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we know that Grignard reagents can react with alylalkoxysilane ,such as: RMgCl + MeSi(OMe)3 → MeRSi(OMe)2 + MeOMgCl (R=phenyl & benzyl)。 But the organozinc reagent such as BnMgBr can’t react with alkylalkoxysilane under the same conditions,even if the additive TMEDA(N,N,N',N'-tetramethyl-ethane-1,2-diamine) or LiCl was used as catalyst 。 enter image description here

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Compare electronegativities of magnesium and zinc and carbon. The difference for magnesium and carbon is $\pu{1.24}$, while for zinc it is $\pu{0.9}$ in Pauling scale. Also, the energy neccessary to oxidize $\ce{M^+}$ to $\ce{M^2+}$ (difference between second and first ionization energy) is about $\pu{715 kJ/mol}$ for magnesium compared to about $\pu{825 kJ/mol}$ for zinc. These informations both tell you that there is much larger negative charge (as in partial charge) on the carbon atom in organomagnesium compounds than in organozinc compounds. Magnesium is also more prone to leaving as a $\ce{MgX^+}$ moiety than zinc as $\ce{ZnX^+}$. In other words, the $\ce{C-Mg}$ bond breaks much more easily than $\ce{C-Zn}$ bond.

In conclusion, magnesium has lower electronegativity (stronger metallic character if you wish) than zinc and that makes organomagnesium compounds much more reactive as nucleophiles than organozinc compounds.

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  • $\begingroup$ Thank you for your warm reply! I assume that TMEDA plays a catalytic role. TMEDA was added to form a complex with Zn to increase the activity of the BnZnBr. But there is no effect 。 . What is the way to make the reaction going smoothly? $\endgroup$ Jan 18 '18 at 11:07

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