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Organic Mechanisms

Electrophilic Addition to Alkenes and Markovnikov's Rule

Why HBr adds to propene the way it does, explained by carbocation stability.

Last reviewed 5 October 2026

Why alkenes attract electrophiles

A C=C double bond contains a π bond: a cloud of electron density above and below the plane of the molecule. Those electrons are held less tightly than in a single bond, so the double bond behaves as a nucleophile and is attacked by electrophiles: electron-poor species such as HX+\ce{H+}.

Key idea

Electrophilic addition has two steps: (1) the π electrons attack the electrophile, forming a carbocation; (2) a nucleophile (such as BrX−\ce{Br-}) bonds to the carbocation. The more stable carbocation forms faster, and that decides the product.

Step by step: HBr + propene

Step 1: a curved arrow goes from the C=C double bond of propene to the H of H–Br, while a second arrow moves the H–Br bonding pair onto bromine. Two carbocations are possible: the secondary carbocation CH3–C+H–CH3, which is more stable and forms much faster, and the primary carbocation CH3–CH2–CH2+, which is less stable and hardly forms. Step 2: a curved arrow from Br− to the positive carbon of the secondary carbocation gives CH3CHBrCH3, 2-bromopropane, the major product. Markovnikov: H adds to the carbon with more H atoms, because that gives the more stable carbocation.
Two possible carbocations; the more stable 2° carbocation leads to the major product.
  1. The π bond attacks the H of H–Br, and the H–Br bond breaks, leaving BrX−\ce{Br-}. The H can add to either end of the double bond, so two carbocations are possible.
  2. Adding H to the end carbon (CHX2\ce{CH2}) puts the positive charge on the middle carbon: a secondary (2°) carbocation. Adding H to the middle carbon gives a primary (1°) carbocation.
  3. BrX−\ce{Br-} bonds to whichever carbocation formed. Since the 2° carbocation forms much faster, the major product is 2-bromopropane.
CHX3CH=CHX2+HBr→CHX3CHBrCHX3\small \ce{CH3CH=CH2 + HBr -> CH3CHBrCH3}

Why carbocation stability matters

Carbocation stability: 3° > 2° > 1° > methyl. Alkyl groups stabilize the positive carbon by pushing electron density towards it (an inductive effect) and by hyperconjugation (neighbouring C–H bonds share some electron density with the empty p orbital). A more stable carbocation has a lower-energy transition state leading to it, so it forms faster.

That is the modern explanation of Markovnikov’s rule (1870): in the addition of HX to an unsymmetrical alkene, the hydrogen adds to the carbon that already has more hydrogen atoms.

Worked example: Predicting the major product

Question: Predict the major product of (a) 2-methylpropene + HCl (b) propene + water with an acid catalyst.

  1. (a) H adds to the CHX2\ce{CH2} end, giving the 3° carbocation (CHX3)X3CX+\ce{(CH3)3C+}; ClX−\ce{Cl-} adds to it: 2-chloro-2-methylpropane, (CHX3)X3CCl\ce{(CH3)3CCl}.
  2. (b) HX+\ce{H+} adds to the end carbon, giving the 2° carbocation; water attacks it and loses HX+\ce{H+}: propan-2-ol, CHX3CH(OH)CHX3\ce{CH3CH(OH)CH3} (the major product).

Bromine: the bromonium ion

BrX2\ce{Br2} has no H, but as it approaches the electron-rich double bond, the Br–Br bond becomes polarized. The near Br is attacked by the π bond, and instead of an open carbocation a three-membered bromonium ion forms. BrX−\ce{Br-} then attacks from the opposite face, so the two bromine atoms end up on opposite sides (anti addition). This reaction is the basis of the bromine-water test: the orange colour disappears as BrX2\ce{Br2} adds across the C=C.

Common mistakes

Common mistake: Putting Br on the carbon with more H

Markovnikov’s rule is about where the H goes. The H goes to the carbon with more H; the Br (or OH, Cl) goes to the other carbon, the more substituted one.

Common mistake: Drawing the arrow from H+ to the double bond

The electrons come from the π bond, so the curved arrow starts at the double bond and points to the H.

More precisely

When a carbocation can become more stable by moving a neighbouring H or CHX3\ce{CH3} group with its electron pair (a rearrangement), it often does, giving unexpected products. In the presence of peroxides, HBr (but not HCl) adds by a radical mechanism and gives the anti-Markovnikov product, 1-bromopropane; this was explained by Kharasch and Mayo in 1933.

Timeline

  1. 1870Vladimir Markovnikov publishes his rule for the addition of hydrogen halides to unsymmetrical alkenes.
  2. 1933Morris Kharasch and Frank Mayo explain the "peroxide effect": with peroxides, HBr adds the opposite way (anti-Markovnikov) by a radical mechanism.
  3. 1937Irving Roberts and George Kimball propose the bridged bromonium ion to explain anti addition of bromine.

Sources and further reading

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