Preparation of Alkanes - Practice Questions & MCQ

Dihydrogen gas adds to alkenes and alkynes in the presence of finely divided catalysts like platinum, palladium or nickel to form alkanes. This process is called hydrogenation. These metals adsorb dihydrogen gas on their surfaces and activate the hydrogen – hydrogen bond. Platinum and palladium catalyse the reaction at room temperature but relatively higher temperature and pressure are required with nickel catalysts.

Diborane(B₂H₆) adds to an olefinic bond forming trialkylborane which on treatment with acetic acid or propionic acid yields the corresponding alkane.

It is an important method for preparing alkane from alkene. Methane cannot be prepared by this method.

Alkyl magnesium halides(RMgX) are called Grignard reagents. These undergo double decomposition reactions with water or ammonia or alcohol or amine having active H atom(attached to strongly electronegative O, N, S or F and triple bond, etc.) to give alkane corresponding to alkyl group of Grignard reagent. The reaction occurs as follows:

Corey House Synthesis
It is suitable for the preparation of alkanes with odd number of carbon atoms by the following S_N² mechanism.

For example:

Reduction by LiAlH₄
In this reaction, alkyl halides are reacted with LiAlH₄ (strong reducing reagent) to reduce to alkanes. The reaction occurs as follows:

Wurtz reaction
Alkyl halides on treatment with sodium metal in dry ethereal (free from moisture) solution give higher alkanes. This reaction is known as Wurtz reaction and is used for the preparation of higher alkanes containing even number of carbon atoms.

Decarboxylation of Fatty acids
When anhydrous sodium salt of fatty acid is fused with soda lime (NaOH +CaO) a paraffin dry ether having one carbon atom less than the fatty acid is acid. The reaction occurs as follows:

For example:

Kolbe's Electrolysis
Sodium or potassium salts of carboxylic acids on electrolytic hydrolysis give alkanes at anode as follows:

For example: