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.
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.
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.
Markovnikov's rule
When a hydrogen halide (HX) adds to an unsymmetrical alkene, the hydrogen atom attaches to the carbon with more hydrogen atoms, while the halogen attaches to the carbon with fewer hydrogen atoms.
This rule ensures the reaction proceeds via the most stable carbocation intermediate.
Applying Markovnikov’s Rule
Let’s examine the addition of HBr to propene ($CH₃-CH=CH₂$):
- Step 1: Protonation of the Alkene
- The $π$-electrons in the double bond attack the partially positive hydrogen atom ($H⁺$) in HBr, breaking the H-Br bond heterolytically.
- This generates a carbocation intermediate.
- If $H⁺$ adds to the terminal carbon ($CH₂$), a secondary carbocation forms: $CH₃-CH⁺-CH₃$.
- If $H⁺$ adds to the middle carbon ($CH$), a primary carbocation forms: $CH₃-CH₂-CH₂⁺$.
- Step 2: Formation of the Major Product
- The bromide ion ($Br⁻$) then attacks the carbocation, forming the final product.
- Since the secondary carbocation is more stable than the primary carbocation, the reaction predominantly forms 2-bromopropane as the major product.
Reaction Mechanism for HBr Addition to Propene:
- Protonation: $CH₃-CH=CH₂ + H⁺ → CH₃-CH⁺-CH₃$ (Secondary carbocation, more stable) OR $CH₃-CH=CH₂ + H⁺ → CH₃-CH₂-CH₂⁺$ (Primary carbocation, less stable)
- Nucleophilic Attack: $CH₃-CH⁺-CH₃ + Br⁻ → CH₃-CHBr-CH₃$ (Major product: 2-bromopropane), $CH₃-CH₂-CH₂⁺ + Br⁻ → CH₃-CH₂-CH₂Br$ (Minor product: 1-bromopropane)
- Why is a tertiary carbocation more stable than a primary carbocation?
- Use Markovnikov’s rule to predict the major product of the reaction between HCl and 1-butene.
