Difference Between Classical and Nonclassical Carbocation

The key difference between classical and nonclassical carbocation is thatclassical carbocations have a carbon atom having sixelectronsin three chemical bonds, whereas nonclassical carbocations have a three-center two-electron structure.

Acarbocationis a chemical species that is a moiety of an organic molecule. It has a positive charge on a carbon atom. A simple example of a carbocation is CH₃⁺. Some carbocations have more than one positive charge, on the same carbon atom or a different atom. Moreover, carbocations are reactive intermediates in organic reactions due to the presence of a positive charge; there are six electrons in a carbon atom, which makes it unstable (presence of eight electrons ensures the stability); therefore it tends to seek electrons.

CONTENTS

1.Overview and Key Difference
2.什么是古典碳水化合物
3.What is Nonclassical Carbocation
4.Side by Side Comparison – Classical vs Nonclassical Carbocation in Tabular Form
5.概括

什么是古典碳水化合物?

A classical carbocation is an ion containing a positively charged carbon atom which has six electrons that take part in three chemical bonds. We can name this carbon atom as a three-coordinate positive carbon.

To ensure maximum stability, the carbon atom should have eightvalence electrons. But in the carbocation, there are only six electrons in the carbon atom having a positive charge. Therefore, it tends to share two more electrons from an electronegative species. This makes the carbon atom stable and neutralizes the positive charge. This is the reason for the high reactivity of classical carbocations. However, the energy of a classical carbocation is low compared to the energy of the corresponding nonclassical carbocation. But this difference in their energies is very small.

什么是非经典碳水化合物？

A nonclassical carbocation is an ion containing a positively charged carbon in a three-center two-electron center. This means, there are three atoms sharing two electrons in these carbocations. This type of electron sharing is named as delocalization of the electrons.

The most common example of a nonclassical carbocation is 2-norbornyl cation. It exists in a less symmetrical three-center two-electron structure. There is very little difference in the energy between classical and nonclassical carbocations. Therefore, it is very difficult to distinguish them experimentally.

What is the Difference Between Classical and Nonclassical Carbocation?

We can classify carbocations into two groups as classical and nonclassical carbocations, depending on the chemical structure. The key difference between classical and nonclassical carbocation is that classical carbocations have a carbon atom having six electrons in three chemical bonds, whereas nonclassical carbocations have three-center two-electron structure. The energy of nonclassical carbocation is higher than the energy of classical carbocation, but the difference between these energies is very small; hence, it is very difficult to distinguish the difference between classical and nonclassical structures.

此外，将经典碳酸盐转化为非经典碳定位或反之亦然的活化能非常小。除此之外，经典的碳环境在碳原子和碳原子周围的点头电子对上具有正电荷，但是在非经典碳化度中，电子在碳原子周围被取代。经典碳酸盐的一个例子是甲基离子，而非经典碳分配的一个例子是2-核心离子。

概括– Classical vs Nonclassical Carbocation

根据化学结构，我们可以将碳化量分为两组为经典和非骨骼碳。经典碳分配之间的主要区别在于，经典的碳原子具有三种化学键中的六个电子，而非经典碳化碳的原子具有三个中心的两电子结构。经典碳酸盐的一个例子是甲基离子，而非经典碳酸盐的例子是2-核离子。

Reference:

1. “Carbocations.” Chemistry LibreTexts, 5 June 2019,Available here.
2.“碳水化合物”。wikipedia.org，2019年，Available here.

Image Courtesy:

1. “The cyclopropyl carbinyl cation” By V8rik at English Wikipedia(CC BY-SA 3.0)viaCommons Wikimedia
2. JCAL730的“ Norbornyl阳离子单井与双重井潜力” - 通过自己的作品（公共领域）通过Commons Wikimedia