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What are alkanes? Write general formula of alkanes and four general methods for preparation of alkanes.

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The general formula of alkanes in Cn H2n + 2 where n stands for number of carbon atoms and 2n + 2 for number of hydrogen atoms in the molecule.
Example : CH4 (methane),
C2H6 (ethane) etc.
General methods for preparation of alkanes :

(1) From unsaturated hydrocarbons : The unsaturated hydrocarbons are converted into alkanes by catalytic hydrogenation in the persence of catalyst such as Ni, Pt or Pd at 150°-200° C.

(2) From alkyl halides : When an alkyl halide is treated with sodium in the presence of dry ether, alkane is obtained. This reaction is called wurtz reaction.

Example: 

(3) By the use of Grignard reagent: Alkyl halide on treating with magnesium in the presence of dry ether forms alkyl magnesium halide. It is called Grignard reagent. It undergoes hydrolysis to from alkane.

(4) By Cleinmensen reduction : When an aldehyde or ketone is refluxed with 40% aqueous hydrochloric acid, amalgamated zinc and an organic solvent like, toluene, it undergoes reduction to form alkane.

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