Methods in which number of carbon atoms increases w.r.t. starting compound
An ethereal solution of an alkyl halide (preferably the bromide or iodide) is treated with sodium, when alkane is obtained. For example,
R1 – X + R2 – X + 2Na ® R1-R2 + 2NaX
In this reaction, two R groups are coupled by reacting RBr, RCl or RI with Na or K, The yields of the product are best for 1° alkyl halides (60%) and least for 3° alkyl 3° halides (10%).
A superior method for coupling is the Corey-House Synthesis which could be employed for obtaining alkanes containing odd number of carbon atoms (unsymmetrical alkanes). An alkyl halide (R–X), which may be primary, secondary or tertiary is first converted into alkyl lithium by treating with lithium. The alkyl lithium is then reacted with cuprous halide to get lithium dialkyl cuprate. The complex is then treated with another alkyl halides R'–X), which must be preferably primary. The reaction follows SN2 mechanism. With secondary alkyl halides the reaction leads, to partly substitution forming alkane and partly elimination forming alkene, with tertiary alkyl halide only elimination takes place
R–X + 2Li ®R– Li+ + LiX
2R–Li+ + CuX ®(R)2CuLi + LiX
R2CuLi + 2R'-X ®2R-R' + LiX + CuXONS
A concentrated solution of the sodium or potassium salt of a carboxylic acid or a mixture of carboxylic acids is electrolysed. For example,
R1CO2K + R2CO2K + 2H2O ® R' – R2 + 2CO2 + H2 + 2KOH
If R1 and R2 are different, then hydrocarbons R1 – R1 and R2 – R2 are also obtained along with R1 – R2. Earlier several mechanisms have been proposed for the Kolbe’s reaction. The free-radical theory is the one now favoured, having strong evidences in support of it.
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