Markovnikov’s rule explained

In the electrophilic addition of HX to an unsymmetrical alkene, hydrogen ends up on the carbon that already has more hydrogens. That is not a superstition — it is “make the more stable carbocation.”

The problem students actually see

A worksheet shows propene plus HBr and asks for the major product. Two carbons of the double bond look similar until you count hydrogens: CH₂ vs CH. Hydrogen adds to the CH₂; bromine adds to the CH. The product is 2-bromopropane, not 1-bromopropane.

Why it works

1. The alkene π bond attacks H–X. Protonation can happen at either carbon.
2. The path that leaves the carbocation on the more substituted carbon is lower in energy (3° > 2° > 1°).
3. The nucleophile (X⁻, or water in hydration) captures that carbocation.

If a rearrangement (hydride or methyl shift) would make an even better cation, the product may not match the carbon that was first protonated. That is still Markovnikov logic — it just includes rearrangement.

Step-by-step on an exam item

1. Find the unsymmetrical alkene (or alkyne).
2. Identify H–X, acid-catalyzed hydration, or oxymercuration (Markovnikov) versus HBr/ROOR or BH₃ then H₂O₂/OH⁻ (anti-Markovnikov).
3. Place H on the carbon with more hydrogens unless the reagent set is an exception.
4. Check for a possible carbocation rearrangement.
5. Assign stereochemistry only if the problem asks; racemic mixtures are common when a new stereocenter forms from a planar cation.

Useful tips

• Memorize the two anti-Markovnikov flags: peroxides with HBr, and hydroboration–oxidation.
• Oxymercuration–demercuration is Markovnikov without rearrangement — that is why instructors love it as a trick.
• Addition flashcards should include reagent → regiochemistry, not just “Markovnikov” as a floating word.

Common mistakes

• Applying the rule to SN1/SN2 problems. It is an addition rule.
• Using it for HBr/peroxides.
• Forgetting that a 1,2-shift can move the cation before the nucleophile arrives.

How Organic Chemistry AI helps

Step-by-step solves name reaction types including Markovnikov, with the product highlighted and reagents called out. Addition flashcards and product-prediction quizzes are the right place to drill this until it is automatic. If a homework scan looks like an addition problem, the solver should say so explicitly — that is the check you want before you submit.

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