Lead tetramethyl, decomposition

Ross and Kistiakowsky, /. Am. Chem. Soc., 56, 1112 (1934). Lead tetramethyl decomposition. Pb(CH3)4 = Pb-f-hydrocarbons. = 2536 =1.1. In trimethyl pentane solution = 0.39. Measurements were made with radioactive lead. 

In 1929, F. Paneth and W. Hofeditz detected the presence of free methyl radicals from the thermal decomposition of lead tetramethyl. The apparatus they used is shown in Fig. 32.4. Lead tetramethyl is a volatile liquid. After evacuating the apparatus, a stream of H2 under about 100 Pa pressure is passed over the liquid where it entrains the vapor of Pb(CH3)4 and carries it through the tube. The gases are removed by a high-speed vacuum pump at the other end. The furnace is at position M. After a short period, a lead mirror deposits in the tube at M, f ormed by the decomposition of the Pb(CH3)4. If the furnace is moved upstream to position M, a new mirror forms at M, while the original mirror at M slowly disappears. 

In water, tetraalkyl lead compounds are subject to photolysis and volatilization with the more volatile compounds being lost by evaporation. Degradation proceeds from trialkyl lead to dialkyl lead to inorganic lead. Tetraethyl lead is susceptible to photolytic decomposition in water. Triethyl and trimethyl lead are more water-soluble and therefore more persistent in the aquatic environment than tetraethyl or tetramethyl lead. The degradation of trialkyl lead compounds yields small amounts of dialkyl lead compounds. Removal of tetraalkyl lead compounds from seawater occurs at rates that provide half-lives measurable in days (DeJonghe and Adams 1986). 

Eltenton141 studied the thermal decomposition of a very dilute stream of tetramethyl lead vapour in He (total pressure = 0.4 torr) in a fast flow system (contact time 0.1-0.001 sec) over the temperature range 400-700 °C. The decomposition was essentially complete at 600 °C. A small portion of the effluent from the reaction zone passed directly into the ionization chamber of a mass spectrometer. The reaction was followed by observing the methyl radical concentration. The rate-controlling step observed under these conditions is probably the loss of the first CH3 group by the reaction... 

The experimentally observed substituent effect on the triplet and singlet quantum yields in the complete series of methyl-substituted dioxetanes, as well as the predicted C—C and 0—0 bond strength for the four-membered peroxidic rings , have led to the hypothesis that a more concerted, almost synchronized, decomposition mechanism should lead to high excitation quantum yields (as in the case of tetramethyl-l,2-dioxetane), whereas the biradical pathway presumably leads to low quantum yields (as in the case of the unsubstituted 1,2-dioxetane)" . However, it appears that this criterion of concertedness is difficult to apply generally to structurally dissimilar dioxetane derivatives. 

For primary and secondary bromides base-catalysis is required, while for tertiary bromides silver acetate or silver oxide are more effective cyclization catalysts. For tertiary substrates dehydrobromination leading to allylic hydroperoxides is a serious side reaction when base-catalysis is employed and, thus, silver ion catalysis is essential. Furthermore, the silver salts must be freshly prepared because metallic silver that might be present due to exposure to light causes decomposition of the dioxetane. The tetramethyl-l,2-dioxetane (7) was the first example prepared in this way (Eq. 13). For primary substrates, abstraction of the base-sensitive dioxetanyl hydrogens are probably responsible for the low yields. For secondary substrates, both side reactions might operate. 

J.B. Homer and I.R. Hurle, Shock Tube Studies on the Decomposition of Tetramethyl Lead and the Formation of Lead Oxide Particles, Proc. Roy. Soc. Lond. A327 (1972) 1568. 

In contrast to these examples, decomposition of 3-diazo-2-oxobicyclo[2.2.2]octane results mainly in the Wolff rearrangement product rather than in /S-C-H insertion (3-oxotri-cyclo[2.2.2.0 ]octane, 5%), Similarly, the 1,3-cyclization of the carbene derived from 2,2,5,5-tetramethyl-4-diazo-3-hexanone (5) by thermal, photochemical or copper(I) chloride catalyzed decomposition leading to 7 is far less efficient than a 1,2-methyl shift. ... 

In 1930, Paneth s experiment allowed the existence of free radicals to be demonstrated. A hydrogen flow (see Figure 9.3) drives away the tetramethyl lead vapors. The wall of the tube in which the vapors circulate is heated at a specific point, A. The thermal decomposition of the tetramethyl lead results in the formation of a lead mirror at point A. If the wall is heated at another point, B, mirror A disappears and a new mirror appears at point B. 

The process can be explained by the formation of methyl radicals, CHs , coming from the decomposition of the tetramethyl lead ... 

Among the early works on free radicals, the importance of experiments of F. Paneth and W. Hafeditz is often quoted. They supplied evidence that aliphatic free radicals occur in the decomposition of metallic alkyls, such as dimethyl-mercury (Hg(CH3)2), and tetramethyl- or tetraethyllead (Pb(CH3)4 or Pb(C2H5)4). Paneth saturated a stream of nitrogen with Pb(CH3)4 vapor. The vapors were then heated to 450 °C. Decomposition of Pb(CH3)4 deposited a lead mirror on the heated part of the tube. When the vapors from the decomposition passed over the deposited lead mirror at 100 °C, the mirror slowly disappeared. It was concluded that the following reaction had taken place ... 

---