Lead, tetramethyl

Tetramethyl-lead.—Calorimetric measurements give S fg, 298.15 K) = (420.4 0.84) J K mol , whereas calculations assuming free rotation give a value 417 8 J mol this discrepancy would be made greater by 

Methylbenzenes.—The barriers to internal rotation in some methylbenzenes are considered as the final examples, since in recent years results from a 

For toluene/ accurate experimental data, and calculations using the vapour phase value for the lowest fundamental, showed that rotation of the methyl group is essentially free. Previous results had indicated an upper bound of 6.7 kJ mol for this barrier and similarly of 4.2 and 3.8 kJ mol for the barriers in m- and p-xylene, respectively. Recent determinations by microwave spectroscopy have given for the 6-fold barrier for toluene the value (58.32 0.42)Jmol with almost identical values for p-fluoro- and p-chloro-toluene. For n-fluorotoluene the same method gives a value of 0.55 kJ mol for the 3-fold barrier, with a comparable but equally small 6-fold component. 

It is relevant that the microwave spectrum of o-fluorotoluene leads to a value of 2.72 kJ mol for the 3-fold barrier. Another recent determination is that by n.m.r. spectroscopy of a barrier of 5 kJ mol for methyl rotation in u-chlorotoluene dissolved in the nematic phase of 4-amino-4-methoxybenzylidene-a-methylcinnamic acid n-propyl ester. The position of minimal energy was inferred to be that in which the chlorine is staggered to the methyl group. This conclusion has also been reached from semi-empirical MO calculations by the extended Hiickel theory, which give a barrier height of 7.41 kJ mol.  

Entropy measurements have been made for 1,2,4- and 1,2,3-trimethyl-benzene, and from these barrier heights of 5.86 and 13.39 kJ mol for the ortho and central methyl groups were deduced, again on the assumption that their rotations are independent. On the same assumption, a neutronscattering peak for 1,2,3-trimethylbenzene at 180 cm corresponds to a barrier of (9.33 0.71)kJmol for the ortho-m ihy groups and an additional broad band with a maximum near 100 cm was attributed to a second set of torsional levels arising from the central methyl group.  

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 apparent first-order rate coefficient is 1.5x 1010 exp(—28,200/RT ) sec-1. This expression has undoubtedly been obtained for a pressure-dependent region. If, as an extreme case, it is assumed that the unimolecular process occurred in the second-order region and if approximately one half of the classical degree of vibrational freedom are active, an upper limit of kx — 1.5 x 1015 exp(—46,000/Rr) sec-1 is obtained. The mean Pb-CH3 bond dissociation energy in tetramethyl lead19,142 is 37.6 kcal.mole-1. Dx should therefore be about 40 kcal.mole-1. 

The rate of decomposition of liquid tetraethyl lead and of solutions of tetraethyl lead in inert solvents, is independent of surface-to-volume ratio but is catalyzed by the finely divided lead formed during the reaction143-146. The 

The autocatalytic effect of lead on the rate of reactions (3) and (4) was demonstrated146 by comparing the pyrolysis of mixtures of tetramethyl lead and hexaethyldiplumbane and of hexaethyldiplumbane and diethyl lead in the presence and absence of lead formed by reaction (4). 

Up to 30 % conversion the rate of decomposition determined on the basis of the percent metallic lead precipitated is zero-order. The rate coefficient for the decomposition in solution is 2.43 x 1012 exp(—35,200/I r) mole.sec-1. If reaction (2) is fast compared with reaction (1), kx = 1.22 x 1012 exp(—35,200/RT) mole. sec-1. The rate of decomposition of hexaethyldiplumbane was also studied. Based on spectroscopic observation of the concentration of the reactants, k3 — 5.89 x 1010 exp(—28,500/RT ) mole.sec-1. On the basis of formation of metallic lead, the rate coefficient was 2.44 xlO10 exp(—28,000/HT) mole.sec-1. Therefore, under the conditions used k3 kx and reaction (3) is followed rapidly by reaction (4). 

---

TMED, (CH3)2NCH2CH2N(CH3)2. B.p. 122 C a hygroscopic base which forms a hydrocarbon-soluble stable chelate with lithium ions and promotes enhanced reactivity of compounds of lithium, e.g. LiAlH4, UC4H9, due to enhanced kinetic basicity of the chelate. Used in polymerization catalysts, tetramethyl lead, TML 5 lead tetramethyl. 

It is useful, nevertheless, to bring to mind their composition and their means of action (Goodacre, 1958). Several components of the same family can in reality be utilized tetraethyl lead, Pb ( 2115)4 or TEL, tetramethyl lead, Pb (CHg) or TML, mixtures of these products or yet mixed chemical components including various combinations of the groups C2Hg and CH3 Pb ( 2115)2 ( 113)2, Pb ( 2115)3 113, Pb ( 2Hg) ( 113)3. 

Two derivatives are used to ensure constant lead content throughout the gasoline boiling range tetraethyl- and tetramethyl lead and their mixtures in variabie proportions. 

Tetraethyl lead Tetraethylene glycol Tetraethyl pyrophosphate Tetrahydrofuran Tetramethyl lead (as Pb)... 

The major use of methyl chloride is to produce silicon polymers. Other uses include the synthesis of tetramethyl lead as a gasoline octane booster, a methylating agent in methyl cellulose production, a solvent, and a refrigerant. 

Other uses of ethylene dichloride include its formulation with tetraethyl and tetramethyl lead solutions as a lead scavenger, as a degreasing agent, and as an intermediate in the synthesis of many ethylene derivatives. 

Organic Lead. Following a single exposure to vapors of tetraalkyl lead compounds (approximately 1 mg/m3 breathed through a mouthpiece, 10-40 breaths of approximately 1 L volume) in four male subjects, 37% and 51% of inhaled tetraethyl and tetramethyl lead, respectively, were initially found in the respiratory tract, but a considerable percentage of these volatile compounds was lost through exhalation (Heard et al. 1979). Approximately 60-80% of the deposited tetraalkyl lead was absorbed by the lungs. 

In a case report of a 22-year-old male exposed to tetramethyl lead, absorption was evident because of elevated urinary lead levels for 4 days after exposure (Gething 1975). 

Relatively few human studies that address the metabolism of alkyl lead compounds were found in the available literature. The dealkylation, mediated by cytochrome P-450, of alkyl lead compounds is thought to occur in the rat, mouse, and rabbit. This step converts tetraethyl and tetramethyl lead to the... 

Organic Lead. Urinary lead levels were elevated for 4 days in a man accidentally exposed to an unknown quantity of tetramethyl lead (Gething 1975). Exhalation of the tetraalkyl lead compounds following inhalation exposure is a major route of elimination in humans. At 48 hours postexposure, 40% and 20% of the initially inhaled tetramethyl and tetraethyl lead doses, respectively, were exhaled with low urinary excretion (Heard et al. 1979). 

Inhaled tetraethyl and tetramethyl lead vapors behave as gases in the respiratory tract and, as a result, their pattern and extent of deposition and absorption differ from that of inhaled inorganic lead particles (EPA 1994a Overton et al. 1987 Overton and Miller 1988). These differences result in a higher fractional absorption of inhaled tetraethyl and tetramethyl lead (Heard et al. 1979). 

Tetraethyl and tetramethyl lead under oxidative dealkylation metabolize to the highly neurotoxic metabolites, triethyl and trimethyl lead, respectively. In the liver, the reaction is catalyzed by a cytochrome P-450 dependent monoxygenase system (Kimmel et al. 1977). Complete oxidation of alkyl lead to inorganic lead also occurs (Bolanowska 1968). 

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). 

---