What is the Difference Between Classical and Nonclassical Carbocation

Classical and nonclassical carbocations are two types of carbocations. Carbocations are positively charged carbon intermediates. They are highly reactive due to their incomplete octet, and their stability increases with increasing alkyl substitution. 

What is the difference between classical and nonclassical carbonation? Classical carbocations are less stable than nonclassical carbocations.

Key Areas Covered

1. What is a Classical Carbocation
      – Definition, Features 
2. What is a Nonclassical Carbocation
      – Definition, Features 
3. Similarities Between Classical and Nonclassical Carbocation
      – Outline of Common Features
4. Difference Between Classical and Nonclassical Carbocation
      – Comparison of Key Differences
5. FAQ: Classical and Nonclassical Carbocation
      – Answers to Frequently Asked Questions

Key Terms

Classical Carbocation, Nonclassical Carbocation

What is a Classical Carbocation

A classical carbocation is a positively charged carbon ion where the positive charge is localized on a single carbon atom. This carbocation has six valence electrons. It is highly reactive because its octet is not complete. The carbon having the positive charge is sp² hybridized. This gives a trigonal planar arrangement of three attached groups.

A carbocation has an electron deficiency, which makes it an electrophile. The stability of a classical carbocation is influenced by the number of alkyl groups attached to the positively charged carbon.

Classical carbocations are formed as intermediates in various reactions, such as in SN1 (substitution nucleophilic unimolecular) and E1 (elimination unimolecular) reactions. These carbocations participate in reactions such as nucleophilic addition and rearrangements. They also participate in addition reactions and polymerization.

What is a Nonclassical Carbocation

Nonclassical carbocations exhibit delocalization of the positive charge over multiple atoms. Due to the overlapping orbitals, delocalization of electrons occurs. This gives rise to a stable intermediate. The geometry of the nonclassical carbocation is mostly nonplanar.

Nonclassical carbocations mostly form in systems with rigid structures, such as bicyclic compounds. These carbocations are involved in many reactions, such as solvolysis reactions and rearrangements. One example of a nonclassical carbocation is the 2-norbornyl cation. This intermediate is formed in the solvolysis of 2-norbornyl derivatives.

Similarities Between Classical and Nonclassical Carbocation

1. Both types of carbocations possess a positive charge on a carbon atom.
2. They are both reactive intermediates formed in various organic reactions.
3. Both classical and nonclassical carbocations act as electrophiles and readily participate in nucleophilic addition reactions.

Difference Between Classical and Nonclassical Carbocation

Definition

• A classical carbocation is a positively charged carbon ion where the positive charge is localized on a single carbon atom. A nonclassical carbocation, on the other hand, is a type of carbocation where the positive charge is not localized on a single carbon atom, but rather is delocalized over multiple atoms. 

Positive Charge

• In classical carbocation, the positive charge is localized on a single carbon atom, while in nonclassical carbocation, the positive charge is delocalized over multiple carbon atoms.

Geometry

• Classical carbocations typically have planar geometry around the positively charged carbon, but nonclassical carbocations often have nonplanar geometry.

Reactivity

• Classical carbocations are highly reactive due to localized positive charge, whereas nonclassical carbocations are less reactive due to delocalization of positive charge.

Stability

• A classical carbocation is less stable than nonclassical carbocations.

Conclusion

Carbocations are positively charged carbon intermediates. They are highly reactive due to their incomplete octet. They are of two different types, namely classical carbocations and nonclassical carbocations. Thus, the main difference between classical and nonclassical carbonation is that classical carbocations are less stable than nonclassical carbocations.

FAQ: Classical and Nonclassical Carbocation

1. Why non classical carbocation is stable?

Nonclassical carbocations are stabilized by delocalization of the positive charge over multiple atoms. This delocalization, often involving bridging structures, lowers the overall energy of the ion, making it more stable compared to classical carbocations with localized charge.

2. Which carbocation has the least stability?

Methyl carbocation is the least stable carbocation. This is due to the lack of electron-donating alkyl groups, which stabilize carbocations through inductive and hyperconjugative effects.

3. What are the factors affecting the stability of a carbocation?

Carbocation stability is influenced by factors like hyperconjugation (alkyl groups donating electron density), inductive effect (electron-donating or withdrawing groups), resonance (delocalization of charge), and hybridization (sp2 > sp > sp3). More stable carbocations form faster and react more slowly.

4. Which carbocation has the lowest energy?

Tertiary carbocations have the lowest energy. More alkyl groups attached to the positively charged carbon stabilize the carbocation through inductive and hyperconjugative effects. This lowers its overall energy.

Reference:

1. “Carbocation.” Wikipedia. Wikipedia Foundation.
2. “Nonclassical Carbocation.” Slide Share.

Image Courtesy:

1. “Trishomoaromaticity non-classical carbocation” By Pillsmarch – Own work (CC BY 4.0) via Commons Wikimedia
2. “The E1 Mechanism for t-Butyl Alcohol” By HGTCChem  (CC BY 2.0) via Flickr