What is a non-classical carbocation?

Question

- What is a non-classical carbocation? - How is it different from a classical carbocation? I am confused as I have come across this term many times on Chem.SE but there seems to be nothing for my level of understanding on the Internet!

Answer 1

Great question! This was actually a hotly-contested area of research for many years, but the succinct answer is that they differ in how electron delocalization occurs to stabilize intermediates.

In greater detail, non-classical carbocations are stabilized via bridged transition states, often via 3-center-2-electron bonding. This is in stark contrast to a classical carbocation, which might be stabilized by simple resonance. A quintessential example of a non-classical carbocation is the 2-norbornyl carbocation, and a benzyl cation is a great example of a classical carbocation.

Why do we care? The existence of non-classical carbocations has profound implications for chemical reactivity, and reaction kinetics can be drastically altered.