Electrophilic addition is one of those IB Chemistry topics that feels easy right up until the question asks for a mechanism, a major product, and a one-sentence explanation of why that product wins.
In the quiet panic of an exam, it’s rarely the definition that slips; it’s the story. Who attacks first? What intermediate forms? Why does the “wrong” product look plausible? Mastering electrophilic addition in IB Chemistry is really about learning to narrate the electrons.
Electrophilic addition in IB Chemistry (one-minute checklist)
Use this checklist before you draw anything:
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Identify the alkene (C=C means a reactive pi bond).
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Identify the electrophile (electron-pair acceptor: H+, polarized Br2, etc.).
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Step 1 is always electrophilic attack (not nucleophilic attack).
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Expect a carbocation (HX additions) or a bromonium ion (Br2 additions).
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Step 2 is nucleophilic attack (X- or Br- attacks the intermediate).
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For unsymmetrical alkenes + HX, use Markovnikov’s rule to predict the major product.
If you want extra targeted practice, the R3.4 Electrophilic addition page is a clean hub for IB Chemistry mechanisms.
What electrophilic addition actually means
In IB Chemistry, electrophilic addition describes a reaction where an electrophile attacks the electron-rich C=C double bond, and two new single bonds form across what used to be the double bond.
Why alkenes? The pi bond is exposed above and below the sigma bond framework. It’s electron-rich, and it’s reachable. Electrophiles don’t need an invitation; they just need electron density.
For quick surrounding context (so you don’t mix mechanisms), RevisionDojo’s types of organic reactions notes place electrophilic addition alongside nucleophilic substitution and other core IB Chemistry patterns.
The two-step mechanism you must be able to write
Most IB Chemistry electrophilic addition questions reward clarity over artistry. Keep it simple and label your steps.
Step 1: electrophilic attack (the slow, deciding moment)
The alkene donates electron density from the pi bond to an electrophile.
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With HX (HCl, HBr), the electrophile is H+.
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This step often forms a carbocation intermediate.
This is where regioselectivity is decided. In other words: the exam is secretly asking, “Which carbocation is more stable?”
Step 2: nucleophilic attack (the finish)
A nucleophile attacks the positively charged intermediate.
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In HX addition, that nucleophile is X-.
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A new sigma bond forms, and the alkene becomes saturated at that site.
To drill the “intermediate controls product” idea, pair electrophilic addition with carbocation stability notes like relative stability of carbocations.
Markovnikov’s rule (and the real reason it works)
In IB Chemistry, Markovnikov’s rule is a prediction shortcut for adding HX to an unsymmetrical alkene:
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H adds to the carbon that already has more H atoms.
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X ends up on the more substituted carbon.
The deeper reason is carbocation stability:
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Tertiary carbocation > Secondary carbocation > Primary carbocation (most stable to least stable).
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More alkyl groups donate electron density (inductive effect), stabilizing the positive charge.
So when you’re asked to “explain the major product,” don’t just quote Markovnikov. Tie it to the more stable carbocation intermediate. That phrasing is pure IB Chemistry markscheme energy.
Bromine addition: the classic unsaturation test
A favorite IB Chemistry practical idea is bromine water as a test for unsaturation.
When an alkene reacts with Br2:
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The electron-rich double bond polarizes Br2 (even though Br2 is neutral).
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A bromonium ion (three-membered ring) can form temporarily.
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Br- attacks, giving a dibromoalkane.
And the observation: bromine water decolorizes as Br2 is consumed.
If you’re revising polymers at the same time, it helps to connect “double bonds react” to “double bonds link up.” RevisionDojo’s addition polymers notes make that leap feel natural within IB Chemistry.
Three exam mistakes that cost easy marks
These show up again and again in IB Chemistry scripts:
Mixing up who attacks first
Electrophilic addition starts with the electrophile. If your first arrow is from a nucleophile to the alkene, you’ve reversed the logic.
Treating “electrophile” as “must be positively charged”
Br2 is neutral, yet it acts electrophilic once polarized. The key is electron deficiency, not formal charge.
Forgetting the purpose of Markovnikov’s rule
It’s not a magic rule. It’s a quick way to pick the pathway that forms the most stable intermediate.
For practice under timed conditions, use the IB Chemistry Questionbank feature and filter by organic mechanisms. It’s the fastest way to make the steps automatic.
How to revise electrophilic addition with RevisionDojo
If you’re preparing for IB Chemistry exams, aim for repetition with feedback:
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Use the IB Chemistry resources hub to jump to the exact syllabus section.
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Do targeted drills in the Reactivity 3 questionbank.
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Rapid-fire definitions with the Reactivity 3 flashcards.
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When a mechanism still feels foggy, use RevisionDojo’s AI Chat and grading tools to check whether your arrows and explanation match what IB Chemistry examiners reward.
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Build stamina with Mock Exams and Predicted Papers, and patch weak spots with Study Notes and Tutors.
Closing: turn the mechanism into a story you can repeat
Electrophilic addition in IB Chemistry stops being intimidating when you treat it like a two-scene story: the electrophile arrives first, an intermediate forms, then the nucleophile finishes the job. Markovnikov’s rule is just the plot twist that rewards the most stable carbocation.
If you want this to feel automatic by exam day, revise it the way the best students do: short notes, lots of exam-style questions, fast feedback. RevisionDojo’s Questionbank, Flashcards, Study Notes, AI Chat, grading tools, Predicted Papers, Mock Exams, Coursework Library, and Tutors make IB Chemistry practice feel structured instead of endless.