Organometalic compound

Organometallic Compounds: -

Compounds that contain a metal-carbon bond, R-M, are known as Organometallic compounds. Organometallic compounds of Li, Mg (Grignard reagents) are the most important organic reagents. other metals have been utilized, for example Cu, B, Na, and Zn.

Importance of Organometallic compounds It provide a source of nucleophilic carbon atoms which can react with electrophilic carbon to form a new carbon-carbon bond.  This is very important for the synthesis of complex molecules from simple starting materials. R-M  =  R^-M⁺ The most important reactions is this chapter are the reactions of Grignard reagents, RMgX, with the carbonyl groups in aldehydes, ketones and esters to give alcohols. However, we will also look at some useful reactions involving Cu, B.

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 = methyl lithium, CH3MgBr = methyl magnesium 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.

Basicity: Organolithium and organomagnesium compounds are strong bases since the negative charge is on carbon. Simple carbanions are strong bases, 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.

Preparations and Reaction of Organometallic Reagents

I.                     Preparation of Organomagnesium Reagents

    Organomagnesiums are formed by the reaction of alkyl halides with magnesium metal.    Typical the reaction is carried in anhydrous diethyl ether or tetrahydrofuran. The alkyl group can be primary, secondary or tertiary. Halide reactivity: I > Br > Cl.                                        
R can be alkyl, vinyl or aryl. The reaction is conducted by adding the organic halide to a
suspension of magnesium in an etherial solvent, which provides ligands required to stabilize the organomagnesium compound. 

Reactions of Organomagnesium Reagents

Reaction with Aldehydes and Ketones

Organomagnesium or Grignard reagents react with the carbonyl group, C=O, in aldehydes or ketones to give alcohols. It react with formaldehyde gives primary alcohol, with other aldehydes gives secondary alcohols and ketones gives tertiary alcohols. Reaction usually carried in ether Et2O or THF followed by acid hydrolysis. Reaction type:  Nucleophilic Addition.