Preparation of Alkanes by the Reduction of Alkyl Halides

Alkyl halides are organic compounds having the generic formula RX, where R stands for alkyl and X for halogen.

Alkyl halide Alkane Halogen Acid containing halogens -H (Alkanes) to – X conversion (Alkyl halides)

Reactivity Order for Halogens

Benzylic, allylic > tertiary > secondary > primary > vinylic, aryl > F2 > Cl2 > Br2 > 1-2 Benzylic, allylic > tertiary > secondary > primary > vinylic, aryl

Alkyl Chloride Preparation from Alkanes:

Reaction in General

Alkyl chlorides or chloroalkanes are formed when an alkane is treated with chlorine in the presence of UV light or diffused sunlight. The process produces a combination of all chloroalkanes that are conceivable.

CI-CI + R-H

Diffused sunlight OR Ultraviolet Light

R – CI +  H – CI = R – CI +  H – CI = R – CI

Alkyl chloride is a kind of alkane chloride. Chlorine of hydrogen

The process is known as photo halogenation of alkanes because it occurs in the presence of UV light or scattered sunlight.

Example 1: 

Methyl chloride preparation

from methane CH4 + Cl2 Ultraviolet Light OR CH3Cl + HCI from diffuse sunlight

Chlorine Methane Chlorine Methyl Hydrogen Chloride

Example -2: 

Making ethyl chloride (Chloromethane) from ethane C2H6 + Cl2 Ultraviolet Light OR Diffused sunshine C2H2Cl + HCI from ethane C2H6 + Cl2 Ultraviolet Light OR Diffused sunlight C2H2Cl + HCI.

Alkyl Bromides from Alkanes: Preparation

Alkyl bromide or bromoalkane is formed when an alkane is heated with bromine in the presence of anhydrous AlBr3 as a catalyst.

R – H+ Br – Br – R – Br + H – Br – R – Br + H – Br

Bromide of alkyl Bromide of hydrogen

Example 1: To make ethyl bromide from ethane, follow these steps:

C2H6 + Br2=C2H2Br+ HBr

Ethane Bromine Bromide of ethyl Bromide of hydrogen

Example 3: Methyl bromide production from methane:

Br2-CH3Br + HBr = CH4 + Br2-CH3Br + HBr

Alkyl Iodide Preparation from Alkanes:

The reaction with iodine is reversible. As a result, we are unable to obtain a high yield of alkyl iodide. As a result, direct iodination is challenging.

R -H + I – R -H + I – R -H + I – R -H + I – R -H + I – R

Alkane Iodine Iodine alkyl Iodine (hydrogen)

The iodoalkane can be produced if the reaction is carried out in the presence of iodic acid HIO3, mercuric oxide HgO, or dilute HNO3 that can oxidise the HI generated. At the mono-iodo state, the iodination process comes to a halt.

Example-1: Preparation of ethyl iodide using HIO3:

5C2H6 + 2I2 + HIO3 – 5C2H5I + 3H2O

Direct halogenation (Photo halogenation) has a number of drawbacks/disadvantages.

Following the formation of an alkyl halide, the reaction progresses to poly-substitution, yielding a variety of di, tri, tetra, and other halo alkanes.

C12+CH4

Ultraviolet Light is a kind of light that is visible in the ultraviolet spectrum Of sunlight diffused

CH3Cl (Chloromethane) + HCl

CH3Cl + Cl2 -> CH2Cl2 (Dichloromethane) + HCl 

CH2Cl2 + Cl2 – CHCl3 (Trichloromethane) + HCl

CHCl3 + Cl2 -> CCl4 (Tetrachloromethane) + HCl

This process proceeds until all of the alkane’s halogens have been replaced by chlorine or bromine, one by one. As a result, we get a combination of halo alkanes such as di, tri, tetra, and so on.

Alkane iodination is a reversible process that necessitates the addition of more iodine.Although alkane iodination is a reversible process, it necessitates the use of additional reagents such as mercuric oxide, iodic acid, or nitric acid in order to get alkyl iodides. As a result, direct halogenation is not an appropriate process for producing alkyl halides.

Alkyl halides Zn-cu Couple

Alkyl halide reduction is accomplished using the Zn/Cu pair in alcohol. In alcohol, the Zn/Cu pair removes the halide from an alkyl halide and replaces it with hydrogen to produce hydrocarbons. In alcohol, the Zn/Cu pair removes iodine from methyl halide and adds hydrogen to produce methane.

Hare and Mc Bee reaction refers to the thermal chlorination of alkanes. It travels at a speed of 673 kilometres per hour. The reaction is a free radical substitution because diffuse sunlight produces homolytic fission of halogen. Monohalogen derivatives of alkanes are the main result when alkanes are employed in excess. Alkyl halides are a kind of alkyl halide.

Benzylic, allylic > tertiary > secondary > primary > vinylic, aryl 

Conclusion

An alkane with a tertiary hydrogen atom is more reactive toward substitution reactions than one with only a primary hydrogen atom or one with both primary and secondary hydrogen atoms. Isobutane would be replaced more quickly than propane or ethane. Fluorination of alkanes does not produce alkyl fluorides since the reaction is exceedingly explosive. On chlorination of propane, a combination of 2 Chloropropane (55%) and 1-Chloropropane is produced (45 percent ) . When butane is chlorinated, n butyl chloride (28%) and sec-butyl chloride (72%) are produced.On chlorination, isobutane yields a combination of tert-butyl chloride (64%) and isobutyl chloride (34%)