General Methods of Preparation of Alkanes

(i)   Decarboxylation:

(a)    Decarboxylation means removal of CO₂ from molecules having - COOH gp.

(b)    Saturated monocarboxylic acid salt of sodium potassium on dry distillation with soda lime gives alkane.

                                 RCOONa   R-H

or                  RCOONa + NaOH   R-H + Na₂CO₃

(c)    The alkane formed always contains one carbon atom less than the original acid.

(d)    The yield is good in case of lower members but poor for higher members.

(e)    Soda lime is prepared by soaking quick lime CaO in caustic soda solution and then drying the products. It is generally written as NaOH + CaO. Its reaction is milder than caustic soda. Otherwise the reaction will occur violently. Also CaO used alongwith NaOH keeps it dry (NaOH is hygroscopic) to aid fusion.

(f)        The decarboxylation of sodium formate yields H₂.

                                        HCOONa + NaOH  H₂ + Na₂CO₃

                                         CH₃COONa + NaOH  CH₄ + Na₂CO₃

                                                                ^methane

(ii)   Wurtz reaction:

(a)    A solution of alkyl halide in ether on heating with sodium gives alkane.

                    R-X + 2Na + X-R  R-R + 2NaX

(b)    An alkyl halide on Wurtz reaction leads to the formation of symmetrical alkane having an even number of carbon atoms.

(c)    Two different alkyl halides, on Wurtz reaction give all possible alkanes.

           CH₃X + Na + C₂H₅X → CH₃CH₂CH₃ + CH₃CH₃ + CH₃CH₂CH₂CH₃

The different steps are:

                    CH₃X + 2Na + C₂H₅X → CH₃CH₂CH₃ + 2NaX

                    CH₃X + 2Na + C₂H₅X →CH₃CH₃ + 2NaX

                    C₂H₅X + 2Na + C₂H₅X → C₂H₅C₂H₅ + 2NaX