2-Norbornyl cation

In organic chemistry, the term 2-norbornyl cation (or 2-bicyclo[2.2.1]heptyl cation) describes one of the three carbocations formed from derivatives of norbornane. Though 1-norbornyl and 7-norbornyl cations have been studied, the most extensive studies and vigorous debates have been centered on the exact structure of the 2-norbornyl cation.

The 2-norbornyl cation has been formed from a variety of norbornane derivatives and reagents. First reports of its formation and reactivity published by Saul Winstein sparked controversy over the nature of its bonding, as he invoked a three-center two-electron bond to explain the stereochemical outcome of the reaction. Herbert C. Brown challenged this assertion on the grounds that classical resonance structures could explain these observations without needing to adapt a new perspective of bonding. Both researchers' views had its supporters, and dozens of scientists contributed ingeniously designed experiments to provide evidence for one viewpoint or the other. Over time, the dispute became increasingly bitter and acrimonious, and the debate took on a personal or ad hominem character.

Evidence of the non-classical nature of the 2-norbornyl cation grew over the course of several decades, mainly through spectroscopic data gathered using methods such as nuclear magnetic resonance (NMR). Crystallographic confirmation of its non-classical nature did not come until 2013. Although most chemists now agree that 2-norbornyl cation itself is non-classical, it is also widely recognized that the energetic landscape for carbocations tends to be "flat", with many potential structures differing only minutely in energy. Certainly, not all bicyclic carbocations are non-classical; the energy difference between classical and non-classical structures is often delicately balanced. Thus, certain alkyl-substituted 2-bicyclo[2.2.1]heptyl cations are now known to adopt classical structures.

The nature of bonding in the 2-norbornyl cation incorporated many new ideas into the field’s understanding of chemical bonds. Similarities can be seen between this cation and others, such as boranes.