Ch 14 : Organometallic compounds

Chapter 14: Organometallic Compounds

Organometallic Compounds

Nomenclature:
Organometallic compounds are normally named as substituted metals, e.g. alkyl metal or alkyl metal halide.
Organomagnesium compounds are generally referred to as Grignard reagents.
Examples: CH₃Li = methyllithium, CH₃MgBr = methylmagnesium bromide.

Physical Properties:

Organometallic are usually kept in solution in organic solvents due to their very high reactivity (especially with H₂O, O₂ etc.)

Structure:

Organosodium and organopotassium compounds are essentially ionic compounds.
Organolithiums and organomagnesiums have a σ bond between a C atom and the metal: C-M
These are very polar, covalent bonds due to the electropositive character of the metals.

Look at the electronegativities of the metals Li, Na, K and Mg compared to C and the other atoms we have seen so far (e.g. N, O, F, Cl etc). In C-L and C-Mg which atom is more electronegative ? What about C-O, C-N or C-Cl bonds ?

H
2.1

Li
1.0

Be
1.5

B
2.0

C
2.5

N
3.0

O
3.5

F
4.0

Na
0.9

Mg
1.2

Al
1.5

Si
1.8

P
2.1

S
2.5

Cl
3.0

K
0.8

Ca
1.0

Br
2.8

Partial Periodic Table with Pauling Electronegativities

Reactivity:



methyl chloride	methyllithium	methylmagnesium bromide
CH₃Cl	CH₃Li	CH₃MgBr

The images show the electrostatic potentials for methyl chloride, methyl lithium and methyl magnesium bromide. The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density. In the alkyl halide, the methyl group has lower electron density (blue), and is an electrophile. In methyl lithium, the methyl group has higher electron density (red) and is a nucleophile. In methyl magnesium bromide, the methyl group is less electron rich that methyl lithium. Therefore, organometallic compounds react as electron rich or anionic carbon atoms i.e. as carbanions, which means they will function as either bases or nucleophiles. It is reasonable to think of these organometallic compounds as R^- M⁺ QUESTION Can you think of other types of anions where this "dual" reactivity is also very important ? ANSWER

Basicity:

The following equation represents the loss of a proton from a generic hydrocarbon forming a carbanion:

Organolithium and organomagnesium compounds are strong bases since the negative charge is on carbon.
Simple carbanions are strong bases, (see pK_a's below) since the C is not very electronegative (compared to N or O)
In the presence of weak acids, RLi and RMgX protonate giving the hydrocarbon.

Implications: RLi or RMgX CANNOT be used in the presence of acidic hydrogens such as -OH, -NH or -SH units.
QUESTION What happens if you do ? ANSWER

Compound	Structure	pKa
2-methylpropane		71
ethane		62
methane		60
ethene		45
benzene		43
ammonia		36
ethyne		25
ethanol		16
water		15.7

The table shows the pK_a's of a selection of representative systems. Note that the hydrocarbons are very weak acids, implying that the carbanions will be strong bases.

QUESTION Explain the pK_a order: ethane > ethene > ethyne ANSWER