The Concept of Strain in Organic Chemistry

“Die Ringschließung ist offenbar diejenige Erscheinung, welche am meisten über die räumliche Anordnung der Atome Auskunft geben kann. Wenn eine Kette von 5 und 6 Gliedern sich leicht, eine von weniger oder mehr Gliedern sich schwierig oder auch gar nicht schließen läßt, so müssen dafür offenbar räumliche Gründe vorhanden sein.… Die vier Valenzen des Kohlenstoffatoms wirken in den Richtungen, welche den Mittelpunkt der Kugel mit den Tetraederecken verbinden, und welche miteinander einen Winkel von 109°28′ machen. Die Richtung der Anziehung kann eine Ablenkung erfahren, die jedoch eine mit der Größe der Letzteren wachsende Spannung zur Folge hat,”[

1 Ring formation is evidently the reaction that can provide the most important information on the spatial arrangement of atoms. Steric reasons should explain the observation that 5- or 6-membered chain undergoes ready closure, whereas a shorter or longer chain undergoes difficult or no closure.… The four valences of the carbon atom act in the directions that connect the center of a sphere with the corners of a tetrahedron and form angles of 109°28′ with one another. The direction of the attraction can deviate, but this in increasing strain as the sizes of the latter increase.

] This is the quintessence of the “ring-strain theory” formulated by Adolf von Baeyer over one hundred years ago. Although it is today only one facet of the many aspects of strain theory, it has repeatedly stimulated experimental and theoretical chemists. Among the most spectacular of the recent successes in synthetic chemistry are the syntheses of tetra-tert-butyltetrahedrane and [1.1.1]propellane. The reasons for the great stability of these two highly strained compounds are completely different. The experimental findings as well as the results of theoretical analysis by means of molecular mechanics and ab initio calculations have contributed decisively to our present state of knowledge of the structure, energy, and reactivity of organic compounds.