Tetramethyllead reactions

Most of the information on this subject refers to solvent effects on the substitution of tetraalkyltins by mercuric chloride (see Chapter 6, Section 1.5, p. 70) and to the iododemetallation of tetraalkyltins and tetraalkylleads (see Chapter 8, Section 6, p. 173). Data on solvent effects for a number of reactions are summarised in Table 12. There is only a partial correlation of the rate coefficients with any of the usual parameters51 of solvent polarity in particular, the solvents acetonitrile and acetone increase the value of the various rate coefficients to a much greater extent than would be predicted on the basis of solvent dielectric constant (e), or Z values, or 2sT values, etc.51. This may be seen especially for the iododemetallation of tetramethyllead, reaction (6) (R = Me)... 

An exchange reaction between bis(trifluorometbyl)mercury and tetrame-thylleud gives trimethyl(mfluoromethyl)plumbane [23] (equation 17) This plum-bane can also be prepared via the reaction of tetramethyllead with tnfluoromethyl radicals produced in a radio-frequency discharge of C2F [24]... 

On the other hand, the presence of a short-lived free radical was confirmed first from an elegant experiment in 1929 by Paneth and Hofeditz [44], although the existence of paramagnetic species was pointed out in the middle of the 19th century by Faraday [45]. When tetramethyllead was thermolyzed, a methyl radical was postulated to be formed as the reaction intermediate (Eq. 2)... 

The classical large-scale method for preparation of tetraethyllead and tetramethyllead is by reaction of alkyl halide with sodium/lead alloy (composition Pb Na 1/1 )38. The product is isolated by steam distillation and yields are high ... 

The initial pathway is the same for tetramethyllead. However, under prolonged electrolysis additional reactions are observed with hexamethyldilead, namely ... 

Further reaction of Ph3P+ and Hg contributes the second ET for the overall two-electron oxidation. This was studied in detail for oxidations of tetraphenyllead, tetraethyllead and tetramethyllead at mercury electrodes in dichloromethane. The rationale of the mechanisms proposed above is based on the following observations122. 

Wei and collaborators180 studied the long-range deuterium isotope effects on the proton chemical shifts of tetramethyllead and chlorotrimethylplumbane. The perdeuterated analogues of these organolead compounds were generated in situ by reaction of equimolar amounts of (CD3)3SnCl and (CH3)4Pb in methanol-d, in equation 118. 

This experiment showed that some volatile component was formed in the thermal decomposition of tetramethyllead and that this compound consumed a cold lead mirror with formation of a volatile product. If, instead, a zinc mirror was first deposited and allowed to be consumed by the volatile product from decomposition of tetramethyllead, dimethylzinc could be identified as the product. Paneth concluded that free methyl radical was formed in the thermal reaction and could determine its half-life to be 0.006 seconds under the reaction conditions employed. Also, free ethyl radicals could be formed in... 

Tetramethyllead (TML). (CH3)4Pb mw 267.33 colorl, dense, strongly refractive liq with a pleasantly sweet odor v toxic f pt -27.5° bp 110° at 10mm d 1.995g/cc. SI sol in benz, ethanol, petr eth insol in w. Prepn is by means of the Grignard reaction of Pb chloride with an absol ethereal soln of methyl Mg chloride. The compd is stable and may be stored for years in... 

Fractional distillation and crystallization were the separation methods used in earlier studies of redistribution reactions. Quantitative equilibrium data were obtained first by Calingaert over a quarter of a century ago from an equilibrated mixture of tetraethyllead and tetramethyllead. Fractional distillation showed that, in addition to the starting materials, all three of the expected redistribution products were found in amounts corresponding to random interchange of methyl and ethyl groups on lead. 

Note added in proof In May 1975 The and Cavell have reported (86b) the successful synthesis of dimethyltris(trifluoromethyl) and trimethylbis(trifluoromethyl) phospho-rane from the reaction of tetramethyllead with suitable fluorophosphorane precursors. The compounds are stable white solids of trigonal bipyramidal structure. 

As was mentioned in Section 1, the first synthesis of an organolead compound was reported by Lowig 210>211), who synthesized tetraethyllead by the reaction of a sodium-lead alloy with ethyl iodide. Some 35 years later, Polis 247,248) prepared the first aryl lead derivative by the reaction of bromobenzene with a sodium-lead alloy. Since 1923, the sodium-lead alloy-ethyl chloride reaction has been used for the commercial production of tetraethyllead. A similar reaction has also been used for the commercial production of tetramethyllead since 1960. The sodium-lead alloy-alkyl chloride reaction is discussed in Section 6. 

These reactions of lead metal and lead alloys with alkyl esters are conducted at elevated temperatures (usually above 80 °C) and at elevated pressure (autogenous pressure of RX), and in the presence of a suitable catalyst, such as ethers, amines, iodides, dependent on the particular system involved. Despite the large number of systems which have been investigated, none has been found to be as economical for the commercial production of tetramethyllead and tetraethyllead as the sodium-lead alloy reaction, with the possible exception of the electrolytic process developed by Nalco Chemical Company for tetramethyllead. Electrolytic processes are discussed in Section 6. 

The mixed methyl-ethyl tetraalkyllead compounds have become quite important commercially in the past several years because of their excellent antiknock effects and volatilities (see Extent of Antiknock Effect). Sometimes tetramethyllead and tetraethyllead are simply mixed together in motor fuel, but usually the compounds are subjected to "redistribution of the methyl and ethyl groups. The redistribution reaction involves the exchange of the organic groups between the compounds, yielding practically a random equilibrium production of all the possible products in a statistical distribution, as shown in the equation ... 

Methylcopper and ethylcopper have been prepared by the reaction of copper(II) nitrate with tetramethyllead or tetraethyllead, respectively, in alcohol at low temperatures 13-15, 76, 80, 125). Copper sulfate can also be used, but not the chloride. Phenylcopper cannot be obtained by a similar reaction with tetraphenyllead 73, 125). From a study of the reaction, Bawn and Whitby 14, 15) proposed the sequence of reactions (2)-(4) ... 

Copper compounds are catalysts for the Michael addition reaction (249), olefin dimerizations (245, 248), the polymerization of propylene sulfide (142), and the preparation of straight-chain poly phenol ethers by oxidation of 2,6-dimethylphenol in the presence of ethyl- or phenyl-copper (209a). Pentafluorophenylcopper tetramer is an intriguing catalyst for the rearrangement of highly strained polycyclic molecules (116). The copper compound promotes the cleavage of different bonds in 1,2,2-tri-methylbicyclo[1.1.0]butane compared to ruthenium or rhodium complexes. Methylcopper also catalyzes the decomposition of tetramethyllead in alcohol solution (78, 81). 

Step (3), attack of methyl radicals on halogen, is exothermic for all four halogens, and for chlorine, bromine, and iodine it has very nearly the same A/f. For these reactions. Fact could be very small, and does indeed seem to be so probably only a fraction of a kcal. Even iodine has been found to react readily with methyl radicals generated in another way, e.g., by the heating of tetramethyllead. In fact, iodine is sometimes employed as a free-radical trap or scavenger in the study of reaction mechanisms. The third step, then, cannot be the cause of the observed relative reactivities. 

The compound R2PbI2 can be converted into PbR4 by an organolithium or Grignard reagent, so that, for instance, tetramethyllead is obtained when methyllithium or methylmagnesium iodide is added to a boiling solution of lead(n) chloride in ether which contains also methyl iodide the reaction may be represented as ... 

The question, however, remained Is there direct evidence for the types of simple, transient (very short-lived) free radicals postulated as reaction intermediates and present in very minute ( steady-state ) concentrations To this question, Paneth and his coworker Wilhelm Hofeditz provided, in 1929, a graphic answer. The weak carbon-lead (Pb-C) single bond in tetramethyllead allows ready dissociation at elevated temperatures. This was a factor in the antiknock properties of tetraethyllead, introduced as... 

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