Draw a structural formula for the more stable carbocation intermediate formed in the reaction shown. – You do not have to consider stereochemistry. – Do not include anionic counter-ions, e.g., I’, in your answer. – For cases in which carbocations of the same or similar stability are expected, draw all of the structures. – Draw one structure per sketcher. Add additional sketchers using the drop-down menu in the bottom right corner. – Separate structures with + signs from the drop-down menu.
Answer
More Stable Carbocation Formed in Alkene Reaction with HBr
Reaction Overview
This reaction involves the electrophilic addition of hydrogen bromide (HBr) to an alkene. The first step is the formation of a carbocation intermediate, and the most stable carbocation is determined based on its degree of substitution.
Mechanism Step-by-Step
- Step 1 – Protonation: The double bond of the alkene reacts with a proton (H⁺) from HBr.
- Step 2 – Carbocation Formation: Following Markovnikov’s rule, the proton attaches to the less substituted carbon, generating a carbocation at the more substituted position.
- Step 3 – Hydride Shift: A hydride shift occurs if it leads to a more stable carbocation, typically converting a secondary to a tertiary carbocation.
Carbocation Stability Comparison
Tertiary (3°) carbocations are more stable than secondary (2°) carbocations due to:
- Hyperconjugation: Delocalization of electrons through adjacent C-H bonds
- Inductive effect: Electron-donating alkyl groups stabilize the positive charge
Final Stable Carbocation Structure
CH₃–C⁺(CH₃)–CH₂CH₃
This is a tertiary carbocation, which is the most stable intermediate formed during the reaction.
Key Takeaways
- Follows Markovnikov’s Rule during protonation
- A hydride shift enhances carbocation stability
- The final intermediate is a tertiary carbocation