Key Concept
When an asymmetric alkene (e.g. propene) reacts with an asymmetric electrophile (e.g. HBr), two possible products can form. The major product is the one formed via the more stable carbocation intermediate.
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.
Tertiary (3°) > Secondary (2°) > Primary (1°) > Methyl (CH₃⁺)
Most stable → Least stable
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:
Option A: H adds to C1
CH₃CH⁺CH₃
Secondary carbocation
More stable = MAJOR product (2-bromopropane)
Option B: H adds to C2
CH₃CH₂CH₂⁺
Primary carbocation
Less stable = MINOR product (1-bromopropane)
Markovnikov's Rule
The Rule
When HX adds to an asymmetric alkene, the hydrogen atom bonds to the carbon that already has more hydrogen atoms. This produces the more substituted (more stable) carbocation intermediate.
Shortcut: "The rich get richer." The carbon with more hydrogens gets the extra H.
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) |
Think About It
When but-1-ene reacts with HCl, what is the major product and why?
H adds to C1 (the terminal carbon, which has more hydrogens), producing a secondary carbocation at C2. The Cl⁻ then attacks C2. The major product is 2-chlorobutane (CH₃CHClCH₂CH₃). The primary carbocation at C1 would be much less stable, so 1-chlorobutane is the minor product.
Common Exam Mistakes
- Stating Markovnikov's rule without explaining why. The reason is carbocation stability. The mark is for the explanation, not just the rule.
- Forgetting that Markovnikov's rule only matters for asymmetric alkenes. With ethene (CH₂=CH₂), both carbons are equivalent.
- Not linking the stability to the inductive effect of alkyl groups.
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