GRIGNARD REAGENTS. ORGANOMETALLICS FROM DIVALENT METALS. Formation of Grignard Reagents. This reaction was discovered by the Frenchman, Victor Grignard -- Nobel Prize in 1912. HALIDE REACTIVITIES. RX + Mg R-Mg-X. RELATIVE RATES :.
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This reaction was discovered by the Frenchman, Victor Grignard -- Nobel Prize in 1912
RX + Mg R-Mg-X
RELATIVE RATES :
R-I > R-Br > R-Cl
but easy to
prepare or buy
Ethers are obligatory solvents for the Grignard Reaction.
The reaction doesn’t work without an ether solvent.
Typical ether solvents are:
Diethyl ether (b.p. 35o C)
Tetrahydrofuran (b.p. 65o C)
Dioxane (b.p. 101o C)
Consider two different leaving groups:
Bromine is more reactive than chlorine.
The second reaction is too slow at 35 °C,
therefore we use a higher boiling solvent.
Formation of this complex is exothermic; the reaction is sufficiently exothermic to boil the solution without having to add external heat!
ETHERS COORDINATE INTO EMPTY 3p ORBITALS
Ethers stabilize the Grignard Complex
empty 3p orbitals
CHEMISTS HAVE BEEN ARGUING ABOUT THE STRUCTURE OF THE
GRIGNARD REAGENT FOR YEARS AND STILL CANNOT AGREE
The complete structure of the Grignard reagent is quite complex. It is probably an equilibrium mixture of the type:
….. and the actual reactive species may be a complex dimer.
While this complex picture may be more correct, it is easier to treat the Grignard reagent as if it were simply R-Mg-X, which is what we shall do in this course.
RMgX is a source of a carbanion ( R:- ) just like RLi.
Therefore we expect Grignard reagents to be
both a strong base and good nucleophile.
PERCENT IONIC CHARACTER
Less ionic character
reactions as alkyllthium compounds.
Grignard reagents are strong bases and react readily with any slightly acidic hydrogen ( compare R-Li).
Any source of H+ will bring about this reaction:
Any -O-H, -S-H, or -N-H bonds
are sufficiently acidic to react.
( or H2O)
This is not a stereospecific synthetic method.
INVERSIONS OF THE GRIGNARD REAGENT
Grignard reagents lose stereochemistry (racemize) when made
from an enantiomerically pure alkyl halide.
Organolithium compounds also usually loose stereochemistry,
however, some RLi compounds retain configuration at low temp-
eratures (-60 oC or below), where the rate of inversion is slow.
Just as with alkyllithiums ( RLi ):
Read the following section only from Chap 16 (16.7)
R-Mg-X + O=C=O R-C-O- MgX+
YOU CANNOT USE
Most Grignard and Alkyllithium compounds are not stable
compounds. You cannot buy them, and you must make
them and use them immediately.
Therefore, I will expect you to show the reaction that forms
these reagents in any synthesis problem.
Do not “pull a Grignard or Alkyllithium out of your hat”
you must show how it is made from a halide.
Also ….. do not forget the hydrolysis step with H3O+
INTERFERING GROUPS !