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Explain various methods of preparation of alkane. |
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Answer» Solution :`1.` Prepartion of alkanes from catalytic reduction of unsaturated hydro-carbons. When a mixture hydrogen gas with alkane or alkyne gas is passed over a CATALYST such as platinum or PALLADIUM at room temperature, an alkane is produced. This process of addition of `H2` to unsaturated compounds is known as hydrogenatione above process can be catalysed by nickel at `298K`. This reaction is known as Sabatier-Senderness reaction Example : `underset("Propane")(CH_(3)-CH=)CH_(2)+H_(2)overset(Pt)(to)underset("Propane")(CH_(3)-CH_(2)-CH_(3))` `underset("Ethene")(CH_(2)=CH_(2))+H_(2)overset(Ni)underset("Ethane")(to)underset("Ethane")(CH_(3)-CH_(3))` `underset("Prop"-1-"yne")(CH_(3)-C-=CH)+2H_(2)overset(Pt)(to)underset("Propane")(CH_(3)-CH_(2)-CH_(3))` `2.` Preparation of alkanes from carboxylic acids: `i)` Decarboxylation of sodium salt of carboxylic acid When a mixture of sodium salt of carboxylic acid and soda lime (sodium hydroxide `+` calcium oxide) is heated, alkane is formed. The alkane formed has one carbon atom LESS than carboxylic acid. This process of eliminationg carboxylic group is known as decarboxylation. Example : `underset("Sodium acetate")(CH_(3)COONa)+NaOH overset(CaO)underset(Delta)(to)underset("Methane")(CH_(4)+Na_(2)CO_(3))` `ii)` Kolbe's Electrolytic method : When sodium or potassium salt of carboxylic acid is electrolyzed, a higher alkane is formed. The decarboxylative dimerization of TWO carboxylic acid occurs. This method is suitable for preparing symmetrical alkanes `(R-R)`. `{:(underset(darr"Electrolysis")(2CH_(3)COONa+2H_(2)O)),(ubrace(H_(3)C-CH_(3)+2CO_(2)+)_("at Anode")ubrace(H_(2)+2NaOH)_("at Cathode")):}`: `3.` Preparation of alkanes using alkyl halides (or) HALO alkanes : `i)` By reduction with nescent hydrogen : Except alkyl fluorides , other alkyl halides can be converted to alkanes by reduction with nascent hydrogen. The hydrogen for reduction may be obtained by using any of the following reducing agents : `Zn+HCl`, `Zn+CH_(3)COOH`, `Zn-Cu` couple in ethanol , `LiAIH_(4)` etc., Example : `CH_(3)-CH_(2)-underset("Propane")underset(CH_(3)-CH_(2)-CH_(3)+HCl)underset([H]darrZn//HCl)underset("Chloro propane")(CH_(2)-Cl)` `ii)` Wurtz reaction : When a solution of halo alkanes in dry ether is treated with sodium metal, higher alkanes are produced. This reaction is used to prepare higher alkanes with even number of carbon atoms. Example : `underset("Methyl bromide")(CH_(3)-Br+2Na)+Br-CH_(3)underset("Ether")underset("dry")(to)CH_(3)-underset("Ethane")(CH_(3)+NaBr)` `iii)` Carey-House Mechanism : An alkyl halide and lithium di alkyl cuprate are reacted to give higher alkane. Example : `underset("Ethyl bromide")(CH_(3)CH_(2)Br)+(CH_(3))_(2)underset("Propane")underset(CH_(3)CH_(@)CH_(3)+CH_(3)Cu+LiBr)underset(darr)(LiCu)` `4.` Preparation of Alkanes from Grignard reagents : Halo alkanes reacts with magnesium in the presence of dry ethers to give alkyl magnesium halide which is known as Grignard reagents. Here the alkyl group is directly attached to the magneisum metal make it to behave as carbanion. So, any compound with easily replaceable hydrogen reacts with Grignard reagent to give corresponding alkanes. Example : `underset("Chloromethane")(CH_(3)-Cl+Mg)overset("Dry ether")(to)underset("Methyl magneisum bromide")(CH_(3)MgCl)` `CH_(3)MgCl+H_(2)Otounderset("Methane")(CH_(4))+Mg(OH)Cl` |
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