Organic Chemistry

Section 12.11 Organometallic Compounds 467

Table 12.3 The Electronegativities of Some of the Elementsa

aFrom the scale devised by Linus Pauling

IA IIA IB IIB IIIA IVA VA VIA VIIA

H

2.1

Li

1.0

Na

0.9

K

0.8

Be

1.5

Mg

1.2

B

2.0

Al

1.5

C

2.5

Si

1.8

N

3.0

P

2.1

O

3.5

S

2.5

F

4.0

Cl

3.0

Br

2.8

I

2.5

Ca

1.0

Cu

1.8

Ag

1.4

Cd

1.5

Hg

1.5

Zn

1.7

Ga

1.8

Sn

1.7

Pb

1.6

Ge

2.0

3-D Molecules:

Chlorobenzene;

Phenyllithium

Organolithium compoundsare prepared by adding lithium to an alkyl halide in a

nonpolar solvent such as hexane.

Organomagnesium compounds, frequently called Grignard reagentsafter their

discoverer, are prepared by adding an alkyl halide to magnesium shavings being

stirred in anhydrous diethyl ether or THF. The magnesium is inserted between the

carbon and the halogen.

The solvent (usually diethyl ether or tetrahydrofuran) plays a crucial role in the

formation of a Grignard reagent. Because the magnesium atom of a Grignard reagent is

surrounded by only four electrons, it needs two more pairs of electrons to form an octet.

Solvent molecules provide these electrons by coordinating (supplying electron pairs) to

the metal. Coordination allows the Grignard reagent to dissolve in the solvent and

prevents it from coating the magnesium shavings, which would make them unreactive.

The organometallic compounds form when the metal (Li or Mg) donates its valence

electrons to the partially positively charged carbon of the alkyl halide.

CH 3 CH 2 + 2 Li

δ− δ+

Br CH 3 CH 2 Li + Li+Br−

CH 3 CH 2 + Mg

δ− δ+

δ+ δ−

δ+ δ−

Br CH 3 CH 2 MgBr

cyclohexyl

bromide

cyclohexylmagnesium

bromide

diethyl ether

THF

vinyl

bromide

CH 2 CHBr

vinylmagnesium

bromide

+ Mg CH 2 CHMgBr

Br + Mg MgBr

hexane

1-bromobutane butyllithium

chlorobenzene phenyllithium

CH 3 CH 2 CH 2 CH 2 Br

hexane

+ 2 Li

Cl + 2 Li Li + LiCl

CH 3 CH 2 CH 2 CH 2 Li + LiBr

BRUI12-437_480r3 27-03-2003 11:51 AM Page 467