Methods of Preparation for Haloalkanes

The most popular haloalkane is bromoalkane. Chloroethane was the first haloalkane to be prepared. Haloalkanes are hydrocarbon halogen derivatives. Haloalkanes contain halogen atom(s) bonded to an alkyl group’s sp3 hybridised carbon atom (s). 

Haloalkanes are classified into three types based on the bond formation of the halogen atom with the carbon chain: primary (1°), secondary (2°), and tertiary (3°). These haloalkanes can be prepared in a variety of ways. Let’s look at the various methods for preparing haloalkanes.

What are haloalkanes?

• Haloalkanes are elements that contain hydrocarbons composed of aliphatic alkanes with one or more hydrogen atoms replaced by halogens.

• It is an alkane that contains one or more halogens bonded to the alkyl group’s sp3 hybridised carbon atom.

• These are commonly represented as R–X, where R denotes the alkyl group(CH3,C2H5) and X denotes the halogen atom(Cl, F, Br).

• These are saturated organic compounds with single bonds connecting all of the chemical bonds to the carbon atom and a single carbon atom to the halogen atom.

• Aliphatic alkanes undergo free radical halogenation to create haloalkanes.

Method of preparation of haloalkanes

Haloalkanes can be prepared in a variety of ways. Alcohol and hydrocarbons can be used to make haloalkanes. Let’s take a look at all of the methods:

Preparation of haloalkanes using alcohol

The most suitable method of producing haloalkanes is through the use of alcohol. When R–OH reacts with appropriate reagents, the reaction causes the formation of R–X. The following reagents are suitable for use in the reaction:

1. Concentrated halogen acids (HX)
2. Phosphorus Halide PX5 or PX3.
3. Thionyl Chloride (SOCl2)

From the above three reagents, the most useful reagent that can easily form haloalkane is Thionyl Chloride.

1. Reaction of alcohol with concentrated halogen acid:

The main product of the reaction between alcohol (R-OH) and halogen acid (H–X) is haloalkanes.

Example: preparing Chloroalkane

The reaction of an alcohol with halogen acid results in the formation of chloroalkane, which is an example of haloalkane formation.

a. We react HCl with primary and secondary alcohol in the presence of catalyst ZnCl2 ( Lucas reagent).

i. CH3CH2OH+HCl→ZnCl2  CH3CH2Cl+H2O (Primary Alcohol)

ii. CH3CH2CH(OH)CH3+HCl→ZnCl2 CH3CH2CH(Cl)CH3+H2O (Secondary Alcohol)

a. In tertiary alcohol, the reaction takes place by simply shaking it with concentrated HCl at room temperature. 

(CH3)3C–OH+HCl→(CH3)3C–Cl+H2O

Reaction of alcohol with concentrated Phosphorus Halide: The halide functional group of phosphorus halides interchanges with the functional group of alcohols (–OH) in this reaction.

ROH+PCl5→RCl+POCl3+HCl

This reaction forms alkyl chloride using PCl5.

Reaction of alcohol with concentrated Thionyl Chloride: It is the most commonly used reagent. This is due to the fact that the byproducts of this reaction are gaseous in nature and therefore can easily escape into the atmosphere, leaving only pure alkyl halide. Darzens halogenation is the name given to this reaction. 

OH + SOCl2 → Cl + SO2↑ + HCl ↑

Preparation of haloalkanes from alkenes (Markovnikov’s rule)

The electrophilic addition reaction can convert alkenes to haloalkanes. R–X is formed when an alkene reacts with HX. The following is the order of reactivity of halides to alkenes:

HI>HBr>HCl>HF

Alkyl halides can be synthesised from alkenes using the following methods:

1. Adding Hydrogen Halide : Markovnikov’s rule governs the addition of halides to alkenes. It says that during the hydrohalogenation of an unsymmetrical alkene, the hydrogen atom of the hydrogen halide molecule forms a bond with the alkene’s doubly bonded carbon atom, which has the most hydrogen atoms.

H3C–CH=CH2 +HBr →H3C–CH2–CH2Br +H3C–CH(Br)–CH3 

Adding halogen : In the laboratory, haloalkanes are also synthesised from alkenes by adding bromine in CCl4 to the alkene. In organic chemistry, this reaction is used to test for unsaturation.

C2H4 + Br2 →  C₂H₄Br₂

Preparation of haloalkanes using free radical halogenation method

In the presence of sunlight, alkanes react with halogens (Cl2 or Br2) to form haloalkanes. This is a free radical substitution reaction that results in a mixture of mono, di, and polysubstituted haloalkanes. 

Example: Methane chlorination produces a variety of products with varying boiling points. As a result, fractional distillation can be used to separate them.

CH4 + Cl2 + light → CH3Cl + HCl

CH3Cl + Cl2 + light → CH2Cl2 + HCl

CH2Cl2 + Cl2 + light → CHCl3 + HCl

CHCl3 + Cl2 + light → CCl4 + HCl

The halogenation of alkanes by free radicals results in the formation of a mixture of haloalkanes. This makes it difficult to isolate a single product. As a result, it is not the preferred method for producing haloalkanes.

Preparation of haloalkanes by halogen exchange reaction

1. Finkelstein’s reaction: Iodo alkanes are formed when chloroalkanes or bromoalkanes are heated with a concentrated solution of sodium iodide in dry acetone. This is referred to as the Finkelstein reaction (SN2 reaction).

R–X+Nal→R–I+NaX

2. Swart’s reaction : When alkyl chlorides or bromides are heated with metallic fluorides such as AgF, SbF3, or Hg2F2, alkyl fluorides are formed. This is known as the Swart reaction.

R–X+AgF→R–F+AgBr

Conclusion

Haloalkanes are made up of halogen atom(s) attached to an alkyl group’s sp3 hybridised carbon atom. These organic compounds have a wide range of applications in both industry and everyday life. They are used as starting materials for the synthesis of a wide variety of organic compounds as well as solvents for non-polar compounds.

There are many methods to prepare haloalkanes like :

• Preparation by halogen exchange reaction

• Preparation using the free radical halogenation method

• Preparation from alkenes (Markovnikov’s rule)

• Preparation using alcohol