Preparation of Alkyl Halides

Introduction to Alkyl Halides

Alkyl halides (or haloalkanes) are the compounds in which one or more hydrogen atoms in an alkane are replaced by halogen atoms (fluorine, chlorine, bromine, or iodine). These are organic compounds with the general formula $RX$, where $R$ denotes the alkyl group and $X$ denotes the halogen (group 17 elements). Alkyl halides and aryl halides (also known as haloarenes) are the two different types of substituted hydrocarbons.

The major difference between both alkyl halides and aryl halides is that haloalkanes are derived from alkanes (open chain hydrocarbons) and haloarenes are derived from aromatic hydrocarbons. Now we will discuss the preparation alkyl halides. Both Haloalkanes and Haloarenes can be prepared from other organic compounds. Some method of preparation of alkyl halides and aryl halides are given below:


Preparation of Alkyl Halides

1. Preparation of Alkyl Halides From Alkenes

The addition of hydrogen halides to alkenes either follows Markovnikov’s rule or the Kharash effect. All the electrophilic addition reactions of alkenes following the Markovnikov rule are known as Markovnikov addition reactions. (In simple definition it states that “Hydrogen is added to the carbon with the most hydrogens and the halide is added to the carbon with least hydrogens”).

General Reaction

$\underset{\text{Alkene}}{R - CH = CH_2} + \underset{\text{Hydrogen halide}}{H - X \to R - CH_2 - CH_2X}$ OR $\underset{\text{Alkyl halide}}{R - CH_2X - CH_2}$

Conversion of $- C = C - (\text{Alkenes}) \text{into} - X (\text{Alkyl halides})$

$\underset{\text{Symmetric alkene}}{R - CH = CH - R } + \underset{\text{Hydrogen halide}}{H - X} \to \underset{\text{Alkyl halide}}{R - CH_2 - CHX - R}$


Preparation of Alkyl Chloride / Alkyl Bromides / Alkyl Iodides:

$\underset{\text{Symmetric alkene}}{R - CH = CH - R} + \underset{\text{Hydrogen chloride}}{H - Cl} \to \underset{\text{Alkyl chloride}}{R - CH_2 - CHCl - R}$

$\underset{\text{Unsymmetric alkene}}{R - CH = CH - R' \,\,\,\, + \,\,\,\,H - X} \to\underset{\text{Hydrogen chloride}}{R - CH_2 - CHX - R'} \to \underset{\text{Alkyl halide}}{R - CHX - CH_2 - R'}$


2. Preparation of Alkyl Halides by Free Radical Halogenation

In free radical halogenation, we get a mixture of mono-substituted, di-substituted, tri-substituted, and even tetra-substituted halo-alkanes (alkyl halides). Since we require only one type of alkyl halide and not all in the form of a mixture, So this method is not used.

$CH_3CH_2CH_2CH_3 \xrightarrow[]{Cl_2 / UVlight} CH_3CH_2CHClCH_3 + CH_3CH_2CH_2CH_2Cl$


3. Preparation of Alkyl Halides from Alcohols

In this reaction of synthesis of alkyl halides, the hydroxyl group of alcohol is replaced with the halogen atom attached to the other involved compound. The reaction requires a catalyst for primary and secondary alcohol whereas tertiary alcohol doesn’t require any catalyst.

$CH_3CH_2OH + SOCl_2 \overset{\Delta}{\rightarrow} CH_3CH_2Cl + SO_2 + HCl$

$CH_3CH_2OH + PCl_2 \overset{\Delta}{\rightarrow} CH_3CH_2Cl + P(OH)_3 + HCl$

$CH_3CH_2OH + PCl_5 \overset{\Delta}{\rightarrow} CH_3CH_2Cl + P(OH)_3 + HCl$

$CH_3CH_2OH + PBr_3 \overset{\Delta}{\rightarrow} CH_3CH_2Br + P(OH)_3 + HBr$


Preparation of Aryl Halides

1. Preparation of Aryl Halides by Electrophilic Substitution Reactions

Aryl halides can be prepared by an electrophilic aromatic substitution reaction of arenes with halogens in the presence of  Lewis acid.

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2. Preparation of Aryl Halides through Sandmeyer’s Reaction

Aryl halides can be prepared by mixing the solution of freshly prepared diazonium salt from the primary aromatic amine with cuprous chloride or cuprous bromide.  In a Sandmeyer reaction, a diazonium salt is reacted with copper $(I)$ bromide, copper $(I)$ chloride, or potassium iodide $(KI)$ to form the respective aryl halide. The diazonium salt can be prepared from aniline by reacting nitrous acid at cold temperatures.


Did You Know?

The order of reactivity of halogen acids towards alcohol is:

Order of Reactivity for Halogen acids:

$HI > HBr > HCl$

In the case of halogen acids, bond length increases from $HCl$ to $HI$. The longer the bond length, the lesser will be dissociation energy, and hence, more will be reactivity.


FAQs (Frequently Asked Questions)

1. What is the difference between primary, secondary, and tertiary alkyl halides in general methods of preparation of alkyl halides?

rimary Alkyl Halide: In a primary (1°) alkyl halide (or haloalkanes), the carbon bonded to the halogen atom is only attached to one other alkyl group.


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Secondary Alkyl Halide: In the case of secondary (2°) alkyl halide (or haloalkanes), the carbon bonded to the halogen atom is joined directly to two other alkyl groups which can be of the same or different group.


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Tertiary Alkyl Halide: In the case of tertiary (3°) alkyl halide (or haloalkanes), the carbon atom holding the halogen is directly joined to three alkyl groups, which can be of any combination of the same or different groups.


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2. What are Alkyl Halides?

Alkyl halides (or haloalkanes) are compounds in which one or more hydrogen atoms in an alkane are replaced by halogen atoms (fluorine, chlorine, bromine, or iodine). Haloalkanes have little or no solubility in water in spite of the polar carbon-halogen bond. The attraction between the alkyl halide molecules is stronger as compared to the attraction between the alkyl halide and water.

3. What is the difference between Haloalkanes and Haloarenes?

The major difference between haloalkanes and haloarenes is that haloalkanes are derived from alkanes (open chain hydrocarbons) and haloarenes are derived from aromatic hydrocarbons (hydrocarbons with sigma bonds and delocalized pi electrons between carbon atoms forming rings).