The Markownikoff rule states that in the addition of a protic acid HX (where X is a halogen) or other polar reagent to an unsymmetrical alkene or alkyne, the acidic hydrogen (H) becomes attached to the carbon with more hydrogen substituents, and the halide (X) group becomes attached to the carbon with more alkyl substituents. In simpler terms, "the rich get richer."
Explanation of Markownikoff's Rule
Markovnikov's rule is a crucial principle in organic chemistry that predicts the regiochemistry of electrophilic addition reactions to alkenes and alkynes. It helps determine which of the two possible constitutional isomers will be the major product. The rule is based on the formation of the most stable carbocation intermediate during the reaction.
How it Works
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Electrophilic Attack: The reaction begins with the electrophile (e.g., H+ from HBr) attacking the pi bond of the alkene.
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Carbocation Formation: This attack forms a carbocation intermediate. According to Markovnikov's rule, the electrophile adds to the carbon atom that will result in the formation of the most stable carbocation. Carbocations are stabilized by alkyl substituents due to the inductive effect and hyperconjugation. Therefore, a tertiary carbocation (three alkyl groups attached to the positively charged carbon) is more stable than a secondary carbocation (two alkyl groups), which is more stable than a primary carbocation (one alkyl group).
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Nucleophilic Attack: The nucleophile (e.g., Br- from HBr) then attacks the carbocation, forming the final addition product. Since the carbocation formed according to Markovnikov's rule is the most stable, the product resulting from the nucleophilic attack at that position will be the major product.
Example
Consider the reaction of propene (CH3-CH=CH2) with hydrogen bromide (HBr):
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Step 1: Protonation. The proton (H+) from HBr adds to the double bond. There are two possibilities:
- H adds to the CH2 carbon, forming a secondary carbocation (CH3-CH+-CH3)
- H adds to the CH carbon, forming a primary carbocation (CH3-CH2-CH2+)
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Step 2: Bromide Attack. The bromide ion (Br-) attacks the carbocation. Because the secondary carbocation is more stable than the primary carbocation, it will be the predominant intermediate. Thus:
- Br- attacks the secondary carbocation (CH3-CH+-CH3) to form 2-bromopropane (CH3-CHBr-CH3). This is the major product (Markovnikov product).
- Br- attacks the primary carbocation (CH3-CH2-CH2+) to form 1-bromopropane (CH3-CH2-CH2Br). This is the minor product.
Anti-Markovnikov's Rule
It's important to note that under certain conditions, the opposite of Markovnikov's rule can occur, leading to what's known as anti-Markovnikov addition. This happens when peroxide is present during the addition of HBr to an alkene. In this case, a free radical mechanism predominates, and the hydrogen adds to the more substituted carbon of the alkene.
Summary Table
| Feature | Markovnikov's Rule | Anti-Markovnikov's Rule |
|---|---|---|
| Reaction | Addition of HX to alkene/alkyne | Addition of HBr to alkene/alkyne in the presence of peroxides |
| Mechanism | Electrophilic Addition | Free Radical Addition |
| Hydrogen Addition | Adds to carbon with more hydrogens (less substituted) | Adds to carbon with fewer hydrogens (more substituted) |
| Halogen Addition | Adds to carbon with more alkyl groups (more substituted) | Adds to carbon with fewer alkyl groups (less substituted) |
| Key Intermediates | Carbocation | Free Radical |
| Major Product | Markovnikov Product (hydrogen on less substituted carbon) | Anti-Markovnikov Product (hydrogen on more substituted carbon) |
