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rrkss
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Iodine is a better leaving group than chlorine so you need a higher boiling point to allow more heat to force the reaction. This is why THF is used
instead of EtOEt.
For ethyl magnesium bromide I intend to use ethyl ether. I will test for peroxides before I use the ether and if the number is too high, will have to
use FeSO4 otherwise I intend to use the reducing nature of the grignard reaction to destroy any peroxide present.
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DJF90
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Nope. The formation of a Grignard reagent is a radical reaction ("oxidative insertion") and so it's nothing to do with "this is a better leaving group
than that". The C-I bond is weaker than the C-Cl bond, but not by much (228kJmol^-1 vs 346kJmol^-1 respectively), and so an iodide will form a
grignard reagent faster than a chloride, given similar conditions.
The difference in the given case is that the chloride is also a *vinyl* halide, which are more difficult to form the grignard reagent from (apparently
they have long been considered unreactive towards magnesium), and hence the higher boiling solvent is necessary. I also suspect a more active
magnesium is used (Riecke magnesium for example).
1,4-dioxane is added to alkyl magnesium halides in order to shift the Schlenk equilibrium, forming the dialkyl magnesium compound and magnesium
dihalide, which is complexed to the 1,4-dioxane and precipitates.
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rrkss
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Very interesting. Thanks for sharing!
Quote: Originally posted by DJF90  | Nope. The formation of a Grignard reagent is a radical reaction ("oxidative insertion") and so it's nothing to do with "this is a better leaving group
than that". The C-I bond is weaker than the C-Cl bond, but not by much (228kJmol^-1 vs 346kJmol^-1 respectively), and so an iodide will form a
grignard reagent faster than a chloride, given similar conditions.
The difference in the given case is that the chloride is also a *vinyl* halide, which are more difficult to form the grignard reagent from (apparently
they have long been considered unreactive towards magnesium), and hence the higher boiling solvent is necessary. I also suspect a more active
magnesium is used (Riecke magnesium for example).
1,4-dioxane is added to alkyl magnesium halides in order to shift the Schlenk equilibrium, forming the dialkyl magnesium compound and magnesium
dihalide, which is complexed to the 1,4-dioxane and precipitates. |
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rrkss
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Well unstabilized ether does form peroxides very fast. I have a level of between 0.5 ppm and 1 ppm on my test strip from less than 5 days of storage.
Normally stabilized ether tests negative for peroxides after a month of storage for me.
Tests negative on iodine starch, only the highly sensitive test gave me a positive reading.
[Edited on 6-14-10 by rrkss]
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