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Unformatted text preview: Alcohols 1 Copyright, Arizona State University Copyright, Arizona State University Organometallic Reagents Carbon-Carbon Bonds 1 Carbon-Metal Bonds We previously looked at the following two disconnections A B Br Br A B Na acetylide anion Na B is clearly better since it generates an ACETYLIDE synthon/reagent with more stable non-bonding electrons on the sp hybridized carbon The acetylide anion is a simple example of an organometallic reagent, i.e. an organic structure with a bond to a metal In this case the carbon metal bond is essentially pure ionic HC C Na electrons stabilized on sp carbon ionic bond acetylide anion Q. Suppose we HAD to do disconnection A above, how can we make a reasonable carbon synthon such as that in A if we don't have an sp A.O. to stabilize the electrons? An ionic blond to a sodium cation will not work if the hybridization has higher p character than sp. A. We must "temporarily" stabilize the non-bonding electrons on the carbon another way, we can out them into a weak bond to a metal atom Compare among different metals for sp3 hybridized carbon sp 3 sp 3 H 2 C H 3 C H 2 C H 3 C HgX very high energy on sp 2 carbon lower energy, weak covalent bond Na mainly ionic bond organosodium, too reactive, can't be formed H 2 C H 3 C lower energy, partial electron sharing Li Li smaller, more covalent character organolithium, reactive, but can be formed Hg less electropositive, more covalent character organomercury, least reactive, very useful! weak covalent bonds with a lot of ionic character mean that the electrons are still relatively high in energy and quite reactive both organolithiums (alkyl and aryl lithiums) and lithium compounds and organomercury compounds Grignard reagents) can be made for carbons with ANY hybridization, including sp3 and and are thus very useful alkyl and aryl lithiums are more ionic and tend to be very strongly basic, but they are still nucleophilic and can be used to make carbon-carbon bonds organomercury compounds in the form of Grignard reagents tend to be the most useful nucleophiles ORGANOMETALLIC reagents are useful carbon anion synthons/reagents Alcohols 2 Copyright, Arizona State University Organolithium reagents R X + 2 Li (metal) R Li + LiX Br + 2 Li + LiBr Li phenyl lithium Grignard reagents R X + Mg (metal) R MgX Br Mg . THF BrMg phenyl magnesium bromide ether or THF the reaction is between an akyl or aryl halide (often bromide) with metallic magnesium in an ether solvent...
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- Spring '12