General Methods of Preparation of Aliphatic Alkanes Notes

Methods of Preparation of Alkanes:

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By Hydrogenation of unsaturated hydrocarbons
By Reduction of Alkyl Halides
By Hydrolysis of Grignard Reagent
By Wurtz’s Reaction
By Decarboxylation of Mono-carboxylic Acids
Kolbe’s Electrolysis

Preparation of alkanes by Hydrogenation of unsaturated hydrocarbons (Sabatier Sendern’s reaction):

Alkanes can be prepared by passing alkenes or alkynes over finely divided nickel or platinum or palladium catalysts. The catalyst changes molecular hydrogen to atomic hydrogen which can break pi-bonds of alkenes or alkynes and will be added to them. The addition of hydrogen to alkenes or alkynes is called hydrogenation and is also called Sabatier-Sendern’s Reaction or S-S Reaction.

While using Nickel, heating is required at 250-300^oC but in the case of Platinum and palladium no heating is required.

2) Preparation of alkanes by Reduction of Alkyl Halides:

Molecular hydrogen cannot reduce alkyl halides, however atomic hydrogen can reduce alkyl halides and gives alkanes. Therefore alkyl halides are treated with Zinc in presence of dil HCl, nascent hydrogen is produced which reduces alkyl halides to corresponding alkanes. LiAlH₄ or NaBH₄ could also be used as a reducing agent.

3) Preparation of alkanes by Hydrolysis of Grignard Reagent:

Alkyl Magnesium Halides (R ─ Mg ─ X) is called Grignard reagents. These are organometallic compounds.

Hydrolysis of Grignard reagents produces corresponding alkanes.

4) Preparation of alkanes by Wurtz’s Reaction:

Wurtz discovered this method in 1885. The reaction of alkyl halides with sodium metal in presence of anhydrous ether produces alkanes and is called Wurtz’s reaction.

In this method, higher alkanes are produced.

5) Preparation of alkanes by Decarboxylation of Mono-carboxylic Acids:

Removal of a carboxylic group or CO2 is called decarboxylation. When carboxylic acids (or fatty acids) are treated with NaOH, the corresponding sodium salt of the acid is produced.

The salt is called sodium carboxylate. This salt on heating with soda lime loses its carboxylic group and produces alkane which is one carbon less than the acid.

In this method alkane having one carbon less than the salt of carboxylic acid, is produced.

Soda-lime is prepared by soaking quick lime in a caustic soda solution and then drying.

Kolbe’s Electrolysis:

When electrolysis of an aqueous solution of sodium or potassium salt of mono-carboxylic acid is carried out, alkanes are produced. This method is used to prepare gaseous alkanes with an even number of carbons.

e.g. Ethane can be produced by electrolysis of sodium acetate (sodium ethanoate).

The mixture of ethane and carbon dioxide obtained at the anode is made to pass through KOH(aq) solution, CO2 will be absorbed by the solution while ethane will pass on and will then be collected over the surface of the water.