Armin Wilson
Bristol Meyers Products Division Hillside, New Jersey
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A Versatile Molecular
Model of Cyclobutane of Dreiding t-butyl groups to positions one and three on the model ring in a cis relationship to each other leads one to speculate on the possibility of the existence of a (semi ?) equatorial, equatorial cis l,&ditertiary butyl cyclobutane. Whether such a form might exist "frozen" is an unanswered question a t presenL5 The model is constructed as follows: Four pieces of brass tubing 1.58 mm in diameter and 45 mm long were bent a t sharp right angles a t the mid-point.' These are coupled into a square 45 mm on a side by telescoping the ends of these right angle tubes into four pieces of brass tubing 2.41 mm in diameter and 30 mm in length. These pieces of larger diameter brass tubing are centered along the sides of the square. They are made permanent by lightly crimping the overlapping tubes together 4 rnrn from each end of the larger pieces of tubing. Torsional bending across the diagonal of
The recent enormously increased interest in ryclobutane chemistry has made necessary the development of adequate molecular models for use in the -chemical and physical studies of this strained molecule. Below we describe the preparation of one such model which we have found heuristically useful. This particular model is conveniently scaled to the Dreiding prototype.' Although this brass copy lacks the elegance and sturdiness of the original, the construction is simple, rapid, and inexpensive. The incorporation in the model of ring bonds that twist in unison permits a display of both the form possessing D4,plane symmetry (Figure la) and the form showing bent dihedral ("gull") Did symmetry (shown in two perspective views in Figures l b and lc). This in turn permits one to observe the total eclipse of all eight hydrogens in the planar form and the
the stable forms of this model. The "C-H bonhs" are prepared by bending 60-mm lengths of the larger
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Figure 1.
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Modekof cyclobutone.
staggered character of the same atoms in the bent form. Further, the trans semi-planar arrangement of the HI-CI-C7-Hz nuclei in the bent form (Figure 2) is clearly visible in the model. This suggests the possibility that some gain in stability could be obtained in cyclobutane and certain of its derivatives by continual inversion of one bent form to the other through the planar form. This idea, as demonstrated by the model, gains additional support from the principle of minimum bending of orbitals of Stewart and Eyring.= The assumption that the simpler cyclobutanes are indeed bent is substantiated by the electron diffraction and X-ray measurements of a number of cyclobutane
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Figure 2.
home in the brass tubing and withdrawn. This sets up a locking notch in the brass tubing. Free rotation is now possible as in the original Dreiding models. The H-C-H arms are attached by positioning them in the corners of the square and soldering them in place. Ring fusions are not possible with this model, since no provision has been made for such attachments. This shortcoming could be remedied where necessary by a skilled machinist. I wish to express my thanks to Andrew Volansky for his excellent ~hotographicwork.
derivative^.^.^
There is, of course, no denying the interim character of such a model. The mime uuruose of this device is fulfilled, however, by suggesting, through its use, many interesting experiments. For example, the attachment 'DEEDING,A. S., Helv. Chim. Ada, 42, 1339 (1959). 'STEWART,G. H. and EYRING, H., J. CHEM.EDUC., 35, 550 (1958). S A ~A,, BASTIANSEN, ~ ~ O., ~ET AL. ~Act. Chen. ~ Scond., ~ J. H., J. Arner. Chrm. Soc., 15, 711 (1961); ef. also BAEWSTER, 81, 5483 (1959). ~ G O L D I E., ~ H J. , CHEM.EDUC.,36, 408 (1959).
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measured for Droner leneth of insertion. and the brass tubing comprising the "C-H bond" is crimped gently at the proper location. The Dreiding fitting is driven
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~We are , unaware of any eyelobutanes containing l-butyl group(s) attached directly to the ring. 6 The brass tubing used is the standard telescoping model airplane type which is available in any hobby shop.
Volume 39, Number 12, December 1962
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649
