Synthesis of Alkanes

Alkanes are saturated hydrocarbons, which means they are made up of hydrogen and carbon atoms held together by sigma bonds. As a result, alkanes may be synthesized utilizing a variety of processes and starting materials. Alkanes are the primary components of crude oil and the primary ingredients of the majority of petroleum products. Alkanes is also known as saturated hydrocarbons. The carbon atoms that comprise the carbon backbone are linked together to create a chain (linear or branching alkanes), a circle (cyclic alkenes), or a combination of the two. The fact that alkanes are entirely saturated with hydrogen distinguishes them from other hydrocarbons. This means that adding more hydrogen atoms to these compounds will ruin the carbon backbone.

Now let us discuss some of the major processes of synthesis of Alkane;

• Alkanes are synthesized by hydrogenating unsaturated hydrocarbons.

The hydrogenation of unsaturated hydrocarbons is a prominent process for the synthesis of alkanes. Double-bonded alkenes and triple-bonded alkynes are examples of unsaturated hydrocarbons. The process of adding hydrogen molecules in the presence of a catalyst is referred to as hydrogenation. Unsaturated hydrocarbons, for example, create alkanes when hydrogenated in a nickel catalyst at high temperatures. As an example, consider the synthesis of ethane from ethene. Ethene is an alkene composed of two carbon atoms linked together by a single double bond. In the presence of a finely ground nickel catalyst, ethene undergoes hydrogen molecule addition. The hydrogenation of ethene molecules occurs only around 150°C. Platinum and palladium are two other metals that can be used as nickel catalysts.

CH2=CH2 + H2 → CH3-CH3

• Alkanes are synthesized by reducing alkyl halides.

Alkanes can also be synthesized by reducing alkyl halide in the presence of a reducing agent like zinc. Except for alkyl fluorides, this reduction process may be used to create all forms of alkyl halides, such as bromides, chlorides, and iodides. In the presence of hydrochloric acid and zinc, alkyl halides are reduced, resulting in the production of the corresponding alkane.

As an example, consider the synthesis of methane from chloromethane.

In the presence of hydrochloric acid and zinc, methyl chloride undergoes a reduction process. As a result, in addition to methane, HCl is produced as a by-product.

CH3-Cl + H2 → CH4 + HCl

• The Wurtz reaction is used to prepare alkanes.

Alkyl halides are organic compounds that can undergo the Wurtz reaction and produce alkanes. In the presence of a Grignard reagent, the Wurtz reaction is a nucleophilic substitution process. The final product has one more carbon atom than the total number of carbon atoms in the reactant.

The general equation for the Wurtz reaction is provided below.

2 R−X + 2 Na → R−R + 2 Na X

Ethane synthesis

In the presence of a dry ethereal solution, methyl bromide undergoes the Wurtz reaction. The alkyl group in the Grignard reagent substitutes for the bromide ion of methyl bromide. Thus, the ethane molecule is formed by the substitution of the bromide ion with the methyl group. The reaction is known as the nucleophilic substitution reaction.

CH3-Br + 2Na + BrCH3 → CH3-CH3 + 2NaBr

• Kolbe’s electrolytic technique for preparing alkanes

Saturated hydrocarbons are produced by the electrolysis of carboxylic acid salts in water (alkanes). As starting materials, potassium or sodium salts of carboxylic acids are used. The carboxylate salt loses carbon dioxide gas and generates alkane radicals during Kolbe’s electrolysis. The saturated hydrocarbon is formed when two alkane radicals combine. The anode produces the ethane molecule, whereas the cathode produces the carbon dioxide gas.

Example: Using Kolbe’s electrolytic process, an ethane molecule is synthesized.

The starting ingredient for the electrolytic production of ethane molecules using Kolbe’s technique is sodium acetate. In the presence of water, sodium acetate generates methyl radicals and carbon dioxide gas. The generated methyl radicals react with one another, resulting in the creation of ethane molecules. 

2CH3COONa+2H2O → CH3−CH3+2CO2 +2NaOH+H2

• Alkanes are synthesized through decarboxylation of carboxylic salts.

Decarboxylation is the process by which an organic substance loses carbon molecules when exposed to a base. Similarly, carboxylic acid salts undergo decarboxylation and give birth to alkanes. The final product has one more carbon atom than the total number of carbon atoms in the reactant.

As an example, consider the synthesis of methane from sodium acetate.

The decarboxylation process uses sodium acetate as the starting material for the creation of methane molecules. Sodium acetate loses carbon dioxide gas and produces methane molecules in soda-lime (sodium hydroxide and calcium oxide). In the presence of heat, the interaction between soda-lime and sodium acetate occurs.

CH3COO-Na+ NaOH → CH4 + Na2CO3

• Alkane synthesis from carbonyl compounds

Alkanes can also be synthesized using carbonyl chemicals such as ketones and aldehydes. Carbonyl compounds undergo a reduction process in the presence of reducing agents and at higher temperatures, yielding alkanes. The Clemmensen reaction occurs when carbonyl compounds undergo a reduction process to create alkanes. The following generic equations depict the reduction of carbonyl compounds into alkanes.  

RCHO → R-CH3

In the presence of Zn-Hg and HCl, the aldehyde ethanal undergoes a reduction. Ethane and water molecules are formed as a result of the reduction process.

Conclusion: Alkanes are organic compounds made up solely of single-bonded carbon and hydrogen atoms with no additional functional groups. Alkanes have the general formula CnH2n+2 and are classified into three types: linear straight-chain alkanes, branched alkanes, and cycloalkanes. Alkanes are saturated hydrocarbons as well.