carbon containing less number of hydrogen atom"

However, in presence of organic peroxide, the addition of halogen acids follow a reverse reaction known as an antimarkovnishs reaction or peroxide effect. This is also called Kharasch's effect.

3. From alcohol.

This is the best method for the preparation of haloalkane in which alcohols are accordingly converted into corresponding haloalkanes by the action of following reagents.,

a. By the action of halogen acid.

Chloro alkane is obtained from alcohols by treating them Luca's reagent (1:1) mixture of Conc. HCl and anhydrous ZnCl₂)

Example

Similarly, the reactivity of halogen follows the order.

Similarly, the reactivity of halogen follows the order.

$$HI>HBr>HCl$$bbvb

b. By the action of phosphorous halide.

When alcohols are refluxed with phosphorous halides, haloalkanes are obtained.

$$R-OH+PCl_5\xrightarrow{Δ}R-Cl+POCl_3+HCl$$

$$3R-OH+PCl_3\xrightarrow{Δ}3R-Cl+H_3PO_3$$

Example.

c. By the action of thionyl chloride (SOCl)₂:

Alcohols reacts with thionyl chloride in presence of small amount of pyridine to form chloroalkane.

This method is regarded as the best methods for preparation of chloroalkanes from alcohol. Because the by-products of this reaction being in gaseous form, escape instantly leaving a pure chloro alkane.

Reference

Bahl, B S, Bahl, and Arun. Advanced Organic chemistry. S. Chand and company Ltd., n.d.

Sthapit, M K, R R Pradhananga, and K B Bajracharya. Foundations of chemistry. Taleju Prakashan, n.d.

Tewari, K S, S N Mehrotra, and N K Vishnoi. A textbook of organic chemistry. Vikash publishing House Pvt. ltd., n.d.

Verma, N K and S K Khanna. Compressive chemistry. 8th edition. Laxmi publications P. Ltd., 1999.