why Grignard reagent gives 1,2 addition while Gill man's reagent gives 1,4 addition?
Answers
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The Grignard reaction, although very well examined, there are still arguments about the mechanism and the overall process. The reagent is usually best described via the Schlenk equilibrium, but that is not only dependent on X, but also on the solvent:
2RMgX↽−−⇀R2Mg+MgX2
There also have been some dimeric structures reported, i.e. the symmetrical, the asymmetrical, and the alkyl bridging dimer. According to Milton Orchin[1] (and references therein) the latter is responsible for the formation of the above equilibrium.
Grignard dimers
There are many mechanism possible, two of them following a non-radical pathway are depicted below. In both cases, the 1,2-addition leads to the major product. The six-membered transition state (top route) has been proposed many times and employs the alkyl bridging dimer.[2]
The bottom route considers only the monomeric Grignard reagent. Whichever pathway is taken, the result is the same: 1,2-addition, resulting in a tertiary alcohol, with the C=C double bond retained.
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