Relative Stability of Carbocations in Electrophilic Addition Reactions
Carbocations: Why Stability is Key
In electrophilic addition reactions, the first step often involves the formation of a carbocation intermediate.
Definition
Carbocation
A carbocation is a carbon atom that carries a positive charge, formed when a bond is broken heterolytically, leaving the carbon electron-deficient.
But not all carbocations are equally stable: this stability directly influences the reaction pathway and product distribution.
Types of Carbocations and Their Stability
- The stability of a carbocation depends on the number of alkyl groups attached to the positively charged carbon.
- Alkyl groups donate electron density through the positive inductive effect, reducing the electron deficiency and stabilizing the carbocation.
- Tertiary Carbocation (Most Stable): Three alkyl groups attached to the positively charged carbon.
- Secondary Carbocation: Two alkyl groups attached to the positively charged carbon.
- Primary Carbocation (Least Stable): One alkyl group attached to the positively charged carbon.
The stability trend can be summarized as: Tertiary > Secondary > Primary.Example
Let’s compare the stability of different carbocations:
- $CH₃⁺$ (Methyl carbocation): No alkyl groups, highly unstable.
- $CH₃CH₂⁺$ (Primary carbocation): One alkyl group, slightly more stable.
- $(CH₃)₂CH⁺$ (Secondary carbocation): Two alkyl groups, moderately stable.
- $(CH₃)₃C⁺$ (Tertiary carbocation): Three alkyl groups, most stable due to maximum electron donation.
- To determine carbocation stability, count the alkyl groups directly bonded to the positively charged carbon.
- More alkyl groups mean greater stability.
Markovnikov’s Rule: Predicting the Major Product
Markovnikov’s rule is a powerful tool for predicting the outcome of electrophilic addition reactions involving unsymmetrical alkenes.
