Chemistry. COORDINATION COMPOUNDS AND ORGANOMETALLICS - 1 Session.
GRIGNARD REAGENTS ORGANOMETALLICS FROM DIVALENT METALS DIVALENT METALS.
-
Upload
katherine-gallagher -
Category
Documents
-
view
244 -
download
0
Transcript of GRIGNARD REAGENTS ORGANOMETALLICS FROM DIVALENT METALS DIVALENT METALS.
GRIGNARD REAGENTSGRIGNARD REAGENTS
ORGANOMETALLICS FROMORGANOMETALLICS FROM DIVALENT METALSDIVALENT METALS
+ Mgether
X
R = 1°, 2°, 3°, aryl
R X R Mg
This reaction was discovered by the Frenchman, Victor Grignard -- Nobel Prize in 1912
Formation of Grignard ReagentsFormation of Grignard Reagents
HALIDE REACTIVITIESHALIDE REACTIVITIES
R-I > R-Br > R-Cl
expensive,not readilyavailable
RX + Mg R-Mg-X
less reactive,but easy toprepare or buy
goodcompromise
RELATIVE RATES :
FAST SLOW
FORMATION OF A GRIGNARD REAGENTFORMATION OF A GRIGNARD REAGENT
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)
CH3 C
CH3
H
Cl
Mg CH3 C
CH3
H
MgClether
+
CH3CH2 O CH2CH3
O O
O
THF
Consider two different leaving groups:
The second reaction is too slow at 35 °C,
Why might you need different Why might you need different solvents?solvents?
Bromine is more reactive than chlorine.
therefore we use a higher boiling solvent.
Br + Mgether
35 °CMgBr
Cl + Mg65 °C
MgClTHF
R
Mg
X
CH3 CH2
O
CH2
CH3CH2
O
CH2CH3 CH3
Formation of this complex is exothermic; the reaction is sufficiently exothermic to boil the solution without having to add external heat!
Ethers stabilize the Grignard ComplexEthers stabilize the Grignard Complex
Mg: [Ne]3s2
3s 3p
3s 3p
Mg. .
3s 3p
R-Mg-X
R X. .+RX
two etherscoordinate
ETHERS COORDINATE INTO EMPTY 3p ORBITALSETHERS COORDINATE INTO EMPTY 3p ORBITALS
two bondsform
promotion
R Mg X
OEtEt ..
..
O
Et
Et ..:
ether moleculescoordinate intoempty 3p orbitalson magnesium
THE GRIGNARD THE GRIGNARD COMPLEXCOMPLEX
Ethers stabilize the Grignard ComplexEthers stabilize the Grignard Complex
The complete structure of the Grignard reagent is quite complex. It is probably an equilibrium mixture of the type:
2 R-MgX R2Mg + MgX2
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.
R-Mg-X IS A USEFUL SIMPLICATIONR-Mg-X IS A USEFUL SIMPLICATION
….. and the actual reactive species may be a complex dimer.
CHEMISTS HAVE BEEN ARGUING ABOUT THE STRUCTURE OF THE GRIGNARD REAGENT FOR YEARS AND STILL CANNOT AGREE
We can view the Grignard as a hybrid reagent.
R MgX R: MgX
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.
R MgX
C Mg C Mg+:-
covalent ionic
PERCENT IONIC CHARACTERPERCENT IONIC CHARACTERcarbanion
- +
C-K 51C-Na 47C-Li 43C-Mg 35C-Zn 18C-Cd 15C-Cu 9
strongest base
best nucleophile
percent ionicbondmost reactive
least reactive
Less ionic characterthan organolithiumcompounds.More covalent.
Grignard reagents are strong bases and react readily with any slightly acidic hydrogen ( compare R-Li).
R MgX + R H + MgX+
R Li + R H + Li+
H+
H+
Any source of H+ will bring about this reaction:
wateracidsalcohols
amines alkynescarboxylic acidsatmosperic moisture
Any -O-H, -S-H, or -N-H bonds are sufficiently acidic to react.
CH3 CH2 CH CH3
Br
CH3 CH2 CH CH3
Mg
CH3 CH2 CH CH3
D
Mg
ether
Br
D2O
This is not a stereospecific synthetic method.
A DELIBERATE SOURCE OF PROTONS OR DEUTERIUMA DELIBERATE SOURCE OF PROTONS OR DEUTERIUM
( or H2O)
R
R+S
C
A
BC
:
Grignard reagents lose stereochemistry (racemize) when madefrom an enantiomerically pure alkyl halide.
INVERSIONS OF THE GRIGNARD REAGENTINVERSIONS OF THE GRIGNARD REAGENT
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.
C
A
BC
: MgX+
A
CBC
X*
chiral halide
scramblesstereochemistry
+..
:.. MgX+
R MgX R C
R
R
O
_
H2O
R C OH
R
R
O
CR R
ether
Reaction with Carbonyl Reaction with Carbonyl CompoundsCompounds
Just as with alkyllithiums ( RLi ):
• formaldehyde primary alcohols• other aldehydes secondary alcohols• ketones tertiary alcohols• carbon dioxide carboxylic acids
Read the following section only from Chap 16 (16.7)
REACTION WITH CARBON DIOXIDEREACTION WITH CARBON DIOXIDE
R-Mg-X + O=C=O R-C-O- MgX+
O
R-COOH
H3O+
ether
CH2 Br CH2 MgBr CH2 COOHMg
1) CO2
2) H3O+ether
(s)
NOTE TO STUDENTS
CAUTIONCAUTIONMost Grignard and Alkyllithium compounds are not stablecompounds. 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.
RX RMgXMg
ether (CH3)2C=O
Also ….. do not forget the hydrolysis step with H3O+
etc