Carbocations are very important in many addition reactions, especially those that follow an electrophilic addition mechanism. Here’s their role explained:
1. Formation of carbocation
- In reactions like hydrohalogenation (adding HX to an alkene), the double bond first attacks a proton (H⁺).
- This breaks the double bond and leaves one carbon positively charged — that’s the carbocation.
2. Stability decides the pathway
- The more stable the carbocation, the more likely it is to form.
- Stability order: tertiary > secondary > primary > methyl (due to hyperconjugation and inductive effects).
- This explains Markovnikov’s rule: the hydrogen attaches to the carbon that already has more hydrogens, because that pathway gives the more stable carbocation.
3. Site for further attack
- The carbocation is very reactive and acts as an electrophile.
- A nucleophile (like Br⁻, Cl⁻, OH⁻, etc.) quickly attacks the carbocation, completing the addition and giving the final product.
4. Possibility of rearrangements
- Sometimes carbocations can shift (hydride shift or methyl shift) to form a more stable carbocation.
- This can change the final product of the reaction.
In short: Carbocations act as key intermediates in addition reactions, guiding where atoms attach, explaining product stability, and sometimes causing rearrangements.