Lead-sodium alloy, reactions

Tetraethyllead can be manufactured by the reaction of ethyl chloride with lead-sodium alloy (see Lead compounds). 

Hence the reaction of alkyl halides with lead-sodium alloy, discovered by Lowig" " opened the way for the industrial production of tetraethyllead. A total of 166,000 tons (1/6 of the US lead production) was used to produce tetraethyllead i . ... 

Sodium-Lead Alloy. Lead-sodium alloy sodium -lead Drynap Dri -Na. Usually contains a minimum of 9.5% active sodium. For tetraethyllead manufacture the sodium -lead alloy is produced in large quantities by making a melt of 90 parts of lead with 10.5 parts of sodium (w/w). The reaction is strongly exothermic and starts at 225. Prepn of the alloy on a laboratory scale Soroos, Ind. Eng. Chem.. Anal Ed 11, 657 (1939). 

Organic halides also afford tetraalkyl- and teraaryl-leads on reaction with a lead-sodium alloy, sometimes with catalysis by a base such as pyridine. 

Sodium metal is used chiefly to produce tetraethyl lead [Pb(C,H,)J or tetramethyl lead [Pb(CHj)J by means of a reaction between lead-sodium alloy (e.g., 90Pb-10Na) and monochloroethane (i.e., ethyl chloride, C,H,C1) or monochloromethane (i.e., methyl chloride, CHjCl) forming NaCl. Actually, lead tetraethyl increases the antiknock rating of gasoline. 

From Alloys and Methyl Halides. One of the commercial procedures now in use for the manufacture of Pb(CH3)4 is based on the reaction of lead-sodium alloy with methyl halide. Actually, the first paper on Pb(CH3)4 already mentioned the preparation from CH3I and a lead-sodium (5 1) alloy [62, 63]. 

Production of Pb(CH3)4 by reaction of PbNa or alkaline earth-metal-lead alloys with CH3X and CH3MgX (X = halide) in ether is also possible under pressure and at elevated temperatures [132, 133]. A procedure to synthesize Pb(CH3)4 or other tetraalkyllead compounds in gasoline by reaction of lead-sodium alloy with CH3CI or alkyl halides, which are introduced or which are eventually obtained by in situ halogenation of lower alkanes in the gasoline, is described in [21]. 

Elemental lead, produced by the reaction of PbNa and C2H5CI, is methylated with dimethyl sulfate in the presence of Pbl2 as catalyst, or with CH3I in the presence of iodine as catalyst with a yield of 65 to 70% Pb(CH3)4 [67]. For methylation of elemental lead, obtained from the industrial reaction of lead-sodium alloy and alkyl halide, see Subsection From Alloys and Methyl Halides , p. 58. 

PbNa is the most reactive lead-sodium alloy for the reaction with C2H5CI [27, 33, 68, 379] and is generally employed in the industrial manufacturing process for Pb(C2H5)4. 

The rate of the reaction of PbNa with C2H5CI is greatly affected by the surface area and the gross structure of the alloy, and therefore by the conditions under which the alloy is made. Consequently, numerous patents describe methods of preparing efficient lead-sodium alloys [29, 54, 64, 74, 75, 102, 104, 114, 115, 117, 124, 150, 175, 176, 221 to 223, 239, 240, 242, 244, 245, 251, 287, 344, 411] see also [126, 731]. Correlations between Pb(C2H5>4 yield and solidification time of molten PbNa [232, 259] and surface type of PbNa [239, 251] are given. 

Good yields of Pb(C2Hs)4 are obtained with alloys that have a high Na content, such as PbNa4, when they are allowed to react with C2H5Br or C2H5I at 20 to 35 C in the presence of water and pyridine or a secondary or tertiary amine as catalyst [23, 26]. Other reactions of lead-sodium alloys and ethyl halides in the presence of water have been described in early patents [14, 21, 22, 32, 33, 39, 41 to 43, 190] and also in the presence of alcohol [22, 42, 43, 51, 52], ether [52], amines [21], pyridine [39, 190], other protic compounds [30, 43, 130, 156, 157], or with a mixture of such additives [21, 22, 39, 42, 43, 51]. This so-called hydrous reaction was used for a short time in the 1920 s for the commercial manufacture of Pb(C2Hs)4 [583]. 

The reaction of lead-sodium alloy with C2H5CI has also been proposed to serve specifically as a source for powdered lead that can be shaped by cold working under pressure [434]. For methods of recovery of elemental lead from the residues, see [231, 271, 346]. 

The presence of tetrahedral Pb4 units In PbNa Is speculated to be largely responsible for the high reactivity towards alkyl halides, while in contrast, PbNa4, containing mutually isolated Pb atoms, has a comparatively small reactivity. Other considerations regarding influences of the structure of lead-sodium alloys and the mechanism of reaction with alkyl halides appear in the original literature [269]. 

Light retards the reactions of PbNa and of lead-sodium-potassium alloys with C2H5CI and this effect is increased by addition of water to the gas phase. Reaction products are assumed to impede penetration of light to the reacting surface of the alloy [267]. The gases produced in the reaction of lead-sodium alloys with ethyl halides contain, aside from C2H6 as the main product, C2H4, n-butane, propane, and minor amounts of other... 

The reaction of alkylchloride or alkylbromide with lead and sodium alloy ... 

The disadvantages of the chemical technique of tetraethyllead production are the low degree of reactant transformation (75% of the lead does not enter the reaction) and secondary formation of sodium chloride. Besides, it is very problematic to extract tetraethyllead out of the spongelike mass, the mixture of lead and sodium alloy and sodium chloride completely. The process is periodic and hence difficult to automate. Electrochemical techniques for the synthesis of tetraethyllead seem to be more promising in this aspect. 

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. 

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