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
Formation of the More Stable Carbocation Intermediate
Reaction Type
This is an electrophilic addition reaction, specifically a hydrohalogenation of an alkene using HCl as the reagent.
Mechanism Breakdown
- Step 1 – Electrophilic Attack: The π-electrons of the alkene attack a proton (H⁺) from HCl, creating a carbocation intermediate.
- Step 2 – Regioselectivity (Markovnikov’s Rule): The H⁺ attaches to the less substituted carbon of the double bond, leading to a more stable carbocation on the more substituted carbon.
- Step 3 – Carbocation Stability: The intermediate formed is a tertiary carbocation, which is stabilized by inductive effects and hyperconjugation.
Order of Carbocation Stability
Final Explanation
According to Markovnikov’s rule, the proton adds to the carbon with more hydrogen atoms, allowing the more substituted carbon to become a tertiary carbocation.
This carbocation is highly stable and represents the major intermediate of the reaction. The chloride ion (Cl⁻) will later neutralize the positive charge, but our focus is only on the intermediate stage.
Final Structure
The stable intermediate is a tertiary carbocation where the central carbon carries a positive charge and is bonded to:
- Two ethyl groups (–CH₂CH₃)
- One methyl group (–CH₃)
- No lone pairs, with an empty p-orbital