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Organic Chemistry 1 · Alkene and Alkyne Chemistry

Electrophilic Addition

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

Alkenes react with electrophiles because their π electrons are exposed and relatively weakly held. In (HX) and (H2O/H⁺), an electrophile (H⁺) adds first, and the nucleophile adds to the carbon that forms the more stable carbocation. summarizes this: hydrogen adds to the less substituted carbon. Carbocation rearrangements can occur, and HBr with peroxides gives anti-Markovnikov addition via a radical mechanism.

Why this matters

underlies the industrial production of many alkyl halides and alcohols, and Markovnikov governs how water and acids add across double bonds in synthetic drug intermediates. The peroxide effect for HBr matters because it flips regiochemistry and thus the identity of a downstream pharmaceutical fragment — the same alkene can be steered to two different products by choosing ionic versus radical conditions.

The college version

1. Pi-Bond Reactivity

The π bond is the reactive site of an alkene: its electrons sit above and below the plane, farther from the nuclei and more available than σ electrons. Electrophilic addition breaks the π bond and forms two new σ bonds.

2. Hydrohalogenation and Markovnikov's Rule

In HX addition, the proton (electrophile) adds to the π bond to form the more stable carbocation; the halide then adds there. Net result: H adds to the less substituted carbon and X to the more substituted carbon — Markovnikov's rule.

3. Hydration, Rearrangements, and the Peroxide Effect

Acid-catalyzed hydration (H2O/H⁺) follows the same Markovnikov pattern, placing OH on the more substituted carbon, and carbocations can rearrange. Adding peroxides to HBr reverses regiochemistry (anti-Markovnikov): Br ends up on the less substituted carbon via a radical chain. The peroxide effect is reliable only for HBr, not HCl or HI.

How it works

  1. The π electrons attack the electrophile (H⁺), breaking the π bond and forming the more stable carbocation.
  2. Check for a hydride or alkyl shift to a more stable carbocation.
  3. The nucleophile (X⁻ or water) attacks the carbocation to form the product.
  4. For HBr with peroxides, run the radical chain instead: Br· adds first, then hydrogen abstraction, giving anti-Markovnikov addition.

Common confusions

Do not confuseWithDifference
MarkovnikovAnti-MarkovnikovH on the less vs more substituted carbon
Carbocation intermediateTransition stateIntermediates are local energy minima; transition states are maxima
HydrohalogenationHydrationX⁻ adds vs OH (from water) adds
Ionic HBrRadical HBr (peroxides)Opposite regiochemistry, different mechanism
Peroxide effect (HBr)General for all HXOnly HBr gives useful anti-Markovnikov addition

Memory aids

"The rich get richer" — hydrogen goes to the carbon that already has more hydrogens (the "hydrogen-rich" carbon), so the positive charge and the incoming nucleophile land on the more substituted carbon.

Quick review

Topic Recap

Electrophilic addition is the signature alkene reaction: the exposed π bond attacks an electrophile (H⁺), forming a carbocation that a nucleophile then traps. Markovnikov's rule predicts H adds to the less substituted carbon (more stable carbocation), and carbocations can rearrange. Acid-catalyzed hydration follows the same pattern, while HBr with peroxides reverses regiochemistry to anti-Markovnikov via radicals — an effect limited to HBr.

Knowledge Check

  1. Predict the major product of propene + HCl.
  2. Why does H⁺ add to the less substituted carbon in Markovnikov addition?
  3. What product forms from 3,3-dimethylbut-1-ene + HBr after ?
  4. Give the anti-Markovnikov product of propene + HBr/peroxides.
  5. Why does the peroxide effect not work well for HCl?

Answers and Rationales

  1. 2-chloropropane — Cl adds to the more substituted C2 carbon (secondary carbocation).
  2. Because that places the positive charge on the more substituted, more stable carbocation.
  3. 2-bromo-2,3-dimethylbutane — a methyl shift converts the secondary carbocation into a tertiary carbocation before Br⁻ adds.
  4. 1-bromopropane — Br adds to the less substituted terminal carbon.
  5. The radical-chain propagation steps for HCl (and HI) include an unfavorable step, so the anti-Markovnikov pathway is not synthetically useful.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Think of an alkene's double bond as a cloud of extra electrons sitting above and below the molecule — an exposed "piggy bank" of electron density. Electrophiles are "electron-hungry" and dive into that cloud first. Once the electrophile grabs the electrons, one carbon is left short (a carbocation), and a nucleophile rushes in to satisfy it.

A comparison: adding HX to an alkene is like a thirsty traveler at a water fountain — the electrophile (H⁺) goes to the carbon that already has more hydrogens (the "hydrogen-rich" carbon), which leaves the positive charge on the carbon with more carbon neighbors, where it is more stable.

Where it stops being exact: Markovnikov's rule is a memory shortcut, not a mechanism. The real driver is carbocation stability — the more substituted carbocation forms preferentially. Also, "the rich get richer" can mislead: hydrogen actually adds to the less substituted carbon (the one with more hydrogens), because that places the positive charge on the more substituted carbon.

Simple Example

Propene + HBr → 2-bromopropane (major, Markovnikov) rather than 1-bromopropane, because the secondary carbocation on C2 is more stable than the primary carbocation on C1.

Worked example

Ionic hydrohalogenation of 3,3-dimethylbut-1-ene with HBr.

  1. A double-headed curved arrow shows the π-bond electrons attacking the proton (electrophile), forming a new C–H bond at C1 while the H–Br bond breaks, giving Br⁻. The positive charge lands on C2, a secondary carbocation.
  2. Check for rearrangement: a methyl group shifts from C3 to C2 (with its electron pair), moving the positive charge to C3, which is now a tertiary carbocation — more stable.
  3. The bromide nucleophile's lone pair attacks the carbocation, forming a new C–Br bond.
  4. Products: without rearrangement, 2-bromo-3,3-dimethylbutane; with the methyl shift, 2-bromo-2,3-dimethylbutane (the observed, rearranged major product).

Anti-Markovnikov HBr with peroxides (radical chain).

  1. Peroxide initiators generate a bromine radical (Br·). Single-headed fishhook arrows show single-electron movement throughout.
  2. Br· adds to the less substituted carbon of the alkene, forming the more substituted (more stable) carbon radical.
  3. The carbon radical abstracts a hydrogen from HBr, giving the anti-Markovnikov product and regenerating Br· (chain propagation).
  4. Result: Br ends up on the less substituted carbon. This works for HBr only because both propagation steps are favorable; HCl and HI each have an unfavorable step, so the peroxide effect is limited to HBr.

Key takeaways

  • High yield: Markovnikov = H adds to the less substituted carbon (more substituted carbocation).
  • High yield: The more substituted carbocation is more stable (hyperconjugation + induction).
  • High yield: Carbocation rearrangements occur in ionic addition — watch for them.
  • High yield: HBr + peroxides → anti-Markovnikov (Br on the less substituted carbon).
  • High yield: The peroxide effect applies to HBr, not HCl or HI.
  • Hydration follows Markovnikov: OH lands on the more substituted carbon.
  • Markovnikov's rule is a prediction shortcut — the mechanism is carbocation stability.

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Practice Organic Chemistry 1

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Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Describe the general mechanism of electrophilic addition to an alkene and explain why the π bond is the reactive site.
  • Apply Markovnikov's rule to predict regiochemistry in hydrohalogenation and acid-catalyzed hydration.
  • Predict carbocation rearrangements and describe the stereochemical outcome of addition.
  • Explain the anti-Markovnikov addition of HBr with peroxides and state its limitations.

Key vocabulary

Electrophilic addition
Electrophile adds to the π bond, then a nucleophile adds
Pi-bond reactivity
π electrons are exposed and weakly held
Hydrohalogenation
Addition of HX to an alkene
Markovnikov's rule
H adds to the less substituted carbon
Carbocation intermediate
Positively charged, 3-coordinate carbon
Rearrangement
Hydride/alkyl shift to a more stable cation
Acid-catalyzed hydration
H2O/H⁺ adds to an alkene → alcohol
Regiochemistry
Which constitutional isomer forms
Stereochemistry
Spatial arrangement of the added groups
Anti-Markovnikov HBr
Br on the less substituted carbon (peroxides)
Peroxide-effect limitations
Works for HBr only

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