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Organometallic Compounds

Reactions of Organometallic Compounds

Carbanions are very strong bases. They are therefore protonated even by weak bases, such as water and alcohols. In this way, alkyl halides are converted into alkanes. The organometallic compound is, first of all, synthesized through treatment of an alkyl halide with a metal, such as lithium or magnesium. Hydrolysis then yields the corresponding alkane.

Fig.1
Conversion of alkyl halides to alkanes.

Due to their nucleophilic alkyl groups, organometallic compounds may alkylate electrophiles, such as carbonyl compounds, alkyl halides, or epoxides.

Fig.2
Reaction of an organolithium compound with an epoxide.

A nucleophilic oxirane ring opening that yields butanol is, for instance, detectable in the reaction of ethylmagnesium bromide with ethylene oxide.

Fig.3
Reaction of ethylmagnesium bromide with ethylene oxide.

The organometallic compounds of other metals may be prepared through transmetallation. Organometallic compounds that are barely obtainable through any other method may be obtained in this way. The conversion of an easily accessible organometallic compound, such as a Grignard compound, with another metal halide in transmetallation leads to the exchange of the alkyl substituents for the halides. The reaction necessitates that the halide's metal, which should be converted into the organometallic compound, displays a higher electronegativity than the metal of the organometallic compound applied. The alkyl group of a Grignard compound (EN of Mg is 1.2) may be transferred to cadmium (EN of Cd is 1.5), though this is not applicable to sodium (EN of Na is 1.0), for instance. If there is a perceivable trivial difference in electronegativity, the reaction does not progress properly, as it is an equilibrium reaction.

Fig.4
Transmetallation between ethylmagnesium chloride and cadmium dichloride.

Transmetallation may also be achieved through treatment of an organometallic compound that contains no halide, such as is the case with R2Hg, in connection with another metal (not a salt!). This reaction requires that the metal, which is to be converted into the organometallic compound, has a lower electronegativity than the metal of the previously applied organometallic compound. These two transmetallation methods complement each other. As a result, virtually all forms of organometallic compounds may be prepared through the application of one of these transmetallation methods. This also applies to solid sodium and potassium alkyls which are obtained by the halide-free transmetallation method.

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