[CONTRIBUTION FROM
THE
RESEARCH LABORATORIES OF CORPORATION]
THE
ETHYLGASOLINHI
STUDIES OF ALKYL LEAD COMPOUNDS GEORGE CALINGAERT
AND
HAROLD SOROOS
Received March 9, 1958
Although the field of alkyl compounds of lead is fairly well known,’-* recent studies undertaken in this laboratory have shown that some compounds not previously reported in the literature exhibit behaviors somewhat at variance from what one might expect from the properties of known related compounds. The present article covers the description and properties of some of these new compounds in this group. METHYLBUTYLLEAD COMPOUNDS
Only two of the four possible trimethylbutyllead compounds have been r e p ~ r t e d . ~From the statements found in the l i t e r a t ~ r e one ~ * ~might ~~ expect that the sec.-butyl isomer would be somewhat unstable, and the tert.-butyl so unstable as to make it doubtful that it could be prepared in a sufficient degree of purity for a study of its characteristics. Actually, it was found that the sec.-butyl isomer could be prepared by the standard method of interaction of a sec.-butyl Grignard reagent and trimethyllead halide. When the preparation of trimetyl-tert.-butyllead was attempted by the same method, with the use, as is customary, of the bromide, a large amount of metallic lead was precipitated, and the only product obtained was tetramethyllead. A systematic study of the preparation resulted in a method using the unstable and seldom used trimethyllead iodide, which gave comparatively high yields (88 per cent.). Contrary to expectations, the material proved to be more stable than any other alkyl lead compound with branched chains, and indeed, in some respects even more stable than a compound containing no branched chain. This is 1 For a comprehensive survey, see KRAUSEAND VON GROSSE, “Die Chemie der metall-organischen Verbindungen,” Borntraeger, Berlin, 1937. 2 CALINQAERT, Chem. Reu., 2, 43-85 (1925). Ref. 1, p. 384; ref. 8, p. 58. Ref. 1, p. 388; GRUTTNER AND KRAUSE, Ber., 60, 574 (1917). 6 GRUTTNER AND KRAUSE, Ann., 418, 338 (1918). 6 Present authors. 535
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GEORGE CALINQAERT AND HAROLD SOROOS
probably related to its similarity in structure with the very symmetrical hexamethylethane.* In the preparation of trimethyl-sec.-butyllead a fair yield of dmethyldi-sec.-butyllead was obtained simultaneously. This is probably to be attributed to the relative instability of the trimethyllead iodide and bromide used in the preparation. The properties of all four trimethylbutyllead compounds are given in Table I. The boiling points decrease regularly from the normal to the tertiary isomer. The secondary shows the exaltation of the refractive index, which has been reported previously for similar compounds.s No further exaltation, however, is evident in the case of the tertiary isomer; indeed, the index of refraction is almost that of the normal isomer. This unexpected result is probably related to the peculiar structure of this compound, a point which will be further discussed. TABLE I TRIMETHYLBUTYLLEAD COMPOUNDS B.P.,
COMPOUND
"c.
(13 MM.)
Trimethyl-n-butyllead3..................... Trimethyl-sec.-butyllead6.. . . . . . . . . . . . . . . . . . Trimethyl-iso-butyllead3. . . . . . . . . . . . . . . . . . . Trimethyl-iso-butyllead e . . . . . . . . . . . . . . . . . . . Trimethyl-tert. butylleade. . . . . . . . . . . . . . . . . .
65 59 57 57 47
"0
1.6777 1.7011 1.6716 1.6822 1.6615
1,5046 1.5133 1.M26 1.5050 1.5089
F.P.,
'c.
5.7
HEXAMETHYLDILEAD
Hexaethyldilead is the only member of the hexaalkyldilead series which has been previously isolated.7 According to the literature, the stability of the hexaalkyldilead compounds increases with the length of the chain, and the product of reaction of Grignard reagents with lead halides contains more and more hexaalkyldilead and less tetraalkyllead as the alkyl groups increase in size.8 It is further stated that in the caseof methyl halides the product is pure tetramethyllead, and that no hexamethyldilead is formed.* In the course of the preparation of tetramethyllead by various methods in this laboratory, it was observed that when the compound was made from methylmagnesium iodide, a residue was present after the distillation of the tetramethyllead. A systematic investiga-
* Dr. Henry Gilman has suggested that the compound might prove useful in the preparation of free tertiary butyl radicals, using the Paneth technique. 7 MIDGLEY, HOCHWALT, AND CALINGAERT, J . Am. Chem. SOC.,46,1821 (1923). * GRUTTNERAND KRAUSE,Ber., 49, 1415 (1916).
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537
tion disclosed that the material was hexamethyldilead, and following this, attempts were made to synthesize the compound by the known methods. The reduction of trialkyl lead halide with sodium in liquid ammonia* gave only low yields, the product being mostly tetramethyllead and metallic lead. It was finally found that by suitable control of the Grignard reaction, using trimethyllead iodide, yields as high as 61 per cent. could be obtained . Hexamethyldilead is a solid, m.p. 38", d 2.38, crystallizing in large hexagonal plates, fairly unstable, decomposing slowly in the dark and more rapidly in diffused daylight, and leaving a residue of metallic lead. This compound and also the trimethyl-tert.-butyllead mentioned above are closely related structurally to hexamethylethane. Indeed, this relation becomes at once apparent if the three compounds are named, in accordance with the latest international convention: tetramethylbutane, Me3(: -CMe3; 2-plumbatetramethylbutane, Me3Pb- CMe3; and 2,3-diplumbatetramethylbutane, Me3Pb.PbMe3. All three show the typical properties of compounds of such structure, among which the camphorlike odor and the relatively high melting point are most familiar. Observations were made with a petrographic microscope of the crystalline structure (of all three compounds, and it was found that hexamethylethane and trimethyl-tert.-butyllead crystallize in the cubic system, while hexamethyldilead crystallizes in the hexagonal system. It is somewhat surprising that the comparatively less symmetrical trimethyl-tert.-butyllead shows a greater degree of crystalline symmetry than the seemingly more symmetrical hexamethyldilead. Thus, in its physical characteristics at least, trimethyl-tert.-butyllead behaves more like a molecule of hexamethylethane in which one quarternary carbon atom has been replaced by lead. EXPERIMENTAL
Trimethyllead iodide.-To 134 g. (0.5 mole) of tetramethyllead in 200 ml. of ether contained in a 2-liter beaker and cooled t o below -60" by the addition of solid carbon dioxide, was added through a dropping funnel with mechanical stirring, 127 g. (1.0 atom) of iodine in 800 ml. of ether. The mixture of trimethyllead iodide, methyl iodide, and ether was allowed to warm to room temperature, and the methyl iodide and ether distilled. The crude trimethyllead iodide, contaminated with Bome lead iodide was extracted with hot ether. The ether was again distilled and the light cream-colored trimethyllead iodide was dried on a porous clay plate. Yield 114 g. (60%). Anal. Calc'd for CaHpPbI: Pb, 54.6. Found: Pb, 54.4, 54.6 Trimethyl-sec.-butyllead.-To 120 g. (0.3 mole) of trimethyllead iodide suspended in 200 ml. of dry ether and contained in a 3-necked round-bottomed flask, fitted with
* This method has been used successfully in this laboratory for the preparation of EtoPbz from EtsPbBr (68 per cent yield).
538
GEORGE CALINGAERT AND HAROLD 80R008
a reflux condenser, vapor-proof mechanical stirrer, and dropping funnel, was added a solution of sec.-butylmagnesium bromide prepared from 66 g. (0.5 mole) of sec-
butylbromide, 12 g. (0.5 atom) magnesium, and 200 ml. of ether. Reaction took place with the lead salt going into solution and the formation of two layers. After refluxing for one hour and allowing to stand overnight, the reaction mixture was decomposed with ice and dilute hydrochloric acid. The ether layer was separated, and after drying and removing the solvent, the residue was distilled from a small Claisen flask. The distillate was fractionated in an atmosphere of nitrogen. The fraction, b.p. 55-56' 11mm., d: 1.6962, upon refractionationgave 39 g. (yield 50%) of trimethylsec.-butyllead, b.p. 59" 13 mm., d: 1.7011, n: 1.5133. Anal. Calc'd for C7H18Pb: Pb, 67.0, Found: Pb, 67.2. Tetramethyllead (16%) and dimethyl-di-sec.-butyllead (22%) were obtained as by-products in the above reaction. When trimethyllead bromide was used instead of the iodide, the yield of trimethyl-sec.-butylleadwas only 28%, together with tetramethyllead (23%), and dimethyl-di-sec.-butyllead (26%). Trimethyl-tert.-6utyllead.-Attempts to prepare trimethyl-tert.-butylleadby the action of tert.-butylmagnesium chloride and trimethyllead bromide resulted in the formation of tetramethyllead and metallic lead. The tetramethyllead was contaminated with hydrocarbons, including hexamethylethane and probably 2,2,4trimethylpentane. The use of trimethyllead iodide in place of the bromide resulted in a high yield of the desired compound. To 107 g. (0.28 mole) of trimethyllead iodide suspended in 250 ml. of dry ether, and contained in a 3-necked round-bottomed flask, fitted with a reflux condenser, vaporproof mechanical stirrer, and dropping funnel, was added a solution of tert.-butylmagnesium chloride prepared from 93 g. (1 mole) tert.-butyl chloride, 24 g. (1 atom) magnesium and 250 ml. of dry ether. After decomposition of the reaction mixture with ice and dilute hydrochloric acid, the ether layer was separated and dried over anhydrous sodium carbonate. After removal of the solvent, distillation of the residue from a small Claisen flask gave 83 g. of material, d, 1.5981, which upon fractionation in an atmosphere of nitrogen, gave 96 g. of trimethyl-tert.-butyllead,b.p., 47-48" 13 mm., d, 1.6570, yield 88%. Upon refraetionation all but a small amount of the material boiled a t 47.0-47.2" 13 mm., and gave the following physical constants: dy 1.6615, n: 1.5089, m.p. 5.7". The material has a camphor-like odor. Anal. Calc'd for C,HJ'b: Pb, 67.0, Found: Pb, 67.6,67.25. Hexamethyldi1ead.-(a) Trimethyllead iodide and sodium i n liquid ammonia.Two and three-tenths grams (0.1 atom) of freshly cut sodium in small pieces was added slorly to a solution of 38 g. (0.1 mole) of trimethyllead iodide in about 150 ml. of liquid ammonia contained in a 500-ml. Pyrex cylinder. After the ammonia had distilled off a t room temperature, the organic lead was extracted with several portions of ether. The solvent was removed by distillation, and the volatile tetramethyllead was distilled under reduced pressure with the bath temperature below 35" to prevent decomposition of any hexamethyldilead, leaving 3.5 g (yield 7%) of a yellow oil, which upon standing a short time, crystallized in large yellow plates. : 82.1, Found: Pb, 81.3. Anal. Calc'd for C s H d b ~ Pb, ( b ) Methylmagnesium bromide and lead chloride.-To a solution of methylmagnesium bromide prepared from 13 g. (0.5 atom) magnesium, 60 g. (0.6 mole) methyl bromide and 250 ml. of ether, contained in a 3-necked round-bottomed flask, fitted with a reflux condenser, vapor-proof mechanical stirrer, and cooled in an ice-saltwater freezing mixture maintained a t -5 to -8" was added in small portions, 139 g. (0.5 mole) of freshly dried lead chloride. Reaction took place with the momentary
STUDIES O F ALKYL LEAD COMPOUNDS
539
formation of a reddish coloration, which changed to a yellow-green, and finally black, caused by the deposition of metallic lead. After stirring for 30 minutes, the reaction mixture was decomposed with ice and water, the yellow-green ether layer was separated and the residue extracted with 2 100-ml. portions of ether. The combined ether solutions were shaken with anhydrous sodium carbonate and filtered. Most of the ether was removed by distillation a t atmospheric pressure and the last traces a t 100 mm. The pressure was then reduced to 2 mm., and the distillation was continued with the bath temperature below 35". Three grams of distillate were collected, and a residue 25.5 g. of large greenish-yellow crystals of hexamethyldilead remained in the distilling flask. Yield of crude hexamethyldilead 25.5 g. @I%), The crystalline residue was dissolved in 25 ml. of ether and filtered from a small amount of black solid. After distillation of the ether under reduced pressure, a residue of light cream-colored soft crystals remained, m.p. 37-8"C., d 2.38. Anal. Calc'd for CsHlgPbt: Pb, 82.1, Found: P b 82.0, 82.6 SUMMARY
Three of the four trimethylbutyllead isomers have been prepared and the properties of the four isomers are compared. Hexamethyldilead was prepared, and its properties are compared with those of the structurally related hexamethylethane and trimethyl-tert.-butyllead.
