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It is generally true that alkanes with even number of carbon atoms can be prepared in good yield by Wurtz reaction. But both butane and isobutane contain four carbon atoms each but butane can be prepared in good yield by Wurtz reaction but not isobutane. Why so ? Explain. |
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Answer» Solution :n-Butane is a symmetrical molecule and hence a pure sample of n-butane is obtained when ethyl iodide is heated with Na metal in dry ether. `underset"Ethyl iodide "(CH_3CH_2I)+ 2Na + ICH_2CH_3 underset"Heat"overset"Dry ether"to underset"n-Butane"(CH_3CH_2CH_2CH_3)+ 2NaI` But isobutane is an unsymmetrical molecule. To prepare isobutane we need two alkyl halides,i.e., methyl iodide and ISOPROPYL iodide. Whenever a mixture of two alkyl halides is heated with Na metal , actually three alkanes are FORMED . This is DUE to the reason that two alkyl halides in addition to reacting with each other also REACT amongst themselves giving a mixture of three alkanes as shown below : `underset"Isopropyl iodide "(CH_3-oversetoverset(CH_3)|CH-I)+2Na+underset"Methyl idodide"(I-CH_3) underset"Heat"overset"Dry ether"to underset"Isobutane"(CH_3-oversetoverset(CH_3)|CH-CH_3+2NaI` `underset"Isopropyl iodide "(CH_3-oversetoverset(CH_3)|CH-I)+2Na+I-oversetoverset(CH_3)|CH-CH_3 underset"Heat"overset"Dry ether"to underset"2,3-Dimethylbutane"(CH_3-oversetoverset(CH_3)|CH-oversetoverset(CH_3)|CH-CH_3)+2NaI` `underset"Methyl iodide"(CH_3-I)+2Na+I-CH_3 underset"Heat"overset"Dry ether "to underset"Ethane"(CH_3-CH_3)+2NaI` Since a mixture of three alkanes is formed which is difficult to separate, therefore, a pure sample of isobutane cannot be prepared by Wurtz reaction . |
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