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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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slide1

GRIGNARD REAGENTS

ORGANOMETALLICS FROM

DIVALENT METALS

slide2

Formation of Grignard Reagents

This reaction was discovered by the Frenchman, Victor Grignard -- Nobel Prize in 1912

slide3

HALIDE REACTIVITIES

RX + Mg R-Mg-X

RELATIVE RATES :

R-I > R-Br > R-Cl

FAST

SLOW

expensive,

good

compromise

less reactive,

but easy to

prepare or buy

not readily

available

slide4

FORMATION OF A GRIGNARD REAGENT

+

ether

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)

THF

slide5

Why might you need different solvents?

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.

slide6

Ethers stabilize the Grignard Complex

Formation of this complex is exothermic; the reaction is sufficiently exothermic to boil the solution without having to add external heat!

slide7

3s

3p

3s

3p

3s

3p

ETHERS COORDINATE INTO EMPTY 3p ORBITALS

Mg:

[Ne]3s2

promotion

.

.

Mg

.

.

+RX

R

X

two bonds

form

R-Mg-X

two ethers

coordinate

slide8

Et

Et

..

O

..

Et

..

:

O

Et

Ethers stabilize the Grignard Complex

ether molecules

coordinate into

empty 3p orbitals

on magnesium

THE GRIGNARD

COMPLEX

R

Mg

X

slide9

R-Mg-X IS A USEFUL SIMPLICATION

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.

slide10

We can view the Grignard as a hybrid reagent.

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.

slide11

d-

d+

-

:

PERCENT IONIC CHARACTER

carbanion

+

covalent

ionic

most reactive

bond

percent ionic

strongest base

C-K 51

C-Na 47

C-Li 43

C-Mg 35

C-Zn 18

C-Cd 15

C-Cu 9

Less ionic character

than organolithium

compounds.

More covalent.

best nucleophile

least reactive

slide12

Grignard Reagents give all the same

reactions as alkyllthium compounds.

R-Mg-X

R-Li

slide13

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:

water

acids

alcohols

amines alkynes

carboxylic acids

atmosperic moisture

Any -O-H, -S-H, or -N-H bonds

are sufficiently acidic to react.

slide14

A DELIBERATE SOURCE OF PROTONS OR DEUTERIUM

R

( or H2O)

This is not a stereospecific synthetic method.

R+S

slide15

A

A

C

C

:

:

B

B

C

C

INVERSIONS OF THE GRIGNARD REAGENT

Grignard reagents lose stereochemistry (racemize) when made

from an enantiomerically pure alkyl halide.

A

*

C

X

B

C

chiral

halide

MgX+

scrambles

stereochemistry

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.

slide16

SYNTHESIS OF ALCOHOLS AND

CARBOXYLIC ACIDS

slide17

Reaction with Carbonyl Compounds

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)

slide18

REACTION WITH CARBON DIOXIDE

O

ether

R-Mg-X + O=C=O R-C-O- MgX+

H3O+

R-COOH

(s)

1) CO2

Mg

ether

2) H3O+

slide19

STARTING HALIDES

YOU CANNOT USE

slide20

NOTE TO STUDENTS

CAUTION

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.

Mg

etc

RX

RMgX

(CH3)2C=O

ether

Also ….. do not forget the hydrolysis step with H3O+

slide21

THESE REACTIONS DON’T WORK …. WHY NOT?

Mg

ether

Mg

ether

Mg

ether

INTERFERING GROUPS !