Alkanes are referred to as the saturated hydrocarbons, that is, hydrocarbons having all carbon atoms bonded to other carbon atoms or hydrogen atoms with sigma bonds only. As the alkanes possess weak Van Der Waals forces, the first four members, C1 to C4 are gases, C5 to C17 are liquids and those containing 18 carbon atoms or more are solids at 298 K. They are colourless and odorless. They are prepared in laboratories and industries through various techniques. Some techniques for the preparation of alkanes are:
Preparation of alkanes from unsaturated hydrocarbon: Alkane can be prepared from alkene and alkyne through the process of hydrogenation. In this process, dihydrogen gas is added to alkenes and alkynesin the presence of finely divided catalysts likeplatinum, palladium or nickel to form alkanes. With the help of nickel as the catalyst, this reaction takes place at an elevated temperature, whereas the reaction takes place at room temperature with platinum as the catalyst.
Preparation of alkanes from alkyl halides:Alkane can be producedfrom alkyl halides predominantly by two ways:
- Alkanes can be prepared from alkyl halides (except fluorides) through reduction with zinc and dilute hydrochloric acid.
- Wurtz reaction: In this reaction, alkyl halides are treated with sodium metal in dry ethereal (free from moisture) solution to produce higher alkanes. This reaction is used for the preparation of higher alkanes containing even number of carbon atoms.
Preparation of alkanes from carboxylic acids:Preparation of alkanes form carboxylic acids mainly happens via two means:
- Alkanes can be prepared from carboxylic acid via the elimination of carbon dioxide. This process is known as decarboxylation. It produces alkane with a carbon atom lesser than that present in carboxylic acid.
- Kolbe’s electrolytic method: In this process, alkane is produced through electrolysis of sodium or potassium salt of carboxylic acid.
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