Halogenation of Alkanes

Chemistry can be naturally defined as the study of chemical substances and compounds. Such compounds are present everywhere, ranging from the food products such as proteins, fats and carbohydrates to clothing materials such as cotton, silk and wool. They also include medicines, drugs, hormones, steroids, vitamins, enzymes, dyes, perfumes and detergents. These chemical compounds are made mostly of carbon and hydrogen.

Alkanes

Alkanes can be defined as the simplest organic compounds made of carbon and hydrogen. They are saturated hydrocarbons bonded to each other by single covalent bonds. Alkanes constitute C-C and C-H strong covalent bonds. They are also known as paraffin. These study material notes on halogenation of alkanes include the various concepts of halogen additions and their respective mechanisms.

Cycloalkanes

Cycloalkanes can be referred to as cycloparaffins. They are defined as saturated hydrocarbons, which involve combining carbon atoms by single covalent bonds to form a cyclic ring structure. They can also be known as alicyclic compounds.

Preparation of cycloalkanes

From dihalides

Cycloalkanes are obtained by treating terminal dihalides like dichloropropane with sodium or zinc metal. It is generally known as the Wurtz reaction.

CH2Cl-CH2-CH2Cl + 2Na→ ◁+ 2NaCl

From calcium salts of dicarboxylic acids

Cycloalkanes can be obtained when heating calcium or barium salts of dicarboxylic acids. It leads to cyclic ketones that can be converted to cycloalkanes by Clemmensen reduction.

(CH2-CH2-COO)2Ca2+ + Zn/Hg + HCl →⬠+ CaCO3

Halogenation of alkanes

Halogenation involves the substitution of hydrogen atoms of alkanes with halogen atoms such as chlorine, fluorine, bromine, and iodine.

Chlorination

Halogenation of alkanes is primarily based on the addition of chlorine. Alkanes react with chlorine in the presence of ultraviolet light, diffused sunlight, or at a temperature of 300-400⁰C, yielding a mixture of products.

For example, here is the reaction of methane with chlorine to give methyl chloride and HCl.

CH4 + Cl2 → CH3Cl + HCl

This process continues until all the hydrogen atoms of methyl chloride can be successively replaced by chlorine atoms.

CH3Cl + Cl2 → CH2Cl2 + HCl

CH2Cl2 + Cl2 → CHCl3 + HCl

CHCl3 + Cl2 → CCl4 + HCl

Mechanism

The chlorination of alkanes occurs through the formation of free radicals. A step-by-step mechanism can further explain this.

Chain initiation step

The chlorine molecule initially undergoes homolytic fission to produce free radicals of chlorine.

Cl:Cl → Cl∙ + Cl∙

Chain propagation step

The process of chain propagation takes place in two steps.

Step 1: The formed chlorine free radical attacks methane to give hydrochloric acid and methyl free radical.

Cl∙ + H:CH3 → H:Cl + CH3

Step 2: The methyl free radical attacks the chlorine molecule to produce methyl chloride and chlorine-free radical.

CH3 + Cl:Cl → CH3: Cl + Cl∙

Chain termination step

The termination step occurs when two free radicals combine to produce a whole molecule.

Cl∙ + Cl∙ → Cl-Cl

CH3 + Cl → CH3 -Cl

Bromination

It is defined as the reaction process of the halogen bromine with alkanes to form the desired product.

Mechanism

The bromination of alkanes occurs similarly to chlorination through the formation of free radicals. A step-by-step mechanism can further explain this.

Chain initiation step

The bromine molecule initially undergoes homolytic fission to produce free radicals of bromine.

Br:Br → Br∙ + Br∙

Chain propagation step

The process of chain propagation takes place in two steps.

Step 1: The formed bromine free radical attacks methane to give hydrochloric acid and methyl free radical.

Br∙ + H:CH3 → H:Br + CH3

Step 2: The methyl free radical attacks the bromine molecule to produce methyl bromide and bromine free radical.

CH3 + Br:Br → CH3: Br + Br∙

Chain termination step

The termination step occurs when two free radicals combine to produce a whole molecule.

Br∙ + Br∙ → Br-Br

CH3 + Br → CH3 Br

Iodination

The process of iodination involves the reversible addition of iodine to alkane molecules. It results in the formation of hydrogen iodide as the byproduct. It is a powerful reducing agent capable of reducing the iodoalkane to the alkane.

Fluorination

The most reactive compound among the halogens is considered to be fluorine. It can react explosively with alkanes under many conditions. Fluoroalkanes can be obtained by diluting fluorine with the addition of nitrogen.

Conclusion

The halogenation of alkanes involves utilising organic chemistry in the field of studying chemical compounds. All chemical compounds are made up of carbon, and it is the basic building block of all elements. Alkanes are the primary type of hydrocarbons that are saturated in nature and easily made up, constituting only carbon and hydrogen attached in single bonds.