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From Unsaturated hydrocarbons

Hydrogen molecule 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 hydrogen molecule 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.

From alkyl halides

Alkyl halides except fluorides react with zinc in the presence of dilute HCl to give alkanes

From Carboxylic acids

  1. Sodium salts of carboxylic acids on heating with soda lime ( mixture of sodium hydroxide and calcium oxide) give alkanes containing one carbon atom less than the carboxylic acid. This process of elimination of carbon dioxide from a carboxylic acid is known as decarboxylation.
  2. Kolbe’s electrolytic method: An aqueous solution of sodium or potassium salt of a carboxylic acid on electrolysis gives alkane containing even number of carbon atoms
Mechanism of Kolbes Electrolysis
The Kolbe reaction is formally a decarboxylative dimerisation and proceeds by a radical reaction mechanism.Kolbe’s electrolysis is ionic (Ist) cum free radical mechanism.

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