Organolead chemistry is the scientific study of the synthesis and properties of organolead compounds, which are organometallic compounds containing a chemical bond between carbon and lead. The first organolead compound was hexaethyldilead (Pb2(C2H5)6), first synthesized in 1858.[1] Sharing the same group with carbon, lead is tetravalent.
Going down the carbon group the C–X (X = C, Si, Ge, Sn, Pb) bond becomes weaker and the bond length larger. The C–Pb bond in tetramethyllead is 222 pm long with a dissociation energy of 49 kcal/mol (204 kJ/mol). For comparison the C–Sn bond in tetramethyltin is 214 pm long with dissociation energy 71 kcal/mol (297 kJ/mol). The dominance of Pb(IV) in organolead chemistry is remarkable because inorganic lead compounds tend to have Pb(II) centers. The reason is that with inorganic lead compounds elements such as nitrogen, oxygen and the halides have a much higher electronegativity than lead itself and the partial positive charge on lead then leads to a stronger contraction of the 6s orbital than the 6p orbital making the 6s orbital inert; this is called the inert-pair effect.[2]
The reaction is accelerated in the presence of dichloroacetic acid, which forms the lead(IV) dichloroacetate as an intermediate.
Other organolead compounds are the halides of the type RnPbX(4-n), sulfinates (RnPb(OSOR)(4−n)) and hydroxides (RnPb(OH)(4−n)). Typical reactions are:[4]
R 4Pb + HCl → R3PbCl + RH
R 4Pb + SO2 → R3PbO(SO)R
R3PbCl + 1/2Ag2O (aq) → R3PbOH + AgCl
R2PbCl2 + 2 OH− → R 2Pb(OH) 2 + 2 Cl−
R 2Pb(OH) 2 compounds are amphoteric. At pH lower than 8 they form R2Pb2+ ions and with pH higher than 10, R2Pb(OH)3− ions.
The reaction requires the presence of a large excess of a coordinating amine such as pyridine which presumably binds to lead in the course of the reaction. The reaction is insensitive to radical scavengers and therefore a free radical mechanism can be ruled out. The reaction mechanism is likely to involve nucleophilic displacement of an acetate group by the phenolic group to a diorganolead intermediate which in some related reactions can be isolated. The second step is then akin to a Claisen rearrangement except that the reaction depends on the electrophilicity (hence the ortho preference) of the phenol.
The carbanion forms by proton abstraction of the acidic α-proton by pyridine (now serving a double role) akin to the Knoevenagel condensation. This intermediate displaces an acetate ligand to a diorganolead compound and again these intermediates can be isolated with suitable reactants as unstable intermediates. The second step is reductive elimination with formation of a new C–C bond and lead(II) acetate.
Abadin, Henry G.; Pohl, Hana R. (2010). "5. Alkyllead Compounds and Their Environmental Toxicology". Organometallics in Environment and Toxicology. Metal Ions in Life Sciences. pp.153–164. doi:10.1039/9781849730822-00153. ISBN978-1-84755-177-1.
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