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.

It is to be noted that highly strained ring of cyclopropane under ring opening upon catalytic hydrogenation to form propane.

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

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

It is to be noted that the B atom attaches to the less hindered carbon atom in the first step while the H atom attaches to the adjacent C atom containing the double bond.

In the next step, the BH₂ group is replaced by the H of the acid (CH₃COOH or H₂SO₄)

The mechanism of this reaction is beyond the scope of the syllabus

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:

A similar reaction occurs with other sources of acidic hydrogen or acids

It is to be noted that Grignard reagents are not stable in Protic Solvents like Water or Ethanol and require aprotic solvents like Ether or Tetrahydrofuran (THF) for their synthesis and reactions.

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

For example:

It is to be noted that in this method of preparation, tertiary halides should be avoided as they may lead to the formation of Alkenes via the elimination mechanism

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

It is to be noted that in this method of preparation, tertiary halides should be avoided as they may lead to the formation of Alkenes via the elimination mechanism

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

Wurtz Fittig Reaction

A modification of the Wurtz reaction is the Wurtz Fitting reaction in which an Alkyl halide and an Aryl halide on treatment with sodium metal in dry ether give substituted aromatic compounds. This reaction involves the coupling of an alkyl halide and an aryl halide.

Frankland reaction

Alkyl halides on treatment with Zinc metal give higher alkanes. This reaction is known as the Frankland reaction and is used for the preparation of higher alkanes containing an 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: