Preparation of Alkanes

Alkanes are a family of saturated hydrocarbons having general formula of CnH2n+2. In alkane each carbon atom form four single sigma bonds with other atoms. Due to their saturated nature and presence of sigma bonds they are very stable or less reactive and are therefore called paraffin’s. Alkanes can be prepared by the following methods. 

Decarboxylation of carboxylic acids

Removal of CO₂ from carboxylic acids is called decarboxylation. Alkanes can be prepared by decarboxylation saturated carboxylic acids. It can be done by reacting sodium salt of carboxylic acid with soda lime which is a mixture of caustic soda and quick lime (CaO). As there is removal of one carbon from carboxylic acid therefore except methanoic acid all carboxylic acids can be used but remember the yield is good in case of using lower carboxylic acids.

Wurtz Reaction

Wurtz reaction is a method for the preparation of alkane from alkyl halides. In this reaction alkane can be obtained by treating alkyl halides with sodium metal in dry ether. Except methane all other alkane can be prepared by this method. By wurts reaction symmetrical alkane of even carbon can be prepared and it is not prepared for synthesis of alkanes of odd carbon atoms.

Frankland method.

This method is similar to Wurtz reaction but here instead of sodium metal zinc is used.

By the Reduction of Alkyl Halides

Alkyl halides can be reduced to alkane by several methods. The most important one is treatment of alkyl halides with zinc and HCl mixture. As a result of Zn and HCl reaction nascent hydrogen is formed which can easily reduce alkyl halides.

Reduction of alkyl halides can also be carried out by using of any one of the following reagents.

Zn + CH₃COOH, Zn-Cu couple in ethanol, Red P + HI, Al-Hg + ethanol or H₂/Pd or LiAIH₄ or by H₂/Ni

From unsaturated hydrocarbons: Sabatier and Senderen’s Method 

Alkanes can be prepared by catalytic hydrogenation of alkenes or alkynes. In this method nickel, platinum or palladium is used as catalyst. The reaction take place at 200oC in presence of Ni catalyst while it may take place at room temperature while using any one of the other two catalysts.  

By Hydroboration of Alkenes

Hydroboration of alkenes may also give alkane. This is a two step reaction in which first trialkyl borane is formed which on further treatment with acetic acid give corresponding alkane.

Kolbe’s Electrolysis Method

Alkane can be prepared by the electrolysis of aqueous solution of salt of carboxylic acid. In this method electricity is passed through the concentrated aqueous solution of potassium slat or sodium salt of saturated carboxylic acids. As a result of electrolysis symmetrical alkanes can be obtained at anode. In case of using mixture of different salts, we can get mixture of alkanes.

By Grignard Reagents

Grignard’s reagents are a group of organometallic compound which contain magnesium metal and are called alkyl magnesium halides (R-Mg-X).

Grignard’s reagents can be reduced to alkanes by reaction with water or ammonia.

By reduction of carbonyl compounds: Clemmensen Reduction

In Clemmensen reduction aldehydes or ketones can be reduced to alkanes by using zinc-mercury amulgum in concentrated HCl.

Wolf-Kishner reduction

In Wolf Kishner reduction aldehydes or ketones can be reduced to alkynes by using hydrazine and alcoholic sodium solution. The reaction take place at 180^oC.

By reduction of alcohol, and carbonyl compounds.

Alcohols and carbonyl compounds can be reduced to alkanes by reacting them with red phosphorus and hydrogen iodide.

By the Hydrolysis Carbides

Al and Be carbides may produce methane by hydrolysis.

Only CH₄ can be obtained by the hydrolysis of Be or Al carbides.

By Corey- House Synthesis

Alkyl chlorides can be changed into alkane by Corey-House reaction. In this reaction alkyl lithium is formed by the reaction of alkyl chlorides with lithium in ether. The alkyl lithium is treated with copper-I Iodide to form lithium alkyl cuprate which on further treatment with alkyl chloride give alkane.