Zinc, organo- reagents reactions with

The zinc organo-lialide is prepared for use by heating together the zinc-copper couple, the alkyl or aiyl halide, ethyl acetate, and toluene for about an hour, the reaction being started if necessary with a ciystal of iodine. All the reagents must be well dried, and the colomdess viscous liquid which results is ntilised by adding to it the acid chloride dissolved in toluene, and when the required amount has been added the intermediate zinc complex is decomposed by the addition of dilute sulphuric acid.1... 

For the Knochel reaction an added Lewis acid is needed to enhance the 1,4-addition otherwise 1,2-addition would take place. A major advantage of the Knochel reagent is that other functional groups (ester, carbonyl, nitrile, Cl, sulfoxide and terminal alkyne) are possible with the organo-zinc starting material. 

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