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Markovnikov's Rule: Why Some Alkenes React Faster Than Others

 

Markovnikov's Rule: Why Some Alkenes React Faster Than Others

If you've ever wondered why HBr adds to an alkene in one specific way and not randomly, this is the rule that explains it — and once you understand why it happens (not just the rule itself), you'll never mix it up again.

The Rule Itself

When a protic acid (like HBr, HCl, or H₂SO₄) adds across a double bond, the hydrogen ends up on the carbon that already has more hydrogens, and the other group (Br, Cl, etc.) ends up on the more substituted carbon.

The old-school phrasing: "the rich get richer" — the carbon with more H's gets even more H's.

But Why Does This Happen?

This is where most textbooks stop, and where most confusion starts. The real reason is about carbocation stability, not some arbitrary rule to memorize.

Here's the mechanism:

  1. The acid's H⁺ attacks the double bond first
  2. This creates a carbocation on one of the two former double-bond carbons
  3. Br⁻ then attacks that carbocation

The H⁺ doesn't attack randomly — it adds in whichever way produces the more stable carbocation. And carbocation stability follows a clear order:

Tertiary > Secondary > Primary > Methyl

More alkyl groups attached to the positive carbon = more stability (through electron-donating inductive effects and hyperconjugation).

So when H⁺ adds to an unsymmetrical alkene, it adds to the carbon that leaves behind the more substituted (more stable) carbocation on the other carbon — which is exactly the carbon that already had fewer hydrogens to begin with.

A Worked Example

Propene + HBr →?

CH₃–CH=CH₂ has one end with 1 hydrogen (the CH connected to CH₃) and one end with 2 hydrogens (the terminal CH₂).

If H⁺ adds to the terminal CH₂ (the carbon with more H's already), the resulting carbocation forms on the middle carbon — a secondary carbocation.

If H⁺ added the other way instead, the carbocation would form on the terminal carbon — a primary carbocation (much less stable).

Since the reaction always favors the more stable intermediate, H⁺ adds to the terminal CH₂, and Br⁻ ends up on the middle carbon.

Product: CH₃–CHBr–CH₃ (2-bromopropane), not CH₃–CH₂–CH₂Br.

Markovnikov addition comparing two possible carbocation pathways CH₃–CH=CH₂ + H⁺ H⁺ adds to middle C CH₃–CH₂–CH₂⁺ primary carbocation not favored H⁺ adds to terminal C CH₃–CH⁺–CH₃ secondary carbocation this path wins Br⁻ attacks CH₃–CHBr–CH₃ 2-bromopropane (product) The more stable carbocation wins — that's the whole rule

When Markovnikov's Rule "Breaks" — Anti-Markovnikov Addition

There's one major exception every student needs to know: HBr in the presence of peroxides follows the opposite pattern (anti-Markovnikov), because the mechanism switches to a free-radical pathway instead of the carbocation pathway. This only applies to HBr — not HCl or HI, since those don't form stable radical intermediates easily.

The One Sentence to Remember

Markovnikov's rule isn't really about "rich get richer" — it's about the reaction always taking the path through the most stable carbocation, and that path just happens to put H on the carbon with more hydrogens already.


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