Carbocation Stability Order
When an asymmetric electrophile (\(\text{HX}\)) adds to an asymmetric alkene (e.g. Propene), two isomeric products are possible. The major product is formed via the more stable carbocation intermediate:
\[\text{Tertiary (3°)} > \text{Secondary (2°)} > \text{Primary (1°)} > \text{Methyl}\]Carbocation Stability
A carbocation is a carbon atom with only 6 electrons (incomplete octet) and a positive charge. Its stability depends on how many alkyl groups are attached.
Why are alkyl groups stabilising?
Alkyl groups are electron-donating via the inductive effect (+I effect). They push electron density towards the positively charged carbon, partially offsetting the charge and stabilising the carbocation. More alkyl groups = more electron density donated = more stable.
The Problem: Asymmetric Addition
Consider propene (CH₃CH=CH₂) reacting with HBr. The H⁺ can add to either carbon of the double bond, producing two different carbocations:
Markovnikov Addition to Propene
Pathway A (Major Product)
\(\text{H}^+\) adds to C1 → Secondary carbocation (\(\text{CH}_3\text{CH}^+\text{CH}_3\)) → 2-bromopropane (MAJOR)
Pathway B (Minor Product)
\(\text{H}^+\) adds to C2 → Primary carbocation (\(\text{CH}_3\text{CH}_2\text{CH}_2^+\)) → 1-bromopropane (MINOR)
Markovnikov's Rule
Positive Inductive Stabilization (+I)
Markovnikov's Rule: The hydrogen atom attaches to the carbon of the double bond that already possesses more hydrogen atoms. Alkyl groups are electron-donating via the positive inductive effect (\(+I\)), dispersing the positive charge and stabilising the carbocation.
Worked Example: Propene + HBr
| Major (Markovnikov) | Minor (anti-Markovnikov) | |
|---|---|---|
| H adds to | C1 (=CH₂, more H atoms) | C2 (=CH, fewer H atoms) |
| Carbocation | Secondary (more stable) | Primary (less stable) |
| Product | CH₃CHBrCH₃ (2-bromopropane) | CH₃CH₂CH₂Br (1-bromopropane) |
Addition of HCl to But-1-ene
When but-1-ene reacts with \(\text{HCl}\), \(\text{H}^+\) adds to C1 to generate the more stable secondary carbocation (\(\text{CH}_3\text{CH}_2\text{CH}^+\text{CH}_3\)), yielding 2-chlorobutane as the major product.
Explaining Markovnikov's Rule
- Always explain WHY: Mention the greater stability of the secondary/tertiary carbocation due to the positive inductive effect of alkyl groups.
- Symmetric alkenes: Markovnikov's rule applies only to asymmetric alkenes (addition to but-2-ene yields a single product).
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