All About Non-Classical Ions

The 2-norbornyl system, which as bare ions unequivocally exhibits such a bridging structure, has been intensively explored in the context of nonclassical ions in particular. 

Nonclassical ions are distinguished by their unusually quick solvolysis rates and the substantial disparities between epimeric esters that they exhibit. 

However, this behaviour is not limited to 2-norbornyl esters, as has been demonstrated with several cyclopentyl and steroidal esters including the tosyloxy leaving group in their structure.

Substitution reactions of secondary esters are mediated by mechanisms that are similar to SN2– or SN1

Only in extremely polar solvents with low nucleophilicity, such as hexafluoroisopropanol (HFIP), can one expect a nearly identical uniform SN1-like process to take place. 

A study of the solvolysis of cyclopentyl and steroidal esters reveals that substantial solvolysis rates and differences between epimers can occur, with rates and differences between epimers exceeding those of the 2-norbornyl system. 

These situations can result in significant delocalization of the positive charge due to vicinal C–C or C–H bonds if these bonds are close to antiperiplanar to the departing group, and the migration results in a more stable tertiary carbocation.

2-Norbornyl cation 

The 2-norbornyl cation has been synthesised by reacting a variety of norbornane derivatives with reagents to create the compound. 

As a result of Saul Winstein’s publication of the first reports of its production and reactivity, there was some debate about the nature of its bonding, as he claimed that a three-center two-electron bond was responsible for the stereochemical conclusion of the reaction.

 Herbert C. Brown argued that traditional resonance structures could explain these facts without the need to adopt a new perspective on bonding, and that this assumption was incorrect. 

Both researchers’ points of view have their advocates, and scores of scientists provided tests that were ingeniously constructed to provide proof for one or the other point of view. 

It was just a matter of time until the discussion became increasingly nasty and caustic, and the debate began to become personal or ad hominem in nature.

Over the course of several decades, increasing evidence of the non-classical character of the 2-norbornyl cation was acquired, mostly through spectroscopic data gathered using technologies such as nuclear magnetic resonance (NMR) (NMR). 

It was not until 2013 that crystallographic proof of its non-classical character was obtained. However, while most chemists generally accept that the 2-norbornyl cation itself is non-classical, it is also widely acknowledged that the energetic landscape for carbocations is flat, with many different possible structures differing just slightly in energy. 

To be sure, not all bicyclic carbocations are non-classical.

The energy difference between classical and non-classical structures is frequently delicately balanced in these structures. 

As a result, it is now known that certain alkyl-substituted 2-bicyclo heptyl cations can adopt traditional structural arrangements.

The form of bonding in the 2-norbornyl cation included several new ideas into the field’s understanding of chemical bonds, which was previously unexplored. 

There are certain similarities between this cation and other cations such as boranes that can be observed.

Nonclassical Carbocation

A nonclassical carbocation is an ion that has a positively charged carbon in a three-centre, two-electron centre structure that is not classical. 

This suggests that there are three atoms in these carbocations that are sharing two electrons. Delocalization of the electrons is the term used to describe this form of electron sharing.

The 2-norbornyl cation is the most prevalent type of nonclassical carbocation found in nature.

 It can be found in a three-centre, two-electron form that is less symmetrical. Between classical and nonclassical carbocations, there is relatively little difference in the amount of energy released.

Conclusion

This non-classical ion, the 2-norbornyl cation, was one of the very first examples of such an ion.

 Non-classical ions can be described as organic cations in which the electron density of a filled bonding orbital is shared among three or more centres and exhibits some sigma-bond character, as opposed to classical ions. 

The 2-norbornyl cation is regarded as the prototypical non-classical ion because of its unusual structure.

 The use of infrared spectroscopy has revealed that other simple cations, such as protonated acetylene (ethynium), protonated ethylene (ethenium ), and protonated ethane (ethanium), are best classified as non-classical.