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Electrophilic Addition & Markovnikov

A double bond is a cushion of loose electrons begging to grab something positive. Watch a pi bond reach out and seize an electrophile, then see why Markovnikov's rule is really a story about building the most stable carbocation.

Why a Double Bond Is Hungry

You already know an alkene from the previous rung: two carbons joined by a double bond, one sturdy sigma bond plus one softer pi bond sitting above and below the line of the carbons. That pi bond is the whole story here. Its two electrons are spread out in a loose cloud, far from the nuclei and weakly held — the highest-energy, most exposed electrons in the molecule. Where the alkanes of an earlier rung were sleepy and only reacted with violent radical conditions, an alkene is the opposite: it carries a pad of electron density on its surface, ready to reach out and donate.

That makes the alkene a nucleophile — literally a 'nucleus-lover,' an electron-rich species looking for something positive to bond to. And what it goes looking for is an electrophile, an 'electron-lover,' an electron-poor atom hungry for a pair. The signature reaction of alkenes, electrophilic addition, is just these two appetites meeting: the electron-rich pi bond reaches out and feeds its pair to an electron-poor partner, and in doing so the double bond becomes a single bond while two new groups bolt onto the two carbons. One pi bond in, two sigma bonds out.

The Cleanest Case: Adding HX

Let us make it concrete with the textbook example, hydrohalogenation — adding a hydrogen halide like HBr or HCl across the double bond. The H-Br bond is polarized: bromine is more electronegative, so it hogs the bonding electrons and leaves the hydrogen with a partial positive charge. That slightly positive proton is the electrophile the alkene wants. Bring HBr near ethene (CH2=CH2) and the pi cloud reaches up toward that hydrogen end. The overall result, with this symmetric alkene, is simply CH3CH2Br — H lands on one carbon, Br on the other.

The reaction happens in two steps, and the split is the key to everything that follows. In the slow first step the pi electrons reach over and grab the proton, forming a new C-H bond. But the pi bond only had one pair to give, so once it spends that pair on the hydrogen, the other carbon is left short two electrons — it becomes a carbocation, a carbon bearing a positive charge and only three bonds. Bromide, kicked loose as Br- when it took the H-Br electrons, then swoops in during the fast second step and bonds to that hungry positive carbon. The carbocation is the hinge the whole reaction turns on.

When the Alkene Is Lopsided: Markovnikov's Rule

Ethene is symmetric, so it does not matter which carbon gets the H. But take a lopsided alkene like propene, CH3-CH=CH2, and a real choice appears. The H could land on the end carbon (CH2) or on the middle carbon (CH). Those two choices give different products — 2-bromopropane versus 1-bromopropane — and experiment says one of them dominates overwhelmingly. Markovnikov's rule is the old shorthand for which: in adding HX to an alkene, the hydrogen goes to the carbon that already has more hydrogens, and the halogen goes to the carbon that has fewer. 'The rich get richer' — the H-poor carbon gets the X.

That rhyme will get you the right answer, but it is a symptom, not a cause — and memorizing it teaches you nothing. The honest statement of the rule is about the carbocation. When the proton adds in step one, it can produce either of two carbocations, and the reaction overwhelmingly takes the path through the more stable one. Carbocation stability follows a clear order: tertiary is more stable than secondary, which beats primary, which beats methyl, written 3 > 2 > 1 > methyl. So 'Markovnikov's rule' really means: the proton adds to whichever carbon leaves the positive charge on the more substituted, more stable carbon.

propene + HBr,  two possible first steps:

  (A) H to CH2 end    ->  CH3-CH(+)-CH3    secondary cation   FAVORED
  (B) H to CH middle  ->  CH3-CH2-CH2(+)   primary cation     not formed

cation stability:  3(deg) > 2(deg) > 1(deg) > methyl

result:  CH3-CHBr-CH3  (2-bromopropane), the Markovnikov product
Propene plus HBr: adding H to the end carbon gives a secondary carbocation (favored); adding it to the middle would give an unstable primary one. The stabler cation decides the product.

Why the More Substituted Carbocation Wins

So why is a tertiary carbocation steadier than a primary one? A positive carbon is electron-starved, and anything that shares electron density toward it soothes that hunger. Two effects do this, and both grow with the number of carbon groups attached. The first is the inductive effect: alkyl groups are mildly electron-donating compared to hydrogen, so each one nudges a little electron density toward the positive center. The second, and more important, is hyperconjugation: the electrons sitting in neighboring C-H (and C-C) bonds can lean into the empty p orbital on the cationic carbon, smearing the positive charge out over a wider region. More attached carbons means more of these neighboring bonds available to lean in and help.

There is a deeper reason the stability of the cation, an intermediate, controls the product at all. The first step is the slow, rate-determining one, and its transition state — the high point on the energy hill — looks a lot like the carbocation it is about to become. By the Hammond postulate, whatever stabilizes that cation also lowers the transition-state energy leading to it, so the lower hill is the faster path. The reaction does not 'know' the product in advance; it simply rolls down the easiest slope, and the easiest slope is the one with the more stable cation waiting at the bottom. That is the real engine under Markovnikov's rule.

The Mechanism, Step by Step

Let us walk the addition of HBr to propene with curved arrows, remembering the golden rule that each arrow tracks a moving pair of electrons, never an atom. Keep your eye on where the positive charge ends up; that single decision in step one fixes the whole product.

  1. The pi bond attacks the proton. A curved arrow goes from the C=C pi bond to the slightly positive H of H-Br, forming a new C-H bond. The H bonds to the end carbon (the one with more hydrogens) — this is the choice that builds the better cation.
  2. The H-Br bond breaks at the same time. A second arrow goes from the H-Br bond onto the bromine, which leaves with both electrons as a bromide ion, Br-. The proton arrived without its electrons, which is why it is the electrophile.
  3. A carbocation appears. The middle carbon, having spent its share of the pi electrons on the new C-H bond, is now left with only three bonds and a positive charge — here a secondary cation, CH3-CH(+)-CH3. This is the slow, rate-determining step.
  4. Bromide closes the deal. A final arrow goes from a lone pair on Br- to the positive carbon, forming the C-Br bond and quenching the charge. The product is 2-bromopropane — bromine on the more substituted carbon, exactly as Markovnikov predicts.

Notice what the mechanism bought you that the rhyme never could. It explains why bromide can attack either face of the flat carbocation, so a stereocenter formed this way comes out as a roughly even mix of both mirror images. It explains why rearrangements happen. And it tells you the rule's real limit: Markovnikov's regiochemistry follows whenever a discrete carbocation forms. The next guide shows the famous exception — radical addition of HBr with peroxides — where no carbocation forms at all, a carbon radical leads instead, and the regiochemistry flips to give the anti-Markovnikov product. Understand the mechanism and the exception stops being a paradox.

Reading the Rule Honestly

It is worth saying plainly what Markovnikov's rule is and is not. It is not a law of nature carved into alkenes; it is a reliable consequence of how carbocations rank in stability, dressed up as a memory rhyme in 1870 before anyone knew about carbocations at all. State it the mechanistic way — the proton adds so as to make the more stable carbocation — and it covers more cases, predicts the rearrangements, and even tells you when it will break. The 'more hydrogens get more hydrogen' version is fine as a quick check, but it is the shadow of the real idea, not the idea itself.

Two clarifications guard against common confusions. First, regioselectivity — which carbon gets which group — is the question Markovnikov answers; it is a separate axis from stereochemistry, which asks which face or which 3D arrangement, so do not conflate the two. Second, the rule is about electron-poor electrophiles adding to electron-rich alkenes; flip the electronics, as in the peroxide-driven radical case ahead, and you flip the outcome. Both HX addition and the upcoming acid-catalyzed hydration follow Markovnikov for the same single reason — a carbocation forms — which is exactly why grasping the cation once unlocks a whole shelf of reactions at the start of this rung.