Preparation of Alkanes

Alkanes are not laboratory curiosities but important naturally occurring compounds -- natural gas and petroleum (crude oil) are their most important natural sources -- though several laboratory methods exist. (1) Catalytic reduction (hydrogenation) of unsaturated hydrocarbons: an alkene or alkyne passed with H2 gas over platinum or palladium at room temperature is reduced to the corresponding alkane; the same addition catalysed by nickel at 298 K is called the Sabatier-Sendersens reaction (e.g. propene + H2 --Pt--> propane; ethene + H2 --Ni, 298K--> ethane; prop-1-yne + 2H2 --Pt--> propane). (2) From carboxylic acids: (i) decarboxylation of the sodium salt of a carboxylic acid -- heating a mixture of the sodium salt with soda lime (NaOH + CaO) eliminates the -COONa group as Na2CO3 and gives an alkane with one carbon fewer than the acid (e.g. sodium acetate + NaOH --CaO, heat--> methane + Na2CO3); (ii) Kolbe's electrolytic method -- electrolysing the sodium or potassium salt of a carboxylic acid causes decarboxylative dimerisation at the anode to give a higher, symmetrical alkane (R-R), releasing CO2 at the anode and H2 + NaOH at the cathode. (3) From alkyl halides (haloalkanes): (i) reduction with nascent hydrogen -- all alkyl halides except fluorides are reduced to the alkane using Zn+HCl, Zn+CH3COOH, Zn-Cu couple in ethanol or LiAlH4 as the hydrogen source; (ii) the Wurtz reaction -- treating a haloalkane in dry ether with sodium metal gives a higher alkane with an even number of carbons (e.g. 2 CH3Br + 2Na --dry ether--> CH3-CH3 + 2NaBr); (iii) the Corey-House synthesis -- an alkyl halide reacted with a lithium dialkylcuprate [R'2LiCu] gives a higher alkane (e.g. ethyl bromide + (CH3)2LiCu --> propane + CH3Cu + LiBr). (4) From Grignard reagents: a halo …