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>> General Methods of Preparation of Alkanes-Part1
General Methods of Preparation of Alkanes
(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.
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
(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