THE CONCEPT
Regiochemistry of HX Addition
When an acid like HBr or HCl adds across an alkene's double bond, both the hydrogen and the halogen end up on the two former double-bond carbons — but which carbon gets which atom isn't random. Markovnikov's rule predicts this precisely: the hydrogen ends up on the carbon that already has more hydrogens attached to it, and the halogen (or –OH, in the analogous acid-catalyzed hydration reaction) ends up on the more substituted carbon.
The reason isn't arbitrary — it comes directly out of the reaction mechanism. The alkene's pi bond acts as a nucleophile, attacking the acidic hydrogen of HX and protonating one of the two double-bond carbons. Protonation happens at whichever carbon leaves behind the more stable carbocation on the other carbon, because that's the lower-energy, faster pathway. Since more substituted carbocations (tertiary > secondary > primary) are more stable, the reaction preferentially protonates the carbon that generates the more substituted cation — which, by definition, is the carbon that already had more hydrogens to begin with.
💡 Memory Trick
The hub's trick is the phrase "the rich get richer." Whichever carbon already has more hydrogens (is already "rich" in hydrogen) is the one that gets the new hydrogen too — it gets even richer. Meanwhile the halogen (or OH) ends up on the more substituted carbon, the one that was "poorer" in hydrogens to start with. Underneath that catchphrase is the real chemistry driving it: the H goes wherever protonation generates the more stable carbocation, and more substituted carbons stabilize a positive charge better through hyperconjugation and induction.
MARKOVNIKOV VS. ANTI-MARKOVNIKOV
When the Rule Flips
1
Standard (ionic) HX addition — Markovnikov
HBr, HCl, or HI adding directly to an alkene proceeds through a carbocation intermediate, so it follows standard Markovnikov selectivity: H to the carbon with more H's already, halogen to the more substituted carbon.Propene + HBr → 2-bromopropane (Br on the middle, more substituted carbon), not 1-bromopropane.
2
Radical (peroxide-initiated) HBr addition — anti-Markovnikov
When HBr is added in the presence of peroxides (ROOR), the mechanism switches entirely — it becomes a radical chain mechanism instead of an ionic one. Now the bromine radical adds first, to whichever carbon gives the more stable free radical (again, the more substituted carbon), which flips the final regiochemistry: Br ends up on the LESS substituted carbon, and H ends up on the more substituted one.Propene + HBr/peroxides → 1-bromopropane instead of 2-bromopropane — the opposite product from the standard reaction.
3
Why only HBr shows this flip
The peroxide-initiated radical pathway is only favorable for HBr specifically — the bond energies and radical stabilities involved don't favor an analogous radical mechanism for HCl or HI under normal conditions. So anti-Markovnikov addition, in practice, is a signal that's specific to HBr with peroxides present; don't expect it by default with the other hydrogen halides.
🧪 Lab Application
You need to synthesize 2-bromobutane from 1-butene, and a labmate accidentally leaves a peroxide-contaminated flask on the bench that could change your product.
1
Identify the target product's regiochemistry. 2-bromobutane has the bromine on the second (more substituted, internal) carbon of a four-carbon chain — that's a Markovnikov-selective outcome from 1-butene.
2
Choose the correct reagent conditions. Plain HBr, with no peroxides present, reacts through the standard ionic mechanism and delivers exactly the Markovnikov product you want: Br on the more substituted internal carbon.
3
Recognize the danger of the contaminated flask. If peroxides are accidentally present, the mechanism flips to the radical pathway, and the major product becomes 1-bromobutane instead — the halogen ends up on the terminal, less substituted carbon, giving you the wrong regiochemistry entirely.
4
Discard and use clean glassware. Given how completely the product distribution can flip, always confirm your HBr and glassware are peroxide-free before running a reaction where you need Markovnikov selectivity specifically.
📌 Exam Application
Exam questions frequently present a reaction and expect you to recognize whether peroxides are mentioned, since that single detail flips the entire product. Train yourself to scan reaction conditions for the word "peroxide" or "ROOR" before predicting any HBr addition product — it's an easy point to lose by defaulting to Markovnikov out of habit.
⚠️ Most Common Markovnikov's Rule Mistakes
The most common mistake is applying anti-Markovnikov logic to HCl or HI additions just because peroxides are mentioned somewhere in the same problem set — the radical anti-Markovnikov pathway is specific to HBr. The second common trap is forgetting that Markovnikov's rule, in its modern form, is really about carbocation stability, not literally about "the atom that already has more of itself" — that phrasing is a mnemonic shortcut, not the actual mechanism.
✓ Quick Self-Test
1) In standard HBr addition to propene, which carbon gets the bromine? 2) What changes about the mechanism when peroxides are present during HBr addition? 3) Why is the more substituted carbocation more stable than a less substituted one? 4) Does the anti-Markovnikov radical pathway apply to HCl addition? Why or why not? 5) What is the modern mechanistic explanation behind 'the rich get richer'?
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