Preparation of alkanes/hydrocarbons

Methods of Preparation:

Methods of preparation of Alkanes:

1) Hydrogenation of alkene or alkynes:

Alkane is prepared by heating alkenes or alkynes with hydrogen in presence of Ni-catalyst at about 200-300ْ C.

CH₃—CH==CH₂   +   H₂    —-(Ni-catalyst)—>  CH₃ — CH₂—CH₃                                                

CH₃ — C  ≡ CH     + 2H₂    —-(Ni/Δ)—-> CH₃ — CH₂—CH₃

2) From alkyl Halides (Wurtz Reaction):

Symmetrical alkane is prepared by treating alkyl halide with sodium metal in presence of dry ether.

R—X   +  2Na  +  X— R   —->  R— R      +   NaX

CH₃—Br  + 2Na  + Br—CH₃  —–> CH₃ — CH₃     +   NaBr

Note:

R—X   +  2Na  +  X— R΄ ——–> R— R  +  R— R΄   +   R΄— R΄   +  NaX

3) Hydrolysis of Grignard Reagent:

Alkyl magnesium halides (Grignard reagent) on treatment with water give alkanes.

CH₃ Mg Br  + HO—H  ——->  CH₄  +  Mg (OH) Br

4) Decarboxylation of Carboxylic acid:

Alkane is prepared by heating sodium salt of carboxylic acid with sodalime (NaOH + CaO).

R—COONa    +  NaOH —-[CaO/ Δ]——>  R— H      +   Na₂CO₃ 

CH₃—COONa   +  NaOH  —-[_{CaO/ Δ]}—->CH₃— H   +   Na₂CO₃

Sodium acetate                                             Methane

5) Reduction of alkyl halides:

Alkyl halides undergo reduction with nascent hydrogen to form alkanes.                                            R—X   + 2[H] —-ZnCl₂/Δ]—-> R— H   +   HX

CH₃—I + 2[H]  —[ZnCl₂/Δ]—-> CH₃— H  +   HI

6) Kolbe’s Synthesis:

Alkane is prepared by electrolysis of a concentrated solution of sodium salt of carboxylic acid.

2R—COONa  —–> 2R— COO^– +   2Na⁺

At anode:  2R— COO^– – 2e  —–> 2[R— COO] ——–> R—R + 2CO₂

At cathode:    Na⁺   + e  ——–> Na

Na +  H₂O ——–> NaOH  + H₂