Markovnikov vs Anti-Markovnikov Addition, Explained by Intermediate Stability

9 min read

When something adds across a carbon-carbon double bond, the two new groups can land two different ways. Markovnikov's rule tells you which arrangement you normally get, and a handful of named conditions deliberately reverse it. Students often memorize the rule as a slogan about the rich getting richer, but the slogan hides the real reason and leaves you helpless the moment a peroxide or a borane shows up.

The reliable way to handle regiochemistry is to stop memorizing outcomes and start asking one question: which reactive intermediate forms, and which arrangement makes that intermediate most stable? Answer that and Markovnikov, the peroxide effect, and hydroboration all fall out of the same logic. This guide builds that habit with the three additions you actually see on exams.

What Markovnikov's Rule Actually Claims

The classic statement is that in the addition of HX to an alkene, the hydrogen goes to the carbon that already has more hydrogens, and the halogen goes to the carbon that has fewer. That description is correct for simple cases, but it is a symptom, not a cause. The modern, general statement is the one worth learning: the electrophile adds so that the more stable carbocation intermediate forms.

In acid-promoted additions such as HCl, HBr, HI, or acid-catalyzed hydration, the first step is a proton adding to one alkene carbon. Whichever carbon does not get the proton becomes the positive center. Because a more substituted carbocation is more stable, the proton adds to the less substituted carbon, leaving the cation on the more substituted one. The nucleophile then adds to that carbon, which is exactly where Markovnikov's rule puts the halogen or hydroxyl.

Propene + HBrH adds to CH2, cation forms on the middle carbon (secondary), Br adds thereproduct is 2-bromopropane, the Markovnikov product

Carbocation stabilitytertiary > secondary > primary > methylthe ranking that decides regiochemistry

Why the More Stable Cation Wins

A more substituted carbocation is stabilized by the electron-donating alkyl groups around it, through both inductive donation and hyperconjugation. The transition state leading to that cation is lower in energy, so that pathway is faster. Regiochemistry in these ionic additions is therefore set by kinetics that trace directly back to intermediate stability.

This same reasoning warns you about a trap. Because these additions pass through a real carbocation, the cation can rearrange by a hydride or alkyl shift if a shift produces a more stable cation. When you see a substrate where a secondary cation sits next to a carbon that would give a tertiary cation, expect a rearranged product. Recognizing the carbocation as the star of the mechanism is what lets you anticipate that instead of being surprised by it.

Markovnikov applies to ionic additions that go through a carbocation: HX and acid-catalyzed hydration.
The proton adds first, to the carbon that leaves the more stable cation behind.
Watch for hydride and alkyl shifts whenever a rearrangement would give a more stable cation.
More substituted alkene carbon ends up bonded to the halogen or the hydroxyl group.

Anti-Markovnikov by Radicals: HBr with Peroxides

Add a peroxide initiator to HBr and the regiochemistry flips. This peroxide effect switches the mechanism from ionic to radical. The peroxide generates a bromine radical, and that radical, not a proton, adds first. A radical follows the same stability ranking as a cation, so the bromine adds to the less substituted carbon to leave the more stable, more substituted carbon radical. A hydrogen atom then caps that radical.

The net result is bromine on the less substituted carbon and hydrogen on the more substituted one, the opposite of Markovnikov. Two details matter for exams. First, the effect is specific to HBr; the chain-propagation steps are not favorable for HCl or HI, so those stay Markovnikov even with peroxides. Second, because the intermediate is a radical rather than a carbocation, these reactions do not rearrange the way ionic additions do.

HBr + ROOR (peroxide)Br radical adds first to the less hindered carbon, giving the more stable carbon radicalanti-Markovnikov: Br on the less substituted carbon

HCl or HI + peroxideno reversal — propagation steps are unfavorablethese remain Markovnikov

Anti-Markovnikov by Hydroboration-Oxidation

The other classic anti-Markovnikov route is hydroboration-oxidation: borane (BH3, usually in THF) adds to the alkene, then oxidative workup with hydrogen peroxide and hydroxide replaces boron with a hydroxyl group. The net transformation is hydration, but the hydroxyl lands on the less substituted carbon, the anti-Markovnikov position.

Two features explain the outcome. Boron is the electrophilic atom, and it adds to the less hindered, less substituted carbon for both steric and electronic reasons, placing hydrogen on the more substituted carbon in the same concerted step. Oxidation then swaps boron for OH with retention, so the hydroxyl ends up where the boron was. Because the boron and hydrogen add to the same face at the same time, the addition is syn, and because there is no carbocation, there is no rearrangement. Hydroboration is the clean way to make the less substituted alcohol from an alkene.

Boron adds to the less substituted carbon; the hydroxyl ends up there after oxidation.
Addition is syn: the new H and OH add to the same face.
No carbocation forms, so no rearrangements — unlike acid-catalyzed hydration.
Use it whenever you need the anti-Markovnikov alcohol from an alkene.

A Reliable Way to Predict the Product

Turn all of this into one short routine. First, identify the intermediate the conditions create. Acidic HX or acid-catalyzed water means a carbocation, so it is Markovnikov, and you should check for rearrangement. HBr with a peroxide means a radical, so it is anti-Markovnikov with no rearrangement. Borane followed by peroxide and hydroxide means concerted syn addition with boron on the less substituted carbon, so it is anti-Markovnikov hydration.

Second, place the groups to make that intermediate as stable as possible, then finish the mechanism. Working this way, you never have to recall a table of outcomes, because the outcome is a consequence of the intermediate. Practice by predicting the product for a handful of alkenes under each set of conditions, then confirm the mechanism arrow by arrow. In Octet you can compare these additions side by side and drill the reagents as flashcards until the intermediate you should expect is the first thing that comes to mind.

Frequently asked questions

What is Markovnikov's rule in simple terms?

In the ionic addition of HX or water to an alkene, the hydrogen adds to the carbon with more hydrogens and the halogen or hydroxyl adds to the more substituted carbon. The real reason is that the proton adds first so the more stable, more substituted carbocation forms.

Why does HBr with peroxides give the anti-Markovnikov product?

Peroxides switch the mechanism to a radical chain. A bromine radical adds first to the less substituted carbon so the more stable carbon radical forms, then a hydrogen caps it. This puts bromine on the less substituted carbon, the opposite of the ionic result.

Does the peroxide effect work with HCl and HI?

No. Only HBr reverses. The chain-propagation steps for HCl and HI are energetically unfavorable, so those additions stay Markovnikov even when a peroxide is present.

When should I use hydroboration-oxidation?

Use it to make the anti-Markovnikov alcohol from an alkene with syn addition and no rearrangement. Boron adds to the less substituted carbon, and oxidation replaces it with a hydroxyl group in the same position.

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