Order of stability of carbocations
The order of stability of carbocations can be determined by considering the factors that stabilize or destabilize the positive charge on the carbon atom.
The following factors contribute to the stability of carbocations in increasing order:
1. Hyperconjugation: This is the most important factor that stabilizes carbocations. It involves the delocalization of the positive charge through the overlap of sigma (σ) bonds between the empty p orbital of the carbocation and adjacent C-H or C-C sigma bonds. The greater the number of hyperconjugative interactions, the more stable the carbocation.
2. Inductive effect: Electronegative substituents adjacent to the carbocation can stabilize the positive charge by pulling electron density away from the carbon atom, reducing the positive charge density. The more electron-withdrawing groups a carbocation has, the more stable it is.
3. Resonance effect: If a carbocation can participate in resonance, the positive charge can be delocalized over multiple atoms, which stabilizes the carbocation. The more resonance structures a carbocation has, the more stable it is.
Using these factors, we can determine the order of stability of carbocations:
1° > 2° > 3° carbocations
Primary (1°) carbocations are the least stable because they have fewer hyperconjugative interactions, few or no adjacent electron-withdrawing groups, and cannot participate in resonance.
Secondary (2°) carbocations are more stable than primary carbocations because they have more hyperconjugative interactions and, in some cases, adjacent electron-withdrawing groups. They may also participate in resonance if adjacent atoms have lone pairs or pi bonds.
Tertiary (3°) carbocations are the most stable because they have the most hyperconjugative interactions, adjacent electron-withdrawing groups, and can participate in resonance with adjacent atoms.
The stability of carbocations can be determined by the following factors in order of importance:
1. Electron-donating groups (EDGs): Carbocations with adjacent electron-donating groups stabilize the positive charge by donating electrons through induction or resonance. This includes alkyl groups (R-), which are electron-donating by inductive effect. The stability increases with the number of alkyl groups attached to the positively charged carbon atom.
2. Resonance: Carbocations with adjacent pi bonds can participate in resonance, wherein the positive charge is delocalized over multiple atoms. This spreads out the positive charge, making the carbocation more stable. For example, allylic and benzylic carbocations have adjacent pi bonds and show resonance stabilization.
3. Hyperconjugation: Hyperconjugation is the overlap of an adjacent C-H or C-C sigma bond with the empty p-orbital of the carbocation. This delocalizes the positive charge and stabilizes the carbocation. More hyperconjugative structures contribute to greater stability. Alkyl groups, especially tertiary alkyl groups, show greater hyperconjugation and stability.
Based on these factors, the order of stability for carbocations is as follows:
Most stable:
1. Tertiary alkyl carbocation (e.g., (CH3)3C+)
2. Secondary alkyl carbocation (e.g., (CH3)2CH+)
3. Primary alkyl carbocation (e.g., CH3CH2+)
Least stable:
4. Methyl carbocation (e.g., CH3+)
It's important to note that these stability trends are guidelines and there can be exceptions depending on the specific substituents and the presence of other factors.