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Alkene Addition Reactions: Predict the Product

Compare hydrohalogenation, hydration, bromination and hydroboration with predictable regiochemistry.

◎ Clear explanations◉ Worked examples▤ Interactive practice⬇ Free worksheet

Understand the concept

Alkenes undergo addition reactions because the π bond can be converted into new σ bonds. Product prediction requires more than remembering a product: you must identify the reagent, mechanism, regioselectivity and stereochemical outcome.

For propene, HBr without peroxides normally follows electrophilic hydrohalogenation and gives mainly 2-bromopropane. With suitable radical initiators/peroxides, HBr addition can favor 1-bromopropane. The peroxide effect is not generally transferable to HCl or HI.

A reliable four-step approach

  1. Identify whether the reagent adds H/X, H/OH, X/X or H/H across the C=C.
  2. Determine whether the pathway uses carbocation, radical, concerted or cyclic halonium chemistry.
  3. Apply regioselectivity only to the appropriate mechanism.
  4. Check stereochemistry, possible rearrangement and any competing products.

Visual explanation

Alkene Addition Reactions: Predict the Product: Compare hydrohalogenation, hydration, bromination and hydroboration with predictable regiochemistry.
Alkene Additions: original educational diagram. Values are illustrative where indicated.

Work through the examples

Worked example 1

Propene + HBr, no peroxides

Protonation generates the more stable secondary carbocation rather than a primary carbocation. Bromide attacks the carbocation.

Result: Major product: 2-bromopropane (Markovnikov addition).

Worked example 2

Propene + BH₃·THF, then H₂O₂/NaOH

Hydroboration adds B to the less substituted carbon and H to the other carbon in a concerted syn addition. Oxidation replaces B with OH with retention at that carbon.

Result: Product: propan-1-ol (anti-Markovnikov alcohol).

Common mistakes

  • Applying the HBr peroxide effect automatically to HCl or HI.
  • Calling hydroboration an anti addition: it is syn addition.
  • Assuming all acid-catalyzed hydration reactions avoid carbocation rearrangements.

Check your understanding

What is the major product of propene + HBr without peroxides?

Normal ionic addition of HBr to propene gives mostly 2-bromopropane.

Complete the interactive practice below to check more questions and review explanations.

Interactive Practice

Answer curated questions, then review the scientific explanation.

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