ORGANOMETAL COMPOUNDS
Some of the popular organometal compounds are:
| IA RLi |
IIA R2Mg |
IB RCu |
IIB R2Zn R2Cd R2Hg |
IIIA R3B R3Al |
IVA Carbon R4Si R4Sn R4Pb |
Beside the organometal compounds the organometalhalides are also important. Examples: RMgX, RHgX, and RAlX2
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The bonding between the organic radical and the metal in organometallic compounds of the alkali and alkaline earth metals is regarded as ionic. The organic radical is the anion known as the carbanion. Example: R‾Li+.
For Groups IV and V elements the metals are closer to carbon in the Periodic Table, so their electronegativities are not so different from that of carbon. Thus the metal-carbon bonds are not so polar.
GRIGNARD REAGENTS
The most important organometal compounds for laboratory preparation are the alkylmagnesiumhalides, known as Grignard reagents. It can be prepared by adding the alkyl halide to an ether (R−O−R') solution of magnesium turnings.
RX + Mg → R-Mg-X ; yield = ca 90%
However the reaction is slow with the chlorides so bromides are more often used. The reaction is exothermic and the ether solvent might evaporate, so cooling is required. Grignard reagents are sensitive (reactive) towards water moisture, protonated solvents, halogens, and oxygen. So the preparation must be conducted in an inert environment (meaning a nitrogen-filled box). The ether, though not a protonated solvent, has an electronegative oxygen and can solvate the organometalhalides yet not react with them.
The other important laboratory organometal compound is RLi prepared in a similar manner.
The other organometal compounds are generally prepared from the Grignard reagent or RLi by ion exchange. Example:
R-Mg-Br + CdBr2 → R2Cd + 2 MgBr2
REACTIONS of GRIGNARD REAGENTS
The carbanion, being negatively charged, is a nucleophile. It will seek out electron deficient centers in the hydrocarbon for an attack.
WITH CARBONYLS: NUCLEOPHILIC SUBSITUTION
The entity C=O in hydrocarbon is known as a carbonyl. The oxygen is more electronegative than carbon so there will be an "unequal" sharing of the electrons in the bond between them; in favour of oxygen. This gave rise to a slight momentary positive charge at the carbon centre. This is enough to attract the attention of nucleophiles.
So for carbonyl with a replaceble functional group attached to the carbon the carbanion will execute a nucleophilic substitution. Examples of such carbonyl compounds are acid halides R−C(=O)Cl or esters R−C(=O)OR� ; represented here as R−C(=O)Z, where Z = R, Cl, and OR'.
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 R'−MgBr |
 ether |
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MgBrZ |
The ketone can futher react with a Grignard reagent. So if an excess of Grignard reagent the final product can be hydrolysed to give the alcohol. Experiment
WITH CYCLIC ETHERS
Of course other oxygen groups attached to carbon are agents to induce positive carbon centers as seen in the previous lesson. However alcohol are protonated solvents, and will execute acid-base reactions with the carbanion before the carbanion can attack the less positive (as compared with proton) carbon center.
This leaves us with ether, a compound with oxygen atom but does not have a displaceable proton. However the oxygen is attached to two carbons and the effectiveness to induce positive carbon centers are greatly reduced. However for cyclic ethers under steric stress, like ethylene oxide, the slightly positive carbon center is sufficient for the epoxide to find an excuse to escape the stress. Experiment
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+ RMgBr |
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MgBr |
H2O |
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WITH CARBON DIOXIDE
Grignard reagents can also react with carbon dioxide to give a carboxylate salt, which upon hydrolysis in acid will give the carboxylic acid.
RMgBr + CO2 � R−COO ‾ +MgBr
R−COO ‾ +MgBr + H+ / H2O � RCOOH + +MgBr
OTHER ORGANOMETALLIC REAGENTS
Alkyllithium
Terminal Alkyne Sodium
CARBANION from α−proton of carbonyl
When aldehydes with an α−proton is stirred with an aqueous sodium hydroxide solution the α−proton is extracted to give a carbanion.
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