9 The Development of Strain Theory A A R O N J. I H D E
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University of Wisconsin, Madison, Wis. To account for the apparent instability of rings with less than five carbon atoms, Baeyer suggested in 1885 a strain of the valencies of carbon away from the normal tetrahedral angle of 109° 28'. The concept proved valuable in inter preting structural problems connected with small rings, bridged rings, and structures found in natural products such as sterols and terpenes. Application to six-membered and larger rings caused problems until it was realized that such rings are strainless owing to their ability to take on a puck ered conformation. Heats of combustion, dipole moments, spectra, as well as chemical evidence have generally been in accord with strain predictions based on examination of models.
T t was commonly believed prior to 1880 that organic compounds be·** longed to only two classes—open chain and aromatic. T h e ring structure of benzene, the parent compound of the aromatic class, had become generally accepted, and many derivatives of benzene were known. Also known were such representatives of the aromatic class as naphthalene, anthracene, phenanthrene, and pyridine—compounds which figured prominently i n many syntheses. A limited number of compounds containing five-membered rings had been purified and studied—e.g., phthalic anhydride, isatin, indigo, indole, furan, pyrrole, and several lactones, but the structural characteristics had not yet been established to everyone's satisfaction. It should be noted that the known five-mem bered rings contained an atom of nitrogen or oxygen. N o rings were known which contained carbon atoms exclusively and which had more or less than six carbon atoms. In 1876 Victor Meyer (37) published a paper in which he considered it highly unlikely that rings of less than six carbon atoms would ever be encountered (Figure 1). H e recalled that reactions which should pre sumably lead to the formation of three-carbon rings had always resulted in producing unsaturated open-chain compounds and pointed out there 140 Benfey; Kekulé Centennial Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
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9.
IHDE
Strain Theory
141
Courtesy of the Edgar Fahs Smith Collection Figure 1.
Victor Meyer
had been less effort toward discovering four- and five-membered rings, but except for one unconfirmed formula for anthracene, there was little evidence to suggest the occurrence of rings smaller than six carbon atoms. Nearly a decade later Adolf von Baeyer (3), known as a brilliant experimenter who was not prone to theoretical speculation, advanced his Spannungs Théorie (Strain Theory) as a short addendum to a paper dealing with preparation and properties of acetylene derivatives ( F i g ure 2). Since the d i - and triacetylenes described in the body of the paper are highly reactive and even explosively unstable, he felt an obligation to deal with the instability of what he chose to consider a ring system. However, a large part of his research had been with aromatic com pounds; hence there was added incentive to ponder the problem of ring stability. Introducing the theory as an apparent afterthought in an ex perimental paper was characteristic of him since he was far more com fortable as an experimenter than as a theorist.
Benfey; Kekulé Centennial Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
142
KEKULE
CENTENNIAL
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After observing that a consideration of spatial arrangements can lead to understanding ring closure, Baeyer summarized what was then known about the nature of carbon atoms and added a statement of his own suggesting that the four valences of carbon make an angle of 109° 28' with one another. He then argued, "The direction of these attractions can undergo a diversion which causes a strain which increases with the
Courtesy of the Edgar Fahs Smith Collection Figure 2.
Johann Friednch Wilhelm Adolf von Baeyer
size of the diversion" (3). The diversion of the bonds was compared with the distortion of elastic springs. Clearly, such distortion led to strain and consequent instability. In open-chain compounds strain was avoided since a zig-zag arrangement of carbon atoms was normal, but if the ends of the chain were joined to form a ring, strain was inevitable. Baeyer went on to show that in no case did ring size permit the normal tetrahedral angle of 109° 28' to exist. The closest approach to this angle was the five-carbon ring where it was 108°. In both larger and smaller rings the angular distortion or strain became greater. He calculated the angular strain for various cycloparaffins, using the equation,
Benfey; Kekulé Centennial Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
9.
Strain Theory
IHDE
143
CH CH
2
CH
2
11
+
CH
540 44'
2
CH2--CH2 + 24° 44'
CH
2
3
CH2" "CH2
CH2 CH2
CH2 CH2
CH2—-CH
