You know that feeling when a reaction just clicks*? You draw the arrow, you see the pieces fall into place, and suddenly the whole mechanism makes sense. Here's the thing — they don't really explain what's happening at each stage, or why the reaction behaves the way it does. Practically speaking, most textbooks show you the curved arrows and call it a day. Electrophilic addition is one of those reactions that rewards you the moment you stop memorizing it and start understanding the why behind every step. That's what we're going to fix here.
What Is Electrophilic Addition
Electrophilic addition is a reaction where an alkene (or alkyne) reacts with an electrophile, and the pi bond breaks so that two new sigma bonds can form. The "electrophilic" part means the species attacking the alkene is electron-loving — it's positively charged or at least electron-deficient, and it gets drawn to the pi electrons sitting above and below the carbon-carbon double bond.
The "addition" part is straightforward: everything adds across the double bond. Now, no atoms leave. No byproducts get kicked out. Two groups land on the two carbons that used to share that pi bond.
This is the bread-and-butter reaction of alkenes. If you understand electrophilic addition, you've basically got the foundation for half of organic chemistry. Halogenation, hydrohalogenation, hydration, oxymercuration — they all fall under this umbrella.
Why This Mechanism Matters
Here's what trips people up. On top of that, they see the curved arrows in a textbook, copy them down, and move on. Why? But when the exam gives them a slightly different alkene or a different reagent, they're stuck. Because they memorized the picture* instead of understanding the story*.
Electrophilic addition reactions follow a general pattern. Once you see it, you can predict the products of reactions you haven't even encountered yet. On top of that, you stop needing to memorize every specific case. The energy changes, the intermediate carbocations, the regiochemistry, the stereochemistry — it all flows from a few core principles.
And this stuff isn't just textbook trivia. But it's how pharmaceuticals get synthesized, how polymers form, how petroleum gets refined. Understanding this mechanism gives you a window into the entire industrial chemistry that built the modern world.
The General Mechanism Step by Step
Every electrophilic addition follows the same three-stage rhythm: the electrophile approaches, an intermediate forms, and the nucleophile finishes the job. Let me walk you through each stage.
Step 1: The Electrophile Attacks the Pi Bond
The alkene is electron-rich. Which means that pi bond sitting between the two carbons is basically a cloud of negative charge just waiting to attack something electron-poor. When an electrophile (let's call it E⁺) drifts close, those pi electrons reach out and grab it.
This is where your first curved arrow lives. Consider this: you draw an arrow from the middle of the pi bond to the electrophile. One bond becomes two — the new sigma bond to the electrophile forms on one of the carbons, and the other carbon gets left holding the positive charge.
That carbon? It's now a carbocation. And carbocations are the central character in this whole story.
Step 2: The Carbocation Intermediate
The moment that carbocation forms, everything that happens next is driven by its need to find electrons. It's unstable. In real terms, it's positively charged. It's hungry for a nucleophile.
Now — and this is where Markovnikov's rule comes in — the carbocation that forms is the most stable* one possible. If the alkene is asymmetric, the proton (or other electrophile) adds to the carbon that leaves the more substituted carbon holding the positive charge. Why? So naturally, because tertiary carbocations are more stable than secondary, which are more stable than primary, which are way more stable than methyl. Hyperconjugation and inductive effects do the work here.
This preference for the more stable carbocation is the entire basis for Markovnikov's rule. It's not magic. It's just the reaction finding the lowest-energy path.
Step 3: The Nucleophile Attacks
Now the nucleophile (let's call it Nu⁻) steps in. Still, it sees that positively charged carbon and attacks it. You draw your second curved arrow — from the nucleophile's lone pair (or negative charge) to the carbocation.
A new sigma bond forms. That's why the positive charge disappears. And you have your final product: the alkene has been turned into a saturated (or partially saturated, in the case of alkynes) molecule with two new groups attached across where the double bond used to be.
That's the whole mechanism in three moves. Arrow, arrow, done.
Walking Through a Real Example: Hydrohalogenation
Let's make this concrete with the addition of HBr to propene. Propene is CH₃-CH=CH₂, and HBr is our hydrogen halide.
The Electrophile Gets to Work
The pi bond in propene attacks the hydrogen of HBr. Hydrogen is the electrophile here — it's partially positive because bromine is more electronegative and pulls electron density toward itself. One of the carbons grabs the hydrogen.
Two Possible Carbocations
Here's where Markovnikov matters. If the hydrogen adds to the terminal carbon (CH₂), the middle carbon becomes the carbocation — and that's a secondary carbocation. If the hydrogen adds to the middle carbon (CH), the terminal carbon becomes the carbocation — and that's a primary carbocation.
If you found this helpful, you might also enjoy the journal of physical chemistry b or when an atom gains an electron it becomes.
Secondary wins. So hydrogen goes to the terminal carbon, and the positive charge lands on the middle carbon.
Bromide Finishes the Job
The bromide ion (Br⁻) — now floating around after losing its hydrogen — attacks the carbocation. Which means it bonds to the middle carbon. The product is 2-bromopropane, the Markovnikov product.
Three arrows. Also, three steps. Day to day, one product. That's the whole dance.
Common Mistakes That Trip People Up
Drawing Both Arrows at Once
I see this constantly. Because of that, students try to draw the entire mechanism as one simultaneous event. But electrophilic addition is a stepwise* process. So the intermediate is real. It exists, even if only for a fraction of a microsecond. Show the intermediate. Give the carbocation its moment. The mechanism is the story of that intermediate, not a one-step magic trick.
Forgetting About Carbocation Rearrangements
Sometimes the carbocation that forms isn't the one that ends up in the product. In practice, hydride shifts and methyl shifts can happen to make a more stable carbocation before the nucleophile attacks. If you're getting the "wrong" product, look for a possible rearrangement. The reaction will always find the most stable carbocation available.
Confusing Markovnikov's Rule with Memorization
"Don't memorize the rule. Understand it.If you understand why the more substituted carbocation is more stable, you'll never need to memorize the rule. " Markovnikov's rule isn't a thing to remember — it's a consequence* of carbocation stability. You'll just know.
Ignoring Stereochemistry
When a new chiral center forms during the addition, you need to think about whether the product is racemic, whether it's syn or anti addition, and what the geometry of the intermediate allows. Think about it: many students draw the product without considering that the nucleophile could attack from either face of the carbocation. Sometimes both faces are equally accessible (giving a racemic mixture). Sometimes they're not.
Practical Tips That Actually Help
Draw every step. Every. Single. Step. The intermediate matters. If you skip it, you don't really understand the mechanism. You're just copying a picture.
Use a consistent arrow convention. Arrow from electron source to electron sink. Always. If you can't identify both ends of the arrow, you don't fully understand the step.
Practice with weird alkenes. Once you're comfortable with propene, try a cyclic alkene. Then try an alkene with an electron-withdrawing group nearby. Watch how the mechanism changes — or how it stays the same.
Predict before you draw. Before you put pencil to paper, ask yourself: where will the electrophile go? Which carbocation is more stable? Where will the nucleophile attack? If your prediction matches your drawing, you've understood it. If not, figure out why.
Learn to recognize the nucleophile and electrophile in disguise. Not every reagent is obviously an electrophile. In oxymercuration, the mercury ion is the electrophile. In acid-catalyzed hydration, the proton from H₃O⁺ is the electrophile. Train yourself to spot who's electron-poor and who's electron-rich.
FAQ
Does electrophilic addition work with alkynes too?
Yes, but it stops at the alkene stage under normal conditions. Alkynes have two pi bonds, so they can undergo addition twice — but controlling the
reaction to stop at the alkene is possible, and the vinyl cation intermediate is generally less stable than a regular carbocation. With excess reagent and harsher conditions, you can push it all the way to a tetrasubstituted carbon.
What makes a carbocation more stable?
Three main factors: hyperconjugation (more alkyl groups = more C–H bonds adjacent to share electrons), inductive donation (alkyl groups are slightly electron-donating), and resonance (allylic and benzylic carbocations are exceptionally stable). Tertiary > secondary > primary > methyl, with resonance-stabilized cations breaking the pattern.
Can a carbocation be too stable?
In a sense, yes. If the carbocation is so stable that the reaction coordinate changes — for example, if a rearrangement happens instead of nucleophilic attack — you'll get unexpected products. But the carbocation itself isn't "too stable" to form; it's just that the system finds a different path forward.
Why does the nucleophile attack the carbocation instead of the double bond?
Because by the time the nucleophile shows up, the double bond is already gone. The nucleophile has no choice but to react with whatever electrophilic center exists. The first step — electrophile addition — broke the pi bond and left a carbocation behind. The pi bond is history.
Is electrophilic addition reversible?
Some steps can be, yes. That's why the deprotonation step that forms the final product is often irreversible because you're forming a strong C–H bond and a stable neutral molecule. But the initial carbocation formation can be reversible under certain conditions, which is why temperature and solvent matter.
