Preparation Of Alkanes: Definition, Preparation Of Alkanes & Sample Questions

Alkanes are acyclic saturated hydrocarbons where they have hydrogen and carbon bond which is bonded by C-C and C-H single bond. Structurally alkanes are linear or branched and are also called paraffin. Each alkane has the suffix “ane” in its name.

Key Terms: Alkanes, Reaction, Bond, Carbon, Compound, Alkyl, Atom.

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Alkanes

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Alkane compounds have same the molecular formula but their structural formulas are different for different compounds. 

• Alkanes are mainly classified into 4 types depending upon the number of alkyl groups attached to the carbon atom. 
• The 4 different types are: Primary alkanes, secondary alkanes, tertiary alkanes, and quaternary alkanes. Examples of alkanes are methane, ethane, propane, butane, hexane, etc.
• The general formula for alkanes is Cn H_{2n +2}. Here “n” = a number of atoms in a chain.
• Alkanes have weak Van Der Waals force, therefore, the first four members i.e. from C1 to C4 are gases, and from C5 to C17 are liquids and compounds which have 18 or more than 18C atoms are solids at 298K. 

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Synthesization Of Alkanes

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Alkanes is synthesized by two means naturally and in the laboratory. 

In the natural method, they are formed through the decomposition of plants and organic matter. In laboratory methods, alkanes are prepared from unsaturated hydrocarbons, alkyl halides, and carboxylic acids.

The laboratory method for the preparation of alkanes are specified below:

1. Preparation of Alkanes from unsaturated hydrocarbons
2. Preparation of Alkyl Halides
3. Preparation of Carboxylic Acids

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Preparation Of Alkanes From Unsaturated Hydrocarbons

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Preparation procedure of alkanes from unsaturated hydrocarbons are discussed in points:

• Alkanes are prepared from the hydrogenation of unsaturated hydrocarbons such as alkenes (double bond) and alkynes (triple bond).

•  Here in hydrogenation, dihydrogen gas is added to the alkenes and alkynes in the presence of a catalyst that is finely divided like nickel, platinum, and palladium.
• In the above-given reaction, the catalysts absorb the dihydrogen gas on their surface activating the H-H bond.
•  Platinum and palladium can catalyze the reaction at room temperature but comparatively, nickel requires higher temperature and pressure to catalyze the reactions.

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Preparation Of Alkanes From Alkyl Halides

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From alkyl halides, alkanes can be prepared by two methods (1) By reduction and (2) by Wurtz reaction.

1. By reduction: All alkyl halides such as bromide, chloride, and iodide can be used for preparation by reduction method except for alkyl fluorides. 
2. The reducing agent mainly used is zinc and Hydrochloric acid. In the presence of the reducing agent's reduction reaction of alkyl halides occur.
3. Wurtz reaction: Wurtz reaction is a neutrophilic reaction. When alkyl halides are treated with sodium metal in a dry ethereal solution that is free from moisture the end product of the reaction is more carbon atoms. 
4. This reaction is used to make higher alkanes having an even number of carbon atoms.

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Preparation Of Alkanes From Carboxylic Acids

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From carboxylic acids alkanes can be prepared by two methods (1) Decarboxylation and (2) Kolbe’s electrolytic method.

1. Decarboxylation: Decarboxylation refers to the removal of carbon dioxide from the compound in the presence of a base. When sodium salts of carboxylic acids are heated with soda lime the end product is alkanes which have one less carbon atom because of decarboxylation.
2. Kolbe’s electrolytic method: In this method, carboxylic acid salts like sodium and potassium salts in the aqueous mixture are used in electrolysis which in turn gives alkanes that have an even number of carbon atoms at the anode.

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Preparation Of Alkanes From Carbonyl Compounds

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Ketones and aldehydes are carbonyl compounds that are used to prepare alkanes. Here, the Clemmensen reaction occurs in which the carbonyl compounds undergo a reduction reaction in the presence of reducing agents at high temperatures.

The general equation of reduction of carbonyl compounds into alkanes is RCHO → R-CH₃.

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Things to Remember

• Alkanes are odorless and colorless. Chain isomerism is common in alkanes.
• Linear alkanes whose carbon atoms are in a straight chain are called normal paraffin, normal alkanes, or n-alkanes. Whereas, the branched alkanes are called isoparaffins or isoalkanes.
• Methane is the simplest alkane. Methane has one carbon atom and four hydrogen atoms. While other alkanes are made from the substitution of carbon atoms for hydrogens.
•  The main source of global occurrence of n-alkanes is petroleum. However, they occur naturally in riverine, estuarine, and marine sediments as well as aquatic organisms
• Alkanes are major constituents of conventional petroleum and petroleum products.

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