Leo Szilard
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Registered: 10-5-2015
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Questions on the Reactivity of Grignard reagents
I understand that alkyl halide reactivity in the formation of Grignard reagents generally follows I>Br>Cl>>F. A question that I have is
how the reactivity of generic Grignards, R-MgX, compare for X=I,Br,Cl,F. For example, is there an appreciable difference in reactivity between R-MgCl
and R-MgI? If so, is the difference largely in nucleophiliity or basicity?
Additionally, I understand that Grignards are generally less basic than organilithiums. But does the basicity of alkyl Grignards follow the same trend
as alkyllithiums, that is: do we expect the trend that alkylmagnesium halides rank as tert>sec>pri in terms of basicity? My guess is that
t-butylmagnesium bromide, for example, is more basic that n-butylmagnesium bromide. But going back to my original question, if we compare
t-butylmagnesium chloride with t-butylmagnesium iodide is there an expectation that one will be more strongly basic?
Lastly, the way in which lithium coordinates to oxygen in addition reactions (or deprotonation reactions) of organolithium reagents with carbonyl
compounds is very important in predicting stereochemistry. Is the same true for Grignards? I guess my question relates to the comparison of the Lewis
acidity (or "coordination ability"? :/) of MgX vs Li. I have read that Grignards are generally less aggregated in ether solutions than
organilithiums...maybe that's the answer.
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zed
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Dunno Leo; must be some papers out there on the subject. But, I don't know where they are.
Much of the selection of which Aryl/Alkyl- Halogen to use to form a Grignard , seems to relate either to impossiblity or expense.
Iodides and Bromides are more expensive. Some Chlorides may be gasses at normal temperatures(making them more difficult to work with). And, directly
creating a Grignard reagent with and Aryl Chloride or Fluoride, may be damn neigh impossible.
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