Preparation of Alkanes


General Method of Preparation of Alkanes (Paraffins):

Alkanes can be prepared by the following methods:
1.    From unsaturated Hydrocarbons
2.    From Haloalkanes
  • By Wurtz Reaction
  • By reduction
3.    By the Reduction of Aldehydes and Ketones
4.    From Grignard’s Reagent
5.    From salts of Carboxylic acids
  • By Kolbe’s electrolytic method
  • By Heating Na-salt of Carboxylic acid
6.    From Metal Carbides
 
 
1.       From unsaturated Hydrocarbons
Alkanes can be prepared by the catalytic hydrogenation of unsaturated hydrocarbons in the presence of catalyst ‘Ni’ or ‘pt’ at 200⁰C to 300⁰C.
general methods of preparation of alkane from unsaturated Hydrocarbons
 
2.       From Haloalkanes
  • By Wurtz Reaction:

When alkyl halides are heated with sodium metal in the presence of dry ether, alkanes are obtained (generally having double number of C-atoms than in alkyl halides). This reaction is known as Wurtz reaction and used for the preparation of symmetrical alkanes.
general methods for preparation of alkane By Wurtz Reaction
  • By Reduction of alkyl halides (RX)

Haloalkanes (R-X) when heated with reducing agents like; LiAlH4/ether, Pd/H2, Pt/H2, Zn/conc. HCl, alkanes are produced.
 R-X chemical preparation of alkanes By Reduction of alkyl halides  R-H + HX
3.       By the Reduction of Aldehydes and Ketones
          Aldehydes and ketones can be reduced into alkanes in the presence of reducing agents: amalgated zinc and conc. HCl.
general method of preparation of alkane By the Reduction of Aldehydes and Ketones
 
4.     From Grignard’s Reagent
        Hydrolysis of Grignard’s reagent in the presence of ether gives alkanes.
                            dry ether
RMgX + H2O —————> R-H + Mg(OH)X
alkyl magnesium halide          (alkane)   (Hydroxy magnesium halide)   
 
5.     From salts of Carboxylic acid
  • By Kolbe’s electrolytic method

 Electrolysis of aqueous conc. solution of sodium or potassium salt of carboxylic acid gives alkanes.
      >>RCOONa—-Sodium salt
      >>RCOOK—–Potassium salt
 
RCOONa——–> RCOO- + Na+
                                          anion             cation
During electrolysis;
           Electrode reaction occurs as
At anode:
       RCOO- - e ——–> RCOO ———> R-R + 2CO2
                                        unstable                alkane
At cathode:
         Na+  + OH- ———> NaOH
 
  • By Heating Na-salt of Carboxylic acid

When Na-salt of carboxylic acid is heated with soda lime (NaOH & CaO), alkane is obtained having one carbon less than salt by removal of a molecule of CO2. This reaction is also known as decarboxylation.
RCOONa + NaOH cao  R-H + Na2CO3
Eg:
Preparation of alkane By Heating Na-salt of Carboxylic acid
Sodium ethanoate                         (methane)
This is the principle reaction for laboratory preparation of methane gas.
 
 
6.        From Metal Carbides
           Metal carbide like aluminium carbide (Al4C3) and beryllium carbide (Be2C) reacts with pure water to produce methane.
                 Al4C3 + 12H2O ———-> 4Al(OH)+ 3CH4

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