Historical Origins of John S. Thayer University of Cincinnati Cincinnati, Ohio 45221
Organometalh Chemistry
I
Part 11, Edward Frankland and diethylzinc
WRhler's conversion of ammonium cyanate to urea ( l a ) and Rolbe's complete synthesis of acetic acid (.I b,) caused a considerable interest in the preparation of organic compounds from inorganic materials. Bunsen's synthesis of cacodyl from potassium acetate and Asz03(g) grew out of this, and stimulated its own line of research. Cacodyl was believed to be a relatively stable organic "free radical," suggesting that other organic radicals could also be prepared and isolated. Bunsen himself continued work in this area for several years after his last paper on cacodyl, and trained many students in his laboratory a t Marburg. Among these was a young Englishman, Edward Franlcland (1825-99). In an attempt to prepare "ethyl," Frankland reacted ethyl iodide with zinc metal ($a, b ) . The results are best described in his own words (Sa) The metal, either finely granulated or otherwise treated so as to expose a large surface, was then introduced and the open extremity of the tube was drawn out to the thickness of a straw; about sn inch of this narrow tuhe was allowed to shrink up until a fine capillav bore was obtained. The narrow tuhe was then bent twice a t right angles. the whale [tube] now being warmed, the open extremity was immersed in the iodide of ethyl, which, by the subsequent contraction of the enclosed sir, was forced into the apparatus in the required quantity . . . (after sealing the tube and heating to 150') white crystals grrtdually encrust the zinc and &ss, while a colorless, mobile liquid remains, equal in volume to only about half the iodide of ethyl employed and very (after two holm) different fram that liquid in appeamnee the reaction appearing to he complete, the tube was removed fmm the heth and allowed to coal. On, afterwards, breaking off its capillary extremity under water, about forty times its volume of gas was evolved, whilst the whole of the mobile fluid, above mentioned, disappeared. . .. On cutting off the upper portion of the tuhe and introducing distilled water, the white mass of crystals dissolved with brisk effervescence, occasioned by the evolution of s considerable quantity of a. gas (similar to the above) . . . . The solution of the crystals thus obtained possesses the properties of a. solotion of iodide of sine and, with the exception of a trace of undecomposed iodide of ethyl, appeared to contain no organic substance.
..
...
The "mobile fluid" was probably a mixture of (C2H&Zn, butane, and ethylene, while the white crystals were C2H5ZnI and ZnI,. Equations (1)-(4) describe the reactions
+ + + +
C2HsI Zn 2CnHd Zn (C,H,)Zn H,O CIH,ZnI 11,O
---
CIHbZnI (C.H&Zn ZnL 2C,H, ZnO C1H6 Zngf OH-
+ +
+
For Part 1, see J. CHIIM.EDUC., 46,442 (1969).
764
/
(1)
+
Journol of Chernicol Education
+ I-
(2) (3) (4)
Further experiments showed that methyl iodide gave similar reaction, and that the zinc derivatives of both reacted violentlv with air (4a) peculiarly penetrating and exceedingly nauseous odor, condensed in the receiver. This liquid spontaneously inflames on coming in contact with sir or with oxygen, burning with a brilliant greenish-blue flame and forming dense clouds of oxide of zinc. the vapom of this compound is highly poisonaos, producing shortlv after its incautious inhalat,ion d l the svmoloms of noison-
..
Frankland quickly extended his reaction and found that zinc also reacted with propyl, butyl, amyl, and phenyl iodides. More importantly, he found that elements such as Sn, Sb, As, and P reacted with organic iodides in analogous fashion (4a, b). Frankland's preparation then was the first example of direct synthesis of organometallic compounds, a process that has since become perhaps the most important preparative method for these species (5). Frankland studied these compounds in some detail, comparing them to each other and to inorganic analogs. His results enabled him to make the first clear statement on the concept of valence, probably his most important contribution to chemistry (6) When the formulae of inorganic chemical compounds are cansidered, even a, superficial observar is struck with the general symmetry of their construction; the compounds of nitrogen, phosphorus, antimony, and arsenic especially exhibit the tendency of these elements to farm compo~mdscontaining three or five equivdents of other elements, and it is in these proportions Without offering any that their affinities me best satisfied. hypothesis regarding the cause of this symmetrical grouping of atoms, it is sufficiently evident.. . that such a tendency or law orevails. and that. no matter what the character of the unitine atoms may be, the combining power of the attracting element is always satisfied by the same number of these atoms.
. ..
This concept laid the basis for much future work in chemistry and led directly to the Periodic Law. Frankland's research on organozinc compounds opened up the area of organometallic chemistry by showing that Compounds between metals and hydrocarbon radicals could exist. At the time of Frankland's initial work, neither eacodyl nor Zeise's salt were well known, and they were not recognized as organometallics until later. Oreanic residues could he transferred intact to and from metal from a bonding viewpoint.
